6. Communications

Software Lab for Communications Creating a Link Budget

COMMS Link Budget

Purpose

  • Understand the role of the telecommunications subsystem in the context of spacecraft as a whole and between other subsystems
  • Calculate the equations of transmission and reception as a function of spacecraft parameters
  • Review different modulation schemes, technologies, and constraints
  • Produce a link budget

Background and Key Concepts to Consider

Application of: 6.7 Communication System Link Budget, 6.9 Communications Analysis, and Link Budget

Artemis CubeSat Kit Specific

The Artemis CubeSat Kit Link Budget may offer you a template to begin with.

Artemis CubeSat Kit description of the communication system:

  • Orbit: Low Earth Orbit (LEO)
  • Spacecraft Receiver: The COMMS subsystem in this kit consists of a long-range radio transceiver (RFM23BW) and an antenna.
  • Ground Station: Kauaʻi Community College (KCC) Ground Station

Links to the Artemis CubeSat Kit Github: https://github.com/hsfl/artemis

Required Materials & Setup

  • Reference Artemis CubeSat Link Budget in Google Sheets/Excel, AMSAT / IARU Annotated Link Model System (Ver 2.5.3)
  • Specifications for Communication System (Section 6.9)
    • Latitude and longitude of the uplink and downlink earth stations.
    • Planned data or information rate.
    • Modulation type (BPSK or QPSK)
    • Forward error correction rate (1/2 or 3/4)
    • Spread Factor – if any (use only for spread spectrum systems)
    • Uplink and Downlink frequencies.
    • Uplink and Downlink antenna sizes.
    • Uplink and Downlink antenna efficiency.
    • Uplink and Downlink transmit and receive gains at frequency.
    • Minimum digital signal strength (EB/No) for desired Bit Error Rate (BER) performance.

Procedure

Preliminary Lab Procedures

Reference the Artemis Link Budget

The Artemis link budget will be used as a reference for this lab. Make your own copy of the Artemis link budget. Find the link to the Hiapo link budget here. With the budget open in Google Sheets, click on “File” and either “Make a Copy” or “Download” as a Microsoft Excel spreadsheet (.xlsx). As needed, modify the spreadsheet accordingly to the spacecraft of interest or budgeting. Note that there are several sheets in the spreadsheet. Sheets are numbered according to the suggested order to follow/view the budget and analysis.

Note: Depending on the readability and functionality of the spreadsheet, Excel may be preferred to using Google Sheets. Instructions in the spreadsheet were originally written for users viewing the file in Excel.

Description of Link Budget Template

“This spreadsheet system is an attempt to provide a new kind of learning tool.  It is intended, clearly, to be a working link model in order to allow satellite system designers to design and then document fully the RF radio links associated with Command (uplink)  and Telemetry (downlink) equipment.  It is, however, also intended to be a tutorial on the RF portion of a satellite system.  The model makes liberal use of “pop-up” notes and “tools” to enhance the understanding (and hopefully the knowledge) of the Link Model Operator (that’s you).  After you use the model for a while, let me know if I have been successful. – Jan A. King, W3GEY and VK4GEY; w3gey@amsat.org”

Link Budget Spreadsheet Organization

The sample spreadsheet provided has been created to help guide users through using the file. Pay attention to the provided notes and instructions per page, especially those in the introductory sheets!

Note: Names and colors correlate to those as created in the original/sample spreadsheet. Categories were created to help identify the kinds of information and data within the entire spreadsheet file. (Add brief description or parameters of each sheet under the bullet points. May need to move information into Main Lab Procedure after skimming sheets/list.)

Introduction & Instructions

  • Title Page
    • Formal documentation for spreadsheet, some instructions
  • I.I.R.R. (Introduction, Instructions for Use, References, Revisions)
    • Detailed instructions on how to use the spreadsheet and the color key on how to read the spreadsheet.
    • These instructions are most relevant for Excel users (not Google Sheets, as some features work slightly differently)

Environmental & Data Parameters

  • Orbit (System Orbit Characteristics)
    • Orbit options: (1) LEO, (2) HEO, (3) GEO, (4) Deep Space
    • Scroll down to the option to use.
  • Frequency (UPLINK & DOWNLINK Frequency Choices)
    • Uplink and downlink selections

Ground Station & Antenna Hardware, Gains & Losses

  • Transmitters (System Transmitters & Line Losses)
    • Uplink Transmitter System (At Ground Station)
    • Downlink Transmitter System (At Spacecraft)
    • Transmitter power, lines, line lengths, and losses
  • Receivers (System Receivers and Line Losses)
    • Uplink Receiver System (At Spacecraft)
    • Downlink Receiver System (At Ground Station)
    • System noise temperature, lines, line lengths, gains, and losses
  • Antenna Gain (System Antenna Gains (Directivities))
    • Antenna gain, directivity, and beam width
    • Considers uplink and downlink
    • Select type of antenna for ground station and spacecraft
      • From a list of options, pick the best match
        • Options 1 – 4 for ground station
        • Options 1 – 7 for spacecraft
        • User-defined – last options in the list
  • Antenna Pointing Losses (System Antenna Pointing Losses)
    • a.k.a Antenna’s Loss in Gain, or “roll-off”
    • How gain or directivity of an antenna changes the further away from the direction of peak gain
    • The function of user input for antenna pointing losses, angle, pointing error
    • Pay attention to figures in the first row (continues to the right)
  • Antenna Polarization Loss (System Polarization Loss and Cross Polarization Isolation)
    • Polarization properties of the ground station and spacecraft antenna
  • Atmos. & Ionos. Losses (Atmospheric and Ionospheric Losses)
  • Modulation-Demodulation Method

Final Results: Uplink & Downlink Budget, Summary

  • Uplink Budget (Command)
  • Downlink Budget (Telemetry)
  • System Performance Summary

Additional Tools

  • Antenna Patterns (Commonly Used Spacecraft Antenna Radiation Patterns)
  • Beam Roll-Off Tool
  • Beam Roll-Off Plot
  • Line Loss Tools & Tables (Transmission Line Loss Tools and Tables)
  • VSWR Loss Tool (Losses Resulting from  Antenna Mismatch – Measured Using Voltage Standing Wave Ratio (VSWR) Method)
  • GEO Azimuth Calc Data
  • Orbit Shape Data

“The notes within the spreadsheet are crucial for using it as a tool. It is important to note that interconnected equations in one worksheet (W/S) may refer forward or back to data located in other worksheets. Loss of this connection could be critical.”

The users will:

  • Begin using the spreadsheet from ‘Title Page’ and ‘I.I.R.R.’ (under Introduction & Instructions).
  • Proceed through each Speciality W/S (under Environmental & Data Parameters, and Ground Station & Antenna Hardware, Gains & Losses), adding data, in sequence. Then select the next tab at the bottom of the W/S.
  • The final results of the model are in the “Uplink”, “Downlink” and “System Performance Summary” sheets.
  • The Tools W/Ss is located beyond the “System Performance Summary” W/S and may be explored and used as they may be helpful to you.

All following sheets after the Introduction & Instructions will be detailed more in the Main Lab Procedures section. Full notes are copied in the References and Other Work section of the lab, to help with the readability of the notes left in the spreadsheet.

Sample Title Page for the Artemis CubeSat Link Budget

Title Page

Review and update information about your satellite program and spacecraft. Use Notes #1 – 8 to help update the information in the cells.

The following lists or summarizes some notes on the Title Page, explaining part of how to use the spreadsheet. The full list of notes is also listed in the References section.

Note #1
    • Use the title page for formal documentation for your satellite program
      • Especially important for tracking changes and versions of generated/modified spreadsheets
    • NONE of the cells are protected – be careful in modifying!
    • Keep a master copy of the spreadsheet as a backup/for creating new versions
    • Color key
  • RED text with a YELLOW background
        • Indicates notes, results, and titles
        • Do not modify
        • The exception to modifying:
          • “University name” and “Project name”
          • On Title sheet ONLY, when filling out sheet for the first time (copies across all sheets)
  • BLACK text
        • Indicates labels or contains formulas
        • Do not modify
  • BLUE text or BLUE text with BLUE background
        • User-input
        • Modify these cells!

Note: This simplified color key is generally applicable throughout! As the user, look out for BLUE cells to edit. More colors are explained in the I.I.R.R. sheet.

Review the I.I.R.R. sheet

(a.k.a. Introduction, Instructions for Use, References, Revisions)

Thoroughly read through the I.I.R.R. sheet for in-depth instructions and the cells color key used throughout the entire spreadsheet. This sheet contains important information!

Note: Remember this information while using the spreadsheet and refer back to it as needed. The colors explained here are used consistently throughout.

Main Lab Procedure

List each cell to modify (blue text or cell background) in the bullet point list per sheet. Screenshot a sample of each sheet for identification.

Environmental & Data Parameters

Orbit
  • Select the orbit in C4: LEO (1), HEO (2), GEO (3), Deep Space (4)
  • Fill in the Applicable Option.
    • Option 1, LEO is the most common orbit for cubesats. Typically, the launch provider provides this information, however, if the payload needs to be in a very specific orbit, filling in this information is very important to be able to determine if your link budget closes
    • Watch this video on Classical Orbital Elements if you want a better understanding of each of these values
  • Earth Radius is preset to the equatorial value
  • Height of Apogee: The point of the orbit at which the satellite is farthest away from Earth
  • Height of Perigee:  The point of the orbit at which the satellite is closest to Earth
  • Inclination: Tilt of the orbital plane with respect to the fundamental plane
  • Argument of Perigee: Indicates orientation of the orbit; where perigee is located
  • Right Ascension of the Ascending Node (RAAN): longitude of the point where the spacecraft crosses the equatorial plane moving from south to north
  • Mean Anomaly: fraction of a elliptical orbit’s period that has elapsed since the orbiting body passed periapsis (point closest to the attracting body)
  • Elevation Angle: the angle above the local horizon at which the satellite is visible and within the interval of 0-90 degrees, regardless of azimuth. Why was 5 degrees selected?
  • “At low elevation angles the target is viewed through the Earthʻs outer atmosphere which absorbs and/or scatters X-rays and hence distorts the spectrum… We have found that data quality degrades with elevation angles less than 5 degrees.” –NASA
Frequency

Select your desired or assigned frequency. Option 4 allows for the user to input their own frequency

Ground Station & Hardware, Gains & Losses

Transmitters/ Receivers
  • These sheets helps to find the line losses on the ground station and spacecraft transmitter systems
  • The Ground Station information comes from the KCC ground station submitted to FCC for the Neutron-1 Mission

    • N1 mission used VHF uplink, UHF Downlink, hence the difference in transmitter power (Max at 100W for VHF frequency, Max 75W for UHF frequency)
    • 0.04 dB cable loss/meter un-verified
    • Filter insertion losses actually 0.04, but needs a decimal place to move one to the right for it to be visible. Actual number is 0.035dB
  • Ground Station transmitter information
  • Satellite transmitter information
Antenna Gain
Antenna Pointing Losses
  • 5 degrees was selected for the Ground Station system. By tracking other signals using RSSI (Radio Signal Strength Indicator) values to calibrate the ground station, pointing errors are usually close to 0 degrees. To add in the margin for rotator error and/or shaking while tracking a satellite, we use 5 degrees to be safe.
  • 20 degrees was selected for the spacecraft. As the satellite does not have pointing control, a larger margin is necessary. Pointing error is usually the same order of magnitude as the Instantaneous Field of View (IFOV) in Earth Observation satellites.

To properly calculate the error, please see this extensive document from ESA: http://peet.estec.esa.int/files/ESSB-HB-E-003-Issue1(19July2011).pdf

Antenna Polarization Loss
  • Axial Ratio (AR) is important in this sheet. Here is some information from Everything RF, the “Axial Ratio (AR) of an antenna is defined as the ratio between the major and minor axis of a circularly polarized antenna pattern. If an antenna has perfect circular polarization then this ratio would be 1 (0dB).” If the polarization is elliptical, then the ratio would be greater than 1 (>0 dB). “The axial ratio for pure linear polarization is infinite because the orthogonal components of the field are zero.”
  • “Maximum signal coupling between stations occurs when both antennas are using the same polarization. In a linearly polarized system, a polarization misalignment of 45 degrees will degrade the signal up to 3 dB. Polarization misalignment near 90 degrees can result in signal degradation greater than 20 dB.” [source]
  • Antenna #1 is the Ground Station, a right-hand circularly polarized antenna. Taking a look at the datasheet for the antenna, 1.5 dB is given as the Ellipticity.
  • Antenna #2 is the satellite, a linear antenna. Since AR for linear polarization is infinity, it is set at 500dB here; for context, the loudest sound possible in the air is about 195 dB [source]
  • Polarization Angle between antennas is set to 90 degrees in the example because it is the maximum misalignment, which results in the maximum polarization loss and Cross Polarization Isolation
  • Downlink reverses Antenna #1 and Antenna #2
Atmos. & Ionos. Losses (Atmospheric and Ionospheric Losses)
  • Minimum Elevation Angle is set to 5 degrees for the same reason 5 degrees is used in the orbit tab. Data quality degrades when the elevation angles are less than 5 degrees.
  • Up/Downlink Loss due to Ionosphere: 0.4 dB was selected because the Artemis kit uses a UHF transceiver, which runs on frequencies closest to 438MHz on the table
Modulation-Demodulation Method
  • Useful link that describes the different modulation/demod schemes [MWRF]
  • The modulation scheme is typically determined by the radio hardware that is selected. It is important to ensure that the transmitter and receiver have compatible modulation

Final Results: Uplink & Downlink, Summary

Uplink Budget
  • The System Link Margin Cell indicates the Ground Station’s ability to transmit a powerful enough signal to the Spacecraft and the Spacecraft’s ability to perceive that signal. Since the parameters are either not well known or fluctuate which accumulates error in our link budget analysis, we want to see the System Link Margin to be above 5 dB to accommodate sources of error or fluctuation.
Downlink Budget
  • The System Link Margin Cell indicates the Spacecraft’s ability to transmit a powerful enough signal to the Ground Station and the Ground Station’s ability to perceive that signal. Since the parameters are either not well known or fluctuate which accumulates error in our link budget analysis, we want to see the System Link Margin to be above 5 dB to accommodate sources of error or fluctuation.
System Performance Summary
  • The outputs of each tab of the spreadsheet are summarized and visualized in this tab. This visualization enables you to see the accumulation of losses and gains along each node of your communication link.

Additional Tools

Antenna Patterns
  • …
Beam Roll-Off Tool
  • …
Beam Roll-Off Plot
  • …
Line Loss Tools & Tables
  • …
VSWR Loss Tool
  • …
GEO Azimuth Calc Data
  • …
Orbit Shape Data
  • …

Test Data Referenced for Link Budget

Clean Up

  • Be sure to save the file and pay attention to file versions and names.
  • If other CASES or TESTS are to be used in the spreadsheet tool, create and rename new versions. Use names that will be descriptive enough for future reference.

Lab Review and Deliverables

Goal(s):

  • Modify the Artemis Link Budget starting with information on the spacecraft and the communication system. Work through the budget to integrate the changes.
  • Calculate the equations of transmission and reception as a function of spacecraft parameters.
  • Produce a link budget with up-to-date information on your spacecraft.

Artemis Communication Requirements

The CubeSat Design Specification Rev. 14 explicitly states in their operational specification:

  • 2.4.1 Operators shall obtain and provide documentation of proper licenses for use of radio frequencies.
  • 2.4.1.1 Note: For amateur frequency use, this requires proof of frequency coordination by the IARU. Applications can be found at www.iaru.org.
  • 2.4.2 CubeSats shall comply with their country’s radio license agreements and restrictions.
  • 2.4.2.1 Note: CubeSat operators should refer to the International Telecommunication Union (ITU) to determine what licenses and approvals are needed for their country

3.5 The CubeSat communications system shall transmit telemetry from LEO

  • 3.5.1 The radio shall transmit detectable telemetry in amateur radio frequency (UHF)
  • 3.5.2 The ground stations shall receive UHF and process true telemetry
  • 3.5.3 The link budget shall have a margin of at least 5 dB

Notes: Title Page

Note #1

  • It is intended that this Link Model can be used as a formal part of the documentation of your satellite program.  The “Title Page” Worksheet (W/S)  of this Excel Workbook is the means by which this is carried out.  If you are using the model in this capacity it becomes important to be able to track changes.  You will find that many versions of the model will be generated as new designs arise and old ones are modified.  This W/S is intended to contain sufficient information to allow the project personnel to clearly identify which version of the workbook you are using at the time.  Of course, self-discipline is always important in documenting software.
  • First, it should be noted that NONE of the cells in this workbook are protected.  So, anything can be changed (or unintentionally erased).  So, the very first thing you should do is make several copies of this model so that there is always a master copy available.  That version should not be changed from the state in which you receive it.  Only use it to make more copies.  The development of this system uses a few basic principles involving the use of color.  These will be explained in detail in the next W/S.  The basics, however, need to be explained in this note.
  • 1) RED text on a YELLOW background is intended to draw your attention to important cells -usually results and titles.  They are not to be modified, except on this sheet, University name and Project name should be modified the first time you use them.  This need only be done on this W/S as these cells will be copied, as appropriate, to the other W/S of the model.  If your project is not associated with a university then, of course, you can simply modify the header of this W/S accordingly BUT, don’t change the cell numbers into which you place your data or the names will not be propagated into the rest of the model correctly.
  • 2) BLACK text should never be modified.  Black text is either used for labels or contains formulas.  Modification of black text will result in the loss of the formula once you hit “Enter.”   That’s not good.
  • 3) Cells with BLUE text are intended for link model operator (that’s you) modification.  If the cell has a blue background, that means it is critical that you enter the correct data for your spacecraft or ground station into that cell.

Note #2

  • Communication System Engineer: This cell should be modified to incorporate the name of the person acting as the communications engineer for your project.
  • Project Manager: This cell should be modified to incorporate the head of the project and the person who will have final authority for the validity of all documents associated with your program.

Note #3

  • Orbit Type: Modify this area to include a very brief but, accurate description of your orbit on just the first line of this box. If the orbit changes you should modify this cell. Also, note that the date in Cell [F13] is not linked to any of the data in the “Orbit & Frequency” W/S so if you change data in that W/S you should change this summary as well. That must be done manually. Cell [F14] is linked to the “Orbit & Frequency” W/S and provides feedback as to whether an LEO, HEO, or GEO orbit has been selected. The choice is made in the “Orbit & Frequency” W/S at Cell [C3].

Note #4

  • Model Under Investigation: In this cell, you should describe the basic characteristics of the spacecraft, its properties, or its frequency that are unique to this particular CASE you are evaluating. Update this cell every time you want to run a new CASE but be careful not to save this on top of the filename you used for the old case.

Note #5

  • Model/ Case No., Rev No.: You are encouraged here to invent a file naming/numbering system that is appropriate for your project. Hopefully, this model will be one of the documents you will use. It should be given a document number consistent with that system and with your project drawing tree. IMPORTANT: Somewhere within this name should be the exact filename which you will use when you SAVE this particular version of the link model. So before you hit the “SAVE” or “SAVE AS” key the last time, make sure you have modified this cell to include that same name.
  • If your project has a documentation specialist or someone responsible for project configuration and control, that person should complete this box. If the link model has entered a formal phase then, once the model is deemed correct and accurate, that person should also check the box at Cell [I20].

Note #6

  • Date Last Modified: Update this date whenever you SAVE the W/S if you have modified any of the BLUE cells.
  • Date W/S Formulas Last Modified: If you choose to modify any of the formulas to improve upon this link model then modify the date in this cell consistent with the date of the change. This will keep you from confusing old and new versions of the link model. Clearly, once the project is under “formal” configuration control (e.g., after CDR) the formulas should not be changed.

Note #7

  • Approvals: If the project is under formal control and this document is used within your documentation system, then the approval boxes below should be used. In the approval process, when appropriate, each responsible project person should review this workbook and then approve it by placing a capital “X” in the cell associated with their function. Upon “ENTER” the cell will change from RED to GREEN. Once all approvals have been granted, the “document released” indicator will be enabled.

Note #8

  • After entering all data, including Approvals, proceed to the “I.I.R.R.” W/S and then the “Orbit & Frequency” W/S.

Notes: Orbit

Note B14

LEO Orbit – Option #1

  • Option #1 is intended for LEO Orbits.  You may find some of the calculations carried out here to be useful for orbit as well as link analysis.  The link model operator should enter the critical values associated with the apogee and perigee height of the orbit.  From that data, the worksheet calculates the semi-major axis and the eccentricity of the orbit, which are proper Keplerian elements.  The operator may also wish to enter the orbit inclination, Argument of Perigee, Right Ascension of Ascending Node, and Mean Anomaly values.  Given this information, the worksheet will determine the period of the orbit and the first derivatives for the AoP and RAAN.  These will determine how the orbit will propagate over time.  Another useful parameter is also calculated by the worksheet.  Assuming only the orbit inclination can be changed, the worksheet determines the correct value of inclination for a sun-synchronous orbit, given the other orbital elements provided.
  • Most importantly for the link analysis, the link model operator must enter the minimum acceptable elevation angle, found to be suitable at the ground station site.  Then, the maximum slant range to the spacecraft will be calculated.  This is the range used by the link model in determining path loss.  In addition, the elevation angle is used in subsequent worksheets to estimate the atmospheric losses.   In fact, any elevation angle can be entered and the corresponding slant range to the satellite is calculated.  This will allow an investigation of link performance as a function of elevation angle or slant range.
  • The figure is provided to help the operator envision the geometry associated with the link.
  • Now that the orbital properties have been selected, move to the “Frequency” W/S to the “Uplink & Downlink Frequency Choices.”

Note C38

High Earth Orbit (HEO) – Option #2

  • Option #2 is intended for High Earth Orbits using elliptical orbits.  The link model operator selects …

Note K68

EARTH ANGULAR DIAMETER ()

  • This is the Earth’s diameter (not counting the atmosphere) as seen from the spacecraft.

Note K69

S/C POINTING VECTOR ()

  • Note that we depict here only the in-orbit plane component.  There is, generally, an out-of-plane component as well.  The pointing vector shown is also to the center of the Earth.  The link model operator may wish to calculate the pointing vector from the antenna boresight axis to the ground station location.
  • To assist in completing this calculation, see Ref. 7 (I.I.R.R. W/S), pages 63-71.

Note K70

WORST CASE SQUINT ANGLE

  • This is the angle from the symmetry axis of the spacecraft antenna to the furthest point away from the spacecraft but still on the Earth’s surface.

Note K71

RX ANTENNA POINTING LOSS

  • The link model operator must calculate and verify that the antenna pointing error is consistent with the S/C pointing vector given in Cell [K69] and takes into consideration the location of the ground station on the Earth.  This is a 3D math problem.  Once this overall angle is determined it may be used in the antenna pointing losses W/S to determine the RX antenna pointing loss.  That will automatically be transferred to this sheet and to the Uplink and Downlink Budget W/Ss.

Note K71

TX ANTENNA POINTING LOSS

  • The link model operator must calculate and verify that the antenna pointing error is consistent with the S/C pointing vector given in Cell [K69] and takes into consideration the location of the ground station on the Earth.  This is a 3D math problem.  Once this overall angle is determined it may be used in the antenna pointing losses W/S to determine the RX antenna pointing loss.  That will automatically be transferred to this sheet and to the Uplink and Downlink Budget W/Ss.

Note D85

Geostationary Earth Orbit (GEO) – Option #3

  • The GEO Orbit Option W/S allows the link model operator to select a GEO Orbit Slot and the location of two satellite users (one for the uplink and one for the downlink).  The W/S calculates the slant range to each user as well as the azimuth and elevation bearing to the satellite from each user.  As an additional output, the Earth central angle from the sub-satellite point to the user location is also provided. The slant range results are forwarded to the frequency W/S for computation of the link path loss.

Note B102

Spacecraft Slot (Longitude)

  • Spacecraft Slot Latitude is 0° by Definition.

Note O96

Downlink Note

  • This data entry allows a downlink to the same user or to a different user location.

Note G115

Deep Space Mission – Option #4

  • The link model operator enters the mission target object and the range to the deep space spacecraft in astronomical units (AU).  No orbital mechanics calculation is carried out.  The computed range (in kilometers) will be used for path loss calculations in the next W/S.
  • After the orbit option has been selected move on to the “Frequency” W/S.

Note F121

Current Range to S/C

  • This value should be estimated or calculated from other available resource data.

Notes: Frequency

Note B7

  • The link model operator enters the uplink and downlink frequency selection at Cell [L10] and Cell [L16] respectively.  If options 1,2 or 3 are not desired, an operator-defined choice is provided at Cell [C13] and Cell [19].  The path loss for each choice is given in column G.  The data from this W/S is forwarded to other subsequent sheets.
  • After the frequencies have been selected move on to the “Transmitters” W/S.

Notes: Transmitters

Note B6

  • This W/S is used to evaluate the losses associated with the ground station and spacecraft transmitter systems.
  • The operator first enters the transmitter power output in watts.  The W/S provides the power converted to dBm and dBW.  The subsequent W/Ss will use dBW.
  • The operator must then estimate the line lengths for all of the cables in series between the transmitter and the antenna.  Line losses can also be determined by using the “Line Loss Toos & Tables” W/S near the end of this workbook.  The operator then must enter the estimated insertion losses for any filters, directional couplers, hybrids or other devices used in-line between the transmitter and the antenna.
  • The W/S calculates the total line losses and the power that is actually delivered to the antennas.
  • Once you know the losses of your transmitter systems and the power delivered to the antennas move on to the “Receivers” W/S.

Notes: Receivers

Note C6

  • This W/S is used to evaluate the losses, noise temperatures, and overall performance of the spacecraft and ground station receivers.
  • The analysis begins by explaining how the total noise temperature of the receiver system is calculated from the individual temperatures and losses that are known.  Just as with the transmitter system, the individual losses of all lines and in-line components must be known, determined, or estimated.  The W/S will walk the operator through each entry that is required.  Cable losses may also be determined by using the “Line Loss Tool & Tables” W/S located near the end of the workbook.
  • There are two separate work areas here.  One is for the spacecraft command receiver and the second is for the ground station telemetry receiver.
  • The “bottom line” of this W/S is the system noise temperature for each receiver.
  • Two tools are also provided in this W/S.  The first allows you to translate from Noise Figure to Noise Temperature or the reverse.  The second is a tool to be used for estimating the sky temperature of a ground receiver.
  • Once you have determined the noise temperature for your receivers move on to the “Antenna Gain” W/S.

Note I28

GLNA = The gain of the LNA in linear (non-dB) units

  • In the equation given here, the gain of the LNA should be expressed in its “unlogged” form (e.g. an amplifier with a gain of 20 dB has a linear gain of 100).  If you know the gain of the amplifier in dB then to get the linear gain:  G=10^((Gain in dB)/10).  This worksheet allows you to enter the gain of the LNA in dB and it calculates the linear gain for you.
  • NOTE:  A classic error made by beginners is to assume that the system performance is increased directly by the gain of the low noise amplifier.  This is incorrect.  Fundamentally, the noise produced by the amplifier can be thought of as being ahead of the gain.  Therefore, the gain of the amplifier increases both the signal and the noise proportionately.  There is a small improvement in overall performance (S/N or Eb/No) given by the gain of the LNA, as can be seen in the last term of the above equation.  But, the amount of this advantage is also dependent upon the noise temperature of the 2nd stage.  For a modern system, the net improvement for, say, an X10 gain increase (10 dB) is usually only a few degrees K, if that.  This is an undetectable improvement.  See for yourself by trying examples using this worksheet.

Note G57

Antenna or “Sky” Temperature

  • The Sky Temperature as seen by a spacecraft must be viewed from its unique perspective.  The antenna at the spacecraft “sees” within its beamwidth (or its “field-of-view”) two possible components    (2020-04-17 11:36:32)
    • The sky itself which is nominally at 2.7 K but, at frequencies below 2 GHz also includes galactic noise (see note at Cell [G128]) which explains how much higher values can occur).
    • The  Earth.  The average Earth temperature used is 290K, however, the Earth may be “warmer” due to man-made noise sources that can be distributed on the surface of the planet.  This is particularly true at lower frequencies in the VHF-UHF range.  The actual value that should be used for the Earth’s temperature is not well understood but, it certainly varies with location and time.
  • There is a math exercise to be accomplished here.  The spacecraft’s Sky Temperature value must be computed as follows.  Determine the fraction of the antenna’s beamwidth that is filled by the Earth.  This fraction of the field-of-view is given a value of (at least) 290K.  Then determine the fractional remainder of the antenna field-of-view.  It will see the actual sky, which is either taken to be 2.7K at frequencies above 2 GHz or some higher value due to Galactic noise at lower frequencies.  The sky temperature will be the weighted average of these two noise components. [For example, if 25% of a particular spacecraft antenna’s field-of-view included the Earth (at 290K) and 75% of the antenna’s field-of-view saw cold sky (at 2.7K) then the Sky Temperature for the spacecraft would be .25(290K)+.75(2.7K) = 74.5 K].  To do this properly, of course, involves Integral Calculus, taking into account the antenna pattern roll-off characteristics and the variations in antenna temperature over the sky within the field of view.  Typically, however, simpler mathematical estimates, as given above, are used.
  • A special case that arises frequently is that of a geostationary satellite using a spot beam antenna that has 100% of its field-of-view filled by the Earth’s surface.  In this case, the first order Sky Temperature value is simply 290K.  However, when the spot beam gets small enough that it illuminates “features” of the Earth’s surface, then determining the local physical temperature of the Earth could have some meaning.  This fine structure exercise, however, is likely not to be productive.  Just use a value of 290K.

Note U56/V52

TO

  • The performance of a low noise amplifier (LNA) [also called a preamplifier] is often expressed in two forms: Its noise figure (NF) in dB or its noise temperature in °K.  The translation between the two depends upon the reference temperature of the system (To).  To can be selected in Cell [J59].  For convenience, it is re-displayed at Cell [U56].
  • This calculator allows you to translate from one parameter to the other, depending upon which parameter is specified to you.

Note I99

GLNA = The gain of the LNA in linear (non-dB) units

  • In the equation used, the gain of the LNA should be expressed in its “unlogged” form (e.g. an amplifier with a gain of 20 dB has a linear gain of 100).  If you know the gain of the amplifier in dB then to get the linear gain:  G=10^((Gain in dB)/10).  This worksheet allows you to enter the gain of the LNA in dB and it calculates the linear gain for you.
  • NOTE:  A classical error made by beginners is to assume that the system performance is increased directly by the gain of the low noise amplifier.  This is incorrect.  There is a small improvement in overall performance given by the gain of the LNA as can be seen in the last term of the above equation.  But, the amount of this advantage is also dependent upon the noise temperature of the 2nd stage.  For a modern system, the net improvement for, say, an X10 gain increase is usually only a few degrees K, if that.  This is an undetectable improvement.

Note H107

Cable or Waveguide “Line” Losses

  • The greatest improvement in downlink system performance can be made at this location within the receiver chain.  The line losses between the antenna and the Preamplifier (LNA) should be minimized by locating the LNA directly at the feed point of the antenna, if at all possible.  Any cable used should be of high quality and the VSWR of the antenna should be as close to 1:1 as possible.
  • It is, however, highly desirable to also include a bandpass filter just in front of the LNA.  This reduces out-of-band signal interference which can otherwise dense the LNA.  The filter, however, also has an in-band insertion loss which must be accounted for in the losses between the antenna and the LNA.

Note G128

Antenna or “Sky” Temperature

  • For a ground station antenna, the Sky Temperature value must include not only the noise intercepted by the ground station antenna coming from the colder sky into which the antenna is looking but, it has to include any terrestrially generated noise that may be generated in the proximity of the station.  This noise most likely enters the system via the sidelobes of the ground station antenna.  This condition is worse when the ground station antenna is at low elevation angles and pointed in the direction of the source of the noise.  Under these conditions, the full gain of the antenna “sees” the noise source at maximum “temperature.”  On campuses around the world these days, the largest source of “sky noise” is generated by the sum of all computers that are within the “radio range” of the ground station.  Finding and eliminating these sources of noise is a major component of the ground station design process.
  • At VHF frequencies (and to a lesser extent, at UHF frequencies) galactic noise can be observed even with small antennas.  The sky brightness is highest in directions that intercept the disk (or plane) of the Milky Way.  At 146 MHz this value can be as high as 1700K and as low as 80K.  For more information regarding this source of noise see Ippolito, Louis J., “Radio Propagation in Satellite Communications,” Van Norstrand Reinhold, pp. 136-138.

Note G136

Cable/Waveguide D Length

  • This is the cable run from the output of the Preamplifier (LNA), down the tower, to the ground station and terminating at the input to the Communications Receiver.  This cable has a moderate impact on the overall noise temperature of the system and should be a high-quality, low-loss cable.  The loss of this cable is modeled as a reduction in the gain of the preamplifier.  This loss and the noise temperature of the Communications Receiver have a small but, measurable impact on the system noise temperature.  The temperature effects at this location in the receiver chain may be improved by 1) increasing the gain of the LNA, 2) reducing the feedline loss, or 3) improving the noise figure or temperature of the Communications Receiver’s first stage of gain.

Note P135

Estimated or Measured Noise Level

  • Using a spectrum analyzer or the receiver’s signal strength meter, try to determine the noise power from the terrestrial source generating the noise.  The receiver’s “white noise” floor, set by its own LNA is likely to be in the range of -130 to -140 dBm in a 10 kHz bandwidth.  This is the noise you would “hear” with no interference and listening to the receiver speaker or on headphones.  You can determine this level by removing the antenna from the LNA input and replacing it with a 50-ohm dummy load (resistor).  You will only be able to measure the terrestrial noise source if the terrestrial noise source power is considerably above the white noise floor set by the receiver’s LNA.  If the terrestrial noise source is small or not detectable then you can ignore this contribution by setting the value here equal to (or less than) the receiver’s white noise floor power level set by its LNA (P = k x TLNA x B).

Notes: Antenna Gain

Note B5

  • This W/S takes some explaining.  Most spacecraft links are made or broken by the antennas used to support them.  Transmitters and receivers are quite straightforward devices (in link modeling terms); antennas are not.  Several of the W/Ss here address various properties of antennas.  This W/S addresses the properties known as gain and directivity.  The directivity of an antenna is a measure of how the antenna concentrates the transmitter’s power in a particular direction in relation to some coordinate system fixed on the antenna.  The directivity of a highly directive antenna is typically taken to be the peak value (sometimes called the boresight directivity).  It is usually measured as the ratio of the power directed in the peak direction divided by the same power when it is radiated isotropically (i.e., equally in all directions).  Some antennas will have losses associated with getting the power from the antenna input to the radiating element.  The gain of an antenna is the directivity (measured in dB above an isotropic radiator [dBi]) minus the antenna feed losses (also measured in dB).  It is sometimes important to be able to make the distinction between directivity and gain.  And, frequently they are confused with one another.  For satellite systems operating in the amateur satellite service, the term gain is more frequently used.
  • This W/S provides the operator with antenna options for both the ground station and the spacecraft.  You should be able to pick among them and find one that is similar to what you will use in your system.  If you are using something different, you may choose the “User Defined” option which is either 4 or 5 depending on if the antenna is a ground station or a spacecraft antenna.  Then, you must enter the gain and beamwidth of the antenna (basically, you must fill in the missing blue values).  So the first action for the operator is to select a generic antenna type for uplink and downlink, spacecraft, and ground station.  Thus, four antennas are involved in your system (at least).  If you are using a simplex satellite transceiver, you may technically have only three antennas.  So, in this worksheet enter both spacecraft antennas as being the same.  The next step is quite educational.  For the ground station, you may design your antenna using the basic parameters given in the various tables in this worksheet.  As always, modify only the blue text cells.  For example,  a crossed yagi antenna is designed here by simply selecting the length of the antenna in wavelengths.  The number of elements required in each plane and the antenna gain and beamwidth is derived in the table.  [BTW, for a yagi design, the gain values achieved here are slightly on the optimistic side, based on experience].  In order to design a helix, you will need to input the number of turns to be used, the turn spacing, and the diameter of the helix in wavelengths.  The outputs are the gain and the beamwidth.  For a parabolic reflector (dish) the inputs are the dish diameter and the aperture illumination efficiency.  The outputs are again, gain and beamwidth.  The frequency used in all cases is the one you selected in the “Orbit & Frequency” W/S.
  • For spacecraft antennas, there are a total of seven options provided.  Option 6 is provided so that the link model operator has at least one high gain spacecraft antenna option.  You may model other high gain antennas as though they were a “dish” antenna by adjusting the diameter and/or aperture efficiency of the dish until the desired gain of your actual antenna is achieved.   Option 7 is a user-defined option.  You must provide the gain and beamwidth values for the antenna you choose in Cells H32 and L32 for the uplink (and Cells H49 and L49 for the downlink) respectively.   For the spacecraft antennas, there are no user-definable design parameters, except for the parabolic reflector (see NOTE at Cell G31).  As most of the options available are primarily Omni antennas, their designs are fairly fixed.  That’s not entirely true for antennas like the quadrifilar helix, however, as the…

Note K11

Uplink Ground Station Polarization

  • Operator Enter RHCP, LHCP, Linear
  • NOTE:  Linear antennas are discouraged.

Note K24

Uplink Spacecraft Polarization

  • Operator Enter RHCP, LHCP, Linear

Note G31

Uplink Spacecraft Parabolic Reflector Gain

  • Link Model Operator Must Set Antenna Parameters at Cells T31 and V31.

Note K31

Uplink Spacecraft Parabolic Reflector Beamwidth

  • Link Mode Operator Must Set Antenna Parameters at Cells T31 and V31.

Note K41

Downlink Spacecraft Polarization

  • Operator Enter RHCP, LHCP, Linear

Note G38

Downlink Spacecraft Parabolic Reflector Gain

  • Link Model Operator Must Set Antenna Parameters at Cells T48 and V48.

Note K38

Downlink Spacecraft Parabolic Reflector Beamwidth

  • Link Mode Operator Must Set Antenna Parameters at Cells T48 and V48.

Note K58

Downlink Ground Station Polarization

  • Operator Enter RHCP, LHCP, Linear
  • NOTE:  Linear antennas are discouraged.

Notes: Antenna Pointing Losses

Note B3

  • This W/S also deals with spacecraft and ground station antennas.  In this case, we are looking at how the directivity (or gain) of an antenna changes as we move away from the antenna’s direction of peak gain.  It is typical to refer to the antenna’s loss in gain, as it might be viewed from a distant location and as the antenna is rotated relative to the observer, to be the gain “roll-off” of the antenna.  If the antenna gain used in the link analysis is the peak gain of the antenna (and it always is) then any roll-off (typically measured in dB) is considered a loss. The beamwidth of an antenna is typically taken to be twice the angle between the boresight and the direction where the power has a roll-off value of a factor of 2 (that is, -3 dB).
  • The gain of an antenna, as a function of some angle (θ) away from the boresight, must be expressed by assigning a coordinate system into which the antenna is placed.  In the case of a spacecraft antenna, which is more-or-less omnidirectional, we primarily care about the location of the antenna with respect to some symmetry axis of the satellite.  Figures 1 and 2 here, define the overall geometry of the uplink and downlink and the antenna placements assumed in this link model.
  • Ground Station Antennas:  For the uplink, the pointing loss of the ground station antenna is due to an error in pointing,θ1.  The antenna is always operated close to the boresight and the gain roll-off starts off slowly but then rapidly increases.  The angle is measured relative to the boresight direction.  It doesn’t usually matter much in which direction the angle varies with respect to the boresight (e.g., in azimuth or in elevation).  The gain of this class of antennas usually falls off in the same manner regardless of the error direction.  Some high gain antenna arrays such as fan beam antennas and offset fed dishes do not have this symmetry property around the boresight so, this comment can’t be totally generalized.  If such antennas are employed, a more complex model might be warranted.  θ4 on the downlink is defined in exactly the same manner as θ1.
  • Spacecraft Antennas:  On the spacecraft side, one doesn’t typically think of the orientation of the spacecraft relative to the remote observer as being a pointing “error” for LEO systems.  Rather, we may think of the ground station observer to be at some vector orientation with respect to the spacecraft’s coordinate system.  The spacecraft antenna itself must also be placed into that same coordinate system.  For the uplink, θ2 is a projection of the vector into one particular plane of the spacecraft.  Generally, it is a plane that contains the symmetry axis of the spacecraft system.  The specific orientation of each of the antenna options as they might be placed on the spacecraft is shown in Figure 3 through Figure 8.  Also shown in these figures is a rough, uncalibrated polar representation of the antenna gain relative to the spacecraft symmetry axis.  θ3 is defined in the same manner as θ2.   It is important to note that all except one of the antenna options have a gain pattern that is symmetric with respect to the Z-axis of the spacecraft body.  The orientation of the antenna options provided is such that if one were to rotate the spacecraft about Z and maintain the projection of the angle to the remote observer in the fixed plane in which we are observing here, then the gain of the antenna would not change.   One antenna, the dipole is actually oriented differently.  It has been assumed that the dipole is mounted perpendicular to the spacecraft Z-axis.  In this one case, the projection of the angle toward the direction to the user cuts the pattern differently, for any arbitrary rotation about Z.  So, we must say that the gain given here is only valid if the remote observer vector were to lie in the X-Z plane.  This particular plane will give the maximum variation in the link performance due to a rotation of the spacecraft.  All other “cuts” of the dipole gain will be less dramatic.  So, this placement for the dipole and the projection…

Note B30

Antenna Loss Determination

  • Notice that below are tables containing the same antenna options as having been presented in the “Antenna Gain” W/S.  However, you will also observe that the cells containing the option selection numbers now have black text.  The tables are related to providing the operator with a clear understanding of the key antenna characteristics already chosen while evaluating the antenna pointing errors and losses.  The operator must now select point errors for the ground station antennas and vector values for the direction of the ground station relative to the spacecraft coordinate system.  These are the angles θ1, θ2, θ3, and θ4.  The tables provide the pointing loss associated with these angle selections.  These loss values are then entered automatically into the “Uplink” and “Downlink” W/Ss.
  • An interesting capability exists using this worksheet.  It is now possible to plot the resultant link performance, say the Eb/No, as a function of the ground station vector in the spacecraft coordinate system as the spacecraft rotates.  This allows one to determine over what fraction of 4p steradians the link will “close.”
  • Once you have selected the four antenna pointing losses, move on to the “Antenna Polarization Loss” W/S.

Note R32

  • Please do not modify numbers or formulas in these columns.
  • This column contains the functional relationships between the angles, θ1 thru θ4, and the gain roll-off.  The equations use a variety of forms and are taken from different sources.
  • You SHOULD modify the equation at Cell R60 and/or Cell R82 if you specify your own “User Defined” antenna.  The equation you provide should define the antenna loss as a function of the viewing angle θ and in the appropriate spacecraft plane.  Currently “plugged” into these cells is data for an isotropically radiating antenna.

Notes: Antenna Polarization Loss

Note B6

  • This worksheet also relates to antennas.  It focuses on characterizing the polarization properties of the ground station and spacecraft antennas.  An antenna may generate E-field and H-field radiation in a fixed relationship relative to the body of the antenna.  Such an antenna is characterized as having linear polarization.  If the E-field is oriented parallel to the local horizon on Earth the antenna is said to be horizontally polarized.  If the B-field is oriented parallel to the local horizon then the antenna is vertically polarized.  If two independent planes of elements are used in an antenna system and the power is divided equally between them and the elements are fed 90° out of phase from one another and if the elements are oriented perpendicular to one another, then a circularly polarized EM wave will be generated.  In such a case, the E-field and the H-field components of the wave will rotate in space as the EM field propagates away from the radiating antenna.  The rotation rate is at the carrier frequency.  So, a 100 MHz circularly polarized signal will make 100,000,000 full rotations per second.  If the phase of the first element leads the phase of the signal to the second element so as to generate a clockwise rotation as viewed from the back of the antenna, looking in the direction of the “launched” wave, then the polarization is said to have right-hand circular polarization (RHCP).  If the phase is swapped, then the rotation will be counter-clockwise in the same reference system and the antenna is said to have left-hand circular polarization (LHCP).
  • There is also an in-between polarization case.  If an antenna has two planes of orthogonal elements as discussed above but, the power delivered to the two planes is not equal, then the antenna will generate elliptical polarization.  The E- and H- fields still rotate at the carrier frequency as in circular polarization but a non-rotating elliptically polarized wave is generated.  The ellipse will have a fixed orientation with respect to the antenna radiators. Usually, the major axis of the ellipse will line up with the element receiving the most power.  If at a distant location, a horizontally polarized element is used to receiver the signal from an elliptically polarized antenna, and if the horizontal antenna can be rotated slowly between horizontal and vertical it will be noticed that a maximum signal attitude and a minimum signal attitude can be found.  These two angles correspond to the major and minor axes (respectively) of the polarization ellipse.  One important issue in link analysis is, how much power loss is associated with the polarization properties of BOTH the transmit and the receive antennas?  The measure of ellipticity for a given transmit or receive antenna is known as its axial ratio.  The axial ratio is equal to 10 times the log to the base 10 of the ratio of the power measured in the major axis divided by the power measured in the minor axis.  Thus, the axial ratio of a perfectly circularly polarized antenna is 10log(.5/.5) = 0.  And a linear antenna has an axial ratio of 10log(1/0) = infinity since all of the power is put into the major axis.
  • Here is an important point:  A spacecraft using a circularly polarized antenna (more or less omnidirectional in gain) DOES NOT typically produce the same axial ratio in all directions.  Thus, to be perfectly general, to characterize the antenna system installed on a spacecraft one must measure (usually in a spherical coordinate system) BOTH the gain of the antenna as a function of two angles (say, θ and j)  AND the axial ratio of the antenna as a function of θ and j.  And, both of these parameters will affect the link performance of the system.  While gain plots are commonly provided or measured, axial ratio plots are not.  Yet, the axial ratio could cause a very large signal loss just as a gain loss can.  Axial ratio plots are typically accomplished (when they are measured at all) by the technique hinted at above.  The spacecraft is placed in a two-axis rotating fixture (to set θ and j) and a…

Note B27

  • These two tables allow you to determine the power loss and isolation between two antennas.  One table is for the uplink and the second is for the downlink.  You must first measure or estimate the axial ratio of all of the antennas in the system at the attitude case you have now selected in the “Antenna Pointing Losses” W/S.  [NOTE:  If this is an early phase of the design, assume either ideal properties such as perfect circularity or pick an axial ratio in the vicinity of say, 1 dB as a starting point].  Enter these values into the tables.  Now estimate the angle between the transmit and receive antenna polarization ellipses (the general case).  See the angle θ in the adjacent figure.  [NOTE:  don’t confuse this angle θ with the spacecraft attitude component θ].  The result will be the polarization loss between the two antennas at that angle.
  • The table also provides the isolation between two cross-polarized circular antennas (one RHCP and one LHCP) using the same axial ratios.
  • The two polarization loss values, given at Cell [F40] and Cell [F60] are automatically transferred into the “Uplink” and “Downlink” budget W/Ss.
  • A table giving example results is provided at Cells [B70:J99].
  • [NOTE:  A linear antenna may be adequately represented by assuming its axial ratio is 30 dB.  From a practical standpoint, this value is approximately correct, as even a dipole will typically have a tiny orthogonal radiation component].
  • After completing the entries to this W/S, proceed to the “Atmos. & Ionos. Losses” W/S.

Notes: Atmos. & Ionos. Losses

Link Losses Resulting from Signals Passing Through Atmospheric Gases

  • Losses due to atmospheric gases (Nitrogen, Oxygen, Carbon Dioxide, Hydrogen, etc.) are nearly independent of atmospheric temperature, mean density, and relative humidity at frequencies below 2 GHz.  Atmospheric absorption depends strongly upon the total number of molecules distributed along the path between the spacecraft and the ground station.  This, in turn, means that the losses from or to the satellite are elevation angle-dependent.
  • The table to the left is a look-up table.  The minimum elevation angle selected in the “Orbit” worksheet is matched against the closest fit from the table and the result is given at Cell [D23] and is automatically inserted into the uplink and downlink budgets.
  • The data used here is taken from “Radiowave Propagation in Satellite Communications” by Louis J. Ippolito, Jr., Van Nostrand-Reinhold, 1986, pp. 33-34, Tables 3-3a-c.
  • One additional interpolated value is added at a 2.5° elevation angle.  This was not taken from Ippolito’s text.
  • If you are using uplink or downlink frequencies above 2 GHz, refer to the referenced text given above to determine the appropriate atmospheric losses.  At millimeter-wave frequencies, the losses can be much higher.
  • Radio waves passing through the ionosphere at VHF, UHF, and Microwave frequencies are influenced far less by this layer of ionized particles than at frequencies in the HF, MF, and LF portions of the radio spectrum.  While there is certainly some correlation between the elevation angle to a satellite and the signal absorption or scintillation experienced, this dependency is nearly masked out by the time variability of effects.
  • There is, however, a frequency dependency that can be quantified, on average.  As transmitter frequencies go below 100 MHz there are times when the attenuation can increase to as much as tens of dB, especially at low elevation angles.  The ionosphere certainly limits the lowest frequency at which satellite communications are feasible.  Below 20 MHz, during solar maximum space signals are usually fully absorbed or reflected by the layers of the ionosphere (D, E, F1, and F2).
  • The values provided in this table are approximate mean values for low earth station elevation angles.
  • It is proposed that these values can be conservatively used in satellite link analyses.  The higher-order statistics of these loss parameters would be interesting to review, however, this effort is more than is necessary for the development of an effective link budget.
  • The losses determined here for the uplink and downlink are based on the operator-selected frequency choice made in the “Orbit” worksheet.  If the “User Defined” option is selected by the link model operator, then the operator must estimate the appropriate ionospheric loss value and manually insert it in either Cell [D34] or Cell [D47] accordingly.
  • Proceed to the “Modulation-Demodulation Method” W/S.

Note D21/F21

Loss due to Atmospheric Gases, Uplink & Downlink Minimum Elevation Angle

  • For this version of this link model, the Link Model Operator must manually change the Minimum Elevation Angle.  This parameter is used for the uplink and the downlink atmospheric losses.  So, for now, the uplink elevation angle and the downlink elevation angle must be the same.
  • I am working on a better version of the model that will independently calculate both the uplink and downlink Atmospheric Losses for different orbits and for transponders as well as TTC links.  But, that version isn’t ready yet.

Note C35

Loss due to Ionosphere, Uplink

  • This is a link model operator selected value.  The value is chosen in the Orbit and Frequency W/S at Cell [C34].

Note C48

Loss due to Ionosphere, Downlink

  • This is a link model operator selected value.  The value is chosen in the Orbit and Frequency W/S at Cell [C40].

Notes: Modulation-Demodulation Method

Note B2

  • Finally, we have arrived at the last point in the link:  the demodulator/decoder portion of the receiver.  The demodulator will extract the data from a modulated signal and deliver it to either an FEC (forward error correction) decoding device (or piece of software) or directly to the ground station computer.  The FEC decoder, if one is used, will remove errors from the data using extra bits that are inserted, either into each byte or into a new form of “frame” (array or data block).
  • Listed in the two tables below (one used for the uplink and the other for the downlink) are various forms of modulation typically used in spacecraft telemetry and command systems.  They are listed from the simplest (and poorest performing) type to the most complex (and best performing type).  The options selectable are:  Audio Frequency Shift Keying on an FM Carrier, A special form of Frequency Shift Keying developed by Mr. James Miller – G3RUH, Non-Coherently Demodulated Frequency Shift Keying, Gaussian Minimum Shift Keying, Binary Phase Shift Keying, and Quadriphase Phase Shift Keying.  You should refer to standard Communications textbooks in order to investigate and understand the properties of these options.
  • Also listed are the type of FEC decoding to be used.  Most of the options show no coding is used.  However, pairing an efficient modulation method such as BPSK with an FEC decoder provides HUGE advantages in terms of link performance.  Convolutional coding operates at the byte level and additional bits are added to each word.   Errors are corrected, however, on a (bit-by-bit) sequential basis.  The most popular of these methods is known as a Viterbi convolutional encoder/decoder system, named for Andrew Viterbi, the inventor.   Two parameters select the degree of coding.  R is the rate of the code (e.g., 1/2, 1/3, 1/6).  The rate defines how many symbols are transmitted per bit of information).  A rate 1/2 code contains two symbols of information for every bit.  The constraint length K is the number of output symbols that are affected by a given input symbol.
  • Another very popular method of FEC coding is known as block coding.  The decoder operates on an entire block of data.  Extra coding bits are added to the end of the block.  The most popular of the block codes is known as Reed-Solomon after the inventors, although there are many other forms of block coding.  In RS coding, two parameters are again used (n and k).  The encoder codes a block of n data information symbols (bits) into a block of k codeword symbols.  Thus, errors are corrected at the block (or frame) level.   Both convolutional and block codes reduce the Eb/No required to achieve a particular bit error rate.
  • Currently, it is common to use two encoders at the transmitter in series (and two decoders in series at the receiver).  Such a process is called concatenated coding.  Usually, the first decoder will be a convolutional decoder followed by a block decoder.  This further reduces the Eb/No required at the demodulator input.   One important reason for considering the use of FEC coding is because contemporary microprocessors are fast enough to allow the decoder to be implemented entirely in software, even at moderate data rates (say up to 50,000 to 100,000 bps).
  • The parameter listed in Column E is the bit error rate (B.E.R.).  It is the average number of errors that occur per bit transmitted.  Put the other way around a BER=1.00E-04 means that, on average, for every 10,000 bits transmitted one bit will be in error.
  • The parameter listed in Column F is the theoretical Eb/No required to achieve the bit error rate given in column E.
  • These two tables allow the link model operator to select a modulation process and bit error rate appropriately matched to the satellite being developed.  As always there is a “User Defined” option, however, the operator must provide appropriate values for the type of modulation, the type of FEC coding used, the B.E.R., and the Eb/No Required.
  • The value “Eb/No Threshold” in Cell [H5] and Cell [H32] are transferred to the “Uplink” …

Note B25

  • The demodulator hardware or software may not be perfect.  The difference between the theoretical value and the actual measured value of the Eb/No required to achieve any particular B.E.R. is known as the implementation loss.  If we add the implementation loss (usually measured for any particular design of demodulator) to the theoretical Eb/No Required we obtain a parameter, defined here as the Eb/No threshold.  If the link does not deliver an Eb/No equal to or greater than the Eb/No threshold, then the B.E.R. specification will be not be met and the link is, therefore, said not to close.
  • As a practical matter, some guidelines can be provided for various types of demodulators.  AFSK and G3RUH/FSK involve two subsequent “concatenated” demodulation processes.  Both of these processes are non-linear.  As such, it is mathematically difficult, given any particular hardware implementation, to theoretically define the required Eb/No.  So, for these two types of demodulators, the Eb/No required is usually measured.  For these choices given in the table (Options 1 through 4) you need not assume any implementation loss (value = 0) since the values provided in Column F are measured performance values and already include the implementation loss.  The values are average for various decoder performance results provided in the literature.  Coherent and non-coherent FSK, GMSK, BPSK, and QPSK all have well-known theoretical B.E.R. vs. Eb/No requirements.  (See Figure 1 below for BPSK vs. B.E.R. theoretical performance).  If one of the Options 5-14 is selected, an implementation loss of 1.0 dB is approximately correct if the demod is implemented in hardware and a value of 0.5 dB is appropriate if a “good” software decoder is used.  For demodulator/decoder options involving FEC, one would hope that the implementation losses would be 0.5 dB or less, otherwise, the performance enhancement of the FEC decoder is being wasted.
  • Once you have selected the two system modulator/ demodulator options and estimated the implementation loss, move on to the “Uplink” W/S.

Note B54

  • Figure 1 below is a plot for a BPSK system of the Eb/No achieved in the link vs. the ideal bit error rate of the system.  This is a classical plot.  BPSK is an ideal modulation method in that it can be lossless.   In fact, it is possible to actually build a matched filter for this type of demodulator.  Without FEC coding, it is not possible to improve on this BER curve performance.  That is why BPSK is such a good choice for a telemetry system.  BPSK, however, does require more bandwidth than almost any other choice of modulator/demodulator.  So, as always, the best performance comes at a price.

Notes: Uplink Budget

Note B1

  • This is the main uplink budget.  Most of the data needed for the link has been provided in prior W/Ss.  As such, this is the final (and formal) uplink budget.  The operator must provide a few user-defined values.
    • If a high microwave or mm-wave frequency is used then rain losses become important.  The formal means of estimating rain losses is a statistically based process.  It is not discussed here.  If rain losses are an issue, then the best that can be done right now is to enter the rain loss that would represent the worst-case loss for (99%, 99.9%, or 99.99% of the time).  These can be obtained from various communications texts for your region of the world.
    • The operator must provide the data rate to be used for the command uplink.
    • If the S/N method of analysis is to be employed, the operator must provide the pre-demodulator bandpass filter bandwidth of the receiver.
    • If the S/N method of analysis is to be employed, the operator must provide the required S/N for the analog or digital demodulation process being used.  This should be specified against some measure of the performance of the link such as B.E.R. or achieve audio S/N or some signal thresholding condition.
  • The comments to the right are intended to help the operator understand each step taken in the link budget.  If this information is not needed, in particular, if you want to print the link budget, click on the “1” in the upper left-hand corner of the first row of this worksheet.  Similarly, if you wish to “hide” either the Eb/No method or the S/N method subtables you may click on the “1” in the upper left-hand corner of the first column of this worksheet.  Alternatively, you can click on the “-” sign at the beginning of Row 47 or Row 69 to hide the data you want.  Clicking on the appropriate “2” box in the upper left-hand corner of this worksheet will restore all of the data.
  • Once you have reviewed the Uplink data, move on to the “Downlink” W/S.

Note B27

S/C Signal-to-Noise Power Density (S/No)

  • Also known as the C/No, this value is already a useful result.  It is equivalent to the signal-to-noise ratio of this link if one were to use a 1Hz wide data filter just ahead of the receiver demodulator.  This is the filter that limits the noise entering the demodulator and passes the signal in its final form.
  • This value is calculated by:
    • S/No = Piso+(G/T)-K
    • where:  K = Boltzman’s Constant

Note B30

Command System Eb/No

  • This is the “Energy per bit to Noise Power Density Ratio.”  It is equivalent to the “Signal-to-Noise Ratio” and is the parameter of choice for digital links.  It is the measure of performance for this link.
  • Once the S/No is known, the Eb/No is simply calculated by:  Eb/No = S/No -10log(R) where R = data rate.

Note B41

Eb/No Threshold

  • This is the result of the Eb/No required theoretically for the modulation method selected plus any additional losses caused by imperfections in the demodulator design.

Note B43

System Link Margin

  • This is the bottom line.  This value must be > 0.0 dB for the link to work or “close.”  A target value should be approximately 10 dB for a low-cost system, 6 dB for a professional system, and 3 dB for a deep space system.

Note E47

Spacecraft Alternative Signal Analysis Method (SNR Computation)

  • This is a more realistic method to use for a spacecraft command receiver.  The Eb/No method assumes the receiver uses a matched filter and assumes a spectral efficiency of 1 bps/Hz of bandwidth.  In this assumption, the bandwidth in Hz is chosen equal to the bit rate (in bps).  As such, no excess bandwidth is assumed.  S/C receivers are typically not coherent and it is best to determine the final bandpass filter bandwidth and use it here in Cell [B57]

Note H57

Spacecraft Receiver Bandwidth

  • If you are using a coherent demodulator at the spacecraft and if a coding option is selected (Uplink Options 15-18 from the “Mod-Demod Method” W/S) then make sure the Spacecraft Receiver Bandwidth chosen includes the bandwidth required for the “Symbol Rate” modulation spectrum.  For example, if Viterbi Convolutional Coding were used with R=1/2 and K=7 then the symbol rate modulation spectrum is exactly twice that which would be occupied by the data only.   The filter must be twice as wide and this means the noise seen at the demodulator is 3 dB higher.  This will result in a reduction in the Signal to Noise Ratio of 3 dB.

Note B61

Signal-to-Noise Power Ratio at G.S. Rcvr

  • This is the S/N result of this uplink.  Note that typically it will be poorer (a lower value) than the Eb/No achieved for the link.  That is because the filter bandwidth is not exactly matched to the data spectral bandwidth.  In order to assure that the data spectrum is contained within the filter and in order to account for some doppler frequency errors on the part of the uplinking ground station the filter has a bandwidth (in Hz) larger than the data rate (in bps).

Note B65

System Link Margin

  • This is the bottom line.  This value must be > 0.0 dB for the link to work.  A target value should be approximately 10 dB for a low-cost system, 6 dB for a professional system, and 3 dB for a deep space system.

Notes: Downlink Budget

Note B1

  • This is the main downlink budget.  Most of the data needed for the link has been provided in prior W/Ss.  As such, this is the final (and formal) uplink budget.  The operator must provide a few user-defined values.
    • If a high microwave or mm-wave frequency is used then rain losses become important.  The formal means of estimating rain losses is a statistically based process.  It is not discussed here.  If rain losses are an issue, then the best that can be done right now is to enter the rain loss that would represent the worst-case loss for (99%, 99.9%, or 99.99% of the time).  These can be obtained from various communications texts for your region of the world.
    • The operator must provide the data rate to be used for the telemetry downlink.
    • If the S/N method of analysis is to be employed, the operator must provide the pre-demodulator bandpass filter bandwidth of the receiver.
    • If the S/N method of analysis is to be employed, the operator must provide the required S/N for the analog or digital demodulation process being used.  This should be specified against some measure of the performance of the link such as B.E.R. or achieve audio S/N or some signal thresholding condition.
  • The comments to the right are intended to help the operator understand each step taken in the link budget.  If this information is not needed, in particular, if you want to print the link budget, click on the “1” in the upper left-hand corner of the first row of this worksheet.  Similarly, if you wish to “hide” either the Eb/No method or the S/N method subtables you may click on the “1” in the upper left-hand corner of the first column of this worksheet.  Alternatively, you can click on the “-” sign at the beginning of Row 46 or Row 67 to hide the data you want.  Clicking on the appropriate “2” box in the upper left-hand corner of this worksheet will restore the data.
  • Once you have reviewed the Downlink data, move on to the “System Performance W/S.”

Note B27

G.S. Signal-to-Noise Power Density (S/No)

  • Also known as the C/No, this value is already a useful result.  It is equivalent to the signal-to-noise ratio of this link if one were to use  a 1Hz wide data filter just ahead of the receiver demodulator.  This is the filter that limits the noise entering the demodulator and passes the signal in its final form.
  • This value is calculated by:
    • S/No = Piso+(G/T)-K
    • where:  K = Boltzman’s Constant

Note B30

Telemetry System Eb/No for the Downlink

  • This is the “Energy per bit to Noise Power Density Ratio.”  It is equivalent to the “Signal-to-Noise Ratio” and is the parameter of choice for digital links.  It is the measure of performance for this link.
  • Once the S/No is known, the Eb/No is simply calculated by:  Eb/No = S/No -10log(R) where R = data rate.

Note B41

Eb/No Threshold

  • This is the result of the Eb/No required theoretically for the modulation method selected plus any additional losses caused by imperfections in the demodulator design.

Note B43

System Link Margin

  • This is the bottom line.  This value must be > 0.0 dB for the link to work or “close.”  A target value should be approximately 10 dB for a low-cost system, 6 dB for a professional system, and 3 dB for a deep space system.

Note H56

Ground Station Receiver Bandwidth

  • If you are using a coherent demodulator at the spacecraft and if a coding option is selected (Downlink Options 15-19 from the “Mod-Demod Method” W/S) then make sure the Spacecraft Receiver Bandwidth chosen includes the bandwidth required for the “Symbol Rate” modulation spectrum.  For example, if Viterbi Convolutional Coding were used with R=1/2 and K=7 then the symbol rate modulation spectrum is exactly twice that which would be occupied by the data only.   The filter must be twice as wide and this means the noise seen at the demodulator is 3 dB higher.  This will result in a reduction in the Signal to Noise Ratio of 3 dB.

Note B60

Signal-to-Noise Power Ratio at G.S. Rcvr

  • This is the S/N result of this downlink.  Note that typically it will be poorer (a lower value) than the Eb/No achieved for the link.  That is because the filter bandwidth may not exactly be matched to the downlink signal modulation spectrum.  In order to assure that the data spectrum is contained within the filter and in order to account for some doppler frequency errors on the downlink the filter often has a bandwidth (in Hz) larger than the data rate (in bps).  In that case, this calculation method is more appropriate.

Note B64

System Link Margin

  • This is the bottom line.  This value must be > 0.0 dB for the link to work.  A target value should be approximately 10 dB for a low-cost system, 6 dB for a professional system, and 3 dB for a deep space system.

Notes: System Performance Summary

Note: A9

  • This is a block diagram of the command and telemetry link systems.  It contains all of the data selected by the link model operator.  It represents a high-level specification for the command and telemetry equipment in the spacecraft and the ground station.  This can be printed as a final result, all or in part, as may be useful to the operator or other project members.
  • Key losses associated with the link are summarized in this system performance summary.  The values are located in the middle of the W/S in the “Radio Link” area.
  • Only one operator selection is needed in this W/S.  At Cell [O12], the efficiency of the transmitter [hTx =(RF Output Power/DC Input Power)X100] is entered.  The W/S calculates the DC power required to be delivered to the transmitter and the amount of heat dissipated by the transmitter (See Cell[O14] and Cell [O16].)
  • Notice the cells in the two regions of the W/S, [E5:G7] and [N107:P109].  They contain the final results of both links.  The color of the results boxes will change, depending on the performance achieved.  The colors are as follows:
    • GREEN:   Link margin > 6 dB.  “Link Closes”
    • YELLOW:  Link margin > 0 dB but, < 6 dB.  “Link Marginal”
    • RED:        Link margin < 0 dB.  “No Link!”
  • Everything else in the “Systems Performance Summary” should be self-explanatory.  After you have reviewed the System Performance Summary, you may want to review some of the design tools that support the Link Model System, including:
  • “S/C Antenna Patterns”, “Beam Roll-Off Tool”, “Beam Roll-Off Plot”, “Line Loss Tools & Tables”, and “VSWR Loss Tool”.  The “Orbit Shape Data” should not be modified by the link model operator.   In a later version, this W/S may be used to provide options for elliptical orbits or planetary (Earth escape missions).

Note O12

hTX

  • Operator Enter Transmitter DC to RF Power Efficiency

Note O14

Tx DC Power

  • This is the DC power required whenever the Spacecraft Transmitter is ON.  This should be passed on to the Spacecraft Power Subsystem analysis process.

Note O16

Tx Dissipation

  • This is the thermal heat that must be dissipated by the transmitter.  This value should be passed on to the Spacecraft Thermal analysis process.

 

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A Guide to CubeSat Mission and Bus Design Copyright © by Frances Zhu is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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