8.9 Pointing Analysis and Budget

Suggested Reading

Just as there are critical loads in the structures system that must be designed around, there are “critical” payloads that the ADCS system must be designed around. The payload with the most demanding pointing or slewing requirements determines the accuracy/precision of the sensors, the capacity and resolution of actuators, and the rigor of the determination and control algorithms. This pointing analysis and budgeting are not very interesting if you only have one instrument or component that needs to point in the entire mission.

1. Define the body axes of your spacecraft by taking a screenshot of your CAD with the coordinate axis. For example, the following image is taken from the structures software lab where the rails lie along the z axis, as dictated by the reference frame in the top right corner:
   
2. Make a copy of the Pointing Budget Template and replace the components in the template with the components in your spacecraft. These components can be copied from a previous budget (like the mass or power budget).
3. Identify which components in your spacecraft mission need to point at a target during operations. Fill in the target.
   For example in the Artemis CubeSat, we have our baseline visible-IR camera. Some satellites have directional antennas and radios but the Artemis CubeSat kit has omnidirectional components. To make this a little more interesting and illustrative, let’s say the radio does need some kind of pointing. Your budget would look something like this:

   Subsystem	Component	Target
   Payload	Camera	Hawaii, Earth
   COMMs	Transmitter (TX)	Ground Station
   	Receiver (RX)	Ground Station
   	Deployment	NA
   EPS	Battery Board	NA
   	Distribution Unit	NA
   	Solar Panels	Sun

    

4. Now we have to generate pointing goals, which would enable each component to achieve its ultimate performance. This process could involve a scientist deriving physical limitations on detecting an event or target, like what is the pointing accuracy so that we can achieve sub-pixel resolution of a faraway star? The scientist would have to account for diffraction limits if there were lenses and the resolution of the charge-coupled device array they chose for the camera.
   

   

We could also include motion constraints here. Some instruments may be sensitive to jitter so the control policies would need to limit jitter. On Ke Ao, a variant of Artemis, the process is very simple. We would like to take a picture of Hawai’i and transmit this picture back. The camera must be within 8 degrees of a straight-on orientation viewing Hawai’i. Let’s say the radio requires a nadir pointing at Earth but just within that hemisphere. The painting requirement for the radio would be within 90 degrees of nadir.

4. Once this chart of requirements and constraints is made, we can identify the most stringent requirements and constraints and pick sensors/actuators around those numbers.

5. Referring to our sensor and actuator typical accuracies, we can identify sensors and actuators that can at least meet our most strict pointing requirement. Since the Ke Ao mission is not demanding, all sensors and actuators meet the requirement.

6. Now we identify which sensors and actuators are realistic to integrate into our spacecraft. For a 1U CubeSat, we are incredibly volume and mass limited. We can utilize coarse sun sensors, magnetometers, and a magnetorquer. The other sensors and actuators are too massive and their higher accuracy performance is not necessary to fulfill the mission.

7. Given our choice of types of sensors and actuators, we go out into the commercial wilderness and pick some real sensors and actuators that meet our desired performance, as specified above.

8. To ensure that these sensors and actuators meet the requirement, we inject the sensor characteristics (noise) and actuator capabilities into a dynamic simulation. The estimator and controller must verify that the spacecraft ultimately points within the pointing requirement, which for Ke Ao is 8 degrees.

a. If not, go back to step 6 to choose better sensors or actuators.

b. If you’ve exhausted all commercial options, go back to step 5 and reconsider the types of sensors and actuators you’ll consider incorporating into the spacecraft design.

c. If these sensors and actuators absolutely cannot fit within the spacecraft, you’ll have to go all the way to step 2 and barter with the principal investigator or chief scientist on relaxing the pointing requirements.

Artemis CubeSat Kit Specific