Real-Time Attitude Control Tutorial

This tutorial demonstrates controlling the Attitude Control and Determination System (ADCS) from a payload. This tutorial follows on from the Tracking an Area Of Interest Tutorial.

The ADCS can be controlled by user code from a payload while in orbit, using the Satellite Bus API. This REST API is provided by the local Spire Linux Agent (which is pre-installed on all Linux payloads), and has accompanying SDKs.

Prerequisites

All tutorials require the steps outlined in the Getting Started Guide. This tutorial should be completed after the Tracking an Area Of Interest Tutorial.

Finding a Transit Time

Use the find_transit script from the Tracking an Area Of Interest tutorial to identify a time window where the satellite transits over an area of interest at a reasonable elevation. The example below is for satellite with Norad Id 51099 (FM1) and an elevation of 80 degrees, but consider reducing the elevation so that a transit can be found for between 24 and 48 hours in the future.

$ python3 aoi/find_transit --sat 51099 --min 80 --hours 168
2021-12-22 14:12:36.630581      773.389237      89.663970
[
    {
        "end": 1640183130.019818,
        "peak_elevation": 89.66397045560218,
        "peak_time": 1639709513.3850813,
        "start": 1640182356.6305811
    }
]

Develop Script

The script adcs_lease_demo queries the API for current ADCS & TFRS information, then requests the satellite to reorient the imaging aperture to track a specific point on the ground. ADCS is polled every second until the end of the window.

The API to command ADCS to track a target:

    cmd = AdcsCommandRequest(command="TRACK", aperture="IPI", target=(lat, lon))
    res = agent.command_adcs(cmd)

Here the script is asserting that ADCS is still following the executed command. Since lat/lon coordinates are of type float, the values are compared with a margin of error of 1 degree (LATLON_ERR_DEG):

def assert_tracking(hk, mode, lat, lon):
    if hk.acs_mode_active == mode \
        and abs(hk.latlontrack_lat - lat) < LATLON_ERR_DEG \
        and abs(hk.latlontrack_lon - lon) < LATLON_ERR_DEG:

Dump the ADCS telemetry to a file in the /outbox, in JSON format:

    with open("/outbox/adcs_data.json", "a") as f:
...
        f.write(json.dumps(adcs.hk))

Deploy Script

The deploy contains the following steps:

1. Uploads adcs_lease_demo
2. Schedules a LEASE_ADCS window to reserve ADCS
3. Schedules a PAYLOAD_SDR window to run adcs_lease_demo

$ adcs-lease/deploy "[YOUR_AUTH_TOKEN]" [YOUR_SAT_ID] [YOUR_START] [YOUR_END]

Review

Once the windows complete and the results are downlinked to AWS S3, they can be analyzed.

For example to view the pointing error over time, download the file and then click "Choose File" below to chart the control_error_angle_deg over time. A sample file can be downloaded here

Next Steps

• Inter-Payload Communications Tutorial