SatGazer is a device that tracks a satellite and points an arrow at it. It does this using a Raspberry Pi Zero, a pair of cheap stepper motors with matching driver boards and some 3D-printed parts.
I had read that you could see the International Space Station with the naked eye if you knew where and when to look. I tried this once when an overhead pass at my location was scheduled but couldn't find the tiny satellite in the large night sky. I wanted someone or something to point out roughly the direction I had to look. Then, a coworker mentioned a device used in Astro-Photography that cancelled out Earth's rotation for the purpose o long-exposure photographs of the night sky. This gave me the initial idea for SatGazer. I also wanted to finally learn how stepper motors are controlled and recently got a 3D printer, so I got to work.
To be able to point in any direction, the device uses two axes.
When starting SatGazer, two threads are running. The main thread becomes the web server thread that waits for incomming requests and commands.
The tracking of the satellite runs in its own thread that periodically (once per second) realigns the SatGazer arrow to point towards the satellite. It also fetches new future positions of the satellite when necessary.
(Implemented in satpoint.py)
The satellite location is fetched from a web API, currently n2yo.com. With one request, the locations for the next N seconds can be obtained. This means that SatGazer doesn't have to request new positions for every update but only when it runs out of previously fetched locations or when the target satellite changes.
Both the user and the satellite location are in the form of latitude, longitude and altitude.
These locations are converted to vectors in an XYZ coordinate system with the Earth's center as the zero vector (0,0,0), the X axis aligned with the prime meridian and the Z axis going through the poles.
A simple vector subtraction gives the vector from the user to the satellite.
This vector gives the direction in which the arrow of the SatGazer should point.
This vector is now transformed to another coordinate system centered on the SatGazer device with the X axis pointing East, Y pointing North and Z pointing skywards.
Using this transformed vector, it's easy to compute the azimuth (aka. azimuth) and altitude angle (aka. elevation angle) of the satellite relative to the SatGazer's location.
(Implemented in motor.py)
The two axes are each driven by a stepper motor.
To talk to these motors through the Raspberry Pi's GPIO pins, the gpiozero library is used.
For each step, the next motor pin in the sequence is activated (i.e. pulled high).
Because the mechanics for the azimuth have a 4:1 gear ratio, it takes four times as long to adjust the azimuth than it takes to adjust the elevation. When the target is "behind" the SatGazer, it is therefore always quicker to point the arrow "backwards" than it is to rotate the top plate by more than 90 degrees. Because of this, it is possible that the arrow may point North with the North indicator pointing South.
To conserve power, the stepper motors are turned off (coasting) when not moving.
The web server serves the GUI under the root URL /.
The GUI is really just a helper to send the proper HTTP requests to the rest of the server's API.
GET /coast turns off tracking and the stepper motors. The device stops moving.
GET /calibrate tells SatGazer that it is in the zero position. See the section about calibration.
GET /track/<lat>/<long>/<sat_id> starts tracking the satellite with the given ID. <lat> and <long> are the coordinates of the device.
GET /driver/<azi>/<ele> turns off tracking and aligns the arrow with to the given azimuth and elevation.
The device consists of the following parts.
3D-printed parts:
- Base plate
- Top plate
- Azimuth gear
- Arrow
Electronics:
- Raspberry Pi Zero
- Transistor array board (2x)
- Stepper motor (unipolar) (2x)
- Battery module (optional)
- Various wires and cables
SatGazer uses n2yo.com's web API to obtain the positions of satellites.
This API requires an API key to be supplied along with the request.
You can obtain such a key for free by following the instructions here.
Once you have your key, store it in a file api.key and place the file in the working directory from which you will launch the tracking script.
Before operation, SatGazer must be brought to a known orientation. Calibration must be repeated after every start of the tracking script.
- Orient the device such that the North indicator on the top plate is aligned to North.
- Orient the arrow such that points straight up towards the sky. There is a small notch at the base of the "arrow tower" to check alignment. Be careful and gentle! Depending on the stepper motors, you may not be able to turn it by hand. In this case use the web GUI to move the arrow.
- Once both axes are aligned, click the
calibratebutton in the web GUI. The device is now in the(0,0)position. - Nice! The device is now calibrated.
When the tracking script starts, it assumes the device is in the (0,0) position.
This means that you can skip calibration if you start the script with the device already in the proper zero position.
To ensure that the stepper motors operate correctly, set both the azimuth and altitude to 90 using the web GUI. Ensure that both axes move smoothly and that the arrow now indeed points East.
If the motors don't run smoothly or at all, check that the wiring corresponds to the pin assignments for the motors.
The order of the pins and wires matters! Also try a longer STEP_TIME.
If the arrow made a full rotation and now points up again, check that you haven't confused the two motors with each other.
If the arrow points West instead of East, check the North indicator of the top plate.
If the indicator points East, the altitude motor must be reversed.
If the indicator points West, the azimuth motor must be reversed.
To do that, add reversed=True when creating the motor, e.g.
# Note: Your pin assignments and step size may vary.
UnipolarMotor(22, 27, 17, 18, 5.625/32, reverse=True)
To track a satellite, you need to know the coordinates of your own location and the NORAD ID of the satellite you want to track. A good way to find this ID is N2YO's search function. Your own location can be found easily using Google Maps, for example.
Enter the information in the web GUI and hit submit. The SatGazer device should not start tracking the satellite.
SatGazer was designed for tracking satellites but could in principle track any object with some code changes.
Ideas:
- Smooth tracking using variable stepping speed
- Automatically determine location using IP
- Autorun script at OS startup
- Improve parsing of coordinates
- Make configuration easier (config file?)
- Prevent drift by measuring device position in steps instead of degrees