The ISS Orbit Tracker (ISSOT) is an Android Things experiment that keeps track of the International Space Station. It calculates and displays the azimuth and elevation during the ISS orbits. This relatively simple device will give you a visual indication of the location of the Space Station, even if you can’t see it.
The ISS conducts operations in the low earth orbit (LEO) about 400 km above the earth’s surface. It travels from west to east on an orbital inclination of 51.6 degrees. Each orbit takes 90-93 minutes, depending on the exact altitude of the ISS. Its location in the sky can be determined by using two “look angles” called, Azimuth & Elevation.
Azimuth (ψ) is the angle measured in the plane parallel to the horizon from the polar north going clockwise. This angle has a value between 0° and 360°.
ψ = atan2((sin Δλ ⋅ cos φ₂), (cos φ₁ ⋅ sin φ₂ − sin φ₁ ⋅ cos φ₂ ⋅ cos Δλ))
Elevation (El) is the angle measured in the plane perpendicular to the horizon going up towards to the ISS. This angle has a value between –90° and +90° such that the horizontal direction is 0°. A vertically upwards is +90° and vertically downwards is –90°. The figure below shows the addtional vectors and angles needed to calculate the elevation angle.
- rS is the vector from the center of the earth to the ISS
- rE is the vector from the center of the earth to the earth station (the tracker)
- d is the vector from the tracker to the ISS.
- γ is the central angle measured between rE and rS
- ψ is the angle measured from rE to d
Built using a Raspberry Pi 3b and the Android Things embedded OS platform, ISSOT utilizes tracking information and motors to point at the space station. To do so, it uses the WTIA REST API in conjuction with the Volley library to grab JSON objects containing the position of the ISS. The motors are driven by a DC & Stepper Motor HAT; which connects to the Raspberry Pi. The stepper motor (geared down with a chain drive) controls the azimuth and the servo controls the elevation. A three wires slip-ring is installed in the bottom of the 6” tube to prevent the cables from getting tangled and allow pulse modulation continuity through the rotating shaft. The Rainbow HAT display shows the azimuth degrees and the LED strip indicates the proximity level of the ISS passing near or above the tracker location.
The device requires to be initialized at true north at the moment of powering it up. After that, it goes through a quick range-of-motion routine before starting to point at the ISS.
- WiFi and Internet access.
- Flash Android Things on the Raspberry Pi 3 (instructions here).
- The following individual components:
Qty(1) - Raspberry Pi 3 Model B
Qty(1) - DC & Stepper Motor HAT
Qty(1) - Pimoroni Rainbow HAT
Qty(1) - 5V 4A Power Supply (or similar)
Qty(1) - Slip Ring - 3 Wire (10A)
Qty(1) - Stepper Motor
Qty(1) - HS-322HD Servo
Qty(1) - Lightweight HUB Horn
Qty(1) - Set Screw Hub - 5mm Bore
Qty(1) - 1” Stainless Steel Tubing (length: 6.00”)
Qty(1) - Standard Servo Plate C
Qty(1) - 0.250” Chain (5 Feet)
Qty(1) - 1” Bore 0.250” Pitch Aliminum HUB Sprocket (# of teeth: 40T)
Qty(1) - 1/2” Bore 0.250” Pitch Aluminum HUB Sprocket (# of teeth: 16T)
Qty(1) - 90° Single Angle Pattern Bracket
Qty(2) - 1” Bore Bottom Tapped Clamping Mount
Qty(2) - 1” Bore Side Tapped Pillow Block
Qty(2) - 5/16” – 3/8” Rubber End Cap
Qty(2) - Large Square Screw Plate
Qty(3) - Flat Dual Pattern Bracket
Qty(1) - Actobotics® Hardware Pack A(optional, but recommended)
Qty(1) - Splitter Power Adapter Cable 5.5 x 2.1mm(optional)
Qty(1) - Wood Rectangle Plaque(optional)
Qty(1) - Chain Breaker Tool(optional)
Special thanks to G. Hillhouse for suggesting the Actobotics® hardware to build the tracker aluminum structural.
Copyright 2018 Al Bencomo
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