Tutorial CubeSat telemetry

Tutorial: CubeSat telemetry

What it is. Many small amateur satellites (CubeSats and others) broadcast telemetry, such as battery voltage, temperatures and status, as short data packets on the 2 m and 70 cm amateur bands. The downlinks are open to anyone with a receiver. OnAir decodes the common packet formats at the same time, logs the frames with callsigns or addresses, names the satellite from an editable table, and can forward the frames to SatNOGS-style tools.

What you need

• Radio. Any radio that tunes 2 m / 70 cm. The mode asks for 2 Msps; the lowest rate that works is 1 Msps (the channel width is 30 kHz). An RTL-SDR is enough. See Supported-radios.
• Antenna. For 145 MHz: a turnstile, a QFH, or a handheld Yagi pointed at the pass; for 435 MHz: a small Yagi or a turnstile. Circular polarisation suits satellites best. An LNA and a band-pass filter for 2 m or 70 cm near the antenna improve weak passes.
• Frequencies from the repository's frequency database. ISS APRS 145.825 MHz (the mode default); ISS voice and SSTV 145.800 MHz; SO-50 downlink 436.795 MHz (uplink 145.850 MHz); AO-91 145.960 MHz; AO-92 145.880 MHz; AO-85 145.980 MHz; ISS cross-band repeater downlink 437.800 MHz; FO-29 downlink 435.800 MHz; RS-44 beacon 435.605 MHz. Satellite frequencies change, so check a current list; the mode's table is editable. The tuning range of the mode is 130 to 1000 MHz.

Before you start

• Doppler: at 435 MHz the shift is up to about +-10 kHz over a pass. The pass planner can correct for it (see below).
• Gain: moderate; keep the passband free of strong pagers and FM broadcast.
• Many CubeSats transmit only part of the time, or only over their ground station's region.

Step by step

1. Set your position, then View > Passes: download TLEs (Celestrak or Keplerpass, or your own server). Add satellites in the planner's own table if needed: Satellite, NORAD, MHz, APRS and an Add button.
2. Mode list, group Satellite & navigation, entry Satellites, 2 m / 70 cm.
3. In the pass planner tick Auto start at AOS, stop at LOS; it starts the receiver at AOS and corrects Doppler. Optionally Auto record IQ, or press Record this pass.
4. Or start by hand: select a satellite in the Satellites tab and press Tune to put the receiver on its downlink.
5. Open Frames and Satellites.

What you should see

A narrow carrier moving through the spectrum with Doppler; frames in the Frames tab with the time, satellite, framing, from / to, number of bytes and a text preview. The status shows frames good, failed their checks and put right by Reed-Solomon, a count per framing, the carrier SNR and offset.

Reading the screen

• Frames. Time, Satellite, Framing, From / to, Bytes, Text. Click a row for hex and ASCII dumps; Copy and Clear.
• Satellites. The editable table: Name, NORAD, MHz, Baud, Callsign and a Tune button; Add a satellite, Restore the defaults. Frames are named after this table: by the AX.25 callsign, else the satellite the pass planner follows, else the frequency tuned. Every framing is decoded whatever the table says.
• Framings decoded, all at once. AX.25 at 1200 baud AFSK and 9600 / 4800 baud G3RUH, GOMspace AX100 Mode 5 (ASM + Golay + Reed-Solomon, GMSK 9600 / 4800) and Mode 6 (Reed-Solomon in HDLC), and CCSDS Reed-Solomon (255,223) frames. The AX100 convolutional code and AO-40 FEC are not decoded.

Options and settings

• Frames to KISS: every decoded frame goes to the KISS TCP server of the data feeds (View > Data feeds), for SatNOGS-style tools: AX.25 frames without their FCS, AX100 frames as the CSP packet, CCSDS frames as the data field.
• CCSDS block: the length of a CCSDS Reed-Solomon block in bytes, 32 of them parity (255 is the full code; shorter is a shortened code).
• Test signal: simulated frames for every framing or one.

Troubleshooting

• Carrier but no frames. Wrong framing or baud; update the table. Weak signal.
• Doppler mismatch. Use the pass planner's auto start so the receiver follows.
• Frames named by frequency only. Add the satellite to the table with its framing.
• Decoded but the CCSDS block fails. Set the CCSDS block length.
• Nothing at all. Satellite not transmitting; many only run over certain regions.

Tips

Check a pass list on SatNOGS or a similar database to see which satellites are active. Short, high passes are best.

Legal and privacy notes

Receiving amateur satellite downlinks is generally legal. Telemetry is open. OnAir only receives here; transmitting to a satellite needs an amateur licence and is a separate matter. See Legal.

Under the hood

AX.25 UI frames with HDLC framing (AFSK 1200 baud Bell 202 style, G3RUH 9600 baud scrambled FSK); GOMspace AX100 modes with Golay-coded length field and Reed-Solomon; CCSDS (255,223) Reed-Solomon frames.

Related

Mode-CubeSat-telemetry, Tutorial-Orbcomm, Tutorial-Meteor-M-LRPT, Tutorial-Weather-satellite-images, Tutorial-Pagers-POCSAG-and-FLEX, Troubleshooting