Tutorial Orbcomm

Tutorial: Orbcomm

What it is. Orbcomm is a satellite data network used for asset tracking and telemetry (containers, trucks, tanks, meters). The satellites are in low orbit and pass over in about ten to fifteen minutes. Their downlinks near 137 MHz carry packets that are visible to any receiver; OnAir decodes the packet framing, sorts the packets by type, follows the Doppler, and puts the satellites on a map from the ephemeris they broadcast. This is a good first satellite mode because a 137 MHz antenna is easy to build.

What you need

• Radio. Any radio that tunes 136 to 139 MHz. The mode asks for 1 Msps; the lowest rate that works is 100 ksps. An RTL-SDR is enough. See Supported-radios.
• Antenna. A 137 MHz antenna with good sky coverage: a quadrifilar helix (QFH), a turnstile (crossed dipoles over a reflector) or a V-dipole. Circular polarisation suits satellites best. A 137 MHz LNA and band-pass filter near the antenna helps a lot and keeps FM broadcast and pagers out.
• Frequencies from the repository's frequency database. Orbcomm downlinks 137.200, 137.500 and 137.800 MHz (137 to 138 MHz band, 4800 baud); the app default is 137.250 MHz. The mode decodes subscriber downlinks 137.2 to 137.8 MHz: every downlink in the passband is found; the presets put a channel near the centre.

Before you start

• At 1 Msps the capture is about 1 MHz wide, so it can cover the 137.2 to 137.8 MHz downlinks when centred between them; a smaller rate sees fewer downlinks.
• Gain: moderate. Strong FM or pagers near the band can desense a dongle: add a filter.
• Doppler is up to a few kHz; the receiver follows it. A test mode sweeps -4 to +4 kHz.

Step by step

1. Install pass data: View > Passes, download TLEs (from Celestrak or Keplerpass) and set your position. Orbcomm satellites appear in the list.
2. Mode list, group Satellite & navigation, entry Satellites, 137 MHz.
3. Press Start shortly before a pass (the pass planner shows AOS, LOS, Max el and Duration; you can have it start at AOS and stop at LOS, and record IQ).
4. Open the tabs Map, Packets, Satellites and Signal.
5. During a pass, packets appear and the Doppler ramp moves across the signal view.

What you should see

A carrier appearing in the spectrum with Doppler drift; on Signal, the downlink list (Channel, MHz) and the Doppler ramp; on Packets, decoded packets and counts by type, bad checksums; on Satellites, a table with frequency (nominal and measured), number of frames, packets and bad packets, and orbit elements (inclination, RAAN, argument of latitude, period); on Map, satellites with a position once their ephemeris was received ("each satellite sends its position once a second").

Reading the screen

• Map. Satellites ("n of n satellites with a position from their ephemeris"), with a follow option.
• Packets. A log by type with a checksum result.
• Satellites. One row per satellite: heard, Doppler, pos, MHz, packets, bad.
• Signal. Number of downlinks decoded at once (a setting: "How many downlinks in the passband are decoded at once"), the Doppler ramp and a clock/sync line ("sat n, frames, ok, bad, sync lost").

Options and settings

• Downlinks decoded at once.
• Doppler ramp for the test signal: sweeps -4 to +4 kHz, 30 s each way.
• The pass planner's auto start/stop and Auto record IQ.

Troubleshooting

• Nothing during a pass. Check the antenna orientation, the polarity and a clear horizon; low passes (under 20 degrees) are weak.
• Many bad checksums. Weak signal or interference; add an LNA and filter.
• No satellite on the map. A position needs the ephemeris packet; stay with the pass longer.
• Frequency not found. Orbcomm downlinks change per satellite; leave the full band in the capture.
• TLEs not loading. Download them from View > Passes (see Privacy).

Tips

Orbcomm and weather satellites (see Tutorial-Meteor-M-LRPT) share the 137 MHz antenna and filter.

Legal and privacy notes

Orbcomm carries commercial telemetry. The packets are partly structured data from customers' equipment; receiving them is legal in many countries but check yours, and do not pass on what you find. See Legal.

Under the hood

137 to 138 MHz downlinks, 4800 bit/s SDPSK, framed packets with a checksum, ephemeris broadcast once per second; satellite positions computed from the broadcast orbit elements.

Related

Mode-Orbcomm, Tutorial-Meteor-M-LRPT, Tutorial-CubeSat-telemetry, Tutorial-Weather-satellite-images, Troubleshooting