OSSA

Open Source Space Awareness · built on STRF

Satellites, tracked by the radio they transmit

A fixed antenna and a wideband receiver record a band's spectrum around the clock. OSSA predicts the Doppler curve of every catalogued satellite over the station, looks for each in the record, matches the signals it finds to the public catalogue, and keeps the ones that match nothing: the objects the catalogue gets wrong or leaves out.

Open the latest spectrogramHow it works

Design phase. The first OSSA station is being built. The spectrograms here were recorded with STRF at PE0SAT and run through OSSA's pipeline.

400.665 MHz400.638 MHz00:15 to 00:31 UTC
KINEIS-4E heard by the search over PE0SAT on 2026-10-05 at 00:23 UTC, at 400.6515 MHz, 64° up. Time runs left to right and frequency up: the S is its Doppler shift as it passes. Open it in the viewer
2.0hours of spectrum
223passes predicted
41heard by the search
421tracks found by detect
21tracks identified
120labels made by hand

How it works

The method is STRF's, run unattended on whole bands: a satellite's transmitter, seen from the ground, drifts in frequency as it passes, and that curve says which satellite it is and where it went.

  1. Record

    A fixed antenna and a wideband receiver write a band's spectrum around the clock, with no schedule, as STRF's rffft does.

  2. Predict

    Every satellite SatNOGS DB lists a transmitter for in the band, propagated from Space-Track's OMM element sets: a Doppler curve for each pass over the station.

  3. Search

    The spectrum is stacked along each predicted curve: the transmitter is heard on its listed frequency, off it, or not at all. FSK is looked for as its two tones.

  4. Identify

    detect finds signals without being told where to look. Each track is fitted against the whole catalogue; one that fits no object is kept as unidentified.

  5. Label

    People check the results in the viewer and mark what was missed, with Doppler curves only an orbit can make. What labels measure corrects the transmitter listings and tells the search where to look.

Spectrograms

Each run opens in the viewer, newest first: drag to pan, wheel to zoom, with the predicted passes and detect's tracks drawn over the spectrum. With a key, label what you see.

PE0SAT 400.0 to 404.0 MHz

2026-10-05, 00:01 to 02:00 UTC · 2.0 h

  • 421 tracks
  • 223 predicted
  • 41 heard
  • 21 identified

Stations

Where the recordings come from. A station's frequency offset is its receiver's own error, measured from the strong carriers it hears; it is taken out of the transmitter corrections.

StationLocatorRecordedFrequency offset
PE0SATSatNOGS station 1466JO21mr2.0 h in 1 spectrogram+1.95 ppm+0.78 kHz at 400 MHz

Built on STRF

OSSA exists because of STRF, the radio frequency satellite tracking toolkit that Cees Bassa started in 2014 and its contributors have grown since. STRF established the method OSSA runs: record timestamped spectrograms continuously (rffft), overlay the Doppler curves predicted from orbital elements (rfplot), and fit orbits and transmitter frequencies to the measured points (rffit).

OSSA reads and writes STRF's formats, so any stretch of its data opens in rfplot. It does not replace STRF and is not a fork of it; OSSA and STRF sets out what OSSA owes STRF and what it promises in return.