OscarWatch SSTV

Tools → OscarWatch SSTV decodes SSTV pictures sent from space, such as the ARISS SSTV events from the ISS on 145.800 MHz FM. It is receive only: OscarWatch never transmits SSTV.

Modes

OscarWatch decodes the modes used from space:

• Robot 36 and Robot 72
• Scottie S1, S2 and DX
• Martin M1 and M2
• Wraase SC2-180
• PD 50, 90, 120, 160, 180, 240 and 290 (the ISS usually sends PD 120 or PD 180)

The mode is picked automatically from the VIS header at the start of each picture. If you need to, choose a mode in the Mode list to decode everything as that mode.

Getting started

1. Connect your receiver audio to the PC (soundcard line in, or the radio's USB audio).
2. Open Tools → OscarWatch SSTV, press Settings on the toolbar and choose the Input device. The same dialogue holds the slant, AFC, Doppler and saving options.
3. Press Start. The status line says when it is listening and which satellite is in focus.
4. Pictures appear line by line as they arrive, and each finished picture is added to Received pictures.

Automatic start

In SSTV Settings, tick Start when the focused satellite rises to listen for the satellite selected in the main window. Listening starts when it climbs through the start elevation, and stops when it sets. The default is −5°, so the radio, Doppler tracking and audio have a few degrees to settle before AOS. You can set the elevation from −15° to 30°. The SSTV window must stay open.

Signal display

The Signal panel shows the receive audio as a spectrum (top) and a scrolling waterfall (below), from 900 to 2600 Hz. Markers sit at 1200 Hz (sync), 1500 Hz (black), 1900 Hz (VIS leader, dashed) and 2300 Hz (white). A picture being sent shows a steady stripe on the 1200 Hz marker with picture activity between 1500 and 2300 Hz. If the stripe sits beside the marker, the audio is off tune; AFC corrects small offsets.

Sync, slant and tuning

• Automatic slant correction measures the line timing from the sync pulses, so clock error in the sender or your soundcard does not lean the picture. With it off, set Clock trim (ppm) by hand. Re-sync redraws the picture with the timing measured across all of it.
• AFC follows the sync tone frequency line by line, so a tuning error or Doppler drift does not shift the colours. The status strip shows the tuning offset and clock error it has measured.
• Find pictures without a VIS header picks up a picture from its regular sync pulses when the header was lost, for example when the satellite is still low at the start of a pass. Such pictures are marked found from sync.
• When the signal goes for about 8 seconds (the sender stops, or the pass ends), the picture is closed and marked partial, so noise is not drawn into the rest of it. Skip picture drops the current picture and listens for the next one.

Doppler

On FM (the ISS and most SSTV from space) a small frequency error does not move the audio tones, so no correction is needed beyond normal FM Doppler tuning. On SSB, rig control tunes in steps and the audio moves between steps. With Rig Doppler correction (SSB) ticked and CAT tracking running, OscarWatch removes that left-over offset from the audio before decoding. The status strip shows the downlink Doppler and, on SSB, the correction being applied.

Where pictures are saved

Pictures are saved as PNG files in:

%AppData%\OscarWatch\sstv\

File names give the UTC time, the satellite in focus and the mode, for example 20241012_073105Z_ISS_Pd120.png. With Save pictures automatically off, select a picture in the list and press Save picture. Open SSTV folder opens the folder.

With Save receive audio ticked, the receive audio is also kept as a WAV file in the same folder (about 1.4 MB a minute), so you can decode a pass again later. These files are not deleted automatically.

Sharing with the OscarWatch.org gallery

Tick Share pictures with the OscarWatch.org SSTV gallery in SSTV Settings to send your pictures to the SSTV gallery, credited to the callsign on your website profile. The option is off by default and stays unavailable until an OscarWatch.org API token is saved in Settings → OscarWatch.

Nothing is uploaded during a pass. Pictures wait until the satellite they came from has set, then go one at a time in the background, so decoding is never disturbed. Only finished pictures from live receive with a sync lock of 50% or more and a satellite in focus are shared. Pictures decoded from a recording are not. The queue is kept if you close OscarWatch and carries on next time. A line under the decoder status in the SSTV window shows what is waiting and the result of the last upload. The gallery accepts up to 20 pictures an hour; anything over the limit is sent later.

Picture library

The grid button beside Received pictures opens the picture library, which shows every picture in the SSTV folder grouped by day, newest first. Filter by period (today, the last 7 or 30 days, or your own dates), satellite and mode. Select a picture to see it full size with its details, double-click to open it, or use Show in folder and Delete. New pictures appear as they are saved.

Decoding a recording

Decode recording on the toolbar decodes a WAV or MP3 recording of a pass, including recordings made by OscarWatch pass recording or other software. The file picker opens in the SSTV folder, where saved receive audio is kept. A recording may hold several pictures; each one is added to the list. Formats other than WAV need ffmpeg on PATH.

Problems?

• Nothing decodes: check the input level moves during a picture, and that the right device is selected.
• Colours are wrong or striped: leave AFC on, and make sure the radio is not on a narrow data filter or audio processing.
• The picture leans: leave automatic slant correction on and press Re-sync after the picture.
• The picture starts half way down: the start was missed, so the decoder joined from the sync pulses. That is normal early in a pass.