RX and TX offsets

Offsets are small shifts added to the nominal downlink and uplink frequencies shown in the frequency panel. They are saved per satellite so your favourite AO-91 spot stays remembered next pass.

What you see in the panel

• Downlink / Uplink — centre frequencies from the satellite database (plus Doppler when tracking).
• RX offset — added to the receive frequency sent to the radio.
• TX offset — added to the transmit frequency.

Doppler correction is separate — OscarWatch moves the “centre” as the satellite passes. Offsets are your choice of where to sit inside the passband.

When to use RX offset

Situation	What to do
Linear SSB/CW — signal not on the nominal frequency	Turn the receive dial until you hear the station, or set RX offset in the panel. OscarWatch can learn from your dial on many rigs.
FM downlink — repeater sounds off-centre	Small RX offset (a few kHz) if the transponder passband is wide and you want a clearer spot.
ISS SSTV / beacon	Only RX matters — adjust RX offset until the picture or beacon is clean.
Testing	Leave offsets at 0 until you are on the air; then tweak once you hear yourself or a reference.

When to use TX offset

Situation	What to do
Linear transponder — you hear yourself off-frequency	Adjust TX offset so your downlink echo lines up with others (often a few kHz on SSB).
FM uplink	Usually leave TX offset at 0 if the database uplink is correct; use offset only if the channel plan for that bird needs a fixed shift.
Split across two radios	Each leg still uses the same logical uplink — TX offset applies to the uplink radio’s frequency.

Offsets vs spinning the dial

On linear passes, many operators hunt by ear on receive first. OscarWatch remembers that shift as an offset so Doppler can continue without losing your spot. Typing offsets in the panel does the same thing if you already know your favourite kHz correction.

On FM with CAT locking the dial, use the offset fields (or Pause CAT) instead of expecting to spin the knob during tracking.

Resetting

• Clear RX/TX offset fields to 0 to return to database centres (Doppler still applies).
• Offsets persist in your settings for that satellite — handy for regular birds, but reset when you change transponder mode.