Doppler tuning

As a satellite passes overhead, its signal shifts in frequency (Doppler). OscarWatch calculates that shift and updates your radio — but it does not need to chatter on the serial port every millisecond. These settings control how often updates are sent, plus two optional helpers for the steep middle of the pass.

Doppler thresholds — the basics

Under Settings → Radio, Doppler threshold FM and Doppler threshold SSB/CW work like SatPC32: OscarWatch only sends a new frequency when the calculated value has moved by more than the threshold since the last CAT write.

• Smaller threshold — more frequent updates, tighter tracking, more serial traffic.
• Larger threshold — fewer updates; fine for wide FM channels, can feel sluggish on linear SSB near TCA.

OscarWatch picks FM vs SSB/CW from the downlink mode in the frequency panel:

Downlink mode	Setting used	Typical use
FM / FMN	Doppler threshold FM	FM cubesats, ISS cross-band FM
USB / LSB / CW	Doppler threshold SSB/CW	Linear voice and CW
DATA-USB / DATA-LSB	Always updates (0 Hz)	Digital modes

Ballpark numbers

• FM (default 350 Hz) — A 5 kHz FM channel is wide; Doppler drift is small compared with the passband. 200–500 Hz is usually fine. Try 150–250 Hz for slightly tighter tracking; go higher if CAT feels too busy.
• SSB/CW (default 50 Hz) — Linear transponders need finer steps. 20–100 Hz is common; 50 Hz is a good default. Set 0 Hz only if you want every correction (heavy CAT load).

Doppler changes fastest near TCA (time of closest approach). If tracking feels “behind” in the middle of a high pass, try a lower SSB/CW threshold before changing the optional extras below.

Adaptive threshold near TCA

Checkbox Adaptive threshold near TCA is on by default for new installs. In plain terms: on the steep part of the pass (and just after TCA), OscarWatch temporarily uses a smaller effective threshold for SSB/CW so the radio catches up when Doppler is changing quickly.

• Your configured SSB/CW value is still the ceiling.
• The effective threshold can drop to about half (minimum 25 Hz when the setting is 50 Hz).
• Turn it off if you prefer fewer CAT writes and are happy with manual offsets near TCA.

Lead Doppler for CAT latency

Checkbox Lead Doppler for CAT latency is also on by default for new installs. Radios and USB serial links are not instant — by the time a frequency command arrives, the satellite has already moved slightly. Lead asks “where will Doppler be in a moment?” and tunes ahead on the steep middle of the pass, tapering off near TCA when the shift crosses zero.

• Gentle legs (typical approach and departure) — lead stays mostly off; you should not notice it at low elevation.
• Steep middle / post-TCA — lead blends in so CAT is less “behind” the signal.
• The frequency overlay Radio row shows the commanded frequency; Sat shows the spacecraft-side value. They match on gentle legs.
• DATA-USB, DATA-LSB, and DATA-FM ignore lead and use the current range rate. SSB, CW, and FM still follow the checkbox.

Lead time and strength

When lead is enabled, two extra fields appear:

Setting	Default (new install)	What it means
Lead time (ms)	40	How far ahead to predict. Set 0 for automatic timing (half your CAT delay, capped).
Lead strength (%)	70	How much of the predicted shift to apply. Lower (e.g. 60–85) if the radio feels ahead of the signal; raise slightly if still behind on fast birds.

Lead does not change CAT pacing delay or the Doppler thresholds — it only adjusts the frequency OscarWatch asks for when lead is active.

Spinning the dial (passband trim)

Thresholds apply to automatic Doppler writes. On linear USB, LSB, and CW, OscarWatch also watches the receive dial when you tune by hand:

• Movement more than about 30 Hz (USB/LSB/CW) from the last CAT receive frequency = you are hunting the passband. Small dial jitter below that is ignored.
• While you turn the dial, automatic CAT pauses.
• After you stop, OscarWatch waits until the dial has been still for Resume CAT after dial (default 800 ms, Settings → Radio) before resuming Doppler.
• Your hunt is remembered for the rest of that overhead pass so Doppler continues around your spot.
• When the satellite sets and rises again on the next pass (same bird still selected), passband trim is cleared automatically. Stored RX/TX offsets in settings are not changed.

ICOM Main/Sub rigs: receive updates on Main when the dial is stable; uplink (Sub) CAT waits for Uplink CAT wait after dial (default 2500 ms, same settings page) after you stop tuning so the radio does not hop to Sub while you scan. Dual radio: only the downlink CAT pauses while you spin the RX radio; uplink keeps tracking Doppler and follows the same NOR/REV passband trim so TX lines up with your RX hunt. FM: no passband-trim path — both legs update from thresholds (Yaesu FM may lock the dial via CAT).

If your hands are off the dial, linear modes track continuously like FM — you are not stuck waiting for the stability timer after every automatic write.

CAT delay (ms)

Separate from thresholds and lead: CAT delay spaces out CI-V and similar commands so the serial link and radio are not flooded. Increase it if commands are missed; it does not replace threshold tuning.

Doppler pass log (CSV)

Optional Log Doppler tuning passes (CSV) writes one file per visible pass under %AppData%/OscarWatch/doppler-logs. Use Open log folder on the same settings page. CSVs older than 14 days are removed automatically (same retention as main app logs). Open Doppler pass insights (Menu → Help) to load a log — the Tuning guide tab gives plain-language advice on adaptive threshold and Lead Doppler settings.

Column DialTracking shows how the rig classified each row: hands_off (linear, Main matches CAT — automatic Doppler), dial_wait (dial moving or stability timer), dial_track (passband hunt), automatic (FM/data). MainDialHz and DialVsCatHz (Main minus last CAT RX) show phantom knob movement. VfoStable is 1 when the receive dial has been still for the configured resume wait. RigTracking is 0 when CAT pause or Standby stopped the loop. SkipReason explains withheld writes (cat_paused, vfo_unstable, below_threshold, doppler_suspend, sub_cooldown). Events cat_pause_start, cat_paused, and cat_pause_end annotate operator CAT pause — no more silent gaps.