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<!DOCTYPE html>
<html lang="en-GB">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Doppler tuning — OscarWatch Help</title>
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</head>
<body>
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<header>
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<a href="index.html" class="brand-home" title="Home"><img src="oscarwatch-icon.png" alt="" class="brand-icon" width="56" height="56" /></a>
<div class="brand-text">
<h1>Doppler tuning</h1>
<p class="tagline">How often the radio updates — and optional extras for sharp passes</p>
</div>
</div>
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<main>
<p class="lead">
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 <strong>how often</strong> updates are sent, plus two optional helpers for the
steep middle of the pass.
</p>
<h2 id="doppler-cat-thresholds">Doppler thresholds — the basics</h2>
<p>
Under <strong>Settings → Radio</strong>, <strong>Doppler threshold FM</strong> and
<strong>Doppler threshold SSB/CW</strong> work like SatPC32: OscarWatch only sends a new frequency when
the calculated value has moved by <em>more than</em> the threshold since the last CAT write.
</p>
<ul>
<li><strong>Smaller threshold</strong> — more frequent updates, tighter tracking, more serial traffic.</li>
<li><strong>Larger threshold</strong> — fewer updates; fine for wide FM channels, can feel sluggish on linear SSB near TCA.</li>
</ul>
<p>
OscarWatch picks FM vs SSB/CW from the <strong>downlink mode</strong> in the frequency panel:
</p>
<table>
<thead>
<tr><th>Downlink mode</th><th>Setting used</th><th>Typical use</th></tr>
</thead>
<tbody>
<tr><td><strong>FM</strong> / <strong>FMN</strong></td><td>Doppler threshold FM</td><td>FM cubesats, ISS cross-band FM</td></tr>
<tr><td><strong>USB</strong> / <strong>LSB</strong> / <strong>CW</strong></td><td>Doppler threshold SSB/CW</td><td>Linear voice and CW</td></tr>
<tr><td><strong>DATA-USB</strong> / <strong>DATA-LSB</strong></td><td>Always updates (0 Hz)</td><td>Digital modes</td></tr>
</tbody>
</table>
<h3>Ballpark numbers</h3>
<ul>
<li>
<strong>FM (default 350 Hz)</strong> — A 5 kHz FM channel is wide; Doppler drift is small compared with the passband.
<strong>200–500 Hz</strong> is usually fine. Try <strong>150–250 Hz</strong> for slightly tighter tracking;
go higher if CAT feels too busy.
</li>
<li>
<strong>SSB/CW (default 50 Hz)</strong> — Linear transponders need finer steps.
<strong>20–100 Hz</strong> is common; <strong>50 Hz</strong> is a good default.
Set <strong>0 Hz</strong> only if you want every correction (heavy CAT load).
</li>
</ul>
<p>
Doppler changes fastest near <strong>TCA</strong> (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.
</p>
<h2>Adaptive threshold near TCA</h2>
<p>
Checkbox <strong>Adaptive threshold near TCA</strong> is <strong>on by default</strong> for new installs.
In plain terms: on the steep part of the pass (and just after TCA), OscarWatch temporarily uses a
<em>smaller</em> effective threshold for SSB/CW so the radio catches up when Doppler is changing quickly.
</p>
<ul>
<li>Your configured SSB/CW value is still the <strong>ceiling</strong>.</li>
<li>The effective threshold can drop to about <strong>half</strong> (minimum 25 Hz when the setting is 50 Hz).</li>
<li>Turn it <strong>off</strong> if you prefer fewer CAT writes and are happy with manual <a href="radio-offsets.html">offsets</a> near TCA.</li>
</ul>
<h2>Lead Doppler for CAT latency</h2>
<p>
Checkbox <strong>Lead Doppler for CAT latency</strong> is also <strong>on by default</strong> 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
<strong>steep middle</strong> of the pass, tapering off near TCA when the shift crosses zero.
</p>
<ul>
<li><strong>Gentle legs</strong> (typical approach and departure) — lead stays mostly off; you should not notice it at low elevation.</li>
<li><strong>Steep middle / post-TCA</strong> — lead blends in so CAT is less “behind” the signal.</li>
<li>The frequency overlay <strong>Radio</strong> row shows the commanded frequency; <strong>Sat</strong> shows the spacecraft-side value. They match on gentle legs.</li>
</ul>
<h3>Lead time and strength</h3>
<p>When lead is enabled, two extra fields appear:</p>
<table>
<thead>
<tr><th>Setting</th><th>Default (new install)</th><th>What it means</th></tr>
</thead>
<tbody>
<tr>
<td><strong>Lead time (ms)</strong></td>
<td><strong>40</strong></td>
<td>How far ahead to predict. Set <strong>0</strong> for automatic timing (half your CAT delay, capped).</td>
</tr>
<tr>
<td><strong>Lead strength (%)</strong></td>
<td><strong>70</strong></td>
<td>How much of the predicted shift to apply. Lower (e.g. 60–85) if the radio feels <em>ahead</em> of the signal; raise slightly if still <em>behind</em> on fast birds.</td>
</tr>
</tbody>
</table>
<p>
Lead does <strong>not</strong> change CAT pacing delay or the Doppler thresholds — it only adjusts
the frequency OscarWatch <em>asks for</em> when lead is active.
</p>
<div class="tip">
<strong>Simple tuning order:</strong> (1) thresholds, (2) adaptive on/off, (3) lead strength, (4) lead time.
Change one thing at a time and try another pass.
</div>
<h2 id="passband-knob">Spinning the dial (passband trim)</h2>
<p>
Thresholds apply to <strong>automatic</strong> Doppler writes. On linear <strong>USB</strong>, <strong>LSB</strong>,
and <strong>CW</strong>, OscarWatch also watches the <strong>receive</strong> dial when you tune by hand:
</p>
<ul>
<li>Movement more than about <strong>30 Hz</strong> (USB/LSB/CW) from the last CAT receive frequency = you are hunting the passband. Small dial jitter below that is ignored.</li>
<li>While you turn the dial, automatic CAT pauses.</li>
<li>After you stop, OscarWatch waits for <strong>eight</strong> stable reads (~800 ms) before resuming Doppler.</li>
<li>Your hunt is remembered for the <strong>rest of that overhead pass</strong> so Doppler continues around your spot.</li>
<li>When the satellite sets and rises again on the <strong>next pass</strong> (same bird still selected), passband trim is cleared automatically. Stored <a href="radio-offsets.html">RX/TX offsets</a> in settings are not changed.</li>
</ul>
<p>
<strong>ICOM Main/Sub rigs:</strong> receive updates on Main when the dial is stable; uplink (Sub) CAT waits
~<strong>2.5 s</strong> after you stop tuning so the radio does not hop to Sub while you scan.
<strong>Dual radio:</strong> 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.
<strong>FM:</strong> no passband-trim path — both legs update from thresholds (Yaesu FM may lock the dial via CAT).
</p>
<p>
If your hands are <strong>off</strong> the dial, linear modes track continuously like FM — you are not
stuck waiting for the stability timer after every automatic write.
</p>
<h2>CAT delay (ms)</h2>
<p>
Separate from thresholds and lead: <strong>CAT delay</strong> 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.
</p>
<h2>Doppler pass log (CSV)</h2>
<p>
Optional <strong>Log Doppler tuning passes (CSV)</strong> writes one file per visible pass under
<code>%AppData%/OscarWatch/doppler-logs</code>. Use <strong>Open log folder</strong> on the same settings page.
Open <a href="doppler-pass-insights.html">Doppler pass insights</a> (Menu → Help) to load a log — the
<strong>Tuning guide</strong> tab gives plain-language advice on adaptive threshold and Lead Doppler settings.
</p>
<p>
Column <strong>DialTracking</strong> shows how the rig classified each row:
<strong>hands_off</strong> (linear, Main matches CAT — automatic Doppler),
<strong>dial_wait</strong> (dial moving or stability timer),
<strong>dial_track</strong> (passband hunt),
<strong>automatic</strong> (FM/data).
<strong>MainDialHz</strong> and <strong>DialVsCatHz</strong> (Main minus last CAT RX) show phantom knob
movement. <strong>VfoStable</strong> is 1 when the eight-sample dial history agrees.
<strong>RigTracking</strong> is 0 when CAT pause or Standby stopped the loop.
<strong>SkipReason</strong> explains withheld writes (<code>cat_paused</code>, <code>vfo_unstable</code>,
<code>below_threshold</code>, <code>doppler_suspend</code>, <code>sub_cooldown</code>).
Events <code>cat_pause_start</code>, <code>cat_paused</code>, and <code>cat_pause_end</code> annotate
operator CAT pause — no more silent gaps.
</p>
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