fix(gps): correct UBX NAV-SVINFO and NAV-SAT satellite decoding#524
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mrosseel wants to merge 3 commits into
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fix(gps): correct UBX NAV-SVINFO and NAV-SAT satellite decoding#524mrosseel wants to merge 3 commits into
mrosseel wants to merge 3 commits into
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The repeated block in UBX-NAV-SVINFO is chn, svid, flags, quality, cno, elev, azim — but the parser started reading at chn, so every field was shifted one byte: satellite IDs were channel numbers, the used flag was svid bit 0, and the C/N0 was the 0-7 quality indicator. Because "cno" was really the quality indicator (>=1 even for idle SBAS/QZSS channels still searching), the sats-seen count was inflated to near the full channel count, and the per-satellite used flags were random — which is why uSat was previously treated as stale and ignored. With the offsets fixed: - nSat only counts satellites with an actual signal (cno > 0) - uSat from SVINFO is valid, so report it as the used count instead of relying solely on NAV-SOL - elevation/azimuth are decoded as signed values per the spec Also update the used count from NAV-PVT numSV, which was parsed but never surfaced — on protVer >= 15 receivers gpsd enables NAV-PVT instead of NAV-SOL, so the used count stayed 0 forever despite a valid fix. And actually set got_sat_update when NAV-SAT arrives so the SVINFO fallback defers to it as intended. Verified against a live MAX-M8 (PROTVER 14) stream: SVINFO uSat now matches NAV-SOL numSV, and IDs/C-N0 match gpsd's own SKY decode. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NrdGJ1s3jmA9qzV8HAsdZs
Newer u-blox receivers (later M8 firmware, M9, M10) get NAV-PVT + NAV-SAT from gpsd instead of NAV-SOL + NAV-SVINFO, so this path is what recent PiFinder GPS units actually exercise. - take "used" from the dedicated svUsed flag (bit 3) instead of inferring it from the quality indicator - only count satellites with an actual signal: NAV-SAT lists every known satellite, so nSat was inflated by idle entries - decode elevation/azimuth as signed values Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NrdGJ1s3jmA9qzV8HAsdZs
During cold start the receiver reports an estimated C/N0 for almanac-predicted satellites it is still trying to confirm, so a cno > 0 filter makes the seen count start around 20+ and sink to the real value as candidates fail to confirm. Gate on the quality indicator instead: only code-locked signals (quality >= 4) count as seen, in both NAV-SVINFO and NAV-SAT. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NrdGJ1s3jmA9qzV8HAsdZs
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@mrosseel is this still waiting on some additional commits, or is it ready to merge? |
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Problem
The satellite counts shown by PiFinder (e.g.
GPS 20/0on the status screen) were wrong on all GPS receiver generations, in two different ways:_parse_nav_svinforead every per-satellite field one byte off. The UBX repeated block ischn, svid, flags, quality, cno, elev, azim, but parsing started atchn. So satellite IDs were channel numbers, the per-sat used flag wassvid & 1(random), and "C/N0" was really the 0–7 quality indicator. Since quality is ≥1 even for idle SBAS/QZSS channels still searching, the seen count was inflated to near the full channel count (the mysterious "20 seen" with nothing tracked), anduSatwas garbage — which is presumably why it was treated as stale and ignored in favour of NAV-SOL.numSVwas parsed but never surfaced, and NAV-SOL never arrives on these units.Fix
_parse_nav_svinfo: shift field offsets by one per the u-blox spec; decode elev/azim as signed; count only satellites with an actual signal as seen. With correct offsets thesvUsedflag is valid, so SVINFO now reports a trustworthyuSat.gps_ubx.py: reportuSatfrom SVINFO instead of the stale-info workaround; update the used count from NAV-PVTnumSV; actually setgot_sat_updatewhen NAV-SAT arrives so the SVINFO fallback defers to it as intended._parse_nav_sat: take used from the dedicatedsvUsedbit (bit 3) instead of the quality heuristic (which also counts tracked-but-unused sats); don't count zero-signal entries as seen; decode elev/azim as signed.Verification
tests/test_gps_ubx_parser.py) with spec-built payloads for both message types, including the idle-channel inflation case.nSat 11 / uSat 6, PRNs, C/N0) exactly matches NAV-SOLnumSVand gpsd's own SKY decode; the old code produced the inflated counts on the same stream.The NAV-PVT/NAV-SAT path is verified against the spec and unit tests only — a sanity check on a unit with a newer (M9/M10) module would be welcome.
🤖 Generated with Claude Code
https://claude.ai/code/session_01NrdGJ1s3jmA9qzV8HAsdZs