twislaandClaude Opus 5.5
f812c62a3d
A temporary `gnss probe` command listens on both pin orders at 115200 and 9600. Only RX 15 / TX 13 at 115200 carries NMEA, as Meshtastic's board file says; M5Stack's GPIO 8/9 are the keyboard's I2C bus. The output format (NMEA 4.10, GSA system IDs, GSV per signal) is recorded in docs/milestones/M2.md. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT
4.3 KiB
M2 — GNSS
Goal: the device knows where it is and what time it is without a network: a GNSS Service in the background, a GNSS App with the position and a sky view of the satellites, the clock set from satellites when there's no NTP, and Tracks recorded to the SD card.
Hardware: the Cap LoRa-1262 carries an ATGM336H-6N (AT6668), multi-constellation (GPS, BeiDou, Galileo, GLONASS, QZSS), with a ceramic antenna. NMEA over UART, 115200 8N1. Measured (step 1, gnss probe): RX GPIO 15, TX GPIO 13, 115200 8N1, as in Meshtastic's board file; M5Stack's page names GPIO 8 and 9, which are the internal I2C bus the keyboard controller sits on. Output is NMEA 4.10 style: GN RMC, VTG and GGA, one GSA per constellation with the system ID (1 GPS, 2 GLONASS, 3 Galileo, 4 BeiDou, 5 QZSS) in its last field, and a GSV sequence per constellation and signal (GP, GL, GA, …) with the signal ID last. RMC carries a time even without a Fix (status V), so only a Fix makes it trustworthy.
Decisions (design round 2026-10-04)
| # | Decision |
|---|---|
| Q58 | Settings has a GNSS On/Off switch, On by default. Off puts the receiver in standby if it accepts a command for it (measured in step 1); otherwise the firmware stops listening. |
| Q59 | The GNSS App has two views, switched with Tab. Position: latitude, longitude, altitude, speed, course, Fix (none / 2D / 3D), satellites used and in view, HDOP, UTC time. Sky: the satellites placed by azimuth and elevation, coloured by constellation, filled when used in the Fix. |
| Q60 | "Radar" in M2 means the Sky view. A radar of other Nodes by distance and bearing needs the mesh: M4. |
| Q61 | The Status Bar shows a GNSS mark: absent when off, muted while searching, normal with a 2D Fix, with the satellite count with a 3D Fix. |
| Q62 | GNSS time sets the clock when NTP hasn't this boot. NTP stays preferred when online. |
| Q63 | A Track is started and stopped in the GNSS App. It's written as GPX to /gnss/tracks/<YYYYMMDD-HHMMSS>.gpx, a point every 5 s when the position moved more than 5 m. It keeps recording with the App closed, with a Toast on start and stop and a Status Bar mark, and gets its own Storage Clean-up category. |
| Q64 | Coordinates in decimal degrees plus the Maidenhead locator; a Settings switch for degrees, minutes and seconds. Metric units only. |
| Q65 | The position never leaves the device in M2. Sharing it over the mesh, and at what precision, is decided in M4. |
| Q66 | Our own NMEA parser, host-tested: RMC, GGA, GSA and GSV, with each talker ID mapped to its constellation. TinyGPSPlus (named in ADR 0001) doesn't track the satellite list across constellations, which the Sky view needs. |
| Q67 | The receiver keeps its defaults (all constellations, 1 Hz). No receiver settings. Time to first fix is measured and recorded here. |
| Q68 | Debug aids: gnss status, and gnss nmea on/off to stream the raw sentences to the console (USB serial and Debug Console). Raw NMEA is never written to the card. |
Done when
- The GNSS Service reads NMEA in the background whatever App is on screen, and a 3D Fix appears outdoors.
- The GNSS App shows the Position and Sky views, both live.
- The Status Bar shows the GNSS mark per Q61.
- With no Wi-Fi, the clock is set from GNSS after the first Fix.
- A Track records while the App is closed, survives the screen turning off, and opens as valid GPX on the PC.
- Settings → GNSS Off stops it (and the Status Bar mark goes away); On brings it back.
- Free heap stays above about 40 KB with GNSS, Wi-Fi, IRC on TLS and the UI running.
Work breakdown
- Hardware check: a
gnss probecommand reads the candidate UART pins and reports which carries NMEA, at what baud rate, and which talker IDs. Then the standby command (Q58) and a first time to first fix. - NMEA parser (host-tested): checksum, RMC, GGA, GSA, GSV across constellations, merged into one GNSS state (Fix, position, time, satellites).
- GNSS Service: UART on its own task, the parser,
gnss statusandgnss nmea, Settings On/Off. - Clock from GNSS (Q62), and the Status Bar mark (Q61).
- GNSS App: Position view, Maidenhead and coordinate formats (host-tested), then the Sky view.
- Tracks: the 5 s / 5 m rule and GPX writing (host-tested), background recording, Clean-up category.