roro9stack/docs/milestones/M2.md

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. Meshtastic's board file for this hardware uses GPIO 15 (ESP32 RX) and 13 (TX); M5Stack's page names GPIO 8 and 9, which the keyboard controller already uses. Step 1 checks on the device.

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

1. Hardware check: a gnss probe command 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.
2. NMEA parser (host-tested): checksum, RMC, GGA, GSA, GSV across constellations, merged into one GNSS state (Fix, position, time, satellites).
3. GNSS Service: UART on its own task, the parser, gnss status and gnss nmea, Settings On/Off.
4. Clock from GNSS (Q62), and the Status Bar mark (Q61).
5. GNSS App: Position view, Maidenhead and coordinate formats (host-tested), then the Sky view.
6. Tracks: the 5 s / 5 m rule and GPX writing (host-tested), background recording, Clean-up category.