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M2: record cold-start time to first fix and the heap measurement

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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT
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twisla
2026-10-04 22:52:07 +02:00
co-authored by Claude Opus 5.5
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docs/milestones/M2.md
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# M2 — GNSS
**Status:** steps 1–6 done on the device (branch `m2`). Open: the heap floor (below).
## Measured
- **Cold start** (`$PCAS10,2`) to a 3D Fix, by a window: **73 s**, 5 satellites used of 8 in view. A restart of the ESP32 alone keeps the receiver's Fix (the Cap stays powered).
- By a window: 3D Fix from GPS, GLONASS, Galileo and BeiDou, up to 14 of 17 satellites used, HDOP 1.0–1.3.
- **Heap, Debug Build, GNSS on:** 82 KB free with Wi-Fi up; with IRC on TLS, **33 KB free and a 12.6 KB low** during the handshake. The floor is 40 KB, and v0.2.1 (before OTA) measured a 79 KB low. Not a GNSS cost: the OTA and Debug Build work added the `update` (8 KB) and `debug` (6 KB) task stacks, the 6 KB console ring, a larger `storage` stack (10 KB, for SD signature checks), 4 KB of serial buffers, mDNS and two TCP servers. To decide before M2 closes.
**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.