M2 step 3: GNSS Service, with standby and the clock from a Fix

The GNSS Service reads the receiver (UART 15/13, 115200, 1 KB buffer)
from its 50 ms tick and feeds the NMEA parser. With a Fix it sets the
clock (GNSS is the most trusted TimeSource) and refreshes it every 10
min. Settings gets GNSS On/Off and the coordinate format (Q64).

Off puts the receiver in standby with $PCAS12,65535, renewed hourly; On
wakes it with a hot start, $PCAS10,0. Both measured on the device: the
output stops within a second, and a command wakes it within a second.

Commands: gnss status (Fix, satellites per constellation, bytes and
sentences), gnss nmea on|off, gnss send <sentence>, gnss restart. The
probe is gone; never drive GPIO 15 (the receiver's output): the first
probe's swapped-pin attempt silenced it until a power cycle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

docs/milestones/M2.md

@@ -8,11 +8,11 @@
 
 | # | 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. |
+| Q58 | Settings has a GNSS On/Off switch, **On by default**. Off puts the receiver in standby. *Measured:* `$PCAS12,<seconds>` (CASIC) stops its output within a second, for up to at least 65535 s, and any command wakes it within a second; Off sends `PCAS12,65535` (renewed hourly), On sends a hot start, `PCAS10,0`. |
 | 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. |
+| Q62 | GNSS time **sets the clock once there's a Fix**, and refreshes it every 10 minutes. *Revised in step 3:* the clock's trust order from M0 (Mesh < NTP < GNSS) already ranks GNSS above NTP, which is right: GNSS time is at least as accurate. So GNSS also corrects a clock NTP set, not only an unset one. |
 | 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. |
@@ -20,6 +20,8 @@
 | 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. |
 
+**Lesson (step 3):** the first probe also tried the pins swapped, driving the receiver's output line from the ESP32 for about a second. The receiver then went silent until a full power cycle (an ESP32 restart doesn't cut the Cap's power). Never drive GPIO 15.
+
 ## Done when
 
 - The GNSS Service reads NMEA in the background whatever App is on screen, and a 3D Fix appears outdoors.