SIM7600: power the active GNSS antenna on every modem start

An active antenna on the roof received nothing until the modem's
antenna supply was switched on (AT+CVAUXV=3050, AT+CVAUXS=1); then +5.5
dB over the passive one. The settings reset on every modem restart and
AT&W doesn't keep them, so interface 03 is reserved as /dev/sim7600-at
(ModemManager ignores it) and a udev-started oneshot re-applies them.
gpsd now re-attaches via gpsdctl after re-enumeration. Verified with a
live modem reset: antenna, LTE and the GNSS fix all recovered unattended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/impl/reports/DEVICE-NAMING.md

@@ -20,6 +20,7 @@ can point at a different device after a reboot or replug.
 | Onboard MTTCAN | platform device `c310000.mttcan` | `can_onboard` |
 | PEAK PCAN-USB | USB `0c72:000c` | `can_pcan` |
 | SIM7600 NMEA port | USB `1e0e:9001`, interface 01 | `/dev/sim7600-gps` (symlink) |
+| SIM7600 spare AT port | USB `1e0e:9001`, interface 03 (ModemManager ignores it) | `/dev/sim7600-at` (symlink) |
 | SIM7600 modem | ModemManager `simtech` plugin (USB IDs and per-interface port probing) | n/a, nothing configured by port name |
 | SIM7600 data | NetworkManager `povo` connection with `ifname '*'` | n/a, follows whatever modem ModemManager exposes |
 

docs/impl/reports/SIM7600-SETUP.md

@@ -22,15 +22,18 @@ date: 2026-10-07
 
 ### USB interfaces (PID 9001)
 
-| Iface | Port | Driver | Role (as ModemManager sees it) |
+| Iface | Port (typical) | Driver | Role |
-|-------|------|--------|-------------------------------|
+|-------|------|--------|------|
-| 0 | ttyUSB0 | option | diagnostics (ignored) |
+| 0 | ttyUSB0 | option | diagnostics (ignored by ModemManager) |
-| 1 | ttyUSB1 | option | **NMEA GPS** → `/dev/sim7600-gps` |
+| 1 | ttyUSB1 | option | **NMEA GPS** → `/dev/sim7600-gps` (gpsd) |
-| 2 | ttyUSB2 | option | AT (primary) |
+| 2 | ttyUSB2 | option | AT, used by ModemManager |
-| 3 | ttyUSB3 | option | AT |
+| 3 | ttyUSB3 | option | AT, **reserved for us** → `/dev/sim7600-at`, ModemManager told to ignore it |
 | 4 | ttyUSB4 | option | audio |
 | 5 | `cdc-wdm0` + `wwan0` | **qmi_wwan** (self-built) | QMI control + network |
 
+⚠ The `ttyUSB` numbers are **not** stable: after a modem reset they shifted (e.g. NMEA on ttyUSB2, AT on ttyUSB3/4) because the old
+nodes were still held open. Use the `/dev/sim7600-*` symlinks or `/dev/serial/by-id/*-ifNN-port0`, never a bare `ttyUSBn`.
+
 ## Problems hit and fixes
 
 ### 1. `sim-missing`: SIM inserted after the modem powered up
@@ -130,9 +133,12 @@ brief NM connectivity-check penalty (`+20000`) after WiFi returns.
 
 - GNSS engine was already running (`AT+CGPS?` → `1,1`). Set to start on boot: `AT+CGPSAUTO=1` (persists in modem).
 - ModemManager location is **not** used. gpsd owns the NMEA port; ModemManager only labels it `(gps)` and leaves it unopened.
-- Stable device name, `/etc/udev/rules.d/99-sim7600-gps.rules`:
+- Stable device names, `/etc/udev/rules.d/99-sim7600-gps.rules`:
   ```
-  SUBSYSTEM=="tty", ATTRS{idVendor}=="1e0e", ATTRS{idProduct}=="9001", ENV{ID_USB_INTERFACE_NUM}=="01", SYMLINK+="sim7600-gps"
+  # if01: NMEA. Re-attach gpsd on every appearance (USBAUTO is off, so gpsd would otherwise lose it after a modem reset).
+  SUBSYSTEM=="tty", ATTRS{idVendor}=="1e0e", ATTRS{idProduct}=="9001", ENV{ID_USB_INTERFACE_NUM}=="01", SYMLINK+="sim7600-gps", RUN+="/usr/sbin/gpsdctl add /dev/sim7600-gps"
+  # if03: spare AT port, reserved for us (ModemManager keeps if02). Powers the active GNSS antenna on every modem start.
+  SUBSYSTEM=="tty", ATTRS{idVendor}=="1e0e", ATTRS{idProduct}=="9001", ENV{ID_USB_INTERFACE_NUM}=="03", SYMLINK+="sim7600-at", ENV{ID_MM_PORT_IGNORE}="1", TAG+="systemd", ENV{SYSTEMD_WANTS}+="sim7600-gnss-antenna.service"
   ```
 - `apt install gpsd gpsd-clients`, `/etc/default/gpsd`:
   ```
@@ -146,7 +152,36 @@ Check: `cgps` or `gpspipe -w -n 10`. On 2026-10-07, tested indoors, gpsd saw `nS
 Next: test with the GNSS antenna (IPEX, labeled GNSS on the HAT) with a clear view of the sky. A cold start can take a few minutes.
 
 **2026-10-09, in the car:** first fix, `mode: 3` (3D) with 4 of 9 satellites used, GPS time valid. Passive GNSS antenna on the dashboard, facing the sky. No setup changes were needed;
-the indoor tests simply lacked sky view. If fixes prove slow or drop out while driving, try an active antenna (check the HAT supplies antenna bias first).
+the indoor tests simply lacked sky view.
+
+#### Active antenna (2026-10-09)
+
+Swapped to an active GNSS antenna (spec 2.7–5.0 V) on the car roof. At first it **received nothing** (8 of 9 satellites at 0 dB-Hz):
+the modem's antenna supply was off. `AT+CVAUXS?` → `0`, `AT+CVAUXV?` → `1700` (mV, below the antenna's minimum).
+
+```
+AT+CVAUXV=3050      # modem range 1700–3050 mV; 3.05 V is inside the antenna's 2.7–5.0 V
+AT+CVAUXS=1         # switch the supply on → fix back within 1 s
+```
+
+| | Passive, dashboard | Active, roof (powered) |
+|---|---|---|
+| Satellites used | 6.3 avg (max 7) | 6.5 avg (max 7) |
+| Signal of used satellites | 31.3 dB-Hz | **36.8 dB-Hz** (peaks 43–47) |
+
+Satellites used barely change; that count looks limited by which constellations are enabled (GPS and GLONASS in use; BeiDou visible in
+NMEA; QZSS/Galileo not checked yet). The gain is ~5.5 dB of margin, which is what matters under trees and between buildings.
+
+**Both settings are volatile.** They reset to off/1.7 V on every modem restart (`AT+CRESET` tested), and `AT&W` does not save them. So:
+
+- `/usr/local/sbin/sim7600-gnss-antenna` sends them over `/dev/sim7600-at` (retrying while the modem boots) and verifies them;
+- `sim7600-gnss-antenna.service` (oneshot, `BindsTo=dev-sim7600\x2dat.device`) runs it, started by the udev rule above whenever
+  interface 03 appears: boot, hot-plug, modem reset.
+
+Verified with `mmcli -m any --reset` while everything was running: the antenna supply was re-applied as soon as the ports returned,
+LTE reconnected ~50 s after the reset, and gpsd re-attached via `gpsdctl` with a 3D fix.
+
+With a **passive** antenna the supply does no harm, but it can be turned off by disabling the service.
 
 ## Rebuilding qmi_wwan after a kernel update
 
@@ -163,6 +198,7 @@ sudo modprobe qmi_wwan && sudo systemctl restart ModemManager
 
 ## Useful one-offs
 
-- Talk to the modem directly: `sudo systemctl stop ModemManager`, then use a terminal on `/dev/ttyUSB2`; start ModemManager again afterwards.
+- Talk to the modem directly: use **`/dev/sim7600-at`**. ModemManager ignores it, so there's no need to stop anything
-  (ModemManager holds ttyUSB2/3 while running.)
+  (e.g. `picocom -b 115200 /dev/sim7600-at`, or `minicom -D /dev/sim7600-at`). Don't send commands that change the data
+  connection behind ModemManager's back.
 - Verbose ModemManager log incl. AT traffic: `sudo mmcli -G debug` then `journalctl -u ModemManager -f` (resets on restart).