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>
This commit is contained in:
mikkeli
@@ -22,15 +22,18 @@ date: 2026-10-07
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### USB interfaces (PID 9001)
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| Iface | Port | Driver | Role (as ModemManager sees it) |
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|-------|------|--------|-------------------------------|
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| 0 | ttyUSB0 | option | diagnostics (ignored) |
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| 1 | ttyUSB1 | option | **NMEA GPS** → `/dev/sim7600-gps` |
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| 2 | ttyUSB2 | option | AT (primary) |
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| 3 | ttyUSB3 | option | AT |
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| Iface | Port (typical) | Driver | Role |
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|-------|------|--------|------|
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| 0 | ttyUSB0 | option | diagnostics (ignored by ModemManager) |
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| 1 | ttyUSB1 | option | **NMEA GPS** → `/dev/sim7600-gps` (gpsd) |
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| 2 | ttyUSB2 | option | AT, used by ModemManager |
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| 3 | ttyUSB3 | option | AT, **reserved for us** → `/dev/sim7600-at`, ModemManager told to ignore it |
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| 4 | ttyUSB4 | option | audio |
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| 5 | `cdc-wdm0` + `wwan0` | **qmi_wwan** (self-built) | QMI control + network |
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⚠ The `ttyUSB` numbers are **not** stable: after a modem reset they shifted (e.g. NMEA on ttyUSB2, AT on ttyUSB3/4) because the old
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nodes were still held open. Use the `/dev/sim7600-*` symlinks or `/dev/serial/by-id/*-ifNN-port0`, never a bare `ttyUSBn`.
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## Problems hit and fixes
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### 1. `sim-missing`: SIM inserted after the modem powered up
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@@ -130,9 +133,12 @@ brief NM connectivity-check penalty (`+20000`) after WiFi returns.
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- GNSS engine was already running (`AT+CGPS?` → `1,1`). Set to start on boot: `AT+CGPSAUTO=1` (persists in modem).
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- ModemManager location is **not** used. gpsd owns the NMEA port; ModemManager only labels it `(gps)` and leaves it unopened.
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- Stable device name, `/etc/udev/rules.d/99-sim7600-gps.rules`:
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- Stable device names, `/etc/udev/rules.d/99-sim7600-gps.rules`:
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```
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SUBSYSTEM=="tty", ATTRS{idVendor}=="1e0e", ATTRS{idProduct}=="9001", ENV{ID_USB_INTERFACE_NUM}=="01", SYMLINK+="sim7600-gps"
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# if01: NMEA. Re-attach gpsd on every appearance (USBAUTO is off, so gpsd would otherwise lose it after a modem reset).
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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"
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# if03: spare AT port, reserved for us (ModemManager keeps if02). Powers the active GNSS antenna on every modem start.
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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"
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```
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- `apt install gpsd gpsd-clients`, `/etc/default/gpsd`:
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```
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@@ -146,7 +152,36 @@ Check: `cgps` or `gpspipe -w -n 10`. On 2026-10-07, tested indoors, gpsd saw `nS
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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.
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**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;
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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).
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the indoor tests simply lacked sky view.
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#### Active antenna (2026-10-09)
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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):
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the modem's antenna supply was off. `AT+CVAUXS?` → `0`, `AT+CVAUXV?` → `1700` (mV, below the antenna's minimum).
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```
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AT+CVAUXV=3050 # modem range 1700–3050 mV; 3.05 V is inside the antenna's 2.7–5.0 V
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AT+CVAUXS=1 # switch the supply on → fix back within 1 s
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```
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| | Passive, dashboard | Active, roof (powered) |
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|---|---|---|
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| Satellites used | 6.3 avg (max 7) | 6.5 avg (max 7) |
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| Signal of used satellites | 31.3 dB-Hz | **36.8 dB-Hz** (peaks 43–47) |
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Satellites used barely change; that count looks limited by which constellations are enabled (GPS and GLONASS in use; BeiDou visible in
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NMEA; QZSS/Galileo not checked yet). The gain is ~5.5 dB of margin, which is what matters under trees and between buildings.
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**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:
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- `/usr/local/sbin/sim7600-gnss-antenna` sends them over `/dev/sim7600-at` (retrying while the modem boots) and verifies them;
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- `sim7600-gnss-antenna.service` (oneshot, `BindsTo=dev-sim7600\x2dat.device`) runs it, started by the udev rule above whenever
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interface 03 appears: boot, hot-plug, modem reset.
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Verified with `mmcli -m any --reset` while everything was running: the antenna supply was re-applied as soon as the ports returned,
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LTE reconnected ~50 s after the reset, and gpsd re-attached via `gpsdctl` with a 3D fix.
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With a **passive** antenna the supply does no harm, but it can be turned off by disabling the service.
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## Rebuilding qmi_wwan after a kernel update
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@@ -163,6 +198,7 @@ sudo modprobe qmi_wwan && sudo systemctl restart ModemManager
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## Useful one-offs
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- Talk to the modem directly: `sudo systemctl stop ModemManager`, then use a terminal on `/dev/ttyUSB2`; start ModemManager again afterwards.
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(ModemManager holds ttyUSB2/3 while running.)
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- Talk to the modem directly: use **`/dev/sim7600-at`**. ModemManager ignores it, so there's no need to stop anything
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(e.g. `picocom -b 115200 /dev/sim7600-at`, or `minicom -D /dev/sim7600-at`). Don't send commands that change the data
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connection behind ModemManager's back.
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- Verbose ModemManager log incl. AT traffic: `sudo mmcli -G debug` then `journalctl -u ModemManager -f` (resets on restart).
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