fix(gps): release base position when selecting GPS output

[HTRamsey]

Receivers previously configured as fixed or surveying bases can remain in that state when switched to navigation output. Release the base position during configuration so navigation can resume.

• UBX: disable time mode on detected M8P, F9P, and X20 receivers for both GPS and GPS+RTCM output, including heading-rover and moving-base configurations. Require survey-stop confirmation before reporting readiness or enabling GPS+RTCM correction output.
• Septentrio: restore Rover PVT mode for navigation and avoid enabling RTCM output in GPS-only mode.
• Femto: stop position averaging and clear the fixed position before enabling navigation output. Buffer fragmented acknowledgments and stop promptly on I/O errors.

PX4 Autopilot impact

PX4 normally selects OutputMode::GPS; its main receiver uses GPSAndRTCM when RTCM communication dumping is selected. Both paths now release stale UBX time mode. This also applies to the rover and moving-base receivers used for GPS heading: they must compute their positions rather than retain a stationary base position. See PX4's output-mode selection.

The additional checks run during receiver configuration, including startup and reconfiguration. UBX requires an acknowledged disable command and stop confirmation, with a roughly three-second confirmation deadline. Femto requires acknowledgments for POSAVE OFF and FIX NONE. If these checks fail, configuration returns an error and PX4 retries or continues protocol detection. Receiver/firmware compatibility with these stricter checks still needs physical validation.

The Femto changes apply when PX4 builds and selects that protocol. The SBF changes do not affect PX4's generic GPS module, which does not compile this repository's sbf.cpp; see its driver source selection.

No PX4 caller API, parameter, or build-system changes were identified as necessary for this PR. Adoption requires updating the GPSDrivers submodule and rebuilding. The PX4 revision linked above pins 1b27986, so it does not yet contain this change. Explicit RTCM base-station configuration retains its existing fixed-position/survey behavior.

Validation

Linux CMake build and all 9 CTest tests passed; git diff --check passed. Regression coverage exercises GPS and GPS+RTCM navigation, stop-before-RTCM ordering, legacy and modern UBX protocols, rejected commands, stop timeouts, read cancellation, and fragmented Femto replies with binary noise.

PX4 firmware builds and physical receiver validation were not performed for this PR.

[Copilot]

🟡 Changes recommended

UBX GPS+RTCM navigation still leaves previously configured time mode enabled.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Releases stale GNSS base-station configuration when returning receivers to navigation output.

Changes:

• Disables UBX, Septentrio, and Femto base-position modes.
• Improves fragmented Femto acknowledgment handling.
• Adds receiver-mode regression coverage.

File summaries

File	Description
src/ubx.cpp	Disables UBX time mode and confirms survey stop.
src/ubx.h	Adds UBX time-mode state and helper declaration.
src/sbf.cpp	Restores rover mode and limits RTCM output.
src/femtomes.cpp	Clears base mode and buffers acknowledgments.
tests/gps-ubx-test.cpp	Covers UBX navigation transitions and failures.
tests/gps-receiver-mode-test.cpp	Tests Septentrio and Femto mode changes.
tests/CMakeLists.txt	Registers the new receiver-mode test.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Balanced

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[HTRamsey]

just copying these in to QGC manually instead