GPS antenna placement

Summary¶

GPS satellite signals arrive at −130 dBm — ten trillion times weaker than a WiFi router. The GPS antenna must have an unobstructed sky view with no electrically conductive material above it, maximum distance from all high-current wiring, and no conducted noise on its supply line. Carbon fibre is electrically conductive and blocks GPS signals entirely — an antenna above a CF plate receives nothing. On libdrone, the M10Q GPS module sits at the top of the nose bracket above the camera, providing a clear 180° sky view and maximum separation from the ESC, battery, and motor wires.

Concept¶

Signal strength and why placement is non-negotiable¶

GPS L1 signals arrive from satellites at approximately −130 dBm. To put this in context:

The GPS signal is 10 billion times weaker than a nearby WiFi router. Any noise or interference that approaches this level can corrupt or prevent a position fix. The GPS receiver achieves this sensitivity using spread spectrum correlation — it knows exactly what signal pattern it expects and can detect it even below the noise floor. But conducted noise on the supply rail or nearby electromagnetic radiation can raise the effective noise floor enough to lose the signal entirely.

Carbon fibre and antenna shielding¶

Carbon fibre is a conductor. The fibres form a conductive mesh. A GPS patch antenna placed above a carbon fibre plate cannot receive signals that must pass through that plate — the plate acts as a Faraday shield, blocking the electromagnetic wave.

This is why: - GPS modules must never be mounted under a CF canopy or top plate - Frames with CF top plates must use an antenna mast or a GPS module on a bracket above the plate - The GPS antenna must face upward with no conductive material in the hemisphere above it (not even at an angle — the satellite geometry uses the full upper hemisphere)

libdrone's PETG sandwich and Platform are non-conductive — no shielding issue. The GPS bracket places the M10Q above the camera, with open sky in all upward directions.

Separation from motor wires and ESC¶

A motor wire carrying 20 A at 30 mm from a magnetometer generates a magnetic field of approximately 130 µT — comparable to Earth's field. A motor wire at 150 mm (the nose-to-ESC distance on libdrone) generates ~2.6 µT — within calibration compensation range.

The GPS module's integrated QMC5883 magnetometer (compass) is what uses this distance separation. The GPS receiver chip itself is primarily affected by conducted noise on its supply and by RF interference near 1.575 GHz.

Antenna orientation¶

GPS patch antennas are directional — they receive best from directly above and reject signals from below. The patch must face upward, parallel to the ground. Any tilt reduces the effective gain toward overhead satellites while increasing sensitivity to multipath reflections from the ground. If the bracket geometry creates any tilt, the Betaflight compass alignment offset must be set to compensate before the maiden flight.

The VTX antenna and GPS clearance¶

The VTX antenna (typically a monopole or cloverleaf mounted at the rear of the drone) must not obstruct the sky view above the GPS patch antenna. On libdrone, the VTX antenna mounts at the tail — maximum separation from the nose-mounted GPS. This positioning is deliberate and must not be changed.

The 5.8 GHz VTX output at 200–800 mW transmit power can also cause desensitisation of the GPS receiver if the VTX antenna is too close — the strong 5.8 GHz signal overloads the GPS receiver's front-end amplifier. At the nose-to-tail separation on libdrone's Platform, this is not an issue.

Reference¶

libdrone GPS module placement¶

Compass calibration requirements¶

After any significant change to the drone's layout — fitting a new payload, changing the battery position, adding a mast — recalibrate the compass in Betaflight. Rotate the drone through all orientations (all 6 faces toward down). Calibrate in an open area away from metal structures and high-voltage power lines. Recalibrate if the drone consistently fails to hold heading in GPS-assisted modes.

Procedure¶

First-time GPS verification¶

1. Arm the drone outdoors in open sky with GPS Rescue enabled.
2. Wait for GPS fix: OSD should show "GPS FIX" and a satellite count ≥ 8 within 90 seconds (cold start). Warm start: within 15 seconds.
3. Walk 10 m from the drone. OSD distance reading should increase to ~10 m.
4. Return to the drone. OSD distance should return to ~0 m.
5. If satellite count does not reach 8 within 3 minutes in open sky: check that the M10Q is connected to UART2 at 57,600 baud in Betaflight Ports tab; verify the GPS cable is not routed through the power channel.

Bracket removal and reinstallation¶

The GPS/camera bracket removes as one unit in under 60 s. After reinstallation: 1. Verify the bracket seats flush against the Platform nose face — no gap. 2. Verify the GPS patch antenna faces directly upward — use a level on the drone body and confirm the antenna surface is parallel to the body. 3. If any tilt is visible, set the compass alignment offset in Betaflight Configurator → GPS → Compass Alignment before the next flight.

Rationale¶

Why GLONASS is disabled¶

Near the eastern borders of the Czech Republic, GLONASS signal jamming has been observed. A GNSS receiver that includes GLONASS will attempt to use GLONASS satellites even at degraded signal levels. At low signal-to-noise ratio, a GLONASS satellite contribution can corrupt the overall position solution computed from all constellations — the receiver trusts a bad measurement more than it should. With GPS + Galileo + BeiDou providing 18–26 satellites, GLONASS adds marginal accuracy benefit while introducing a jamming vulnerability. The constellation is disabled.

Connections¶

requires: - emc-noise-sources - power-signal-separation related: - gnss-gps - barometer-magnetometer - ferrite-beads leads_to: - conformal-coating