GNSS and GPS

Summary¶

A GNSS receiver calculates geographic position from the measured travel time of signals from multiple satellites. Four satellites are the minimum to solve for three position coordinates plus receiver clock error simultaneously. libdrone uses the Matek M10Q-5883, configured for GPS + Galileo + BeiDou with GLONASS disabled. EGNOS SBAS augmentation reduces position error from 2–5 m to 0.5–1.5 m CEP over Europe. GPS Rescue requires a valid home point (recorded at arm time) and a minimum of 8 satellites — fly only after GPS fix is confirmed.

Concept¶

How GNSS positioning works¶

Each satellite continuously broadcasts a precisely timestamped signal. The receiver measures the difference between the time the signal was sent (encoded in the signal) and the time it arrived. Travel time × speed of light = distance from that satellite.

With known distances from three satellites, the receiver's position lies at the intersection of three spheres — which gives two candidate points (one on Earth's surface, one in space). Four satellites eliminate the clock error ambiguity: the receiver's clock is imprecise, adding an unknown timing offset. Four range equations with four unknowns (latitude, longitude, altitude, clock error) yield a unique solution. More satellites improve accuracy by over-constraining the solution.

Constellations and why multiple constellations matter¶

GPS (US, 31 satellites), Galileo (EU, 28 satellites), BeiDou (China, 35+ satellites), and GLONASS (Russia, 24 satellites) all broadcast at overlapping frequencies. A dual- or triple-constellation receiver sees more satellites from any location, giving better geometry (lower PDOP), faster fix acquisition, and redundancy if one constellation is unavailable.

libdrone configuration: GPS + Galileo + BeiDou enabled. GLONASS disabled.

GLONASS is disabled because near the eastern borders of the Czech Republic, GLONASS jamming has been observed. A GLONASS satellite at degraded signal level can corrupt the combined position solution even without providing a valid fix. Disabling the constellation removes the attack vector. With GPS + Galileo + BeiDou and clear sky, 18–26 satellites are typically visible — more than sufficient for an accurate, fast fix.

EGNOS augmentation¶

EGNOS (European Geostationary Navigation Overlay Service) is the European Satellite-Based Augmentation System. A network of precisely surveyed ground stations across Europe continuously measure their own GPS-derived positions, compare them to their known exact positions, and broadcast the difference as correction data via geostationary satellites at 1575.42 MHz (same frequency as GPS L1).

Receivers that use EGNOS apply these corrections, reducing position error from the typical raw 2–5 m CEP to approximately 0.5–1.5 m CEP. For air quality mapping where sensor readings are correlated with GPS coordinates, this matters directly: a 1 m position accuracy means the pollution map correctly locates readings to specific streets and buildings.

EGNOS is enabled by default in the M10Q-5883 configuration for libdrone. SBAS signals are available across most of continental Europe. Over UK, Iceland, or areas at the edge of EGNOS coverage, performance may be reduced.

Compass and GPS interaction¶

The M10Q-5883 integrates both the GNSS receiver (M10 chip) and a magnetometer (QMC5883). The compass provides heading for GPS navigation — GPS alone cannot determine which direction the drone is pointing (only where it is). GPS Rescue uses the compass to point the drone toward home. Position hold uses it to maintain heading in wind.

The compass is mounted at the front of the drone on the GPS/camera bracket — maximum physical separation from the ESC, battery leads, and motor wires. A single motor wire carrying 20A at 30 mm distance creates a magnetic field of approximately 130 µT at the compass — comparable to Earth's field (~50 µT). Placement at the nose is mandatory. After any significant electronics layout change (adding a payload, different battery), re-run compass calibration.

Reference¶

Matek M10Q-5883 specification¶

GPS Rescue configuration¶

Home point is set when the drone arms, not when GPS fix is acquired. If the pilot moves after arming (while walking to take-off position), the home point reflects where the drone was standing, not where the pilot is standing. This is important for return-to-home accuracy in the field.

Procedure¶

GPS module configuration (initial setup)¶

1. In Betaflight Configurator → Configuration: enable GPS, set protocol to UBX.
2. In Ports tab: set UART2 to GPS, baud 57,600.
3. Power cycle. In GPS tab: verify satellites are detected and increasing.
4. Configure constellations via UBX config or BetaflightGPS passthrough: enable GPS, Galileo, BeiDou; disable GLONASS.
5. Enable SBAS/EGNOS.
6. Verify in GPS tab: fix acquired, satellite count ≥ 8 in open sky.

Compass calibration¶

1. In Betaflight Configurator → Calibration tab: initiate compass calibration.
2. Rotate the drone slowly through all orientations: nose up/down/left/right, all roll angles. Aim for smooth, continuous rotation covering the full sphere.
3. Keep all electronic components in their normal flight positions — payload, battery, everything. Calibrate with the full payload fitted if the payload contains any magnets or high-current wiring.
4. Save calibration. Verify in attitude display that heading rotates correctly as the drone is turned.

Rationale¶

Why GPS fix is required before arming (not just recommended)¶

→ See power-sequencing for the full rationale. In summary: without a GPS fix at arm time, GPS Rescue has no home point and cannot function. In any environment with obstacles — trees, buildings, pylons — a drone without GPS Rescue is a single-point-of-failure system. The 30–90 second wait is not optional for safe field operations.

Connections¶

requires: - flight-controller-hardware related: - barometer-magnetometer - power-sequencing leads_to: - piloting-operations