7.6 GPS Components, Errors and Electromagnetic Hazards

The Three Segments of a GNSS

GPS is the United States constellation; GNSS (Global Navigation Satellite System) is the generic term covering GPS, the Russian GLONASS, the European Galileo and the Chinese BeiDou. Modern RPA receivers are multi-constellation, which is why satellite counts on a ground station often exceed twenty.

Schedule 4 topic 8(a) asks for the components of a GPS, and the standard answer is three segments:

How a fix is computed

Each satellite transmits its identity, its orbital position and the exact time of transmission. The receiver measures how long the signal took to arrive and multiplies by the speed of light to get a range to that satellite. One range puts the receiver on a sphere; two ranges intersect in a circle; three narrow it to two points, one of which is absurd. In practice a fourth satellite is needed to solve for the receiver's own clock error, so:

• Three satellites → a 2D fix (latitude and longitude, no reliable altitude).
• Four or more satellites → a 3D fix including altitude.
• Six or seven or more, well distributed, → a robust fix suitable for automated flight.

Accuracy and DOP

Dilution of precision (DOP) expresses how good the satellite geometry is, independent of how many satellites there are. Satellites spread widely across the sky produce well-crossing range spheres and a low DOP; satellites clustered in one part of the sky produce shallow intersections and a high DOP. HDOP is the horizontal component and is the figure most ground stations display.

Standard GNSS gives horizontal accuracy of a few metres. RTK (real-time kinematic) and PPK (post-processed kinematic) systems use a base station or correction service to reach centimetre accuracy, and are standard on survey and mapping aircraft.

Factors That Degrade GNSS Accuracy

Schedule 4 topic 8(c) lists them, and the practical versions are:

• Number of satellites available. Fewer satellites means a weaker solution. Deep valleys, urban canyons and dense canopy all cut the visible count.
• Path interference and multipath. Signals reflecting off buildings, cliffs, water or vehicles arrive slightly late, and the receiver may compute the range from the reflection rather than the direct path. Multipath is worst near tall buildings and over still water.
• Type of software / receiver processing. Multi-constellation, multi-frequency receivers with good filtering resolve a better fix than a basic single-frequency unit.
• Signal availability. Ionospheric disturbance, deliberate jamming, and any physical obstruction of the antenna reduce it. Never place a payload, a hand or a metal bracket over the GNSS antenna.
• Indications of faulty equipment. Erratic satellite count, a fix that will not settle, a home point recorded metres from the actual launch position, position wander in a hover with no wind, or an altitude reading that drifts while the aircraft is stationary.

GNSS-Denied and Degraded Operations

If the fix is lost in flight the flight controller cannot hold position. Typical behaviour:

• The aircraft drifts with the wind rather than holding station.
• Many systems fall back to attitude (ATTI) mode, holding a level attitude and altitude but leaving position to the pilot.
• Return-to-home becomes unavailable or unreliable, because RTH navigates to a stored GNSS home point.

The pilot's job is to know their aircraft's GNSS-loss behaviour before they need it, to keep the aircraft within visual line of sight so manual recovery is possible, and to be current on manual attitude-mode flying. An operator whose pilots have never flown in ATTI mode has an unpractised emergency procedure.

Electromagnetic Hazards: EMI, Powerlines, LTE and Wi-Fi

Schedule 4 topic 9 names three hazards specifically.

Electromagnetic interference (EMI) is any unwanted electromagnetic energy that disturbs a system. On an RPA it comes from outside — transmitters, industrial equipment, powerlines — and from inside, where the ESCs and motor leads radiate noise into the compass and GNSS antenna a few centimetres away.

Powerlines. High-voltage transmission lines and substations are a triple hazard:

1. A physical strike hazard. Conductors are thin, low-contrast against terrain and sky, and effectively invisible from the aircraft's camera. Powerline strikes are among the most common RPA accidents.
2. A magnetic hazard. The current flowing in a conductor produces a magnetic field that can deflect the aircraft's compass, corrupting heading.
3. A radio hazard. High-voltage lines produce corona discharge and broadband electrical noise that raises the noise floor for the C2 link.

The mitigation is separation. Keep meaningful lateral distance from conductors, and if an inspection task requires close work, plan for compass disturbance and reduced link margin rather than being surprised by them.

LTE and Wi-Fi. Mobile networks and Wi-Fi share or sit adjacent to the 2.4 GHz and 5 GHz bands used by most consumer and commercial RPAS links. In a dense urban environment the 2.4 GHz band can be almost fully occupied. Symptoms are video stuttering, rising latency, intermittent telemetry and a falling link-quality reading. Mitigations: select a less-congested channel or band where the equipment allows it, use a directional ground antenna, get physical height and clear line of sight, and move the operating position away from access points and cell sites.

The Compass: The Most Interference-Prone Sensor

The magnetometer senses the earth's magnetic field to determine heading, and the earth's field is weak — which makes local ferrous and electromagnetic sources disproportionately influential.

Common sources at a typical work site: reinforced concrete with steel rebar, a vehicle roof or bonnet used as a launch pad, buried services and pipelines, steel structures and shipping containers, powerlines, and the aircraft's own motors and power leads.

Why it matters more than it looks: the flight controller uses heading to compute both position-hold corrections and the RTH bearing. A compass reading 40 degrees in error sends the aircraft 40 degrees off course on return — the classic flyaway.

Defences:

• Calibrate after changing airframe components or battery type, after travelling a significant distance from the last calibration site, and whenever the ground station reports a compass error.
• Never launch from a metal surface, a vehicle, a manhole cover or a rebar-reinforced pad. Use a portable landing pad on open ground.
• Run the on-site interference check the ground station offers, and treat a high interference value as a reason to move the launch point rather than a nuisance warning to dismiss.
• Watch the first hover. A steady drift with no wind and no stick input is a compass or IMU problem. Land and investigate rather than pressing on.