robotics//sensor//GNSS receiver

A GNSS receiver is a radio sensor that computes its position, velocity and time from the signals of navigation satellites (GPS, Galileo, GLONASS, BeiDou), and it is the absolute reference that keeps the inertial estimate of a drone, a car or a ship from drifting away. Each satellite broadcasts its orbit and the exact time of transmission; the receiver measures how long each signal took to arrive, which gives a distance to that satellite. Since the receiver's own cheap clock is off by an unknown amount, it needs at least four satellites to solve for three coordinates and its clock error, and the velocity comes from the Doppler shift of the carriers.

A GNSS receiver is a radio sensor that computes its position, velocity and time from the signals of navigation satellites (GPS, Galileo, GLONASS, BeiDou), and it is the absolute reference that keeps the inertial estimate of a drone, a car or a ship from drifting away. Each satellite broadcasts its orbit and the exact time of transmission; the receiver measures how long each signal took to arrive, which gives a distance to that satellite. Since the receiver's own cheap clock is off by an unknown amount, it needs at least four satellites to solve for three coordinates and its clock error, and the velocity comes from the Doppler shift of the carriers.

In the loop of a drone it is the slowest important sensor: a fix every 100 to 1000 ms (1 to 10 Hz), and a delay of tens to hundreds of milliseconds between the instant described and the instant the message leaves the UART. A typical standalone fix is good to a few metres. Near buildings the signals bounce (multipath) and the error jumps, under trees or in a canyon of streets satellites disappear, and in a tunnel the fix stops.

It does not drift, which is its whole value. Fused with the IMU it bounds the error of dead reckoning; fused naively it ruins the estimate, because 100 ms of latency at 10 m/s is a metre of error if the fix is treated as current (delayed measurements).

RTK GNSS (real-time kinematic) reaches centimetres by measuring the phase of the carrier wave and taking corrections from a base station or a correction service a few kilometres away. It is what lets a show of thousands of drones fly prearranged paths without collision, and what surveyors and agricultural machines use.

Its time is the best part. The pulse per second marks the start of each second to tens of nanoseconds, and many systems that never need a position use a GNSS receiver as their clock (time synchronization).

It can be lied to. Its signals arrive at the power of a faint whisper, so a transmitter nearby can drown them or forge them, and a spoofed receiver reports a smooth, plausible and wrong position (GNSS spoofing). Estimators defend themselves by checking each fix against what the inertial sensors predicted (innovation).