Outdoor robots need to know where they are before they can plan safe and useful motion. A GNSS receiver, often called GPS in everyday language, estimates position by using radio signals from satellites such as GPS, Galileo, GLONASS, and BeiDou. This lets a rover navigate fields, roads, construction sites, and test tracks without relying only on local landmarks.
The result is a global position estimate that can be placed on a map and used by the robot navigation system.
A GNSS receiver does not measure position directly, but estimates distance to several satellites from signal travel times. With at least four satellites, the receiver solves for latitude, longitude, altitude, and its own clock error. Real robots combine GNSS with an IMU and wheel encoders because satellite signals can be noisy, blocked, delayed, or reflected by buildings and trees.
Sensor fusion lets the robot stay smooth and stable between GNSS updates and recover when the satellite position jumps.
Understanding Robotics: GPS and GNSS Receiver
Each navigation satellite carries very accurate atomic clocks and sends a repeating radio code. The receiver makes its own copy of that code, then compares the two patterns. A small timing difference means a large distance difference because radio waves travel at the speed of light.
The receiver must know where each satellite was when it sent its signal. It gets this information from navigation data included in the transmission.
Satellite orbit information is called ephemeris data. A receiver often needs several minutes after starting before it has collected enough data for a strong position estimate.
Timing errors are one reason satellite navigation is harder than it first appears. The receiver clock is far less accurate than an atomic clock, so even a tiny clock offset would create a large range error. The calculation estimates that clock offset along with location.
Signals slow down slightly as they pass through charged layers high above Earth and through the lower atmosphere. A signal can even bounce from a wall, parked vehicle, wet ground, or glass building before reaching the antenna.
This reflected path is called multipath. It makes the signal seem to have travelled farther, which can shift a robot position by metres.
Accuracy depends strongly on the surroundings and the equipment. In open farmland, a basic receiver may provide a position accurate to a few metres. That can be enough for tracking a delivery robot or keeping a mower inside a large area.
It is not enough for a tractor that must follow crop rows precisely. For higher precision, many robots use correction data from a fixed base station or a correction network. The base station knows its surveyed position and measures the errors affecting satellite signals nearby.
It sends corrections to the moving robot. A method called real time kinematic positioning can then reach centimetre level accuracy when conditions are good.
A robot still needs to treat a satellite position as one piece of evidence, not absolute truth. A navigation program checks reported accuracy, number of visible satellites, signal quality, and whether the new location is physically believable. If a rover appears to jump across a fence in one update, the software should not immediately steer toward the false location.
An inertial measurement unit tracks turning and acceleration at a fast rate. Wheel measurements estimate short travel distances, though they become wrong when wheels slip. A filter combines these sources and gives each one more or less trust based on its current reliability.
Students should pay attention to coordinate frames as well. Satellite coordinates describe a location on Earth, while the robot needs local directions such as forward, left, and up to control its motors safely.
Key Facts
- GNSS stands for Global Navigation Satellite System and includes GPS, Galileo, GLONASS, BeiDou, and other satellite systems.
- Distance from satellite to receiver is estimated by pseudorange: range = cΔt, where c is the speed of light and Δt is the measured signal travel time.
- At least 4 satellites are needed to solve for 3D position and receiver clock error.
- Trilateration finds position from distances to known satellite locations, not from angles.
- Robot localization often fuses sensors using an estimate such as pose = position + orientation, often written as x = [px, py, pz, roll, pitch, yaw].
- Wheel odometry estimates distance from wheel rotation: distance = wheel radius × angle turned, or s = rθ.
Vocabulary
- GNSS
- GNSS is the general name for satellite navigation systems that provide timing and position signals to receivers on Earth.
- GPS
- GPS is the United States GNSS constellation and is one of several satellite systems a modern receiver may use.
- Trilateration
- Trilateration is the method of finding position by using measured distances to multiple known points.
- IMU
- An IMU is an inertial measurement unit that measures acceleration and rotation rate to help estimate robot motion.
- Odometry
- Odometry is the process of estimating a robot's change in position using motion measurements such as wheel rotations.
Common Mistakes to Avoid
- Treating GPS and GNSS as identical, which is wrong because GPS is only one GNSS system while many receivers use several constellations together.
- Assuming three satellites are always enough, which is wrong because a practical receiver also must solve for clock error, requiring at least four satellites for a 3D fix.
- Confusing trilateration with triangulation, which is wrong because GNSS uses distances from signal timing rather than measured angles.
- Trusting raw GNSS position without sensor fusion, which is wrong because reflections, signal blockage, and noise can cause sudden jumps that an IMU and wheel odometry help smooth.
Practice Questions
- 1 A GNSS signal arrives 0.070 microseconds later than expected. Using c = 3.0 × 10^8 m/s, what distance error does this timing error cause?
- 2 A rover wheel has radius 0.15 m and rotates through 20 radians with no slipping. Using s = rθ, how far does the rover travel?
- 3 A robot drives beside tall buildings and its GNSS position suddenly jumps 5 m to the side, but its IMU and wheel encoders show smooth straight motion. Explain why the navigation stack should not immediately trust the GNSS jump.