RTK GPS, or Real Time Kinematic GPS, lets robots know their position much more precisely than ordinary GPS. This is especially important for agricultural robots that must drive between crop rows, seed accurately, spray only where needed, or return to the same path day after day. Standard GPS may drift by several meters, which is too large for many field tasks.
RTK GPS can reduce that error to the centimeter level when conditions are good.
Understanding Robotics: RTK GPS
An RTK receiver does more than read satellite positions. It measures the carrier wave, a very short repeating radio wave sent by each satellite. Measuring the wave phase gives much finer detail than measuring signal travel time alone.
The hard part is that the receiver cannot initially tell how many whole wave cycles lie between it and a satellite. This is called resolving the integer ambiguity. The receiver compares measurements over time and uses data from several satellites to find the correct whole number of cycles.
Once this solution is fixed, its position becomes highly precise. If the solution is only float, it is less certain and should not be treated as full centimetre accuracy.
The base and rover work well because many errors affect both receivers in nearly the same way. Satellite clock errors, small orbit errors, and delays in the upper atmosphere are similar for receivers that are not too far apart. The base measures these shared errors from its surveyed location and sends corrections to the rover.
Distance matters. A rover far from its base sees more different atmospheric conditions, so the corrections become weaker.
For large areas, robots may use a network RTK service. This service combines data from many permanent base stations and creates corrections for the region.
A precise position is useful only when it is connected correctly to the robot. The antenna may sit on a mast above and behind the robot's centre point. Software must account for that offset when it turns antenna coordinates into the location of the wheels, tool, or camera.
It must use the right map coordinate system and the right height reference. A map can be shifted even when the RTK receiver is accurate if these settings do not match. Heading needs care too.
A single moving antenna can estimate direction from motion, but this becomes unreliable when the robot is stopped or moving very slowly. Two antennas can give a more direct heading measurement.
Real fields and streets create conditions that can break a good solution. Trees, buildings, grain silos, and hills can block satellites. Signals can bounce from metal, walls, or wet ground before reaching the antenna.
This multipath error makes the signal seem to travel farther. Radio dropouts or delayed cellular corrections can leave the rover using old information. Students should watch the receiver status, correction age, number of tracked satellites, and reported accuracy instead of trusting one position value.
A robot should slow down, pause, or switch to a safer mode when it loses a fixed solution. Update rate matters as well.
At two metres per second, a robot moves twenty centimetres between ten updates each second. Control software must predict motion between updates and combine RTK with wheel encoders and inertial sensors for smooth steering.
Key Facts
- RTK GPS uses a fixed base station and a moving rover receiver to improve position accuracy.
- The base station knows its exact location and calculates GPS error by comparing measured signals to its known position.
- The rover receives correction data from the base station by radio, cellular link, or another communication channel.
- Carrier-phase measurements use the phase of the GPS signal wave, not just the arrival time, to estimate position very precisely.
- RTK position error is often about 1 cm to 3 cm horizontally under good satellite visibility.
- Position update distance while moving is d = vΔt, so a robot traveling at 2 m/s with 10 Hz RTK updates moves 0.2 m between updates.
Vocabulary
- RTK GPS
- A positioning method that uses real-time correction data and carrier-phase measurements to achieve centimeter-level GPS accuracy.
- Base station
- A fixed GPS receiver at a known location that calculates correction data for nearby rovers.
- Rover
- The moving GPS receiver on a robot, tractor, drone, or vehicle that uses corrections to find its precise position.
- Carrier phase
- The measured fraction and number of cycles of a GPS radio wave used to estimate distance very accurately.
- Correction data
- Information sent from the base station to the rover to reduce errors caused by satellite clocks, atmosphere, and orbit uncertainty.
Common Mistakes to Avoid
- Treating RTK GPS as the same as ordinary GPS. Ordinary GPS often has meter-level error, while RTK uses base station corrections and carrier phase to reach centimeter-level accuracy.
- Placing the base station at an unknown or moving location. The base station must have a stable known position, otherwise its errors are passed to the rover.
- Ignoring line of sight to satellites. Trees, buildings, hills, and machinery can block or reflect signals, which reduces accuracy or causes RTK fixes to drop.
- Assuming GPS alone is enough for all robot control. Robots often combine RTK GPS with IMUs, wheel encoders, cameras, or lidar because GPS can update slowly or fail in obstructed areas.
Practice Questions
- 1 A field robot moves at 1.5 m/s and receives RTK position updates at 5 Hz. How far does it travel between position updates?
- 2 A standard GPS reading has a typical horizontal error of 2.0 m, while an RTK system has a horizontal error of 0.02 m. How many times smaller is the RTK error?
- 3 Explain why an RTK rover can be more accurate when it is close to its base station than when it is very far away.