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.
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.