What is RTK?

     RTK stands for Real Time Kinematics and is another technique to improve the accuracy of GPS position measurements, however, it is one of the most difficult to understand and most intensive to implement.

     Standard Positioning Services (SPS) were the first position fixes implemented by the GPS system and they use C/A code. The accuracy of SPS can be improved if the time delay of the differentially corrected satellite signals can be eliminated. Positional accuracy of about 40 centimeters is fairly normal using DPGS. RTK is the next step in DGPS, and it is available in two versions: RTK Floating Point, which achieves decimeter-level accuracy, and RTK Integer, which achieves centimeter-level accuracy.

     To understand how these two versions work, it is important to understand two things.

First, RTK breaks the rules of what we should be able to accomplish with a civilian GPS receiver. It is not a technology designed into the system to help us. Instead, it is a method that GPS manufacturers came up with to achieve greater accuracy than expected using C/A codes.

The second thing to understand is that it is based on the carrier itself, not on the C/A code or navigation information that the carrier carries.

So how does it work?

     In our "What is SPS" page, we discussed how SPS is based on pseudo-range measurements calculated using C/A code. The receiver does this by synchronizing its clock with the C/A code. The GPS satellite then generates its own version of the C/A code for each satellite it can see. If it has to delay its own copy of the C/A code by 7 milliseconds in order to match the C/A code received by the antenna, then it knows that the signal from that satellite has a travel time of 7 milliseconds, and it can then calculate how far away that satellite is.

     The ultimate goal of RTK is to determine how many carriers are between the antenna and the satellite. The reason is simple. Each satellite broadcasts a unique C/A code consisting of 1,023 bits. The code is sent at 1.023 Mb/s, which means that one bit is sent every microsecond. In one microsecond, the radio signal from the satellite covers a distance of about 300 meters.

    However, the frequency of the carrier wave to which the C/A code is modulated is much higher - 1575.42 MHz. this means that a single wave covers about 19 centimeters. If we could calculate how many full waves there are between the satellite and the antenna, then it would be possible to calculate the distance more accurately. In fact, if we knew how many full waves there were and could also measure partial waves (phase angle) then we could be very accurate.

RTK Integer and RTK Floating Point Solution can be thought of as two separate (but related) algorithms running simultaneously in the system. If the RTK integer has a valid solution, the system will switch to that solution (but the RTK floating-point solution algorithm will continue to run in the background). If the RTK integer then becomes invalid, the system reverts to the RTK floating-point solution and does not need to start up again as long as the carrier phase lock is not lost.

     So what is the difference between the two algorithms? The purpose of RTK Float is to use statistical methods to determine your probable position (improving on the accuracy of current DGPS). It requires at least four satellites common to the base station, and (in layman's terms) looks for points of satellite rotation in a circle around the current position measurement. Unlike RTK Integer, the Float algorithm never attempts to resolve ambiguities - it only tries to determine the most probable position within the circle drawn around the current position estimate.The accuracy of RTK Float starts at around 40 cm, but increases to a maximum of 20 cm.

     RTK Integer, on the other hand, is designed to resolve ambiguities, and it is used only after they have been resolved. It requires five common satellites and, when a valid solution is found, the system knows that there are n carriers and any partial waves between it and the satellites. It could align its measurements to 0.6 per cent of the 19-centimetre wavelength, thus providing an accuracy of about 1 centimetre.