RTK Systems

Fundamentals
Title	RTK Systems
Author(s)	GMV
Level	Basic
Year of Publication	2011

There are DGNSS techniques used by high-precision navigation/surveying applications, based on the use of carrier phase measurements. This is the case of the Real Time Kinematics (RTK), where the differential GNSS measurements are computed in real-time by specific GNSS receivers if they receive a correction signal using a separate radio receiver. When referring to GPS in particular, the system is also commonly referred to as Carrier-Phase Enhancement, CPGPS.

RTK Systems

Most of the currently available RTK receiver systems use double-differenced GPS and/or GLONASS carrier phase measurements to determine the position of the roving receiver. The typical nominal accuracy for dual- frequency systems is 1 centimetre ± 2 ppm (horizontal) and 2 centimetres ± 2 ppm (vertical). The latency for position outputs varies from less than 20 ms to 100 ms for different receiver systems. If the receiver works in the synchronised RTK mode, the latency depends, of course, on the latency of the radio data link.

The position accuracy drastically decreases if the double-differenced ambiguities are not resolved or are resolved incorrectly. In the latter case, errors in the order of one metre can easily occur. Therefore, reliable ambiguity fixing is probably the most important design aspect for an RTK system. All other error sources - such as unmodeled atmospheric effects, (carrier) multipath, orbital errors, or receiver measurement noise - are of the order of several millimetres up to a few centimetres.

In practice, RTK systems use a single base station receiver and a number of mobile units. The base station re-broadcasts the phase of the carrier that it measured, and the mobile units compare their own phase measurements with the ones received from the base station. There are several ways to transmit a correction signal from base station to mobile station. The most popular way to achieve real-time, low-cost signal transmission is to use a radio modem, typically in the UHF band. In most countries, certain frequencies are allocated specifically for RTK purposes. Most land survey equipment has a built-in UHF band radio modem as a standard option.

This allows the units to calculate their relative position to millimeters, although their absolute position is accurate only to the same accuracy as the position of the base station. The typical nominal accuracy for these dual-frequency systems is 1 centimetre ± 2 parts-per-million (ppm) horizontally and 2 centimetres ± 2 ppm vertically.

Over the last few years, permanent reference station installations have emerged in many countries. This is quite appealing, since in areas with considerable GPS surveying activity, a number of users might share the infrastructure and the associated costs. Some of the installations are operated by companies and provide a service to the surveying community. Installations can be just :

• single reference stations,
• a number of single reference stations,
• or networking reference stations.

A single reference station set-up within up to 20-30 km is typically required if a user is operating in baseline mode. Otherwise the performance, accuracy, and with some systems the reliability of user's RTK is degraded. This is quite appealing, since in areas with considerable GPS surveying activity, a number of users might share the infrastructure and the associated costs. Some of the installations are operated by companies and provide a service to the surveying community. Installations can be just single reference stations, a number of single reference stations, or networking reference stations.

The integration of several reference stations into a combined network provides benefits for the user by improving the accuracy and increasing the overall user system performance. For the reference station operator, networking reduces the number of stations that are needed to provide a given level of accuracy to the rover users. These permanent reference station networks are requiring real-time communication to a networking computation center and real-time estimation of biases between reference stations.

A key factor of success is the distribution of the information generated within the networking computation center to the roving user in the field. Some of the installations are relying on proprietary computation algorithms and possibly formats and restricting themselves with the field equipment. However, in general it is in the interest of service providers to supply the service for more than a single type of RTK field equipment. Therefore, the detailed understanding of the supplied information such as applied corrections or the way of processing is absolutely mandatory.

Today, installations are supplying the information basically in two ways: the so-called FKP-approach (FKP stands for the German word of spatial correction parameter) and the VRS approach (Virtual Reference Station). Both approaches deliver observations that are supposed to be operational with modern RTK equipment. However, as noted above, the ways the computational algorithms running at the networking computation center are proprietary. Optimal interoperability is not guaranteed, since the definition and an interface mechanism is missing. While the roving user equipment might work optimally with one vendor's networking software providing a service, it might have degraded performance with another vendor's software.