What are rtk?

RTK (Real-Time Kinematic) is a highly accurate technique used to determine the position of a receiver using the signal received from satellite-based positioning systems like GPS, Galileo, BeiDou, and GLONASS. RTK is based on the carrier phase measurement technique that uses the phase of the carrier signal to determine the location of the receiver. As a result, it is more accurate than traditional timing-based GNSS solutions found in devices like smartphones and wearables.

Traditional GNSS receivers, like the ones in smartphones or wearable devices, receive signals directly from GNSS satellites and estimate their location using the differential in times transmitted from multiple satellites. The accuracy of these systems is usually around 1 - 4 meters. However, GNSS receivers using RTK can provide centimeter-level accuracy. The phase measurement technique is also not impacted by weather conditions so can be more reliable than the timing approach.

Since it provides higher accuracy, the RTK technique is useful for carrying out land surveys, hydrographic surveys, and for other applications that required very accurate positioning information. RTK is particularly suited for measuring the relative position of several moving and stationary objects, making the technique very useful in the test and verification of ADAS systems. It is the most popular GNSS-based precise positioning technique available. To operate, RTK requires at least 5 satellites in view for initialization. Tracking 5 satellites provide insurance against losing one abruptly; also it adds considerable strength to the results.

RTK (Real-Time Kinematic) involves the use of one stationary reference receiver, called the base station, and one moving receiver, called the rover. The Base Stations are stationary and their location is known. The rover is the GNSS Receiver whose location needs to be determined. Rovers can be moved from point to point, stopping momentarily at each new point.

How does it work?

An RTK system consists of a Base Station and a Rover. The base station is a stationary receiver whose location is known. The base station calculates its location by using the signal received from the GNSS Satellites based on the carrier phase measurement technique. It then compares this location to its known location to identify any errors and generate a correction signal.

This correction signal is transmitted in real-time to the rover. The Rover uses this correction data to improve its own computed position from the GNSS constellations to achieve centimeter precision. The rover also uses the carrier phase measurement technique to determine its position. The data radio transmitter (base station) consists of an antenna, a radio modulator, and an amplifier. The modulator converts the correction data into a radio signal. The amplifier increases the signal’s power, which determines how far the information can travel.

It is also important to note that it takes some time for the base station to calculate corrections, and it takes some time for it to put the data into packets in the correct format and transmit them. Then the data makes its way from the base station to the rover over the data link. It is then received by the rover and decoded. The time this takes is called the latency of the communication between the base station and the rover. It can be as little as a quarter of a second or as long as a couple of seconds. And since the base station corrections are only accurate for the moment they were created, the base station must send a range rate correction along with them. Using this rate correction, the rover can backdate the correction to match the moment it made that same observation.

The base station is usually fixed at a particular location and can provide this correction data to multiple rovers within a certain range. All the receivers involved observe the same satellites simultaneously. The base receivers are stationary on control points. The rovers move from point to point, stopping momentarily at each new point. The rover and the base station must be tuned to the same frequency for successful communication.

It is advisable to keep the distance of 6-12 miles or less between the base station and the rover, as while transferring the correction data (from base to rover), the signal may get hampered due to the different environmental conditions present at rover’s and base station’s place. RTK receivers can be single or multi-frequency receivers with GNSS antennas, but multi-frequency receivers are usual because RTK relies on carrier phase observations corrected in real-time. In other words, it depends on the fixing of the integer cycle ambiguity, and that is most efficiently accomplished with a multi-frequency GNSS receiver capable of making both carrier phase and precise pseudorange (distance between a satellite and a navigation satellite receiver) measurements.

Requirements

RTK requires a real-time wireless connection to be maintained between the base station and the rover. The radio receiving antennas for the rovers will either be built into the GNSS antenna or be present as separate units. Usually, the radio antenna for the data transmitter and the rover are omnidirectional whip antennas; however, at the base, it is usually on a separate mast and has a higher gain than those at the rover. The typical gain on the antenna at the base is 6 dB.

The position of the transmitting antenna affects the performance of the system significantly. It is usually best to place the transmitter antenna as high as is practical for maximum coverage, and the longer the antenna, the better its transmission characteristics. It is also best if the base station occupies a control station that has no overhead obstructions, is unlikely to be affected by multipath, and is somewhat away from the action if the work is on a construction site. It is also best if the base station is within the line of sight of the rovers. If the line of sight is not practical, as little obstruction as possible along the radio link is best.

The base station transmitter ought to be VHF, UHF, or spread spectrum-frequency hopping or direct to have sufficient capacity to handle the load. UHF spread spectrum radio modems are the most popular for DGPS and RTK applications.

Communication Between the Base Station and Rover

Most radios connected to RTK GPS surveying equipment operate between UHF 400-475 MHz or VHF 170-220 MHz, and emergency voice communications also tend to operate in this same range, which can present problems from time to time. That is why most radio data transmitters used in RTK allow the user to use several frequency options within the legal range. Many countries define a specific frequency band to be used for RTK communications between base stations and rovers.

The usual data link configuration (b/w base station and rover) operates at 4800 baud or faster. The units communicate with each other along a direct line-of-sight. The transmitter at the base station is usually the larger and more powerful of the two radios. However, the highest wattage radios, 35 Watts or so, cannot be legally operated in some countries. Lower power radios, from 1/2 W to 2 W, are sometimes used in such circumstances. The radio at the rover has usually lower power and is smaller. The Federal Communications Commission (FCC) is concerned with some RTK GPS operations interfering with other radio signals, particularly voice communications. It is important for GPS surveyors to know that voice communications have priority over data communications.

The FCC requires cooperation among licensees that share frequencies to minimize interference. For example, it is wise to avoid the most typical community voice repeater frequencies. They usually occur between 455-460 MHz and 465-470 MHz. Part 90 of the Code of Federal Regulations, 47 CFR 90, contains the complete text of the FCC Rules including the requirements for licensure of radio spectrum for private land mobile use. The FCC does require an application be made for licensing a radio transmitter.

Other international and national bodies also govern frequencies and authorize the use of signals elsewhere in the world. In some areas, certain bands are designated for public use, and no special permission is required. For example, in Europe, it is possible to use the 2.4 GHz band for spread spectrum communication without special authorization with certain power limitations. Here in the United States, the band for spread spectrum communication is 900 MHz.

Advantages of RTK

Disadvantages of RTK

Real-time kinematic positioning (RTK) is the application of surveying to correct for common errors in current satellite navigation (GNSS) systems. It uses measurements of the phase of the signal's carrier wave in addition to the information content of the signal and relies on a single reference station or interpolated virtual station to provide real-time corrections, providing up to centimetre-level accuracy (see DGPS). With reference to GPS in particular, the system is commonly referred to as carrier-phase enhancement, or CPGPS. It has applications in land surveying, hydrographic surveying, and in unmanned aerial vehicle navigation.

The distance between a satellite navigation receiver and a satellite can be calculated from the time it takes for a signal to travel from the satellite to the receiver. To calculate the delay, the receiver must align a pseudorandom binary sequence contained in the signal to an internally generated pseudorandom binary sequence. Since the satellite signal takes time to reach the receiver, the satellite's sequence is delayed in relation to the receiver's sequence. By increasingly delaying the receiver's sequence, the two sequences are eventually aligned.

The accuracy of the resulting range measurement is essentially a function of the ability of the receiver's electronics to accurately process signals from the satellite, and additional error sources such as non-mitigated ionospheric and tropospheric delays, multipath, satellite clock and ephemeris errors.

RTK follows the same general concept, but uses the satellite signal's carrier wave as its signal, ignoring the information contained within. RTK uses a fixed base station and a rover to reduce the rover's position error. The base station transmits correction data to the rover.

As described in the previous section, the range to a satellite is essentially calculated by multiplying the carrier wavelength times the number of whole cycles between the satellite and the rover and adding the phase difference. Determining the number of cycles is non-trivial, since signals may be shifted in phase by one or more cycles. This results in an error equal to the error in the estimated number of cycles times the wavelength, which is 19 cm for the L1 signal. Solving this so-called integer ambiguity search problem results in centimeter precision. The error can be reduced with sophisticated statistical methods that compare the measurements from the C/A signals and by comparing the resulting ranges between multiple satellites.

The improvement possible using this technique is potentially very high if one continues to assume a 1% accuracy in locking. For instance, in the case of GPS, the coarse-acquisition (C/A) code, which is broadcast in the L1 signal, changes phase at 1.023 MHz, but the L1 carrier itself is 1575.42 MHz, which changes phase over a thousand times more often. A ±1% error in L1 carrier-phase measurement thus corresponds to a ±1.9 mm error in baseline estimation.

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 observes, and the mobile units compare their own phase measurements with the one 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. RTK provides accuracy enhancements up to about 20 km from the base station.

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

Although these parameters limit the usefulness of the RTK technique for general navigation, the technique is perfectly suited to roles like surveying. In this case, the base station is located at a known surveyed location, often a benchmark, and the mobile units can then produce a highly accurate map by taking fixes relative to that point. RTK has also found uses in autodrive/autopilot systems, precision farming, machine control systems and similar roles.

The RTK networks extend the use of RTK to a larger area containing a network of reference stations. Operational reliability and accuracy depend on the density and capabilities of the reference-station network.

A Continuously Operating Reference Station (CORS) network is a network of RTK base stations that broadcast corrections, usually over an Internet connection. Accuracy is increased in a CORS network, because more than one station helps ensure correct positioning and guards against a false initialization of a single base station.

A Virtual Reference Network (VRN) can similarly enhance precision without using a base station.

RTK is a technique used to improve the accuracy of a standalone GNSS receiver. Traditional GNSS receivers, like the one in a smartphone, could only determine the position with 2-4 meters (7-13 feet) accuracy. RTK can give you centimeter accuracy.

GNSS receivers measure how long it takes for a signal to travel from a satellite to the receiver. Transmitted signals travel through the ionosphere and atmosphere and are slowed down and perturbed on the way. For example, travel time on a cloudy day and in clear sky conditions would be different. That is why it is difficult for a standalone receiver to precisely determine its position. RTK is a technology that solves this issue.

This video will show you how RTK technology works.

Two receivers are used in RTK. One of them is stationary, another moves freely. They are called base station and rover.

The base's mission is to stay in one place and send corrections to a moving receiver. Rover uses that data to achieve centimeter precise position. Any number of rovers can connect to one base if their input settings match the base's output.

You do not necessarily need a second unit for RTK all the time. Usually, there are local services that share base corrections over the Internet. This technology is called NTRIP.

NTRIP is a good option for areas with strong 3G/LTE coverage and a vast network of NTRIP bases nearby. In other cases, using the second receiver as a local base station has two advantages:

Roughly speaking, there are two types of receivers: single-band (L1) and multi-band (L1/L2 or more). Their differences come from how much data they can receive from satellites.

For example, it helps to increase the maximum distance between base and rover, which is also called baseline:

Multi-band receiver is also way more robust when it comes to sky view. It can maintain centimeter precision even if you survey in challenging conditions: forest, city, mining sights, quarries, etc.

Real-time kinematic (RTK) is a surveying technology that measures the relative positions using two Global Navigation Satellite System (GNSS) antennas in real-time with better accuracy. The errors found in GNSS results are determined and corrected using RTK technology.

This article discusses the working and benefits of RTK surveying in the construction industry.

An RTK setup consists of two receivers. One of them, called the base station or GNNS receiver, is stationary, while the second receiver called rover moves freely.

The base station is a static point whose coordinates are fixed and determined by other precision methods in surveying. It uses GNSS to compute errors in this point by comparing it to its precise location. The error determined is transmitted in real-time to the rover. The objective of the antenna at the base station is to remain at one point and send the corrections to the moving receiver.

Rover uses these corrections to improve its computed position from the GNSS and achieve centimeter-level precision. More than one rover is used and can be connected to one base if their input settings match with the output from the base.

Traditional GNSS receivers measure the time taken for a signal to travel from a satellite to the receiver. These receivers determine the position with 2-4 meter accuracy. But the incorporation of RTK gives the relative position in centimeter accuracy.

RTK is mainly used for construction applications that require higher frequency like cadastral survey, drone navigation, and other construction activities.

The benefits of RTK in construction are:

RTK technology is unavailable in marine areas, lands with obstructions (trees, mountains, etc.), or projects that disrupt communication. The system requires a pre-surveyed base station with known coordinates.