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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
The disadvantages of this system have caused new approaches to emerge. Long base RTK is one of the approaches. GSM modems are used in long base RTK instead of radio modems used in the traditional RTK Method. Thanks to GSM modems, the distance between base and rover stations reached approximately 100 kilometres.
Radio communication poses several disadvantages in RTK applications. Firstly, UHF/VHF and higher frequencies are limited to line of sight range. Radio range may also suffer from attenuation due to atmospheric conditions, antenna response and RF interference from other users in the frequency band. Often these bands are crowded and external interference can deteriorate communication quality and cause data loss. This is especially true in urban environments where the airwaves are dense with a variety of RF transmissions. In most jurisdictions, RTK radios must be licensed prior to use [Gao et al, 2002]. Radio communication is also regulated by federal and international bodies and available signal bandwidth continues to diminish because of the myriad demands for usable slots, many of these also shared by a large portion of the regional population.
Applications of RTK
RTK is mainly used for applications that require higher accuracy such as cadastral surveying, construction activities, and drones navigation. Surveying with GNSS RTK receiver saves time, energy, and cost allowing more work to get done with accurate results. The GNSS-RTK approach has been applied to various types of topographic surveys, including those done for the utility industry, forestry and natural resources, land management, landscaping, precision farming, civil engineering, land-deformation monitoring, and open-pit mining.
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