RTK stands for Real Time Kinematic and is another technique that improves the accuracy of GPS position measurements—however, it’s one of the hardest to understand and the most intensive to implement.
Standard Positioning Service (SPS) is the first position fix that GPS systems achieve, and they do this using the C/A code. If differential corrections are available, the SPS accuracy can be improved as timing delays on the satellite signals can be removed. Using DGPS position accuracy of around 40 cm is fairly normal. RTK is the next step on from DGPS, and there are two versions of it; RTK float, which achieves decimetre level accuracy and RTK fixed, which achieves centimetre level accuracy.
To understand how either version works it’s necessary to understand two things:
- The first is that RTK is bending the rules of what we should be able to achieve using a civilian GPS receiver. It is not a technology that was designed into the system to help us. Instead, it’s a method that was figured out by GPS manufacturers to achieve greater accuracy than was intended using the C/A code.
- The second thing to understand is that it is based on the carrier waves themselves, not on the C/A code or navigation message they carry.
How does RTK work?
In our ‘What is SPS’ page, we discussed how SPS is based on pseudo-range measurements calculated using the C/A code. The receiver does this by synchronising its clock with the GPS satellites, then generating 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, say, seven milliseconds in order to match the C/A code being received at the antenna, then it knows the travel time of the signal from that satellite is seven milliseconds and it can then work out how far away the satellite is.
The ultimate aim of RTK is to establish how many carrier waves there are between the antenna and the satellite. The reason for this is simple. Each satellite broadcasts a unique C/A code made up of 1,023 bits. The code is sent at a rate of 1.023 Mb/s, which means one bit is sent about every microsecond. In one microsecond the radio signal from the satellite covers a distance of about 300 metres.
The carrier wave that the C/A code is modulated onto is at a much higher frequency however—1575.42 MHz. This means a single wave covers about 19 cm. If we could work out how many full waves there are between the satellite and the antenna, then it would be possible to calculate the distance much more accurately. In fact, if we know how many full waves there are, and it’s possible to measure partial waves (the phase angle) too, then we can be very precise.
RTK Fixed vs RTK Float
RTK Fixed and RTK Float can be considered as two separate (but related) algorithms running concurrently within a system. If RTK Fixed has a valid solution, the system switches to it (however the RTK Float algorithm continues running in the background). If RTK Fixed subsequently becomes invalid, the system reverts back to RTK Float, which does not need to start again as long as the carrier phase lock is not lost.
So how are the algorithms different? RTK Float aims to identify your probable location (improving on the current DGPS accuracy) using statistical methods. It requires a minimum of four common satellites with the base station, and (in lay-terms), looks for a point in a circle around the current position measurement about which the satellites are revolving. Unlike RTK Fixed, the Float algorithm never attempts to solve the ambiguity problem—it only tries to identify the most probable position within a circle drawn around the current position estimate. The accuracy for RTK Float starts around 40 cm, but increases to 20 cm at best.
RTK Fixed, on the other hand, aims to solve the ambiguity problem and is not used until it has done so. It requires five common satellites, and when a valid solution is found the system knows there are n carrier waves plus any partial wave between it and the satellite. It can align its measurements to about 0.6% of the 19 cm wavelength. When multiple measurements are combined into a navigation solution this can give an accuracy of about 1 cm.
Now we’ve reached the point in our ‘What is GNSS?‘ series where we’ve discussed how GNSS systems are generally easy to use, don’t drift and can achieve high levels of accuracy. But nothing in life is perfect, is it? So, what’s the down side? What most improves accuracy, and what are the limitations of GNSS?
GPS vs RTK GPS
The difference between GPS and RTK GPS is simple to understand. Traditional GPS systems use data from satellites alone to calculate a position. RTK GPS systems however supplement standard GPS data with additional information from a nearby base station.
Using an RTK GPS system can improve accuracy to within centimetres which is required for applications such as ADAS testing, mapping and autonomy.
RTK FAQs
How accurate is RTK?
Over the years as GNSS/INS technology has improved, so to has RTK accuracy. In favourable conditions, customers using an OxTS GNSS-aided Inertial Navigation System like the RT3000 v4, can expect 1 cm RTK accuracy, making the measurements suitable for applications such as automotive testing, mapping and autonomous vehicle navigation. On occasions where GNSS is unavailable, such as in an urban canyon or indoors, additional sensor technology can be employed to constrain position drift and retain accuracy for as long as possible.
What is the difference between RTK and PPK in GPS measurements?
On the face of it, the difference between RTK and PPK is fairly straightforward. As mentioned previously in this blog, RTK stands for real time kinematic. It it the process by which position data is corrected and as the name suggests the process takes place in real-time. Where PPK, or post-processed kinematic, differs is that although the data is still corrected, it is done so after the data collection has taken place.
PPK can be more accurate than RTK, however there maybe some instances where it makes more sense for the data to be corrected in real-time, such as in autonomous navigation.
What is RTK for drones?
Any project that requires something to navigate autonomously, or provide its user with accurate, instantaneous position updates requires RTK data. Whether the object is a car, boat or a drone the most important factors to consider relate to your project’s requirements. Ask youself what level of accuracy you need, and what effect this will have on your project goals. Many drones come equipped with RTK positioning technology, however these can generally be more expensive than drones with standard GPS positioning sensors. One way to cut down cost is to build your own payload with the relevant onboard sensors. While this can take time to build, it offers the user added flexibility when deciding on the sensor combination for the job.
OxTS GNSS/INS devices like the xRED3000, provide drone integrators with RTK position accuracy in an OEM board form for ease of integration.
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We have been developing highly-accurate GNSS/INS solutions for over 25 years.
Our solutions are used in a range of industries across the world to provide engineers with accurate position, navigation and timing information in both open-sky and GNSS-denied environments.
Read the OxTS Company Brochure to find out how our RTK capable GNSS/INS hardware could help you.
