What is RTK

What is RTK

RTK (Real-Time Kinematic)

RTK is a process where GPS signal corrections are transmitted in real time from a reference receiver at a known location to one or more remote rover receivers. The use of an RTK capable GPS system can compensate for atmospheric delay, orbital errors and other variables in GPS geometry, increasing positioning accuracy up to within a centimeter. Used by engineers, topographers, surveyors and other professionals, RTK is a technique employed in applications where precision is paramount. RTK is used, not only as a precision positioning instrument, but also as a core for navigation systems or automatic machine guidance, in applications such as civil engineering and dredging. It provides advantages over other traditional positioning and tracking methods, increasing productivity and accuracy. Using the code phase of GPS signals, as well as the carrier phase, which delivers the most accurate GPS information, RTK provides differential corrections to produce the most precise GPS positioning.

The RTK process begins with a preliminary ambiguity resolution. This is a crucial aspect of any kinematic system, particularly in real-time where the velocity of a rover receiver should not degrade either the achievable performance or the system's overall reliability.

Thales Navigation/Magellan has developed two innovative, high-performance technology solutions for RTK ambiguity resolution; KART (Kinematic Applications in Real Time) for single-frequency receivers and LRK® (Long Range Kinematic) for dual-frequency receivers.

KART (Kinematic Applications in Real Time)

KART is a kinematic method that allows any initialization mode, from a static or fixed reference point, to On The Fly (OTF) ambiguity resolution, when performing single-frequency GPS positioning.

This unique technology introduces a fundamentally different strategy than that of other traditional methods, and delivers results that are substantially more reliable with single-frequency GPS L1 receivers.

Conventional ambiguity resolution follows this procedure:
1. Define a search area based on an approached solution and its uncertainty.

2. Test all potential solutions within the area statistically.

3. Select the most likely solution among the possible solutions, according to a minimal variance criterion.

4. Validate the chosen solution according to statistical criteria or by comparison with the second best candidate. In theory, this process is correct and perhaps reasonable under certain circumstances. However, Thales Navigation has found it to be inadequate when applied to single-frequency operations in the field for two fundamental reasons:
1. When only GPS L1 frequency data is available, the search area contains numerous potential solutions positioned closely together. The actual position may appear to have the minimal variance for some time.

2. The statistical tests and criteria that are used cannot provide reliable results unless the functional and a priori stochastic models used can correctly represent reality, which in practice is not the case.

Any reliable ambiguity resolution in real-time using this approach with a single-frequency receiver is nearly impossible. That is why Thales Navigation has created a new approach.

KART operates by a simple and reliable method:
1. Calculate approximate position using the EDGPS process, which offers no ambiguity, is unbiased, and tends towards the true position.

2. Re-calculate the EDGPS result from the unambiguous pure phase solution, providing the minimal risk of error in ambiguity resolution.

3. Validate the solution position using the residuals of the least-squares adjustment.

4. Confirm the solution by observing its stability over time. Resolutions of the conventional method will tend to drift. KART technology does not depend on an a priori stochastic model to achieve and confirm a kinematic solution, making it a faster, easier, more reliable and cost-effective method. KART makes real-time kinematic operation possible with single-frequency receivers in applications otherwise impossible without dual-frequency measurements.

LRK (Long Range Kinematic)

Thales Navigation's LRK technology is a sophisticated kinematic method that optimizes the advantages of dual-frequency GPS operation. Other conventional methods use the dual-frequency only during initialization. LRK makes solving ambiguities during initialization easy and continuous dual-frequency kinematic operation possible at distances up to 40 kilometers.

Conventional dual-frequency kinematic operation is limited to about 10 kilometers, using a combined observation on GPS L1 and L2 frequencies to produce an initial wide lane solution, ambiguous to around 86 centimeters. During a second phase, the conventional kinematic method uses measurements from the L1 frequency only. This method only allows for kinematic operation as long as the de-correlation of atmospheric errors is compatible with a pure phase single-frequency solution. Similar to the KART process, LRK is a simple and reliable method that allows any initialization mode, from a static or fixed reference point, to On The Fly ambiguity resolution, when performing dual-frequency GPS positioning. LRK technology reduces initialization times to a few seconds by efficiently using L2 measurements in every mode of operation. LRK maintains optimal real-time positioning accuracy to within a centimeter at a range up to 40 kilometers, even with a reduced number of visible satellites.