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How To Use GNSS Simulators To Test RTK Positioning Systems

Drone over city

Traditionally the preserve of precision land and marine surveying operations, Real Time Kinematic (RTK) systems are becoming cheaper and smaller, making them more attractive to manufacturers of other commercial devices looking to enhance positioning accuracy, such as drones or unmanned systems.

What is RTK?

RTK systems are used in conjunction with a GPS or other global navigation satellite system (GNSS) receiver to significantly increase the accuracy of the positioning, navigation and timing data obtained from the GNSS signals. When paired with an RTK system, a GNSS receiver can accurately pinpoint a location to within one centimeter horizontally and two centimeters vertically.

An RTK system typically comprises a fixed base station, positioned at a known (surveyed) location, and one or more mobile receivers. The base station receives phase signal data from GNSS satellites, applies error correction, and re-transmits the resulting highly accurate position data to the mobile receivers. This allows the mobile stations to very accurately determine their location in relation to the base station, whose own highly accurate position has already been established.

The Radio Technical Committee for Maritime (RTCM) SC-104 standard specifies the code and carrier correction data messages for differential GPS (DGPS) applications. RTCM messages are commonly used by DGPS and RTK systems for transmitting correction data to users. There are different ‘Types’ of RTCM messages, some common ones are:

  • Type 1: Code differential corrections
  • Type 3: Reference station information
  • Type 9: Similar to Type 1
  • Types 18-19: RTK Carrier corrections
  • Types 20-21: RTK PR corrections

Testing the corrections

As the purpose of RTK systems is to provide centimeter-level position accuracy and position integrity for business-critical operations, they should be tested to ensure they function as expected in all of the situations users will put them.

Many organizations rely on live-sky testing to verify a receiver. This approach offers a realistic test environment, but it has significant drawbacks:

  • It can be expensive to transport staff and equipment to different field test locations
  • The real-world RF environment doesn’t include all conditions in which the system may be used – in particular, you will want to understand how the system performs in the presence of interference of different types, which may not be present at the test site
  • The real-world environment offers you, the tester, very little control over the test conditions (e.g. you can’t test how your system will perform with minor degradations to the GNSS signal)
  • The test environment may be polluted with RF noise or other types of interference that will potentially provide misleading results
  • The test environment is continuously changing, so the same precise conditions can’t be re-created for repeat testing purposes (unless you use a record and playback solution, see below)
  • Field testing only allows you to test with signals from satellites present at the test location – so can’t test performance with non-present regional satellite systems or future systems

Using your GNSS signal simulator to test RTK systems

Lab-based simulation is a much more efficient and reliable way to test the performance of your RTK systems.

Using a GNSS signal simulator, you can test the accuracy and integrity of RTK correction signals in the same way as you would test your GNSS receivers. By manually entering the location of an RTK base station, you can test GNSS and correction signals in a repeatable, reliable environment. You can also select message types per satellite, scheduling \ rate and edit base station data quality & errors(clock, multipath) to test comprehensively. RTCM messages can be transmitted in real-time to the receiver test directly over a RS232 link.

And when it comes to testing real-world performance, you can use a Record & Playback system (RPS) to record the complete signal spectrum at your chosen test location. You can then replay this signal recording to your system back in the lab to fully test how your receiver and RTK systems perform in a real environment. Using an RPS saves time and money, and it increases the reliability of your test results because you are always testing under the same conditions.

Talk to Spirent

Whether you’re developing RTK systems for survey purposes or exploring new applications of RTK, you need to know your systems are broadcasting accurate correction data whenever it’s needed.

For more information on how to test your RTK system using a Spirent simulator and/or RPS, how to use Virtual Referencing Systems in your testing, or which Spirent solutions are right for your needs, get in touch.

 

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