How many times have you been in the midst of gathering positioning data only to find you’ve lost your connection to RTK corrections and have to start over?
RTK is a powerful positioning engine that can provide centimetre-level solutions—if you have access to RTK correction services, which typically come from UHF radios or cell phones. Of course, radios require line of site, which is not always possible, and cell phones are prone to dropped signals. The loss of line of site or a dropped signal, even for a few seconds, degrades RTK performance.
Obstructions, dead spots and baseline limitations are familiar foes of every RTK operator.
Subscription-based RTK extension services are available to help a receiver in RTK mode maintain centimetre-level positioning accuracy during periods of RTK correction outages. These services are typically delivered over L-Band so they are accessible with standard GNSS hardware and independent from the RTK corrections.
With an RTK correction extension service, an operator’s receiver will automatically transition to the L-Band source for correction data when an RTK signal is lost. The receiver will utilize that L-Band correction until normal RTK corrections are recovered, a timeout is reached or position accuracy degrades beyond a defined threshold. All corrections are handled seamlessly in the background and these services automatically compensate for datum differences and related biases between correction sources. As with any carrier phase positioning, position jumps may sometimes occur depending on the quality of the ambiguity fix prior to the dropout.
While durations may vary depending on the service provider, users are typically able to maintain RTK level performance and accuracy during an outage for up to 20 minutes as long as there is L-Band tracking and the GNSS receiver has had at least one RTK fix prior to the outage. Some providers also offer RTK correction outage bridging that operates at a sub-decimetre level of accuracy.
There are a few limitations with RTK correction extension services. Any obstructions (i.e., grain tanks, tree lines, etc) that can degrade or block the L-Band satellite signals will affect performance. An L-Band signal block will cause the position error to increase over time. If L-Band signals are lost for more than 2 minutes during the RTK extension period, then RTK-level accuracies cannot be maintained and the correction extension service will end.
It’s also worth noting that accuracy degrades the longer the RTK correction outage period, but it will level out at approximately 4-5 cm. That’s because providers like NovAtel rely on global Precise Point Positioning (PPP) correction data to extend the RTK positioning solution during the outages. While global PPP data provides a consistent solution to all users regardless of location, it has the disadvantage of not containing the local correction information that enables a true RTK solution. Also, if the RTK outage occurs in the first 30 minutes of receiver operation, further degradation to approximately 20 cm may be observed while operating with an RTK extension service.
RTK is the accepted solution in many applications that require instant, centimetre-level positioning. Applications ranging from agriculture to survey & mapping, to unmanned vehicles and more all depend on RTK to some degree. RTK correction
extension services extend RTK operation for any of these applications, which improves productivity by reducing the downtime caused by RTK correction signal outages. As long as RTK positioning has commenced prior to the outage, a correction extension service can help maintain continuous operation and ensure maximum productivity.