Tool tips on GPS latency and RTK "fixes."
Q: I have just started using RTK (Real-time Kinematic) GPS. Why is it important to pay attention to latency? I understand that this is the lag between the time the GPS measurement is made and the time the result is shown on my display. But isn't it better to have a measurement and a position result, even if the latency is high?
A: Many people pay attention to latency after getting "burned" from not paying attention to it. You are correct in that this is the time delay between the GPS signal measurements (from every one of the satellites when a position is calculated) and the results sent to the user. These measurements include phase of the L1 and L2 carriers and phase of the C/A and P/Y codes from each satellite. GPS systems are designed to simultaneously make these measurements at the reference station and rover station receivers. For RTK to work, the reference station measurements must be transferred over a link, usually radio, which involves "packaging the data" and sending it to the digital modem of the link, which then sends it on in a predictable way (transmission format) to the digital modem at the other end of the link. The received data is then converted into a form that the processing algorithms running on the roving receiver can combine with its GPS measurements. Assuming the ambiguities have already been resolved, there is still a further time delay due to the computations for determining the changes since the last epoch and then converting the result into a coordinate in the system that the user is working. Often this result is shown as the difference between a desired position (if the surveyor is involved in a stakeout operation) and the current position. Most RTK systems typically update (or have epochs) at a rate that is faster than 1 Hz (one update per second).