A wordy explanation describing the problem using RTK for mobile mapping.
If you lose GPS lock while using RTK, and you will especially when you need it most, you are totally messed up with no recovery except to post process (and there goes that RTK advantage and price). No matter what, your range is extremely limited with RTK unless you use VRS (not sure you have that where you are). With a radio modem or VRS you will lose coverage (from blockage, etc.) and when you do your positioning degrades from the few centimeters you typically enjoy to several meters. That is completely unacceptable. If you lose it because you are behind a building then unless you move your radio base station to within LOS you will never be able to collect in this obstructed area. And good luck moving your base station many, many times during the day – not very productive.
In addition, your heading is adversely affected by GPS outages and by the limitation of processing only forward in time. As a consequence, the positional accuracy is greatly reduced using only the RTK solution. This means a poorly constructed point cloud.
Lastly, the entire INS (Inertial Navigation System using GPS/IMU) solution is processed with Kalman Filter error states. When using RTK it is only possible to process forward in time thus limiting the number of states (i.e. parameters) that can be utilized. If your GPS measures at 2Hz then the entire computation and adjustment needs to be computed within 0.5 seconds before the next GPS epoch comes in (faster at 5Hz means even less time). The CPU can only process data so fast so the number of parameters must therefore be reduced.
Since RTK is a forward only solution, during GPS outages a combined forward and backward solution is not possible, so no smoothing of the data can be performed. This means the drift forward cannot be smoothed with a reverse solution which would largely negate the most profound drift errors. The graphic below is a very rough drawing used simply to illustrate the forward, reverse and combined solutions in regards to drift.
Of course, the advantage of RTK is you save 15 minutes at the office updating your INS solution (a rather small task compared to everything else) and you save $10,000 to $15,000 on post processing software.
