5.2 DGPS Correction Methods
There are two different ways the corrections in DGPS can be applied.
If they are applied at the time they are created, these are called real time corrections, while corrections applied later, usually in the office, are called post processed corrections.
There are advantages and disadvantages to both approaches.
Real time corrections
Real time corrections can be applied by communicating the correction message through a radio link or a telecommunications connection, such as a mobile phone, that is configured to communicate with the GNSS receiver.
These corrections may come from your own base station, or they may be broadcast from a commercial provider, where users pay a subscription fee to access the correction service. Usually commercial services require you to use a specialised receiver that is programmed to process the correction service signals.
These DGPS commercial systems sit at the bottom of the GNSS hierarchy; they are generally ground based augmentation systems (GBAS), where the corrections are broadcast from a ground system such as radio or a telecommunication network. Occasionally, they will be satellite based augmentation systems (SBAS), where the correction is broadcast via a communications satellite.
The obvious advantage of real time corrections is getting the corrections immediately. Real time corrections also allow the rover to be used for navigation activities, such as finding a particular mark or location of an object, and also marking out coordinates of something (at the appropriate accuracy).
The disadvantages of real time corrections are a generally higher expense – the cost of the radio or communications link, and potentially the commercial service cost, along with any specialised receiver costs. Real time corrections can also mask system errors that happen over longer timeframes.
Post processed corrections
Post processed corrections are applied at some point after the collection of positions is completed, and requires specialised software to determine the corrections at the base and then apply them to the rover dataset. This processing of applying the corrections is referred to as post processing.
Both the base station and the rover must log its data to memory if users are planning on post processing their data.
Post processing has the benefit of allowing a model of the entire time frame that the GNSS receivers were logging data to be created, which will often remove more errors than real time corrections. The disadvantage of post processed corrections is the lack of corrected data available in real time, so activities like navigating to specific points or using coordinates to mark out something are not able to be undertaken.