What is GPS Post Processing?

Nearly all GPS devices output NMEA sentences, these are readable lines of text that contain information on not only your position, time, date, altitude etc. but also the number of satellites, their position in the sky and a lot of other info., the sentences are generated by your GPS after processing the raw data it has received from a set of satellites, however the raw data itself is not usually available to the user of a hand held unit and you might think that it would be of little use even if it was available.

Professional survey quality GPS units have always made this 'raw data' available as it is possible to work on it in order to get more accurate positioning data, however there are two main reasons that this is not done instantly to get a real time improvement in accuracy, the first is that the unit couldn't cope with the calculations needed in real time and the other is that external inputs are sometimes used with the raw data and that this external data isn't available at the time.

Post Processing simply means working on stored data from a GPS unit after the event to produce better positioning accuracy, sometimes as much as 100 or 1000 times better, in the same way as a standard GPS can get better positioning data by averaging a number of readings taken at a single location, post processing can work its magic on a block of data to come up with better results

Not all of the improvements in accuracy seen on professional survey GPS equipment is due to post processing, this is because these units also employ various other techniques not available to early GPS units, they include :-

• GNSS satellites in addition to GPS such as Glonass, Galileo, Beidou
• SBAS satellites e.g. WAAS, Egonos, MSAS, Gagan, SNAS, SDCM, WADGPS
• Dual Band / Frequencies from each satellite e.g. L1 + L2 +L5
• Differential GPS radio data feeds from other measurement equipment
• RTK carrier phase tracking + RTK base stations

History
Until recently I thought that post processing in non survey quality GPSs was nonexistent due to the fact the raw data was simply not available to the end user, however I have discovered that undocumented commands have always been available to Garmin GPS owners, these so called 'async messages' were sent via the RS232 serial port of units such as the GPS12 unit back in 1998, there were even programs to convert the 'async messages' to the more portable Rinex format, see :-

Obtaining raw data from some (Older) Garmin units
Log Garmin Asynchronous Binary


Processing Rinex Data
With modern smartphones and a few hand held GPS/GNSS units such as the Garmin GPSMap66 and Garmin Montana 700 it is possible to store / extract raw satellite data in Rinex format and then post process it into usable possition data, one program that will carry out this post processing is :-

RTKLIB

Useful Links
Hand Held Units with GNSS Raw Data Capability
Ordnance Survey Rinex Data (UK)
Report on postprocessiing of Rinex data from GPSMap66