What is required for post-processing differential correction? (And can Garmin units provide it?)

Question

2 very closely related questions about differential correction with a Garmin unit:

1). Some data was collected using a Garmin GPSMAP 76Cx unit and the data collectors are interested in whether post-processing differential correction is possible. Does anyone know if this model of unit supports this kind of functionality?

If I'm understanding this right (and please correct me if I'm wrong), various brands of GPS units collect a whole slew of information behind the scenes as they record points and save this in a proprietary format, such as SSF for Trimble. This is then convertible to a universal RINEX format using software (sometimes open-source?), which is then used to do post-processing differential correction. Does the Garmin GPSMAP 76Cx produce this kind of file, equivalent to, say, an SSF file on a Trimble Juno SB? I can't seem to find any such information in its detailed specs.

2). If there is no such background info collected by the unit, is any kind of post-processing differential correction possible to data collected by this unit? It may even be the case that the data collectors didn't create any points on the unit but simply wrote down the time and coordinates it displayed. As I understand it, RINEX / SSF / whatever files contain a lot of other info -- the exact "GPS time" (as opposed to local or UTC time) when each point was taken, satellite configuration and pseudo-range, a Doppler shift based on whether a satellite is approaching or retreating, number of satellites detected at time of collection, etc. etc.

Presumably if you don't have a unit that's doing all this for you, or you don't even bother to log the point on the unit but just write down the coordinate & time, you're out of luck if you want to do post-processiong correction?

Would be great if you all could confirm! Thanks so much.

Answer 1

As you said, differential correction is based on the correction of the position of EACH satellite viewed in order to compute a new position. So I confirm that it is NOT possible to do differential correction if you only have the coordinates, even if you had the exact time.

If you want to improve the precision of your measurements with a unit that does not store all information, there are 2 things that you can do :

• Plan your measure : without going into detail, the more satellites you use for measure your position, the more precise your are (but you can still be a little inaccurate, and the location of the satellites also affect the results). You can predict on a short time period the best hour of the day to do your measurement (minimum GDOP is the best).

• Repeat your measure: there is a temporal correlation of the errors, but if you take many measurement (if possible at diferent times o the day) and take the average, you increase the precision.

Note : - the highest precision with a GPS is achieved by using the carrier phase. This is more than an issue about storing the right information in memory: your GPS also need to be able to "read" the phase.

• it was not mentioned earlier, but differential GPS means that you also need the data from at least one other GPS (located not too far and working at the same time)