Hi Gabriel -
I've seen this error when there are multiple SSF files in the same project and the one you've chosen does not span the entire duration of time you were collecting GPS data.
Ideally, there should be just one SSF file contained in the checked-out data for ArcPad folder and it will be called GPSCorrect.SSF. Look at the ArcPad data folder in Windows Explorer and see if you have multiple SSF files. If so, you can add them all in the differential correction wizard by clicking the black plus-sign icon and navigating to their location. It might work, but if you've been out collecting data in the same checked-out project for multiple days, it might not work.
The recommended workflow here is to check out your data for ArcPad, go into the field to collect your GPS data, then transfer the data to your PC, check in the AXF and SSF files and do your post-processing. Repeat this the next time you go out into the field to collect data. You should NOT go back out into the field later that day and definitely not on another day to continue collecting data in the same checked-out folder. To do so increases the likelihood that your data will no be able to be checked in and/or post-processed properly.
