I will answer this question from a "technology of radio" perspective, because half of the question is very much related to the technology of radio. The other half is related to the decoding of a data stream from multiple GPS satellites, and that is beyond the scope of this site.
In order to receive the data from the GPS satellites, your best bet would be to ...
Xastir isn't bad at all but I recently encountered something called YAAC (Yet Another APRS Client). On it's site it states the following:
YAAC can be used as a stand-alone APRS client, an APRS RF-Internet
gateway (I-Gate), or as a AX.25 digipeater. It supports 16 different
data reporting views as delivered, and can be extended with
Start with a GPS receiver that outputs NMEA data. This data will be made available through a serial connection such as RS-232, USB, or Bluetooth. Pick a serial connection that you can use with your MAC.
On the MAC you will need a serial terminal emulation program (not to be confused with the MAC terminal program). A program such as Serial would do the job.
MultiScan 3B works fine in terms of decoding. This being said I had some issues making it work with my external sound card for transmitting SSTV. For some reason transmitted images turned out to be cropped. I accidentally figured this out using cqsstv.com.
Eventually I ended up using a Linux laptop and QSSTV software. Still if you are interested only in ...
Check the sample rate configuration in the software
you are using. Older Apple products used a hardware sample rate of 44.1 ksps, whereas newer Apple products all seem to use a hardware audio sample rate of 48k (or a multiple thereof).