[GPSNavX]

To determine if NASA AIS receiver is a switching model:

Start GPSNavX or MacENC
Connect to the AIS receiver where traffic is present
Open the AIS monitor panel
Push "Start" button
Now look at the !AIVDM messages.
If the letter following the message is an "A" then it's receiving on channel "A". if it's a "B" then it's receiving on channel "B".

If your NASA receiver switches channels every 36 seconds it will alternate channels "A" and "B" in the NMEA monitor.

I believe the Mark II did not ship until last October.