Does either the Raymarine 33STV or 37STV have auto skew?
They mention this in an install manual.
Why would you need to connect to an external GPS if the unit has it's own internal GPS.
Text from the 37STV manual:
Connecting the system to a GPS
For improved satellite tracking, you can connect your satellite TV
system directly to your boat's NMEA 0183 GPS system. To do
this you will need two lengths of cable suitable for connecting to
your GPS system and the green 2-way ACU GPS connector
supplied with your 37STV Satellite TV System.