Can I use an external GPS antenna?
The VMS units are using some of the latest SiRF4 chips that are
very good at getting a signal in a weak spot, eg. in a car. However
if you add to that an obscured view of the sky, for example,
limiting the view with building or tree canopy this will really
limit what the unit can do.
If you are constantly under very heavy tree cover then one
possibility is a re-radiating antenna. This is a repeater type
setup for GPS signals (we use one here in the VMS office!). They
consist of a receiver antenna mounted outside the car and a
transmitter antenna inside the car. They will draw a very small
amount of current, so if one is purchased then wiring it to the
accessories in the car would be a prudent thing to do.
There are a number of suppliers around, one of these is GPSOZ.
http://www.gpsoz.com.au/External_Aerials.htm