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.