QUOTE(toby.c13 @ Dec 21 2008, 03:42 PM)
wow.. well i do heard before of tinting on the windows does affect the gps signal strength..

but never heard that it affect smartag before..
could it be that smartag uses infrared and the beam cant go through the tinted glass?

Well I have heard the opposite - tinting films affect smartag and not GPS navigation devices. You see, tinting films are meant to filter out heat from sun. A sunshine's heat originates from a wide spectrum of electromagnetic rays from visible light all the way to infrared regions. In fact about 40% and above of the heat comes from infrared spectrum. So tinting films are specifically designed to block these infrared. Unfortunately thats the same spectrum our smartag uses which is infrared.
GPS signals however does not use infrared but microwaves. They require direct line of sight to work and that is the main criteria for location pinpointing. Having there direct lines emitted on a location will give you triangulation. If your signal is not straight but disperse like lower frequencies, you will not get an accurate signal. A perfect example is the use of GSM networks to pinpoint location. They can only give an accuracy of about 1km radius. Their signal is dispersed which is a criteria for covering as much city ground as possible so we get good mobile network coverage.
Unfortunately this means GPS signals are hard to detect if you drive in an urban jungle where tall buildings cover the sky. This is where a good and sensitive GPS receiver will come in handy. A lot of things affect GPS reception including weather, trees, availability of satellite at their current orbit/hemisphere, clouds, etc.