GPS Obstructions

Leddy · 06-06-2004, 02:31 PM

I notice a lot of threads about metal interfering with GPS signal. I was curious about a few of the other obstructions that may/may not interfere with the signal because of a recent experience.

I had my car parked in the garage playing with my laptop. I plugged in my GPS connection(Onstar modified) and fired up Streets and Trips. I turned on the car and clicked on track position. The second I did that I had a locked position. I know most units can take up to 30 seconds to lock. So my question is using the onstar system(with the antenna) makes that much difference that I can get an instant lock while parked in my garage?

I don't have another GPS that I can use to test signal inside the garage, but would "normal" gps setups work inside garages?

beaverpants · 06-06-2004, 02:58 PM

if your in a bilding you wont get a lock

Leddy · 06-06-2004, 06:53 PM

A lock is defined as knowing my Lat/Long? Or is there more to it than that?

rando · 06-06-2004, 07:40 PM

The simple answer is, "It depends." I have an Earthmate USB and sometimes get a signal in the garage. My garage is underneath the 2nd story of my also. I'm also ocassionally able to get a signal inside the house. In most cases, the time to lock onto a signal inside the house, in the garage, or in other obstructed locations is much longer than normal.

GPS lock times depend on several factors. The most important is if the receiver has been completely powered down or not. This scenario is called cold boot and most GPS chips require 30+ seconds to get a lock (assuming a strong signal) in this situation. Dependong the specifics of the design, it might be able to produce valid data even sooner if it is able to retain it's last state and thus already know which sats are in view. This is referred to as a warm boot. For this to work, the unit needs remain powered (full or reduced), or have non-volatile memory, or be able to receive commands to feed it some of the data that it normally must discover upon a cold boot.

It's posisble that your OnStar based unit retains power at all times and thus would, in most cases, very quickly lock onto available sats and produce a valid stream.

Ormo · 06-07-2004, 04:55 AM

Or it could be that your GPS software simply "remembered" where you were when you last stopped using it. This would mean it would place you on it's map before your GPS has locked onto a signal.

Laidback · 06-07-2004, 05:06 AM

Yeah, I know that mine keeps sending the last few signals it received (at the garage door) over & over. Also depends on what your garage is made from - plastic roof?, no problem!

danon · 06-07-2004, 05:30 AM

Quote: Originally Posted by Ormo

Or it could be that your GPS software simply "remembered" where you were when you last stopped using it. This would mean it would place you on it's map before your GPS has locked onto a signal.

my rand mcnally mouse gps also "remembers" the last lat/long it recorded. after unplugging, it seems that the power that was left in its circuitry is enough to do this for a short period of time.

danon

6GenX · 06-10-2004, 07:42 PM

What about tinted windows would that interfere with the siginal? I saw a car identical to mine, (even tinted windows), and he had his gps on the dash rather than on the rear deck. Just curious.