Dumb question re: GPS trackers

Ordinarily we think of multipath reception--a GPS signal which gets to the receiver not by direct line of sight from the satellite but by bouncing off something--as purely a bad thing as it is a primary source of position error.

But I'll guess that for a receiver suspended under a car it may be an enabler. The GPS system signal strength reception margin is nowhere near enough to allow direct path reception through any of the metal parts of the car--body sheet metal might as well be a perfect shield for this purpose. So I'm guessing (and it is just a guess), that even though pavement is probably not a very good reflector at 1.6 GHz, it may sometimes be good enough to propagate a reflected signal a few inches to a receiver under the car.

And since the error term is path length difference dependent, in this case the added position error would be negligible.

But I'm guessing from general principles--be happy to hear from someone with more direct knowledge.

Of course, maybe they are not always actually under the car. Many locations in the cabin will usually get a signal--if you can figure out how to hide the unit. The windows let through signal just fine--as do most dashboard materials...

One of my novelty items I have is a "Trackstick" which is about 1 inch wide and 4.5 inches long.
It records your route, speed, altitude and time of travel. I use it to see where I had been and where I stopped. It is faster than my Garmin or not available with my car GPS. All one has to do is plug it into a USB plug, save it as KML and open automatically in Google Earth.
I usually place it in the back seat of the car clipped to the seat belt so it is out of the way.
So far it has not failed in record keeping.

On rental cars, the GPS tracker antenna is often installed under the plastic vent area between the hood and windshield.

Don't forget they can track you by your cell phone too. I'd be more worried about that.

Short answer: you gotta be able to see the sky.

GPS uses circular polarization of the transmitted signal. Our receivers use antennas with right hand circular polarization (RHCP) to pick up these signals.

Think about a mirror -- reflections have things reversed.

A multipath signal, one coming down from a bird and hitting a reflecting surface such as the metal side of the huge truck you're passing, gets its polarization reversed -- just like a reflection in a mirror. And that reflected signal is by design attenuated by the RHCP antenna in the GPS receiver, as it's the wrong polarization. Makes our receivers less susceptible to multipath. (And Perseus not susceptible to the Medusa, as he was looking at her reflection -- similar principle.)

It takes two (or generally, an even number) of reflections to get a signal that the GPS antenna can "see," and each reflection greatly weakens the signal (which is below the noise level to start with) unless those reflective surfaces were designed for use with GPS signals.

(I hope you like it when I talk dirty like this!)

Then maybe I'll know where I'm going or went !

I think that's a little like the hurrier I go the behinder I get !

The Medusa reference was brilliant

talk dirty like this anytime

Are you sure? Non-polarized antennas cost less and are smaller and there is always the big push to reduce cost. For most automotive applications multipath will not really hurt much, especially if the software can filter out temporary discrepancies in the data. I know my older Nuvi has serious filtering software - the newer ones are probably better.

My guess is that the trackers use better receivers. A few years ago I bought a cheap ($50) receiver for work. No software, no power supply, nothing - had to write/acquire my own software just to get started. Inside an older commercial office building (i.e. lots of steel and concrete) I was getting about 9 satellites. Wrapping the entire receiver in a conductive electrostatic shielding bag got it down to 4 or 5 satellites. That is way better performance than I see from my Garmins.

My guess is that the Garmins are good enough for the application, but you can do better if you want.