How Can This Happen?

[Didjerrydo]

I've been trying to see if my Garmin 60CSx suffers from the reported "failure to regain location after losing signal" syndrome by letting it locate outdoors and then covering its antenna with an inverted narrow stainless steel cup which extended about halfway down the display screen completly enclosing the antenna.

To my surprise, I never could get it to drop signal by doing this!

My understanding is that the radio waves upon which GPS operate have to hit the antenna via "line of sight" in order to triangulate properly, but in this case, doing this would have been seemingly impossible yet the unit retained signal level and position even several minutes (actually it never dropped as long as I tried to get it to).

What's going on here and how can this happen?

[peter]

What's going on here and how can this happen?

Diffraction and reflections. The GPS radio waves have a wavelength of a little under 8 inches. Like other waves with which we're familiar, e.g. water waves, they don't travel in perfectly straight lines when encountering an obstruction, but will have some bending around the edges of that obstruction. Think of a harbor entrance protected by a breakwater but with an opening for the boat traffic. Ocean waves hitting the opening in the breakwater don't travel in a perfect line-of-sight path into the harbor. Instead you'd see the waves spreading out behind the opening. And then they reflect off various objects in the harbor (boats, shoreline, docks, etc.) with the result that some amount of wave energy is present throughout the harbor and not just in portions that have a clear line-of-sight view to the incoming ocean waves. But the waves will be much weaker than would be the case without any breakwater protection.

 

If the obstruction is much larger than the wavelength, then the line-of-sight model is pretty accurate. But it breaks down as the sizes become more similar. The opening of your steel cup is probably smaller than the wavelength of GPS waves so diffraction effects will be very significant.

 

Have you tried wrapping the antenna area with aluminum foil?

[+rpminmn]

I have a 60cx and it will maintain a lock under amazing conditions, like the windowless basement of my house. I've occasionally lost a signal down there, but when I walk back upstairs, it'll pick it up again. On the other hand, when I park in the underground ramp at work, the signals are lost and don't pick up again when I go up and outside unless I poweroff and on.

 

I'm thinking that if you keep a satellite or two locked, but not enough to triangulate and give you a position, it'll come back when you get into an open area. But if you lose contact with all the satellites, you need to poweroff and on to acquire them again. Just a hunch.

 

-bob

[+Sputnik 57]

Also, it is my understanding that the "failure to regain signal" issues arises only if you have WAAS enabled.