ARCHIVE: Australian Ham Radio Discussion Forum ( AHRDF )

Like some others about the place I have been toying with an eBay GPS receiver module and cobbling together some software to read it.

I wanted to test the behaviour of my code when the GPS lost signal.

I covered the unit with a metal bowl and connected to the unit with a terminal program (PuTTY)  Surprisingly I did not lose any signal.

I then placed the unit under two metal bowls and again little loss of signal - I lost 2 of the original 11 satellites.

I then clipped the two bowls together and connected them to earth (Faraday shield style).  Still nine satellites, GPS strength 1 Fix.

"Faraday Enclosure" :-
[attachment=171]

Screen capture from PuTTY.  The highlighted areas are:
A = Active GPS fix.
1 = GPS fix 1 (this is a positive fix)
09 = Number of satellites

[attachment=172]

How can this be?

I disconnected the GPS unit and then restarted it thinking it may be buffering, but this made no difference.  I've run it for a while now in the 'shield' but it is still reporting.

I wish my old handheld GPS unit were this sensitive 

I noticed the same thing, they are quite sensitive but I know you can measure the difference if you have a uBlox GPS. There's a nice GUI for windows users that allows you to check (and change) almost every aspect of your GPS. In here you can check the receiver AGC action which will give you a better indication of the effect the shielding is having on the receiver.

Hi Jou,

It's *supposed* to be a uBlox receiver, so I am grabbing the software as I type.

Thanks - I'll post an update one way or the other.

I was able to make it lose fix - two layers of foil, the metal bowls, all earthed and then I put the lot in the bottom draw.

Even then it still picked up the odd valid sentence.

My GPS receiver (U-Blox 6-based) continues to pump out data even in the absence of actual GPS signals and creates basic GPGGA and GPRMC strings based on its internal clock. Sure, it is more accurate when GPS is available but is still pretty close if it is not.

There are a number of the data points that you can use to see if the GPS is actually seeing sats and the quality of the fix but using the U-Centre software and checking the AGC is a good option for actual signal level, as suggested previously.  Actual signal level is probably not necessary for most position and timing applications but if you want to dig deep, why not ?   My GPS2Time application uses only the GPGGA and GPRMC streams.

It seems that the support of reading that AGC level is a U-Blox proprietary command sequence so will only work with that brand device, ie. it is not generic to all GPS receiver devices, something others thinking about using such a feature need to be aware of before they get too far into it.

My 2.2c ... (includes GST)

Doug