Martin McCormick wrote:

> Russell McMahon writes:
> 
>>Completely different suggestion - would GPS meet your need?
>>Limited indoor use due to shielding by buildings may be a killer. 
>>Modules getting very cheap.
>>Extremely easy to interface to.
> 
> 	That is a great suggestion.  If it wasn't for the indoor
> signal reception problem, I wouldn't even consider the LF radio
> solution.  Maybe I should buy a module and see if enough signal leaks
> in to our house through the windows or walls to be reliable.

You would be quite disappointed.

I have a fairly normal wood frame house and several different
brands of GPS and GPS modules, none of which worked well except outside.
(Magellan, Garmin, Trimble, Microsoft/Pharos, and the like).

If I put the GPSs in a window I can get reasonable signals, but
only for satellites in the direction of the window. Behind a wood
wall (with vinyl siding) I get very weak signals, and only from
satellites slightly past 90 degrees down to 45 degrees. e.g. ones
that would read otherwise read full strength.
I was looking at ways to get cheap timing for a radio astronomy
application. The correct solution is an outdoor GPS antenna,
with either weatherproof timing GPS puck or an amplified antenna
feeding coax or twisted pair (1pps signal) to the frequency/time
reference.

VLF time does not appear to work well indoors because there
are just too many sources of in-band noise (SVGA monitors for
example), and the signal fades during the day.

You may be able to put up a GPS passive relay antenna to
reradiate indoors, but what's the point? You might as well
put the active antenna or puck outdoors and have a reliable
signal.

If you have a high speed connection, you might be able to
use NTP (actually the successor protocol which works better)
to get within a millisecond or so of the correct time on a
PC. There are also commercial $$$ cards to get nanosecond
accuracies on a PCI bus box.

Check out Brook Shera's page for a good PIC based solution

http://www.rt66.com/~shera/index_fs.htm
"GPS-Controlled Frequency Standard"

His site is a wealth of useful info on timing and frequency references.

Check out TAPR as well for inexpensive GPS interface boards.
http://www.tapr.org/tapr/index.html
http://www.tapr.org/tapr/html/Fgarminib.html

Robert

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