[time-nuts] GPS antenna setup: how good?

[Bob Camp]

Hi

As always with any simple question - the answer is "that depends ...".

What are you using for a receiver? Is it "sensitivity challenged"?
A modern receiver will be a *lot* more sensitive than the receiver in an old
HP GPSDO. The next layer to that onion is feed line and splitters (or not).
All that is made more complex by the fact that you can put to much gain in
front of some receivers.

Where is the antenna located? Is it "multipath challenged"?
An antenna on the top of a 100 foot tall mast with no obstructions anywhere
has a very different situation than one at ground level with dozens of
reflecting objects nearby. What is and isn't a reflector at 1.5 GHz is not
always obvious. There are a wide variety of multipath situations you can run
into. Some receivers are better at rejecting stuff than others. 

What are you trying to do? Are you after carrier phase data or conventional
reception? Carrier phase is less forgiving in general.

Is your objective timing or navigation? I'll assume it's fixed location
timing. How does your GPSDO (or what ever) handle the incoming data? How
good is it at rejecting odd looking inputs?. All of that can vary depending
on what the recent history into the unit has been.

How good can you measure? Put another way, is there a "good enough" timing
value below which a change could not be seen? Are you averaging readings
over a day or over a few minutes? What are you comparing the timing to? Are
you comparing to an independent standard or to another GPS?

Are you sky view challenged? Are you far enough down in a hole that only a
portion of the sky is visible? Some common urban settings are quite poor for
seeing more than a couple satellites all the time. Some very poor ones are
lucky to catch a couple every so often.

Are you overload challenged? Antennas have a wide variety of filtering in
them. Some receivers have a lot of filtering, others have very little. If
you are located next to a cell tower, filtering in both is a good thing.
These days there are a surprising number of gizmos that come and go spraying
RF all over the place. The kid next door who talks on the el-cheapo cell
phone for 4 hours at a time is one....

Hard to believe, but that's far from a complete list. Some of what's there
is pretty specific, other stuff is going to apply to just about every
setting.

In some cases you could compare the best antenna made to a bent coat hanger
and the difference while very measurable would not be statistically
significant. In other cases a metric that is multipath focused would be so
very well to make a decision. In still other cases it's sensitivity and only
sensitivity. 

Simple answer - no one number, no 60 second test, no free lunch :)....

More than you would ever really *want* to know about GPS orbits and the
like:

http://www.kowoma.de/en/gps/index.htm

A good look at what you *could* be doing with GPS, but might not be:

http://www.boulder.nist.gov/timefreq/general/pdf/1424.pdf

They seem to like tracks in the 15 minute range. 

Simple experiment:

Take two (hopefully) identical receivers, two fixed and widely separated
crummy antenna locations. Similar sized antennas with different stuff inside
them. Run antenna A in location A, B in B. Collect pps output data for a few
weeks. Look at the pps wander around. 

Swap A to B and B to A. Collect data for a few more weeks. 

Swap A back to A and B back to B. More data.

Repeat A to B and B to A.

Look for an obvious winner. 

The winner changes depending on which data sets I use, and how I look at the
data. That's not to say that the antennas are equal. They most decidedly are
not. One is better at some things, the other at other things. Which one wins
is situational. 

No simple answer....

Bob



-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Hal Murray
Sent: Wednesday, March 24, 2010 2:00 PM
To: time-nuts at febo.com
Subject: [time-nuts] GPS antenna setup: how good?


How do I tell which of two setups is better?  For example, how much does 
adding a pie pan help?

Is there some simple parameter I can look at that tells me an antenna 
goodness value?  If not, what's a reasonable recipe to come up with a number

or compare two antennas?

What's the appropriate time scale to use when thinking about that problem?


I haven't (yet?) looked at any of the satellite position and signal strength

data.

Can I do something like wait until a satellite is about to go directly 
overhead and plot the signal strength while occasionally swapping the 
antennas?  Are the signal strength readings reasonably noise free and/or 
repeatable from run to run which may be a few days apart?

Assuming it goes high overhead, how long is a satellite within view?  
(ballpark)






-- 
These are my opinions, not necessarily my employer's.  I hate spam.




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