SiRF III: is sensitivity better than accuracy?

Greater sensitivity does not equate to better accuracy. In fact, they
can be opposing forces. Greater sensitivity can simply mean the
receiver accepts noiser data, which means less accurate data.

Don't ignore the antenna quality either. No matter how good a receiver
is, if the antenna is poorly designed the receiver will perform
poorly.

My experience with Holux products has been poor. The ability for the
one I used to hang onto WAAS when you are moving is nearly non-
existent.

sat_alight@yahoo.com wrote:

-All too true... The first Garmin SIRF receivers had poor
accuracy, but they were able to upgrade their firmware to
fix the problem... Here are our test results:
http://gpsinformation.us/gps60c/g76Creview.html#NOTE

This also indicates that the non-SIRF GPSmap 76C is still
relatively inaccurate under moderate tree cover...
--
Jack

Get general GPS information at:
http://www.gpsinformation.net/

On May 10, 5:57 pm, sat_ali...@yahoo.com wrote:

That does not sound correct. If a receiver has more sensitivity, that
means it will pull the signal out of the noise better (actually it
makes less noise on its input and so can hear lower level signals).
What you are describing is just gain; amplify the input, both signal
and noise. Gain is necessary, but not sufficient to have a good
receiver.



Yep, but from what I can see, most units have very similar if not
identical patch antennas. I was told by two GPS module reps to avoid
the quad helix antennas, but they did not give specifics.



Can you give specifics? What unit did you have? Where did you use
it? Did losing WAAS make a difference in the accuracy of the fix?

Actually, after reading what the SiRF reps are saying, I am starting
to believe that WAAS is not really a significant improvement.

I did finally get the Holux 240 to work with WAAS. I had is
programmed correctly using GPS NMEA Monitor and sending binary
commands. But I could not really see what was going on since it does
not display any results in binary mode. Dennis Gr=F6ning sent me an
email with very detailed instructions on how to use SiRFDemo to set a
bluetooth receiver to work with SBAS. Then I could see the results
and realized that I needed to move the antenna to get a better
signal. I am now picking up SV135 and have a DGPS coorection.
Woohoo! or not...

In response to Dan Anderson; higher sensitivity does not mean it works
better in urban canyons or any other canyons. The problem there is
multipath, where a signal travels a path to the antenna *other* than a
straight line which is what the calculations assume. In fact,
multipath if often a lower level that is better off ignored or comes
in at a low angle which the antenna could ignore. There are some
fancy receivers that make use of multipath to actually boost the
received signal, but I doubt that those techniques are used in
handheld or bluetooth GPS receivers. They tend to be processor
intensive and use extra power.

Ok. You'll have to read about GPS signal processing. I'm not writing
that sarcasticly, just worth the read if you want to understand it.


There's a reason there are $300 patch antennas and $20 patch antennas
and $2 patch antennas. They vary widely in quality. The cheaper they
get, the less the designer cares about accuracy.


I used their CF GPS receiver. GR-270 I think. Took forever to get WAAS
and very little to lose it.

WAAS makes a difference in accuracy. But WAAS wasn't designed to
improve accuracy. It was designed to improve integrity.

Whether the accuracy improvement is significant or not depends on what
you're using the position for.



Depends on what you are using it for. Most likely you won't care or
even know if you're getting 7M or 3M accuracy. But practically
speaking, even WAAS-enabled receivers aren't using WAAS a lot of the
time because the line-of-sight to the WAAS satellites is blocked.
Marine and airborne users would be the exception.


Low-level multipath is the reason it was recommended to you that you
stay away from helical antennas. Plus, the ergonomics suck.

On May 11, 1:38 am, sat_ali...@yahoo.com wrote:

Perhaps you can point me to some references you have read?



My experience has been that with any product you buy there is a wide
range of pricing. Often the pricing reflects how much money is
desired to go into the pockets of the sellers. Can you give specifics
of what is better about a $300 antenna than a $20 and what exactly can
be done to make an antenna for only $2?

The Sarantel antennas are machine manufactured and cost over $10, even
in quantity. What do you mean by "accuracy", accuracy of what? I
can't recall ever seeing a spec for an antenna called "accuracy".



So did you *see* a difference in accuracy when you used WAAS? I am
being told by the receiver manufacturers that it can only improve the
accuracy by up to 2 meters. That is not very much compared to the
other errors that are typical in normal operation.



I am at N 40 degrees on the US East coast and I can see SV122 at 10
degrees elevation, SV135 at 17 degrees, SV138 at 34 degrees and that
is indoors. I don't seem to have any trouble finding SBAS satellites
unless I am behind a large hill. So I don't agree that WAAS
satellites are typically blocked.



You say a lot of things that don't make sense. The difference between
patch and helical antenna patterns is rather moot in most practical
applications because of the lack of consistent orientation. As to the
ergonomics, that greatly depends on the application, no? The design I
worked on actually *required* the helical antenna because the unit
would be attached to the users chest or side of his shoulder. Had a
patch antenna been used the end product would have been larger
(although seeing the Holux 240 makes me think we just didn't do enough
research into available antennas) and the antenna would have been
pointed at the horizon most of the time.

I am pretty confident that in another 5 years, the signal processing
will be low power enough to provide for signal enhancement in
multiplath environments. You will actually *look* for multipath as a
way to boost gain!