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ZL3AI > APRDIG 29.05.03 09:06l 232 Lines 9771 Bytes #999 (0) @ WW
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Subj: TAPR Digest, May 26, 2/4
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From: ZL3AI@ZL3VML.#80.NZL.OC
To : APRDIG@WW
Subject: RE: My wandering car
From: Gerry Creager N5JXS <gerry.creager@tamu.edu>
Date: 26 May 2003 10:33:43 -0500
X-Message-Number: 7
I'm catching up late on this thread, but here goes.
Selective Availability (SA) has been off for some time, at the order of
the White House. There have been a couple of reported episodes of
diminished accuracy, but the official line is, "No SA."
I'd love to know antenna configurations, and receiver types (by private
e-mail, please), and if I can get time, I'll talk to the receiver
manufacturers I am not already familiar with, to see if the receivers in
question are maintaining old, potentially expired ephemeris data when
satellites come back into view.
On a similar, but also different note, I've seen, in some of my
research, a situation arising when extremely long point occupations
occur using geodetic receivers. It's now been documented, and the
causes are being investigated, that double-difference processing, even
with precise ephemeris data, results in a marked increase in position
noise when a pair of receivers is left in place for 12-17 hours. The
accuracy is pretty good in the 4-11 hour range, and gets better again in
the 17-24 hour period.
We posit an element of orbital harmonic interaction, occurring at the
point where the original satellite constellation is reintroduced into
the processing matrix. This effect should be mitigated by the
processing software and the use of precise ephemeris data, but does not
appear to be completely resolved.
The purpose of this inclusion is to suggest that, yes, it could have
something to do with the satellite constellation and geometry, and it
could, as well, have to do with multipath, if you're in a high or
variable multipath environment when the car's parked.
Gerry
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Subject: RE: My wandering car
From: "Herb Gerhardt" <hgerhardt@charter.net>
Date: Mon, 26 May 2003 09:57:04 -0700
X-Message-Number: 8
From my experience is that yes, GPS location errors still exist but they are
much less than they were before Selective Availability was shut off.
Before SA was shut off, if I placed my GPS on the ground for an hour and
recorded the track, it showed that it had wandered up to 1/4 mile between
extreme points. Now that SA is off, this distance has been reduced to
around 100 yds or so.
For all practical purposes that we Hams and Outdoor Enthusiasts use a GPS
for, that is just phenomenal and serves all of my needs. I give GPS classes
to our local SAR group and I tell them not to believe all the published GPS
spec's on accuracy. From my experience, they are consistently accurate
within 100 yds but not with 10 or 15 ft! I use the Garmin III+ GPS and have
not played with the latest ones with the latest units with WAAS technology.
Here is what Garmin states on their web site:
"Garmin GPS receivers are accurate to within 15 meters on average.
Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System)
capability can improve accuracy to less than three meters on average. No
additional equipment or fees are required to take advantage of WAAS. Users
can also get better accuracy with Differential GPS (DGPS), which corrects
GPS signals to within an average of three to five meters. The U.S. Coast
Guard operates the most common DGPS correction service. This system consists
of a network of towers that receive GPS signals and transmit a corrected
signal by beacon transmitters. In order to get the corrected signal, users
must have a differential beacon receiver and beacon antenna in addition to
their GPS. "
Now I have had some real strange occurrences with my GPS units. On occasion
they hick-up when left on for days and my APRS location moves as much as 20
MILES, with altitudes of 40,000+ ft and speeds of over 100 mph. On everyone
of those occurrences when I got back to my truck, the GPS display read to
Power Down to Re-Initialize. The first time it happened was sort of scary
since I was camped in the mountains and my truck moved close to 20 miles! I
was sure someone stole my truck and was stupid enough to leave my tracker
on, but I was relieved to find it right where I had left it with a hung up
GPS. Of course my wife knew that something was broke since I hardly ever
even drive 55 mph!
Herb, KB7UVC
NW APRS Group, West Sound Coordinator
Our WEB Site: www.nwaprs.org
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Subject: Re: I-GATEing?
From: "Dakota Summerhawk" <dakotas@dotorg.org>
Date: Mon, 26 May 2003 11:19:54 -0600
X-Message-Number: 9
In the files section of tapr there is a program called WinAPRS_Igate.exe is
where I got that from I was not aware that the regular version of Winaprs
would gate the feed to net, guess I need to outfit my PC with a NIC card and
get it into the hub to be able to get an I gate up and running in Cheyenne
WY. Would be nice to have one IN town instead of 40 miles away.
Dakota
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Subject: DB-9 pin outs
From: John Kraus <flyfisher452@cox.net>
Date: Mon, 26 May 2003 13:37:18 -0400
X-Message-Number: 10
I am reworking my Garmin GPS-35 HVS / Tiny Track II setup. I am wondering
if there is a safe way to tap 12V from the TTII input and power the GPS
with only the one connection. I currently have the power connection split
out at the DB-9 with a power pole to supply both the radio/TTII and a
separate one for the GPS.
Here is the setup:
Icom IC-T2H sport with the TTII installed in a AA alkaline shell with a
power pole glued to the outside that feeds both the radio and the TTII.
There is a DB-9 mounted in the side of the AA pack that serves to connect
to both the GPS and the PC for programming.
I am worried about sending 12V into the computer when I connect the TTII
for programming. Is there a safe pin to apply 12V to on a standard DB-9 port?
By the way this makes a great 6 watt tracker radio setup. AES sells them
for < $xxx and often < $xx. If you need more power add a brick. Need a
voice HT for ARES or other uses buy an extra AA shell and add batteries. I
keep a spare shell in the car with the tracker,a rubber duck, and a mag
mount 1/4 wave. With an 18 Ah jump pack in the trunk and 24 AA's in the
glove box I am set for a while. I use three series 6 amp diodes to drop
the 12v to close to the 9.6v the radio requires.
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Subject: RE: My wandering car
From: "Herb Gerhardt" <hgerhardt@charter.net>
Date: Mon, 26 May 2003 12:00:41 -0700
X-Message-Number: 11
Not sure how you are determining an accuracy of 9 ft but I would not believe
the accuracy number on any GPS screen other than smaller is better. That
can give you an indication of drift but it is best to turn on the track
recording and let it sit stationary for an hour or two, then see how much
drift you see on your screen.
The thing you really need for accuracy is when you return to a previous
waypoint, how close are you this time. I still say within 100 yds is OK at
least for me. Usually I am within 75 ft or so, but not always. Also
depends on your terrain. I use mine in the mountains from to peaks to the
valleys.
Just my 2 cents worth.
Herb, KB7UVC
NW APRS Group, West Sound Coordinator
Our WEB Site: www.nwaprs.org
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Subject: D700 Question
From: "Dave Sloan" <desloan@earthlink.net>
Date: Mon, 26 May 2003 13:02:28 -0600
X-Message-Number: 12
I run the D700 mobile with a Garmin 45XL. The GPS reports my current
altitude at 6557 feet. But, the D700 reports that I am at 95 feet. What do I
have set wrong? Or, do I have an equipment problem?
Thanks,
Dave N0EOP-14
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Subject: Re: D700 Question (height ques nmea)
From: "Andrew Rich" <andrew.rich@bigpond.com>
Date: Tue, 27 May 2003 06:03:38 +1000
X-Message-Number: 13
How many GPS satellites were in view ?
I think it takes 3 sats for 2D, 4 sats for altitude and 5 is even better.
Were you in the clear ? Mutilpath GPS sigs can confuse.
Barring that, capture your nmea into a log file and send it to me for
analysis.
Andrew VK4TEC
www.tech-software.net
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Subject: RE: My wandering car
From: "Spider" <spider@rivcom.net>
Date: Mon, 26 May 2003 13:09:10 -0700
X-Message-Number: 14
This link will help you find Geodetic Survey Markers and the dates they were
verified. They will also give the Datum to set your GPS too!
http://www.ngs.noaa.gov/cgi-bin/ds_desig.prl
My GPS usually says an EPE of less than 3 feet. If I go to the 5 markers in
my area, I am always within a few feet of them. I stay there about 10 to 15
minutes and walk up to a maker from different directions, if I can.
You can find your markers on the web page above and check yours! I've had
friends check theirs and they are close but not as close as mine! I think I
got lucky with my Magellan XL3000 as I find it outperforming some of the
newer units!
Another neat trick is to find a road crew that is surveying a road or
highway with those super Trimble units. Normally, when they start their
survey, they start it at some geodetic marker nearby and go from that. I've
walked up to these folks and asked them to give me a reading right were we
stand....and my GPS has been within a few feet of theirs.
If you do any Geocache hiking, you really do need a GPS that works or you'll
have a really hard time finding the hidden treasure!
Jim, WA6OFt
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