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'[OT]: (Report of) GPS anomalies'
2002\11\19@214651 by Jim

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- - - - - - - - - - - - - - - - - -

Short description of the event:

On 7-mar-98 between 21:11:57 and 21:31:46 I noticed GPS anomalies that
I've never seen before. These anomalies resulted in position errors in
latitude, longitude and altitude up to 600 meters. I have data to support
the event in the form of track and waypoint files recorded by pcx5
downloaded after the event from a Garmin 45. I'm interested whether the
event was local because of RFI or global because of a GPS problem.

Long description of the event:

The event started around 21:11:57 GMT where the G45 displayed the message
"power down and re-init" which according to the manual refers to a
situation where the GPS 45 is not able to calculate a position due to
abnormal satellite conditions. The manual suggests to turn the unit off
and to verify the last position by other means.

I did this several times and it didn't help. I tried several modes (normal
and autolocate), in both cases the GPS 45 reinitialized itself, appeared
to be running for a minute or so and came back with the same messages.
Eventually I ran a self diagnostics test (enter/on simultaneously). The GPS
45 did not appear to complain about anything abnormal during the
diagnostics.
At 21:31:46 things went back to normal and the receiver operated as usual.

The track file that I've downloaded from the receiver (0710398.trk) shows
errorneous position readings from a car on the interstate between Antwerpen
and Breda. The North to South track along that interstate was recorded in
the morning where everything seemed to be ok which I can tell because
I've driven this part many times. Earlier I've recorded waypoints which are
stored in the file 0710398.wpt. The morning track ran nicely through the
waypoints.

The evening track goes South to North along the same interstate. For most
part you can see that the tracks run parallel. This is not anymore the case
near the locations where the receiver started to display the "power down
and re-init" messages. There is evidence for this between 21:11:57 and
21:18:02 GMT and also between 21:24:16 GMT and 21:24:29 GMT.

- - - - - - - - - - - - - - -

More at:

Report:
http://pocus.geo.tudelft.nl/~schrama/Public/gps/g45/anomaly.txt

Track file:
http://pocus.geo.tudelft.nl/~schrama/Public/gps/g45/0710398.trk

RF Jim

{Original Message removed}

2002\11\20@022737 by rad0

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oh yeah, that was the day the ET ship flew by and
caused a temporal wave disturbance,

just wait til the conspiracists get hold of this....


{Original Message removed}

2002\11\20@092055 by Chris Loiacono

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Back during the gulf war, in order to support the US military action dubbed
'desert storm' a number of satellite orbits were adjusted significantly. The
result was that areas of the US mainland went without normal coverage for a
period.
Is it possible that anomalies like this can be due to similar adjustments
being made these days? With all the international intrigue and the war on
terrorism, the Iraq situation, etc, one would think that satellites are
being moved as needed.??

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2002\11\20@093932 by Michael Rigby-Jones

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> -----Original Message-----
> From: Chris Loiacono [SMTP:spam_OUTchrisTakeThisOuTspamMAIL2ASI.COM]
> Sent: Wednesday, November 20, 2002 2:28 PM
> To:   .....PICLISTKILLspamspam@spam@mitvma.mit.edu
> Subject:      Re: [OT]:  (Report of) GPS anomalies
>
> Back during the gulf war, in order to support the US military action
> dubbed
> 'desert storm' a number of satellite orbits were adjusted significantly.
> The
> result was that areas of the US mainland went without normal coverage for
> a
> period.
> Is it possible that anomalies like this can be due to similar adjustments
> being made these days? With all the international intrigue and the war on
> terrorism, the Iraq situation, etc, one would think that satellites are
> being moved as needed.??
>
I thought the entire operation of the GPS system relied on the satellites
were in known, fixed locations?  Surely they can't just be moved around as
and when desired.

Mike

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2002\11\20@093933 by rad0
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I thought the satellites were in geo synch?



----- Original Message -----
From: "Chris Loiacono" <chrisspamKILLspamMAIL2ASI.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Wednesday, November 20, 2002 8:28 AM
Subject: Re: [OT]: (Report of) GPS anomalies


> Back during the gulf war, in order to support the US military action
dubbed
> 'desert storm' a number of satellite orbits were adjusted significantly.
The
{Quote hidden}

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2002\11\20@103215 by Dave Tweed

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Chris Loiacono <EraseMEchrisspam_OUTspamTakeThisOuTMAIL2ASI.COM>
> Back during the gulf war, in order to support the US military action dubbed
> 'desert storm' a number of satellite orbits were adjusted significantly. The
> result was that areas of the US mainland went without normal coverage for a
> period.

That doesn't make any sense; what's your source for this information?

The GPS satellites are in 12-hour circular orbits that are inclined at
an angle of 55 degrees in order to provide even coverage over the entire
earth (except near the poles). The key to GPS accuracy is knowing the
precise orbits of the satellites, and to accomplish this the system needs
to measure them over a period of days, if not weeks. Arbitrarily "adjusting"
the satellite orbits to support a particular area would destroy the
accuracy of the system everywhere for quite a long period of time, until
the new orbits could be measured to the necessary precision.

-- Dave Tweed

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2002\11\20@105500 by Bob Ammerman

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They are not in geo synch, nor in absolutely fixed orbits. The satellites
transmit their orbital parameters ("ephemeris data", IIRC) to the ground
station, along with a very accurate clock. The ground station then can
figure out the satellite is right now.

I don't expect that they move the orbits around much though. I don't know
where they'd get the delta-V to do so for long.

Bob Ammerman
RAm Systems

{Original Message removed}

2002\11\20@112933 by M. Adam Davis

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The GPS system was designed for full coverage at a certian number of
satellites in the air (24, IIRC).  The original spec called for many
more for redundancy and additional accuracy, but there are only a few
'extra'.

When extra precision is needed in a particular portion of the world, I
have no doubt that they'll change their orbits slightly so there are
more in the area needed at one given time.  This would necessarily mean
fewer in other areas.

I doubt, however, that they would change the orbits so much that you
couldn't easily 'see' at least 4 from any given point on the globe at
one time, since that would degrade their military capability elsewhere

Furthermore, there may be additional instrumentation on these satellites
that is not public.  It could be these instruments needed to make more
frequent fly-bys of the area in question.

But this is all wild speculation.  <knock, knock>...  Hmmm....  I wonder
who that could be?

@ f#W
$%
NO CARRIER

-Adam

Q: How many exploits have been found in Microsoft's IIS?
A: Let's just say they keep getting their ASP handed to them on a platter...

Michael Rigby-Jones wrote:

>>{Original Message removed}

2002\11\20@143610 by Chris Hunter

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----- Original Message -----
From: "rad0" <rad0spamspam_OUTATTBI.COM>
To: <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Wednesday, November 20, 2002 2:38 PM
Subject: Re: [OT]: (Report of) GPS anomalies


> I thought the satellites were in geo synch?

They are.

Chris

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2002\11\20@145307 by Bob Barr

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On Wed, 20 Nov 2002 19:35:51 -0000, Chris Hunter
<KILLspamchrisehunterKILLspamspamBLUEYONDER.CO.UK> wrote:

>----- Original Message -----
>From: "rad0" <RemoveMErad0TakeThisOuTspamATTBI.COM>
>To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
>Sent: Wednesday, November 20, 2002 2:38 PM
>Subject: Re: [OT]: (Report of) GPS anomalies
>
>
>> I thought the satellites were in geo synch?
>
>They are.
>

No, they're not.

See: http://sirius.chinalake.navy.mil/satpred/

This page predicts which satellites will be visible at a given time
and location. If the satellites were geo-synchronous, the results
would always be the same for any given location.


Regards, Bob

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2002\11\20@150353 by Spehro Pefhany

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At 07:35 PM 11/20/02 +0000, you wrote:
>----- Original Message -----
>From: "rad0" <TakeThisOuTrad0EraseMEspamspam_OUTATTBI.COM>
>To: <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>
>Sent: Wednesday, November 20, 2002 2:38 PM
>Subject: Re: [OT]: (Report of) GPS anomalies
>
>
> > I thought the satellites were in geo synch?
>
>They are.


Here's a Shockwave tutorial on GPS:

http://www.trimble.com/gps/


GPS Satellites
Name: NAVSTAR
Manufacturer: Rockwell International
Altitude: 10,900 nautical miles
Weight: 1900 lbs (in orbit)
Size:17 ft with solar panels extended
Orbital Period: 12 hours
Orbital Plane: 55 degrees to equitorial plane
Planned Lifespan: 7.5 years
Current constellation: 24 Block II production satellites
Future satellites: 21 Block IIrs developed by Martin Marietta


They are much lower than geosynchronous satellites, about half the
altitude. This is MEO (medium earth orbit).

Russia has a similar system called GLONASS:

The operational space segment of GLONASS consists of 21 satellites in 3
orbital planes, with
3 on-orbit spares. The three orbital planes are separated 120 degrees, and
the satellites within
the same orbit plane by 45 degrees. Each satellite operates in circular
19,100 km orbits at an
inclination angle of 64.8 degrees and each satellite completes an orbit in
approximately
11 hours 15 minutes.

I recall issues with GPS during the Gulf War, they delayed some of the
activities there because the full constellation was not yet in place.
Don't recall any effect on US GPS services, but could have missed
that, it was early days for the system.

A friend was in South Asia not long ago, during a period of high tension
between India and Pakistan. He reported his GPS didn't work at all.
Perhaps it was being jammed.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffEraseMEspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2002\11\20@154029 by Herbert Graf

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> > I thought the satellites were in geo synch?
>
> They are.

       No, they are in polar orbits. TTYL

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2002\11\20@170342 by Chris Hunter

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Whoops - 12 hour orbits....  Brain not functioning too well today....

Chris

----- Original Message -----
From: "Bob Barr" <EraseMEbbarrspamCALIFORNIA.COM>
To: <RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU>
Sent: Wednesday, November 20, 2002 7:49 PM
Subject: Re: [OT]: (Report of) GPS anomalies


{Quote hidden}

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2002\11\20@171213 by Nate Duehr

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On Wed, 2002-11-20 at 07:38, rad0 wrote:
> I thought the satellites were in geo synch?

No they're not that high... they're almost LEO's.

Nate

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2002\11\20@171836 by Bob Blick

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> No they're not that high... they're almost LEO's.

As long as they stay up and don't decide to become Aquariuses :)

-Bob

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2002\11\20@172459 by rad0

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yeah, I remember now that the little handheld gps units
have depictions showing them coming into and out of view.


rad
{Original Message removed}

2002\11\20@172914 by Nate Duehr

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On Wed, 2002-11-20 at 13:38, Herbert Graf wrote:
> > > I thought the satellites were in geo synch?
> >
> > They are.
>
>         No, they are in polar orbits. TTYL

Which is one of many reasons that most of them are launched out of
Vandenberg on Titans.  :-)

Cape Canaveral is not the greatest place to do polar launches from...
too many people in the way if something goes wrong.

Some of the most spectacular Shuttle launch videos are the very few
where Shuttle was the "the right tool" for polar launches and big
payloads -- which are usually military payloads.  The "roll program"
after the Orbiter clears the tower is quite impressive... big long roll
to put the Orbiter heads-down and pointed generally North -- over the
city which is what gets the locals all up-in-arms.

Nate

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2002\11\20@195445 by Jim

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Nope, sorry, they aren't Chris.

RF Jim

----- Original Message -----
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To: <PICLISTSTOPspamspamspam_OUTMITVMA.MIT.EDU>
Sent: Wednesday, November 20, 2002 1:35 PM
Subject: Re: [OT]: (Report of) GPS anomalies


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2002\11\20@200707 by Barry Gershenfeld

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>> I thought the satellites were in geo synch?
>
>No they're not that high... they're almost LEO's.


Which is why the Space Shuttle takes them up...
Bzzt!  Wrong answer.

LEO = 200 miles, 120 minutes.

Geosynchronous = 22,000 miles, 1 day.

Navstar GPS = 11,000 miles, 12 hours.
  -- orbit inclined 55 degrees, not "polar"

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2002\11\20@220912 by Lee Jones

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>>>> I thought the satellites were in geo synch?
>
>>> They are.

>> No, they are in polar orbits. TTYL

> Which is one of many reasons that most of them are launched out of
> Vandenberg on Titans.  :-)

Doesn't hurt that you can launch due south over water (in case you
have to blow up the bird in case it starts wandering).

Downside to polar launch is higher delta-V needed (for a given weight)
because use can't use Earth's rotation for part of rocket's velocity.
It's even worse in a westerly launch (ballistic missle defense tests).


> Cape Canaveral is not the greatest place to do polar launches from...
> too many people in the way if something goes wrong.

> Some of the most spectacular Shuttle launch videos are the very few
> where Shuttle was the "the right tool" for polar launches [snip]
> big long roll to put the Orbiter heads-down and pointed generally
> North -- over the city which is what gets the locals all up-in-arms.

Are you talking about Vandenberg or Cape Canaveral?

I don't think a north launch from Vandenberg would be tried because
the booster or any debris field would fall on northern California
(or Oregon on Washington state).  Rockets leaving Vandenberg head
due south through west (maybe northwest).

I also don't think a due north launch from Cape Canaveral would
happen because the booster(s) or any debris field would fall on
the eastern seaboard.  Rockets leaving Cape Canaveral head
northeast through southeast.

                                               Lee Jones

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2002\11\21@010229 by Nate Duehr

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On Wed, 2002-11-20 at 17:52, Barry Gershenfeld wrote:
> >> I thought the satellites were in geo synch?
> >
> >No they're not that high... they're almost LEO's.
>
>
> Which is why the Space Shuttle takes them up...
> Bzzt!  Wrong answer.

I didn't say anything of the sort.  This isn't a game-show, so I'll
assume you're just immature in your dealings with other adults since you
added the Bzzt.

> LEO = 200 miles, 120 minutes.
>
> Geosynchronous = 22,000 miles, 1 day.
>
> Navstar GPS = 11,000 miles, 12 hours.
>    -- orbit inclined 55 degrees, not "polar"

Yes... yes... yes... 55 degrees is quite inclined for most stuff and
some may consider them "almost polar".  
I also said "Almost LEO" because I didn't remember that MEO was more
appropriate... and it was in response to someone saying they were
geosynch which was way way way wrong.

And... since you're so worried about it... here's an example with real
data... just so you'll shut up and leave me alone.  Here's one of the
GPS constellation that's in it's "parking" orbit.  Only 35 deg of
inclination and the altitude varies greatly from 194 km to 10,892 km.

-------------
NAVSTAR GPS 29

The orbit data is extracted from the following two-line orbital
elements,

1 22849U 92089D   02323.50989757  .00070023  32009-7  83912-3 0  6258
2 22849  35.0156  71.4313 4487263 167.5640 208.9596  6.66984491142323

Epoch (UTC): 12:14:15 PM, Tuesday, November 19, 2002
Eccentricity: 0.4487263
Inclination: 35.0156&#176;
Perigee Height: 194 km
Apogee Height: 10,892 km
Right Ascension of Ascending Node: 71.4313&#176;
Argument of Perigee: 167.5640&#176;
Revolutions per Day: 6.66984491
Mean Anomaly at Epoch: 208.9596&#176;
Orbit Number at Epoch: 14232
------------------------

And here's NAVSTAR 26 which is more "normal" and hits your numbers.

------------------------

The orbit data is extracted from the following two-line orbital
elements,

1 22275U 92089A   02322.23234028 -.00000053 +00000-0 +00000-0 0 01394
2 22275 055.6376 073.1194 0080547 256.9115 102.1674 02.00570761072615

Epoch (UTC): 5:34:34 AM, Monday, November 18, 2002
Eccentricity: 0.0080547
Inclination: 055.6376&#176;
Perigee Height: 19,967 km
Apogee Height: 20,395 km
Right Ascension of Ascending Node: 073.1194&#176;
Argument of Perigee: 256.9115&#176;
Revolutions per Day: 02.00570761
Mean Anomaly at Epoch: 102.1674&#176;
Orbit Number at Epoch: 07261

-----------------

And if you look at the ground-track of something in this type of orbit
you quickly see why they're about 55 deg.  There's not much need to
cover the poles with GPS data...

And here's Echostar-5 just as an example of a geosynch... it's MUCH
higher than the GPS birds...
-----------------

The orbit data is extracted from the following two-line orbital
elements,

1 25913U 99050A   02322.06825178 -.00000039  00000-0  10000-3 0  4913
2 25913   0.0638  77.8830 0000812  68.1685 185.2525  1.00271715 11551

Epoch (UTC): 1:38:17 AM, Monday, November 18, 2002
Eccentricity: 0.0000812
Inclination: 0.0638&#176;
Perigee Height: 35,783 km
Apogee Height: 35,789 km
Right Ascension of Ascending Node: 77.8830&#176;
Argument of Perigee: 68.1685&#176;
Revolutions per Day: 1.00271715
Mean Anomaly at Epoch: 185.2525&#176;
Orbit Number at Epoch: 1155

Better, Sir?

Sheesh...

Nate

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2002\11\21@010925 by Nate Duehr

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There was one very northerly launch out of the Cape with Shuttle... I'm
trying to remember the STS number.... seems to me like it was 1989 or
so... secret payload, military crew, big stink with residents of cities
quite a ways north.... all due to the reasons you described... and the
payload was nuclear-powered... which made all the environmentalists and
safety people hopping mad too.

That's the one to see the video from.

Nate

On Wed, 2002-11-20 at 20:10, Lee Jones wrote:
{Quote hidden}

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2002\11\21@025244 by Roman Black

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> Chris Loiacono <spamBeGonechrisspamKILLspamMAIL2ASI.COM>
> > Back during the gulf war, in order to support the US military action dubbed
> > 'desert storm' a number of satellite orbits were adjusted significantly. The
> > result was that areas of the US mainland went without normal coverage for a
> > period.


Yes but not GPS satellites, those relocated
were mainly sensor and camera types. :o)
-Roman

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2002\11\21@103325 by Hazelwood Lyle

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>-----Original Message-----
>From: Nate Duehr [TakeThisOuTnate.....spamTakeThisOuTNATETECH.COM]
>Sent: Thursday, November 21, 2002 1:05 AM
>To: TakeThisOuTPICLISTKILLspamspamspamMITVMA.MIT.EDU
>Subject: Re: [OT]: (Report of) GPS anomalies
>
>
>There was one very northerly launch out of the Cape with Shuttle... I'm
>trying to remember the STS number.... seems to me like it was 1989 or
>so... secret payload, military crew, big stink with residents of cities
>quite a ways north.... all due to the reasons you described... and the
>payload was nuclear-powered... which made all the environmentalists and
>safety people hopping mad too.

I remember a Northerly night launch of the shuttle. Quite a view from
about
80 miles North of there in Daytona Beach. It's one of those sights that
I'll probably never forget. It's hard to appreciate how much fuel is
being
spent until you see it light up the night sky.

Lyle

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2002\11\21@132315 by Chris Hunter

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You're right, Jim.  Brain switched partially off at the time....  They're in
12 hour orbits, at about 55 degrees.

Chris

{Original Message removed}

2002\11\22@044304 by Alan B. Pearce

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>The GPS system was designed for full coverage at a certian number
>of satellites in the air (24, IIRC).  The original spec called
>for many more for redundancy and additional accuracy, but there
>are only a few 'extra'.

My understanding too, except I believe they have extra ones in orbit, but
idle rather then active so they have immediate availability of a spare if an
active satellite should fail.

>When extra precision is needed in a particular portion of the
>world, I have no doubt that they'll change their orbits slightly
>so there are more in the area needed at one given time.  This
>would necessarily mean fewer in other areas.

I suspect that any extra accuracy or coverage required would be done by just
making a standby satellite active, without shifting the orbit of any others.

For those with Motorola GPS units, it is worth using the Motorola utility on
their website to track the satellites in view. It produces a nice polar
picture of where it believes the satellites are.

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2002\11\22@212836 by Jim

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M. Adam Davis wrote:
 "When extra precision is needed ..."

You are aware that they:

1) have implemented WAAS (a wide area broadcast 'message' that
contains real-time corrective and status information) and that

2) the military (and other qualified users) have access to
a 'chip rate' ten times what civilian users have with approximately
ten times the accuracy commonly availiable on 'civilian' GPS
receivers - aren't you?

RF Jim

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2002\11\23@105707 by Dave Tweed

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RF Jim <TakeThisOuTjvpollspamspamDALLAS.NET> wrote:
> 2) the military (and other qualified users) have access to
> a 'chip rate' ten times what civilian users have with approximately
> ten times the accuracy commonly availiable on 'civilian' GPS
> receivers - aren't you?

This is a common misconception.

Actually, the chip rate has very little if anything to do with the overall
accuracy; the quality of the correlators and local clock is the real
limitation. The main reason military receivers do better is because they
get access to two different carrier frequencies, which allows them to
measure and compensate for atmospheric refraction in real-time.

-- Dave Tweed

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2002\11\23@111400 by Jim

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  "Actually, the chip rate has very little if anything to do with
   the overall accuracy;"

That's not how it has been expressed in the past (at the time GPS
NAVSTAR program was coming about as a DOD activity) ... perhaps
*newer* techniques have yielded *better* accuracy using

      "COURSE/ACQUISITION (C/A) code" (chip rate 1.023 MHz)

alone - but the C/A code was *initially* a means to 'bootstrap up'
and acquire the

       "PRECISION (P) code" (chip rate 10.23 MHz)

which was actually intended for precise position location by users of
the system.

The use of the L2 frequency (in the 1200 some MHz area) does allow for
some compensation due to atmospheric-imposed anomolies on the RF signals
as they make their way to the user ...

RF Jim



{Original Message removed}

2002\11\23@112446 by Jim

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"common misconception" begone ...

+ + + + + + + + + + + + + + + +


GPS CAPABILITIES

< snip >

GPS provides two levels of service, Standard Positioning Service and the
Precise Positioning Service .

The Standard Positioning Service (SPS) is a positioning and timing service
which will be available to all GPS users on a continuous, worldwide basis
with no direct charge. SPS will be provided on the GPS L1 frequency which
contains a coarse acquisition (C/A) code and a navigation data message. SPS
provides a predictable positioning accuracy of 100 meters (95 percent)
horizontally and 156 meters (95 percent) vertically and time transfer
accuracy to UTC within 340 nanoseconds (95 percent).

The Precise Positioning Service (PPS) is a highly accurate military
positioning, velocity and timing service which will be available on a
continuous, worldwide basis to users authorized by the U.S. P(Y) code
capable military user equipment provides a predictable positioning accuracy
of at least 22 meters (95 percent) horizontally and 27.7 meters vertically
and time transfer accuracy to UTC within 200 nanoseconds (95 percent). PPS
will be the data transmitted on the GPS L1 and L2 frequencies. PPS was
designed primarily for U.S. military use. It will be denied to unauthorized
users by the use of cryptography. PPS will be made available to U.S. and
military and U.S. Federal Government users. Limited, non-Federal Government,
civil use of PPS, both domestic and foreign, will be considered upon request
and authorized on a case-by-case basis, provided:


< snip >


GPS SIGNAL CHARACTERISTICS

The satellites transmit on two L-band frequencies: L1 = 1575.42 MHz and L2 =
1227.6 MHz. Three pseudo-random noise (PRN) ranging codes are in use.

The coarse/acquisition (C/A) code has a 1.023 MHz chip rate, a period of 1
millisecond (ms) and is used primarily to acquire the P-code.

The precision (P) code has a 10.23 MHz rate, a period of 7 days and is the
principal navigation ranging code.

The Y-code is used in place of the P-code whenever the anti-spoofing (A-S)
mode of operation is activated.

The C/A code is available on the L1 frequency and the P-code is available on
both L1 and L2. The various satellites all transmit on the same frequencies,
L1 and L2, but with individual code assignments.

Due to the spread spectrum characteristic of the signals, the system
provides a large margin of resistance to interference. Each satellite
transmits a navigation message containing its orbital elements, clock
behavior, system time and status messages. In addition, an almanac is also
provided which gives the approximate data for each active satellite. This
allows the user set to find all satellites once the first has been acquired.

+ + + + + + + + + + + + + + + + + + + + + + + + + + + +

From: http://tycho.usno.navy.mil/gpsinfo.html

RF Jim





{Original Message removed}

2002\11\23@120006 by Dave Tweed

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RF Jim <jvpollEraseMEspamDALLAS.NET> wrote:
> ... but the C/A code was *initially* a means to 'bootstrap up'
> and acquire the "PRECISION (P) code" (chip rate 10.23 MHz)
> which was actually intended for precise position location by users of
> the system.

Yes, that was the original intention, but actual use proved otherwise.
If you have a clock and correlators that are capable of using the P code,
they'll do just as well on the C/A code. The key is how well you can align
on the transitions between chips, not how far apart they are.

-- Dave Tweed

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2002\11\23@120438 by Jim

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I guess at this point you havn't read the bit I posted
from a *much* more authoratative source than I ...

RF Jim (Veteran of GPS system work back in the late 70's)


{Original Message removed}

2002\11\23@130004 by Dave Tweed

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RF Jim <RemoveMEjvpollEraseMEspamspam_OUTDALLAS.NET> wrote:
> I guess at this point you havn't read the bit I posted
> from a *much* more authoratative source than I ...

Actually, I did, but it was still talking about intentions, not practice.

> RF Jim (Veteran of GPS system work back in the late 70's)

Interesting. Did you actually build and test receivers?

-- Dave Tweed

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2002\11\23@133537 by Jim

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"Dave Tweed" expressed the following thought:
   "but it was still talking about intentions, not practice."

I don't see where you can come to a conclusion like that when the
document reference I cited uses phrases like "The GPS is a ..."
and "GPS provides ..." and "The satellites transmit ..".

These are statement of fact, not 'wish lists' or desires for system
capabilities ...


I suggest you do some research starting here:
http://www.navcen.uscg.gov/gps/geninfo/default.htm

Notably this document:

2001 GPS SPS Performance Standard Final (PDF)
http://www.navcen.uscg.gov/gps/geninfo/2001SPSPerformanceStandardFINAL.pdf


And this site:
NAVSTAR Global Positionaing System Joint Program Office
http://gps.losangeles.af.mil/

and notably this page:
GPS Technical Library
https://gps.losangeles.af.mil/gpsarchives/1000-public/1300-lib/default.html


I think you will find that even stated INTENTIONS on an activity
like GPS AREN'T very often VAPORWARE like 'product' touted by the
loud-mouthed publicity-seeking hucksters in industry - these people
who engineerind GPS/NAVSTAR are the same class 'brains' (the private
contractors who do work for the DOD and NASA) that put men on the
moon in the 60's and they are fully capable, indeed, they achive,
their objectives.

Costly, large-scale programs like NAVSTAR/GPS also take years
to accomplish and were well-proven early-on with field trials.
They aren't subject to the 'whims of marketing' as product from
say, Intel, Microsoft or GM are - and programs like GPS/NAVSTAR
are accomplished in stages after much study and due concern for
every facet of operation. It *was* and still is a series
undertaking ...

And yes, the company I was with (we) built and tested receivers
and we tested them at Yuma proving grounds on a 'range' when there
were only a couple of NAVSTAR 'birds' in the air giving limited
periods of coverage each day ...

RF Jim

{Original Message removed}

2002\11\23@145226 by Dave Tweed

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RF Jim <@spam@jvpollRemoveMEspamEraseMEDALLAS.NET> wrote:
> "Dave Tweed" expressed the following thought:
>     "but it was still talking about intentions, not practice."
>
> I don't see where you can come to a conclusion like that when the
> document reference I cited uses phrases like "The GPS is a ..."
> and "GPS provides ..." and "The satellites transmit ..".
>
> These are statement of fact, not 'wish lists' or desires for system
> capabilities ...

Look, I don't see what you're getting so hot about. Nothing I said
contradicts anything in the references you cite.

However, everyone seems to assume (and you stated) that a C/A-only receiver
CANNOT perform as well as a P-code receiver, all other things being equal,
based soley on the chip rate of the two signals. Although this might have
been the case in the early days, it is simply not true now, and has not
been for quite some time. Furthermore, the availability of DGPS and WAAS
have all but completely obliterated the performance differences between
even inexpensive C/A receivers and P(Y) receivers, which is one of the main
reasons that the government has turned off SA pretty much permanently.

In fact, SA was turned off for a good part of the Gulf War, because the
military couldn't get their hands on sufficient quantities of P(Y)
receivers, and was issuing COTS C/A receivers to many of the ground forces.

And that's the last I'll contribute to this thread.

-- Dave Tweed

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2002\11\23@222805 by Jim

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  "Nothing I said contradicts anything in the references you
   cite."

A statement was made earlier to the effect about a 'common
misconception' regarding the higher chip-rate P-code
and I have laid down authoratative material and cites
countering that statement. Now we all know that the PPS
(Precision Positining Service using the higher chip-rate
P-code) has been used and is the *more* accurate GPS signal
emanating from the NAVSTAR birds (making use of that is
another matter - as one must be granted access to it).

Even the documents I cited show a greater absolute accuracy
(using GPS alone) with PPS (using P code) versus SPS (Standard
Postioning Service using the civilian-acessable "C/A code").

Matter closed.

RF Jim



{Original Message removed}

2002\11\25@172427 by Chris Loiacono

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> Chris Loiacono <EraseMEchrisspam@spam@MAIL2ASI.COM>
> > Back during the gulf war, in order to support the US
> military action dubbed
> > 'desert storm' a number of satellite orbits were adjusted
> significantly. The
> > result was that areas of the US mainland went without
> normal coverage for a
> > period.
>
> That doesn't make any sense; what's your source for this information?

"GPS Made Easy" Third Edition, 2001, by Lawrence Letham pg. 31 last
paragraph.
This he cited under a sub-heading 'Satellite Geometry' and the context was
different receiver's mask angle abilities and Satellite PDOP, or Position
Dilution of Precision.
He noted that during the Gulf War, people in California had noticeably
different results from their receivers.

Chris

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