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'[PIC] DGPS'
2005\12\07@204526 by R. I. Nelson

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Has any one done any thing with Differential Global Positioning System.  
I was told it can improve GPS accuracy to 1' or less.

I works on the principle of setting one Gps on a known surveyed marker.  
The gps data goes into a computer which is compared to the known
location.  A correction factor is then sent out to second gps and laptop
that is mobil.  The correction factor is applied  and  amore accurate
location is  established.  Sounds like a PIC and GPS unit.

But then in the world of  Deg., Minutes and seconds how long is a Tenth
of a second in Feet or meters?



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2005\12\07@205812 by Richard Prosser

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On 08/12/05, R. I. Nelson <.....rindesignsKILLspamspam@spam@charter.net> wrote:
> Has any one done any thing with Differential Global Positioning System.
> I was told it can improve GPS accuracy to 1' or less.
>
> I works on the principle of setting one Gps on a known surveyed marker.
> The gps data goes into a computer which is compared to the known
> location.  A correction factor is then sent out to second gps and laptop
> that is mobil.  The correction factor is applied  and  amore accurate
> location is  established.  Sounds like a PIC and GPS unit.
>
> But then in the world of  Deg., Minutes and seconds how long is a Tenth
> of a second in Feet or meters?
>

I can't answer the first part other than that's my interpretation of
the situation also - but the answer to the second question depends on
where on the surface (or beond it) you are.
Also interrelated is whether the difference is lat or long.
RP

2005\12\07@210656 by Lee Jones

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face
> Has any one done any thing with Differential Global Positioning System.  
> I was told it can improve GPS accuracy to 1' or less.
>
> But then in the world of  Deg., Minutes and seconds how long is a
> Tenth of a second in Feet or meters?

One minute is one nautical mile at the equator (definition).

So 1/10 of a second of arc would be roughly 10 feet
(6075 ft/min  /  60 sec/min  /  10).
                                               Lee Jones

2005\12\08@071514 by Jan-Erik Soderholm

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Aren't most sold GPS devices today actualy D-GPS devices ?

Jan-Erik.



2005\12\08@080922 by Michael Rigby-Jones

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>-----Original Message-----
>From: piclist-bouncesspamKILLspammit.edu [.....piclist-bouncesKILLspamspam.....mit.edu]
>Sent: 08 December 2005 12:15
>To: EraseMEpiclistspam_OUTspamTakeThisOuTmit.edu
>Subject: RE: [PIC] DGPS
>
>
>Aren't most sold GPS devices today actualy D-GPS devices ?
>
>Jan-Erik.


I thought that differential GPS required two GPS receivers, one fixed at a known location and the other mobile receiver.

Regards

Mike

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2005\12\08@084533 by Jan-Erik Soderholm

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Michael Rigby-Jones wrote :

> >
> >Aren't most sold GPS devices today actualy D-GPS devices ?
> >
> >Jan-Erik.
>
>
> I thought that differential GPS required two GPS receivers,
> one fixed at a known location and the other mobile receiver.
>
> Regards
>
> Mike

Yes, but you don't have to own both yourself.

In Sweden that state runs "fixed" GPS'es that resends
the diff-info and many/some GPS receivers sold in Sweden
has an built in recevier for this correction info, so they
are true DGPS devices. There are also option receivers
for this correction signal that can be attached to a
non-DGPS receiver, if I'm not wrong.

The correction signal is also sent over the RDS (Radio
Data System) on the FM band all over Sweden.

Anyway, I've also read that DGPS isn't realy needed
anymore since the "error" in the GPS signal has been
removed.

Jan-Erik.



2005\12\08@090223 by Alan B. Pearce

face picon face
>There are also option receivers for this
>correction signal that can be attached to
>a non-DGPS receiver, if I'm not wrong.

Most OEM receivers I have looked at seem to have the option to have DGPS
info fed into them via a serial link to allow the receiver to make the
correction.

>The correction signal is also sent over the RDS
>(Radio Data System) on the FM band all over Sweden.

Handy way of doing it.

>Anyway, I've also read that DGPS isn't realy needed
>anymore since the "error" in the GPS signal has been
>removed.

Well, I guess that depends on the accuracy you want. The basic GPS signal
that is used by the common receivers is still "low accuracy" even after the
scrambling was turned off. The error goes to about 10% what it was with the
scrambling turned on IIRC.

AFAIK the receivers used by the likes of surveyors and construction gangs
use a higher accuracy signal that comes from the same spacecraft, but I
think ordinary Joe Bloggs cannot get the info on this signal stream.

2005\12\08@094955 by olin piclist

face picon face
Jan-Erik Soderholm wrote:
> Anyway, I've also read that DGPS isn't realy needed
> anymore since the "error" in the GPS signal has been
> removed.

The deliberate error was removed some years ago.  Today DGPS corrects for
systemic errors from to the satellites bouncing up and down due to
gravitational anomolies and other errors that are unpredictable or can't be
corrected for without a large database beyond the capability of low cost
receivers.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\08@101159 by Joe McCauley

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Certainly many OEM GPS modules are DGPS ready. This means that there is a
fixed location broadcasting the correction info as has been said. This info
is received by the end user using a radio reciever tuned to the DGPS station
and the correction info input to the GPS module. As far as I remember (it's
been a while since I looked at this) you are supposed to use the nearest
DGPS station to you for best accuracy. Selective availability (intentional
errors introduced by the US military) was turned off in 2000 as far as I
remember. This could, of course, be turned on again at any time if they
needed to, so I would not be throwing out the DGPS rx unit yet...

Joe


2005\12\08@111435 by Rolf

face picon face
Also worth investigating is WAAS (in north america...).

http://www.garmin.com/aboutGPS/waas.html

Surveyors, etc. who need high precision can be licensed to use equipment
that uses the military bands available from GPS satellites. This allows
for greater precision, although there are accuracy problems. The
accuracy problems are solved by "averaging" multiple readings, thus, it
typically involves a couple of hours with very expensive equipment to
get centimeter types of accuracy.

Additionally, many of these systems use post-processing to further
refine the numbers.

In other words, even the military grade signal does not give
instantaneously useful highly accurate readings.

Atmospheric conditions are the primary cause of accuracy issues now with
both WAAS, and GDPS.

Rolf

Jan-Erik Soderholm wrote:
{Quote hidden}

2005\12\08@113739 by Herbert Graf

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face
On Thu, 2005-12-08 at 15:11 +0000, Joe McCauley wrote:
> Certainly many OEM GPS modules are DGPS ready. This means that there is a
> fixed location broadcasting the correction info as has been said. This info
> is received by the end user using a radio reciever tuned to the DGPS station
> and the correction info input to the GPS module. As far as I remember (it's
> been a while since I looked at this) you are supposed to use the nearest
> DGPS station to you for best accuracy.

FWIW, the reason DGPS can be so helpful is local variations in the
atmosphere, which is why they recommend using the closest station. These
variations cannot be predicted, so it has nothing to do with how
"affordable" a receiver is. TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

2005\12\08@130328 by Enrico Schuerrer

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----- Original Message -----
From: "Jan-Erik Soderholm" <jan-erik.soderholmspamspam_OUTtelia.com>
To: <@spam@piclistKILLspamspammit.edu>
Sent: Thursday, December 08, 2005 2:45 PM
Subject: RE: [PIC] DGPS


[DGPS devices]
|
| Yes, but you don't have to own both yourself.
|
| In Sweden that state runs "fixed" GPS'es that resends
| the diff-info and many/some GPS receivers sold in Sweden
| has an built in recevier for this correction info, so they
| are true DGPS devices. There are also option receivers
| for this correction signal that can be attached to a
| non-DGPS receiver, if I'm not wrong.
|
| The correction signal is also sent over the RDS (Radio
| Data System) on the FM band all over Sweden.

As in Austria, too.

|
| Anyway, I've also read that DGPS isn't realy needed
| anymore since the "error" in the GPS signal has been
| removed.

Only half of the truth. There are 3 variants of the GPS signals: a public
one, a commercial useable one and a military signal. The public signal is
not encrypted, the other two signals are (this is only the simplified
description - if you google for GPS you will find the exact story). For the
commercial signal with higher accuracy you have to pay for, the military
signal is - as the name implies - only for military use.

If you want higher accuracy (down to cm-range) with the "simple" public
receiveable signal you have to use DGPS. Even in commercial applications
(f.i. surveying of digged cables) with use of the commercial signal
technicians use DGPS corrections due to higher accuracy.

Regards
Enrico

2005\12\08@161143 by Howard Winter

face
flavicon
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Mike,

On Thu, 8 Dec 2005 13:09:20 -0000, Michael Rigby-Jones wrote:

> I thought that differential GPS required two GPS receivers, one fixed at a known location and the other
mobile receiver.

That's the whole system (with a link between the two) but the practical arrangement is that you use someone
else's fixed receiver.  That calculates the error and it's sent by radio of one sort or another, and your
mobile GPS has a receiver for the Differential data which is then passed to the GPS itself, which then uses
the error data to provide the corrected fix.

In some places the Differential data is provided free (around the coasts of the USA, I believe) and in other
places you have to pay for it, either getting it from a broadcast radio signal that carries the data as part
of RDS (encrypted so only paid-up subscribers can use it) or on some other radio system, possibly set up just
for the purpose.  I believe in the UK if you want 10cm accuracy DGPS data, you have to pay many thousands of
pounds a year for it.

Every GPS unit I've seen in recent years will receive DGPS data on its serial port (I think there's a NEMA
sentence specifically for it), and do the corrections automatically.  The tricky bit is getting the data in
the first place!

Cheers,


Howard Winter
St.Albans, England


2005\12\08@161147 by M Graff

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face
Olin Lathrop wrote:
> The deliberate error was removed some years ago.  Today DGPS corrects for
> systemic errors from to the satellites bouncing up and down due to
> gravitational anomolies and other errors that are unpredictable or can't be
> corrected for without a large database beyond the capability of low cost
> receivers.

I belive we have Regan to thank for that, btw.  Must have been between naps.

Selective Availability is turned off now, but the military reserves the
right to enable it again without warning.  The military also taught the
sats to turn SA on only when over certain areas, such as, well, whoever
we're illegally bombing today.  However, practically, they can't enable
it globally without planes crashing.

This is one reason the FAA helped sponsor WAAS, which does three things:

       They act like normal, civilian-grade L1 transmitters.

       They transmit a DGPS-like signal.

       They transmit atmospheric correction information.

So, with WAAS, a large part of the atmosphere is "fixed" -- the
propagation delay through parts of it is given to a receiver in
real-time, so it can correct in real-time.

For more info on WAAS, see http://gps.faa.gov/FAQ/index.htm

However, it is still fairly vague on resoloution of this data since it
would be impossible to transmit (say) 1 meter details fast enough to matter.

There are some receivers that use the civilian band to get a rough fix,
and then do some tricks with the military band to get a more accurate
fix.  Since the L1 band is unencrpted, and the L2 band is the military
encrypted one, I don't know what sort of tricks they can pull with it,
but who knows.

I believe the military receivers use both L1 and L2, probably using L1
for course position and L2 for fine-tuning.  The one military unit I saw
also had a "kill switch" -- hitting two specific but badly marked
buttons at once would wipe the decryption key from the unit.

--Michael

2005\12\08@161400 by M Graff

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face
Enrico Schuerrer wrote:

> Only half of the truth. There are 3 variants of the GPS signals: a public
> one, a commercial useable one and a military signal. The public signal is
> not encrypted, the other two signals are (this is only the simplified
> description - if you google for GPS you will find the exact story). For the
> commercial signal with higher accuracy you have to pay for, the military
> signal is - as the name implies - only for military use.

Can you provide a reference to this "commercial signal" please?  It's
the first I've heard of a commercial band.  I know only of the military
and civilian one -- not a public, commercial, and military one.

--Michael

2005\12\08@162547 by olin piclist

face picon face
M Graff wrote:
> I belive we have Regan to thank for that, btw.

I'm pretty sure it was Clinton.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\08@163436 by Robert Rolf

picon face


Olin Lathrop wrote:

> M Graff wrote:
>
>>I belive we have Regan to thank for that, btw.
>
>
> I'm pretty sure it was Clinton.

Yep.

clinton3.nara.gov/WH/EOP/OSTP/html/0053.html
President Clinton Orders the Cessation of GPS Selective Availability -
May 1, 2000

2005\12\08@164001 by David Van Horn

picon face
> I believe the military receivers use both L1 and L2, probably using L1
> for course position and L2 for fine-tuning.  The one military unit I
saw
> also had a "kill switch" -- hitting two specific but badly marked
> buttons at once would wipe the decryption key from the unit.

I had some fun with that years back, storing DES keys in SRAM.
Interesting thing, when the keys had sat in SRAM for months, it was
actually very hard to get them to erase just by pulling power.  We
changed the code to store them in something like a circular buffer, and
to circulate the bytes regularly.




2005\12\08@170250 by Peter

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 while the remaining parts are likely unreadable without MIME-aware tools.

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On Wed, 7 Dec 2005, R. I. Nelson wrote:

> But then in the world of  Deg., Minutes and seconds how long is a Tenth of a
> second in Feet or meters?

A tenth of a second is about 46 meters (150 feet) at the equator in
navigation terms.

The relationship between feet, degrees, and time is as follows:

The duration of a day is about 24 hours. This can be divided into hours,
minutes, seconds, etc. One full rotation is obviously 360 degrees.
Therefore each second of time is equivalent to 360/(60*60*24) = 1/240 =
0.0041... degrees. Degrees can also be subdivided like hours into
minutes and seconds (60 * 60), instead of fractional degrees. The
display of a GPS etc unit shows such units for sub-degree divisions
(unless set otherwise).

Now the length. At the equator the earth is about 29,400 (statute) miles
long (about 40,070 km). This is equivalent to 360 degrees and nominally
24 hours (86400 seconds). Therefore the 'length' of a second at the
equator is 0.34... (statute) miles (about 460 meters). If you use a
sextant or other sky-viewing instrument to find out where you are and
everyting works right to the nearest second then you should be able to
know where you are to within 1/4 statue mile or so. On a moving platform
this is worse (see Wikipedia article below).

At various latitudes the length of a rotation-second is different. One
nautical mile is about 1/21636 of the equator length. 21636 is fairly
close to 21600 = 24 * 60 * 15. So one nautical mile is about equivalent
to four seconds of (sky observation) at the equator (and at the same
time is also one arc minute = 1/(360*60)). Four seconds was considered
fast enough to take a star sight and a time reading at the time when the
nautical mile was 'stadardised'.

See here: http://en.wikipedia.org/wiki/Celestial_navigation (under
Angular Measurement)

Normal GPS is meant to improve this by at least 20 times (200+ times
without dithering and 2000+ times with DGPS).

One geographical mile is exactly equivalent to one arc minute at the
equator.

Peter
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2005\12\08@170747 by Enrico Schuerrer

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----- Original Message -----
From: "M Graff" <RemoveMEexplorer-piclistTakeThisOuTspamflame.org>
To: "Microcontroller discussion list - Public." <spamBeGonepiclistspamBeGonespammit.edu>
Sent: Thursday, December 08, 2005 10:13 PM
Subject: Re: [PIC] DGPS


| Enrico Schuerrer wrote:
|
| > Only half of the truth. There are 3 variants of the GPS signals: a
public
| > one, a commercial useable one and a military signal. The public signal
is
| > not encrypted, the other two signals are (this is only the simplified
| > description - if you google for GPS you will find the exact story). For
the
| > commercial signal with higher accuracy you have to pay for, the military
| > signal is - as the name implies - only for military use.
|
| Can you provide a reference to this "commercial signal" please?  It's
| the first I've heard of a commercial band.  I know only of the military
| and civilian one -- not a public, commercial, and military one.
|

The L2 signal has as far as I know 2 encrypted parts - one part is for
commercial purposes and you have to pay for the key, the other part is
military.

Enrico

2005\12\08@171955 by M Graff

flavicon
face
Enrico Schuerrer wrote:
> The L2 signal has as far as I know 2 encrypted parts - one part is for
> commercial purposes and you have to pay for the key, the other part is
> military.

I doubt this is the case.  The military is unlikely to share a data
stream with lowly commercial types when they clearly work so hard to
keep it military only.

All the documents I can find refer to civilian (L1) and military (l2)
bands.  There is no distinction made between general public and
commercial use.  Even the FAA does not use the military signal, after all.

--Michael

2005\12\08@173812 by Enrico Schuerrer

picon face
----- Original Message -----
From: "M Graff" <TakeThisOuTexplorer-piclistEraseMEspamspam_OUTflame.org>
To: "Microcontroller discussion list - Public." <RemoveMEpiclistspamTakeThisOuTmit.edu>
Sent: Thursday, December 08, 2005 11:19 PM
Subject: Re: [PIC] DGPS


| Enrico Schuerrer wrote:
| > The L2 signal has as far as I know 2 encrypted parts - one part is for
| > commercial purposes and you have to pay for the key, the other part is
| > military.
|
| I doubt this is the case.  The military is unlikely to share a data
| stream with lowly commercial types when they clearly work so hard to
| keep it military only.
|
| All the documents I can find refer to civilian (L1) and military (l2)
| bands.  There is no distinction made between general public and
| commercial use.  Even the FAA does not use the military signal, after all.
|

Now we have to go inside the signal... The known P-Code and the unknown
W-Code will be combined to the new Y-code. This code can be decrypted by
different keys - one with the result of a lower accuracy and one with the
result of a higher accuracy. The latter is known as the military code, the
former as the commercial one. There are even other methods of L2 tracking
available which allows higher accuracy as  the civilian signal, but not as
accurate as the military code.

I've learned this in a seminar for surveying combined with DGPS which allows
an accuracy up to 10cm with long term measurements. Due to the fact that my
documentation was a handout of the instructor I am afraid I can't name no
reference weblinks.

Enrico

(and sorry for my English, I'm no native speaker)

2005\12\08@175648 by Enrico Schuerrer

picon face
In addition I found a link were the L2C (C for civic) signal is specified:
http://www.navcen.uscg.gov/gps/modernization/TheNewL2CivilSignal.pdf

regards
Enrico

2005\12\08@215440 by M Graff

flavicon
face
Enrico Schuerrer wrote:
> In addition I found a link were the L2C (C for civic) signal is specified:
> http://www.navcen.uscg.gov/gps/modernization/TheNewL2CivilSignal.pdf

Thanks muchly.  I love learning new things.  :)

--Michael


'[PIC] DGPS'
2006\03\09@115000 by Joe Mailer
picon face
Hello,

Do you have reference parts, providers for DGPS
modules, also compatible with EGNOS or WAAS ?

Regards,
Joe


               
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