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'Extra GPS receivers?'
1999\07\27@095113 by Adam Davis

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Has anyone purchased any extra gps receivers from bgmicro that they'd be willing
to part with?  Or has anyone seen any other surplus gps units for sale
elsewhere?  I'd like to stick with the same unit, as I know that in the same
area they will (mostly) pick up the same satellites.  My application is mapping
bike trails, and even a 10M error makes a difference.  It would be nice to have
about 1M resolution, as that's a little shorter than the length of the bike.
Better resolution would be nice for other uses, but not necesary.  So I'm
planning on having one sit at my car (fixed location) with a data logger, and
another on my bike.  I'll run the two sets of data through the computer later
and normalize the bike data according to the car data.  That should eliminate
most of the problem with SA.  Relative mapping is all that's necessary, I don't
need to know the absolute locations of the trails.

I live in the great lakes region, so I could use the dgps signals sent around
300kHz, but the receivers (both do it yourself and ready made) are more
expensive than the surplus gps receivers, and only gives about 5M resolution.

Thanks for your time (and your gps receivers!!!)!

-Adam

1999\07\27@123302 by Reginald Neale

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 Adam said:

>Better resolution would be nice for other uses, but not necesary.  So I'm
>planning on having one sit at my car (fixed location) with a data logger, and
>another on my bike.  I'll run the two sets of data through the computer later
>and normalize the bike data according to the car data.  That should eliminate
>most of the problem with SA.  Relative mapping is all that's necessary, I don't
>need to know the absolute locations of the trails.
>
>I live in the great lakes region, so I could use the dgps signals sent around
>300kHz, but the receivers (both do it yourself and ready made) are more
>expensive than the surplus gps receivers, and only gives about 5M resolution.
>


 Adam:

 Be aware that this system only works well when both receivers have
 acquired the same set of satellites. Even if they are right next to
 each other this is not guaranteed.

 Reg Neale

1999\07\27@124527 by Adam Davis

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I realize that.  I am hoping that by using the same receivers, and the same
antennas, I will stand a better chance of doing so.  I will be less than a mile
from my vehicle, so I should be seeing the same satellites.

Alternately, maybe the receiver can tell me the raw data it is receiving from
the satellites.  If so, I could record that for each satellite, then make a
program to pick the right satellites for any given point, and calculate that.
But I don't know if it's worth that amount of trouble, and I doubt the receiver
puts that data out... but I haven't researched it.  Maybe that's what tsip can
put out?

-Adam

Reginald Neale wrote:
>
>   Adam said:
>
> >Better resolution would be nice for other uses, but not necesary.  So I'm
> >planning on having one sit at my car (fixed location) with a data logger, and
> >another on my bike.  I'll run the two sets of data through the computer later
> >and normalize the bike data according to the car data.  That should eliminate
> >most of the problem with SA.  Relative mapping is all that's necessary, I don
't
{Quote hidden}

1999\07\27@150819 by Jeff King

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Adam Davis wrote:

>  I'd like to stick with the same unit, as I know that in the same
> area they will (mostly) pick up the same satellites.

They would have to pick up exactly the same satellites to work, and the
newer units (the 12 channel) also make some assumptions and/or Kaman
filtering that may not give you the results you expect. The ones BG Micro
was selling (the trimble CM-6?) are actually one of your best choices for
this type of application. This is only a 6 channel receiver and it tries to pick
the best 4 satellites (lowest HDOP) to make its fix on. So this one would be
the most repeatable in your application.

>  My application is mapping
> bike trails, and even a 10M error makes a difference.

You do realize that S/A gives you a +- 100 meter accuracy? And more importantly,
S/A is different on every satellite. So your not going to get 10 meter accuracy
very often without some sort of post correction and/or dgps.

>    So I'm
> planning on having one sit at my car (fixed location) with a data logger, and
> another on my bike.  I'll run the two sets of data through the computer later
> and normalize the bike data according to the car data.  That should eliminate
> most of the problem with SA.

You'll have to insure that your seeing (and using) the same satellites. Possibly
if you use psedoranging data (like from a Garmin 25) and do you own
calculations. I wish this was easier,  as I also have a interest in this as
well.

>  Relative mapping is all that's necessary, I don't
> need to know the absolute locations of the trails.

Right, but as I said, SA is different on every satellite, hence unless
your logging the same satellites *AND* the receiver is not trying to
second guess you, then the relative mapping will be in error.

The Trimble GeoExplorer II already does everything you want:
www.trimble.com/cgi/giprod.cgi/geoexplore.htm
You log the path you want, then you can download the post correction data
off the internet for the time in question. It is in the $3000 price class howeve
r
but you might be able to rent one from a local dealer.

Jeff King
Aero Data Systems

1999\07\27@181849 by Craig Lee

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I am in the final design of a product very similar to these.  It utilizes
8 channel receivers, but will accept any NMEA receiver.  Also, supports
JRC binary format as it is already efficiently pre-compressed.  I have done
this with the PIC 16F84 with a non-interrupt driven software UART.  However,
the final product will most probably utilize the 16F877.  I'm a little TIGHT
on resources about now.  Especially for a C program!

The JRC GPS receivers I'm using pick the first 3 or 4 strongest satellites
to get there fix on as well.

Our scheme is to place a logger unit at a surveyed location close to the
mapping site, and then go out logging with a rover.  The satellites visible
in the mapping area will typically be the same for the fixed station and the
rover.  The magic happens at post correction.  First calibration is done with
the fixed station data and the surveyed position numbers.  Then corrolation to
the rover data is processed.  We have not evaluated accuracy yet, however I
suspect it is sub-meter.  Definetely closer than the post correction data you
strip off the web.

It does data logging, post correction, custom or general attribute logging,
and output in map format.  The data logging is done in PC file format on
readily attainable SRAM type 2 cards in a compressed format to maximize
storage capability.  All for less than $3000 CAN now and dropping with volume.

Also, for those interested, I still have a number of JRC micro GPS units
with integrated antennas and cables for $100 CAN each.  Also, I have a bunch
of cca-290 units.  These units require an external antenna, and come with
no cables, plus they are over twice the size of the micro gps units.  The
cca-290 units I will let go for $50 CAN each.  I'm using the micro gps units
without the sealed chassis because of it's compactness including antenna
(1.9x1.5x0.5 inches)!

Curriously, Mitch supplied me with gifs of the two units.  Email privately
for more info.

Regards,

Craig

> {Original Message removed}

1999\07\27@232452 by Anne Ogborn

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Adam Davis wrote:
>
> Has anyone purchased any extra gps receivers from bgmicro that they'd be willi
ng
{Quote hidden}

Running two sets of data will not remove the SA reliably unless you
move slowly (much more so than normal mtn bike speed)
and take LOTS of data, in which case you can ignore the SA
anyway. The jitter is not directly in the position, it's a time jitter
in the sat signal. So unless the fixes are within millisec of each other
and are from the same sats this won't work.

And 1M resolution is unrealistic unless you're willing to stay at each
station for 1/2 hour or longer.

Hate to disappoint, but this doesn't sound do-able from my experience with
tracking busses with GPS.

--
Anniepoo
Need loco motors?
http://www.idiom.com/~anniepoo/depot/motors.html

1999\07\28@102537 by Adam Davis

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Correct me if I'm wrong, but I was under the impression that the gps receivers
were set up to take their position at 1 second intervals.  Since their time is
set by one of the gps receivers in the sky, then they should have taken the
readings at the beginning of each second, which will be within msec of each
other.

Aside from that, the gps receivers select 4 satellites, one to get the time, and
three the triangulate the position.  Both receivers would have to select the
same satelite for time, and the same three for triangulation, in the same area
for very good correlation.  There is a great chance that they would select the
same four satellites, but not necessarily assign the same roles to the same
sats.

So, yes, I know I'm not going to get great positioning.  However, it will be
significantly better than the 100M the SA affords.

Secondly, 1M resolution is unrealistic(ie, low probability of being within 1
meter of the real position given by the mean of the plots) even if you are
willing to stay in one place for an hour or longer.  You can average and get to
5M accuracy if you take one reading every ten minutes for 24 hours, and after 36
hours you can assume 1M.  After 36 hours the results don't improve much UNLESS
you have a receiver which is significantly better than mine.

More info on position averaging (which seems to be the thrust of your
suggestion) can be seen on sci.geo.satellite-nav in the threads
Position averaging, 7/3/99
Averaging, 7/11/99

-Adam

Anne Ogborn wrote:
{Quote hidden}

1999\07\28@113229 by Tjaart van der Walt

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Adam Davis wrote:
>
> Correct me if I'm wrong, but I was under the impression that the gps receivers
> were set up to take their position at 1 second intervals.  Since their time is
> set by one of the gps receivers in the sky, then they should have taken the
> readings at the beginning of each second, which will be within msec of each
> other.
>
> Aside from that, the gps receivers select 4 satellites, one to get the time, a
nd
> three the triangulate the position.  Both receivers would have to select the
> same satelite for time, and the same three for triangulation, in the same area
> for very good correlation.  There is a great chance that they would select the
> same four satellites, but not necessarily assign the same roles to the same
> sats.

The reciever doesn't use one for time and three for positioning.
There are four variables, X,Y,Z and the time difference between
GPS and local time. In order to resolve the navigation solution,
the receiver needs to have four equations to solve for four unknowns.

Another misconception, is that SA consists of jitter in
the timing from the space vehicles. SA is made up by
artificially added jitter, as well as artificial errors
in the ephemeri (space vehicle position).

The reason that two receivers *have* to use the same sats
for the navigation solution with DGPS, is that the correction
in position at the base, is reversed into corrections in the
pseudoranges to the four space vehicles. These 'corrections'
to the pseudoranges only approximate the corrections that
should be made to the ephemeri and clock. If one or more
of the space vehicles that the rover uses differ from the
set used by the base, you can end up (theoretically) with
an even worse position. This doesn't happen often, of
course. Neither does the fact that some navigation solutions
also have a second solution *outside* the constellation
come up often.

One thing about GPS that gets to me, is when people refer
to the receivers as "GPS's". It is like calling your car
an "internal combustion", or your cell phone a "GSM".

There, I said it.

--
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|--------------------------------------------------|

1999\07\28@113427 by Matt Bennett

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At 09:23 AM 7/28/99 , you wrote:
>Correct me if I'm wrong, but I was under the impression that the gps
receivers
>were set up to take their position at 1 second intervals.  Since their
time is
>set by one of the gps receivers in the sky, then they should have taken the
>readings at the beginning of each second, which will be within msec of each
>other.

There really is nothing that says a GPS receiver has to make 1s updates-
the PN
code that is used for SA is just 1ms long, so conceivably, you could make 1ms
position updates with a receiver that just uses code phase.

>Aside from that, the gps receivers select 4 satellites, one to get the
time, and
>three the triangulate the position.  Both receivers would have to select the
>same satelite for time, and the same three for triangulation, in the same
area
>for very good correlation.  There is a great chance that they would select
the
>same four satellites, but not necessarily assign the same roles to the same
>sats.

It is a bit more complicated- time is derived from all the satellites.  The
receiver
adjusts its clock to best fit the signal coming down from all the SVs (GPS
term for
satellite).  It is an iterative process- it makes a good guess, and
iterates to minimize
the time errors.  4 is the absolute minimum for a full 3-D solution, but
the DOP term
(dilution of precision) can shoot up if the SVs are in poor geometry (all
directly above,
for example).  If you use more SVs, you can make your time estimate more
accurate, plus
you can reduce the errors that occur when you lose or gain a SV. This one
reason why 12 channel receivers are becoming so popular.

>So, yes, I know I'm not going to get great positioning.  However, it will be
>significantly better than the 100M the SA affords.

True differential GPS is pretty hard to cobble up- because you have to have
continuous
code (or carrier) phase information.  You can do a pretty good, cheap
pseudo differential
GPS if your two receivers are close enough that they are using the same SVs
to generate
their solution.  At that point you can just fix one station and subtract
the two positions. This will only work well if you are using *the exact
same* SVs.  This becomes problematic in the real world because trees or
buildings may be blocking one or more of the SVs.

>Secondly, 1M resolution is unrealistic(ie, low probability of being within 1
>meter of the real position given by the mean of the plots) even if you are
>willing to stay in one place for an hour or longer.  You can average and
get to
>5M accuracy if you take one reading every ten minutes for 24 hours, and
after 36
>hours you can assume 1M.  After 36 hours the results don't improve much
UNLESS
>you have a receiver which is significantly better than mine.

To my knowledge the DOD has never published the actual statistics of the SA
clock
dithering.  With this knowledge you would be able to get an analytic answer
of how
good a position you can get with averaging.  All of the results I've seen are
from observations.

I second the fact about receiver quality.  Most consumer GPS receivers
don't have precision components, since SA limits the precision much more
than their receivers.  Long term averages are going to be skewed pretty
badly by the lack of ionospheric corrections unless you happen to happen to
have a dual channel (L1/L2) receiver with a really big memory.  The effect
of the ionosphere can be as much as a 40m error in the distance to the
SV, which varies with elevation angle above the horizon.  The Ionospheric
time delay is maximum at about 1400 local time, but it can vary by as much
as 25% (RMS) from the monthly
mean.  There are some corrections made, but it can still have a signifigant
effect.

Matt Bennett
.....mjbKILLspamspam@spam@arlut.utexas.edu
--
Matt Bennett
mjbspamKILLspamarlut.utexas.edu
512-835-3867

1999\07\28@125555 by Anne Ogborn

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Adam Davis wrote:
>
> Correct me if I'm wrong, but I was under the impression that the gps receivers
> were set up to take their position at 1 second intervals.  Since their time is
> set by one of the gps receivers in the sky, then they should have taken the
> readings at the beginning of each second, which will be within msec of each
> other.
>

not necessarily, or even all that likely. Have you tested this?

> Aside from that, the gps receivers select 4 satellites, one to get the time, a
nd
> three the triangulate the position.  Both receivers would have to select the
> same satelite for time, and the same three for triangulation, in the same area
> for very good correlation.  There is a great chance that they would select the
> same four satellites, but not necessarily assign the same roles to the same
> sats.
>

GPS receivers try to get as many sats as possible. 3 is minimum for a shakey 2D
fix,
4 is minimum for a shakey 3D fix, more is better, although a sat low on the hori
zon
can add very little - they also have to have a favorable geometry.

> So, yes, I know I'm not going to get great positioning.  However, it will be
> significantly better than the 100M the SA affords.
>

> Secondly, 1M resolution is unrealistic(ie, low probability of being within 1
> meter of the real position given by the mean of the plots) even if you are
> willing to stay in one place for an hour or longer.  You can average and get t
o
> 5M accuracy if you take one reading every ten minutes for 24 hours, and after
36
> hours you can assume 1M.  After 36 hours the results don't improve much UNLESS
> you have a receiver which is significantly better than mine.

I was tracking buses - of extreme precision I know not.

--
Anniepoo
Need loco motors?
http://www.idiom.com/~anniepoo/depot/motors.html

1999\07\28@130616 by Adam Davis

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Well, I jumped in both feet first, and beleived a lot of what I've read from
various websites and newsgroups.  It would be difficult for me to track down the
places I got different information from, however, I realize a lot of it was
either wrong, or I put it together incorrectly.  I appreciate everyone's help in
GPS receiver info.

-Adam

1999\07\31@040518 by White Horse Design

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<x-flowed>At 19:57 27/07/99 , Jeff King wrote:

>You do realize that S/A gives you a +- 100 meter accuracy? And more
>importantly, S/A is different on every satellite.

The pseudo-range information from each satellite is in error by differing
amounts, yes. Two receivers next to each other, tracking the same
satellites, should produce very similar position fixes due to S/A.

Regards

Adrian
---
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WWW WW WWW   White Horse Design
WWWWWWWWWW   +44-385-970009 (Mobile/SMS), +44-118-962-8913/4 (voice/fax)
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---
Developers of GPS satellite-based tracking systems for vehicles/helicopters

</x-flowed>

1999\07\31@040527 by White Horse Design

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<x-flowed>At 04:14 28/07/99 , Anne Ogborn wrote:

>The jitter is not directly in the position, it's a time jitter in the sat
>signal.

S/A used to be implemented by varying the time (satellite clocks) but is
now implemented by varying the satellite ephemeris data (lying about where
it is in space).

This change was made, apparently due to the fact that mobile base stations
(cell phones) use GPS to measure the time of calls for billing purposes.

Regards

Adrian
---
WWW    WWW   Adrian Gothard
WWW WW WWW   White Horse Design
WWWWWWWWWW   +44-385-970009 (Mobile/SMS), +44-118-962-8913/4 (voice/fax)
WWWW  WWWW   EraseMEwhdspam_OUTspamTakeThisOuTzetnet.co.uk, http://www.users.zetnet.co.uk/whd
---
Developers of GPS satellite-based tracking systems for vehicles/helicopters

</x-flowed>


'Extra GPS receivers?'
1999\08\01@023413 by Eric Smith
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White Horse Design <whdspamspam_OUTZETNET.CO.UK> wrote:
> S/A used to be implemented by varying the time (satellite clocks) but is
> now implemented by varying the satellite ephemeris data (lying about where
> it is in space).

I have no reason to doubt you about that, but...

> This change was made, apparently due to the fact that mobile base stations
> (cell phones) use GPS to measure the time of calls for billing purposes.

Whoever came up with that excuse is full of it.  The amount of time variation
in the time broadcast by the satellite that would be necessary to account for
S/A would be under well under a microsecond.  GPS receivers that are used as
time sources generally provide a 1 PPS output with better than 100 ns
accuracy.  Cell phone billing doesn't have to be more accurate that one second
at best (and usually it's in minutes).

ObPIC: I'm mostly using my Garmin GPS-25 OEM module as a time source for
an NTP daemon on my server, but I'm also planning to connect the serial
output to a PIC to drive a character LCD module.  Once it's working I'll put
the code on my web site.

1999\08\01@151427 by Steve

picon face
Maybe the original poster was mistaken..  It sounds more feasible that the
GPS timebase would be used for T1 or T3 timebase synchronization, which must
remain perfectly locked across long nationwide and international links to
make sure there is not a bit error on long telecommunication links.  The
phone companies typically use one master source, which is synced with all
the other phone companies so they can communicate between each other without
buffering and retiming the bitstream.  It seems reasonable that GPS is used
for this these days, and realistic that as the bitrates increased, the
accuracy became that important.  I agree it probably has nothing to do with
billing.

-Steve


{Original Message removed}

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