Sub-Meter Accuracy

You do not receive WAAS, but EGNOS from Switzerland. And if you trust
the EPE display of your receiver, you're lost.
Currently, EGNOS does not yet improve any enabled GPSR accuracy.

- Carsten

 

Interesting... Enable WAAS == enable WAAS option in the handheld. This connects
to the two available sources (AOR-E = 33 and IOR = 44, I believe) of EGNOS.
The handheld has one option for the three systems (WAAS, EGNOS and MSAS).

I have no valid reference to validate the accuracy of the estimation, only
consecutive measurements from the unit itself. And they look like being
consistent. Could you explain why EGNOS is not working as it should? As far
as I know, from April 1st we should have the corrective factor uncertified
but already enabled...

Thanks,

 

Trimble's GeoXT replaces the older GeoExplorer III and is more capable.
Both feature Carrier phase data collection.

 

Hi Roberto--Are you referring to measured statistical error, or receiver
deceiver displayed EPE? Thanks.
Regards,
-Sam

 

WAAS is a regional augmentation of GPS in North America and
is operational at this time. EGNOS is regional augmentation
for Europe and is in intial testing stages.

An Essential Element of EGNOS, like WAAS -- spatial correlation.

"Differential GPS involves the cooperation of two receivers, one
that's stationary and another that's roving around making position
measurements. The stationary receiver is the key. It ties all the
satellite measurements into a solid local reference." -- Trimble
Navigation Ltd.

In the case of EGNOS (and WAAS), the "reference receiver" is a
network of ground stations. The WAAS differential corrections
based on that reference network would be valid for a EGNOS (and
WAAS) enabled GPS receiver that is within or spatially close to
the reference network. One would not expect valid differential
corrections from a reference network that exists on another
continent.

-Sam Wormley
http://edu-observatory.org/gps/dgps.html

 

Actually the GeoXT replaced the GeoExplorer III, which has now been
discontinued. The GeoXT is more expensive and more accurate.

 

If you think that WAAS is superior (all things being equal), then why do you
find it so hard to believe that the GeoXT can give submeter accuracy using
only WAAS corrections? Is it because the GeoXT is L1 only?

What software did you use to post-process the data with the GeoExplorer III
to get 1 cm RMS?

 

Interesting... Enable WAAS == enable WAAS option in the handheld. This connects

Logging your position with SA Watch will show you that EGNOS doesn't
improve the reliability (i.e. dispersion is the same and sometimes
worse than without it)
___________________
Keith
www.gpspassion.com
All things GPS and PocketPC

 

This premise is flawed. WAAS (WADGPS) is almost always better than DGPS
if the receiver is capable of taking advantage of its features. the idea
that it is only general is highly flawed. DGPS is also only general
unless you are setting right next to the beacon tansmitter. Lets look at
the data a bit. Note that consumer GPS receivers do not take full
advantage of the WAAS abilities so this discussion does not necessarily
apply to a consumer unit.

DGPS does not analyze the incoming data but only generates a single
correction for pseudo range data based on its location and sends
corrections for upto 9 SV's max. It does not know the makeup of the
error components.

WADGPS breaks down the error sources by removing but not sending
corrections for tropospheric errors. Understands and sends separate
corrections for clock errors and ephemeris errors. It sends a correction
for ionospheric errors that is based on a sophisticated analysis of not
just one location but all the locations in the vicinity and will be
highly accurate near a ground station reference but also accurate
between ground station references by integrating data from a number of
sources providing information not available to DGPS beacon transmitters.

Capable receivers can apply their own tropospheric error corrections
based on their own capability. They can correct for multipath
separately, they can use the main three error souces in appropriate ways
that can't even be approximated by the DGPS solution. For example, if
they have better ionospheric information they can compensate for this
and still use ephemeris and clock corrections. One such compensation
could be time of day for example.

So if you want sub

If you mean by that you can supply your own beacon transmitter within
very close proximity then I would agree but if you are talking about the
availabilty of the government beacon transmitters then see above. Also
this is not real time by any stretch.

Also not real time. Kinematic receivers can do this in real time, not
post processing.

Dale

 

Yes. They should better call it 'SBAS', but since Garmin is a US company
and WAAS is a US system (and was the first of these SBAS systems in
operation), they just do not want to irritate their US customer base.
Enabling WAAS enables WAAS, EGNOS, MSAS (automatic selection of
satellites).

EGNOS is not working as it should, because it does not even exist
currently. What your receiver get's from AOR-E and IOR is ESTB signals.
You can easily check the quality of the EPE indication by logging the
NMEA output of the receiver, or create a time-triggered tracklog of a
static receiver. You will be surprised.

I have access to two identical receivers and thus can compare
ESTB/non-ESTB logs of the same timespan. Currently there is NO ESTB
correction quality visible, only spurious over very short periods of
time, but not in general. They are still deploying RIMs and improving
the algorithms. Wait for next year to see real improvements. What they
recently did was just changing the msg format so that most consumer
receivers are now able to receive the ESTB test signals. There will be a
compliant msg format change once the system goes into EGNOS operation.

Currently it seems that all consumer GPSRs simply seem to 'weight' their
ordinary EPE figures by some sort of 'WAAS' factor (e.g. dividing by two
or so, or weighting by the number of corrected satellites). Maybe it's
not so easy to calculate a more reliable WAAS EPE. Application of SBAS
corrections is not a 1:1 standard procedure - manufacturers have to
develop their own correction code and make their own experiences. There
is a learning curve.

- Carsten

 

Receiver display and some samples from time to time, nothing statistical.
Plus some news gathered around the 'net concerning the same brand/model
of handheld.

Ciao,

 

Submeter didn't surprise me. It was how far under "submeter" you were.
I just haven't seen any test numbers, other than yours, showing accuracy
at the 0.6 meter (95%) level. That doesn't mean I think your numbers
are wrong - just that I'm looking for more data from others. If you take
your GeoXT anywhere in the WAAS service area can you expect the
same level of accuracy or will it vary and by how much? I expect
it will vary given the nature of WAAS. So was your 0.6 meters the
best you can hope for, about average, or rather poor compared to the
complete service area? More numbers from others would help
answer that.

I typically use Waypoint Consulting's GrafNav/GrafNet software
as it works well with a variety of manufacturers equipment. Any software
capable of "fixed-integer" processing will do it.

 

Thanks! Will do some experimeting myself.

 

John,

I checked out the GrafNav/GrafNet
website and it looked great. The only problem is that I couldn't find a
price anywhere for either of their packages.

Also, I would like to pick your brain, since you obviously have experience
doing carrier phase post-processing and I don't! Since I potentially have a
unit that could acquire 1cm accuracy, I would like to be able to use it for
that, at least for some applications. Most of what I do, frankly, doesn't
need that level of accuracy, but there might be some tasks that would
benefit from it. One that springs to mind is the mapping of wetland flags
on some of the wetland delineations we do. This would involve only
capturing a series of points along a woodland or open canopy line. Some
questions that spring to mind are:

1) Roughly how much do the various GrafNav/GrafNet software packages go for?
If it's ridiculously expensive (like $2000 or so),
then I might reconsider whether or not it's worth it.

2) How long do you have to average to acquire 1cm accuracy (or close to
that), especially in a wooded environment?

3) Which of the GrafNav/GrafNet software packages would I need? I know they
have a lite version, but it doesn't guarantee 1cm accuracy.

4) How much of a learning curve will I have to climb to be able to use this
software with my unit? I read a bit of the online manual, and I was not
familiar with some of the terms.

Thanks in advance for your time and knowledge,

 

I only have a german pricelist, it may be more expensive over here:

GrafNav/GarfNet (kinematic) - about 5000US$
static only - 3500US$
Lite - 2500US$

So, it is ridiculously expensive - but it also seems ridiculously
capable.

Does anyone know how much the Trimble/Terrasat software is?

If you only want to go for static single points, maybe some of the
online services will also do. GrafNet and similiar packages will
certainly allow more analysis and a better insight in positioning
reliability.

- Carsten

 

The software is ridiculously expensive. The numbers Carsten posted
seem about right. Software costs have always been the biggest part
of cm-level GPS costs. The hardware can be quite cheap with OEM
boards going for as little as ~ $100US.

There is a significant learning curve for getting comfortable with
carrier phase processing. Like anything it depends on a host of
variables, but you're not going to get really proficient in days or
even weeks.

Remember that there are many more limitations to collecting
data for cm-level processing, especially L1-only processing. The
basic requirement for ~ 1cm accuracy is to keep carrier lock on at
least 5 satellites for ~ 10 minutes. Once that is accomplished a
good software package can usually "fix the integers" and give
you cm-level accuracy. If the software is sophisticated enough
it can do this even when the receiver is moving. Then, as long
as you never drop below 4 satellites you maintain the cm-level
accuracy. If you do drop below 4 sats the initialization
process needs to be restarted. Note that when post-processing
you can process the data backwards in time so if you dropped
below 4 sats for only a short period in the middle of an hour
long session you might only loose cm-level accuracy for that
short period.
The above description is approximate. I can't go into every
detail here, but the point is that since you need 10+ minutes of
continuous carrier lock for cm-level accuracy it's not going to
happen while walking around under even fairly light canopy.
It is possible for static sessions under light canopy though.

One last point on carrier phase processing. There is what's
called a "float" solution and a "fixed" solution. Most processing
packages start off with a float solution. Here the accuracy
starts at meter-level and works its way down to a few cm
over time. The time period for this to happen can be
anywhere from 10 minutes to an hour or more. This is
all some packages provide so while they can provide
accuracy at the few cm level you may not want to wait that
long. It's also why you see specs like 30cm for your receiver
with carrier phase processing. Fixed solutions make some
guesses, and, if correct, accuracy goes immediately to
cm-level.

A package like GrafNav does make learning post-processing
easier, but since you don't really need cm-level accuracy you
may not want to spend several thousand dollars. I'm guessing
that you would just like to see how accurate your receiver can
be so the thing to do would be to look for some cheaper
software. What you might find will certainly be more limiting,
but it might get you a start.

Terrasat use to provide a free "float" version of their software.
They were bought out by Trimble and I'm not sure if the offer
still stands. It's worth checking out.

Ashtech's old MSTAR program does provide both a float
and fixed solution for static baselines. You can download it.
ftp://ftp.thalesnavigation.com/pub/software/mstar/MSTARsoftware/

It works with RINEX data. The carrier phase processing
isn't state of the art, but it's something you can play with. In
general it needs longer time periods and it may need fast
data collection rates. Something like at least every three
seconds. I don't remember exactly. NGS's INTERPO.EXE
program available on their web site can come in handy
here. Interpolating carrier phase data can be tricky though.

There may be other relatively inexpensive programs, but they
will probably be "float" only and also probably static only.

 

Carsten,

Thanks for the prices. I'm afraid that's a little more than I had in mind
for what I'm going to do with post-processing.

 

There are I guess 4-5 free or low cost packages that should do nicely at
least for static applications. There are also some internet based
services (but some of them need dual frequency rover data).

- Carsten

 

John,

Thanks a million for all of your help and suggestions. You're right about
the ridiculously expensive part, but businesses that must have that kind of
accuracy will pay that much without batting an eye .

My sessions would be static.

Well, it sounds like I need to do some more homework, before leaping into
this facet of GPS. Once again, thanks for all of your tips and suggestions.
I think I will start with the least expensive solutions, just to see if I
get any significant improvement and then gradually work my way into more
rigorous treatments.

 

You get 1cm rms on a Geo III? I'm impressed. And I'm fairly sure the guy
who designed it will be when I tell him

A general note on the way the XT accuracy is specified: It's a mixture
of the theoretical accuracy and a large amount of measurment and
testing. Rather oddly the two do tie up fairly well. Typically a unit is
left on the roof of the building or outside somewhere with fairly clear
skies and run terrasync. A position is logged every second for either 24
hours or until the battery is flat (about 10 hours). The standard
deviation of the points logged is then calculated after the appropriate
type of post processing.
So a claim of around 30cm carrier corrected accuracy means around 66% of
all locations will be within 30cm of the average. I'm not sure if this
is used to calculate the specs or just to check the calculations are
correct but it is what the specification means in the real world.

It's not all purely an attempt to sell more expensive recievers, honest.

Actually a trimble survey reciever downgraded to the capabilities the XT
(L1 only non-RTK etc..) and the Geo XT do give very similar results,
they are afterall similar basic designs. The survey reciever is however
very slightly better (typically ~2-3% lower SD), there is just more
space to spread the electronics out and so there is slightly less noise
from the recievers processor interfering with the GPS.
With the survey recievers you're also paying a lot for the RTK features,
even discounting the extra hardware required that's a lot of
development on a relativly low volume product, they have to make the
cost back somehow.


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