Recommendations for procurement of GPS logger

Hi group,

First post here - needs some advice

I recently bought two rather inexpensive GPS loggers (Holux M-1200)
for use in an application where I do some offline analysis of GPS
logged data in NMEA format and compare the results with another less
accurate sensor (the GPS data are supposed to be a relibale "ground
truth reference").

It has been a somewhat dissapointing though as I had anticipated that
the exported NMEA data would be fully valid. However, they are not:
the fields for
the number of satellites, horizontal dilution of precision and WGS 84
geoid height in the exported GPGGA NMEA sentences are void (zero).

Moreover I have done some tests with two loggers laying side-by-side
in the open terrain and found that the easting and northing between
the two loggers drift by more than 10 m during a period of 2-3 hours.
Moreover, some shifts in position are abrupt and not easy to couteract
in postprocessing. Even worse the drifts are sometimes anticorrelated
between the two loggers, so i cannot use one static logger to debias a
second mving logger.

For my application I need
* Horizontal accuracy <= 1.5 m (may transiently be higher if reliably
indicated in horizontal dilution of precision)
* Vertical accuracy <= 5 m
* Sample rate of 1 Hz (or higher)
* The ability to store a log lasting for several hours at 1Hz sample
rate
* Easy to operate (preferably just a power on/off switch) - no need
for a display or anything like that
* Reliable quality measures in GPGGA NMEA sentences (I do not need
these data live, OK, they are exported in this format using some tool
afterwards)
* Fairly ruggedized mechanics
* Form factor. Preferably not larger than a walkie talkie or "mug"
* Data interfaces: USB and/or Bluetooth for offline data transfer
following data acquisition.
* Price: <400$
* Operational latitudes -70 deg south to +70 deg north

Is it possible find a GPS logger, which meets these requirements?

Best wishes,

Kim

Slaunger wrote:

Welcome to a fascinating field.


It's not as simple as it looked, is it ;)
There is an old saying, "Cheap/fast/accurate - choose two."
It almost becomes "Cheap/fast/accurate/convenient - choose one."

I would suggest some background reading:

"David L. Wilson's GPS Accuracy Web Page"
http://users.erols.com/dlwilson/gps.htm

"GPS Errors & Estimating Your Receiver's Accuracy", Sam Wormley
http://www.edu-observatory.org/gps/gps_accuracy.html

"GPS ACCURACY MONITOR (Garmin 12XL)", Dennis Milbert
http://home.comcast.net/~dmilbert/handacc/accur.htm

Doing separate Google searches for "DGPS" and "WAAS" will also
prove enlightening.

You listed some desired technical specifications. BUT you never
described what you wished to accomplish.

HTH

Thank you.

No ;) I initially bought the Holux M1200E because I naively thought it
would provide the fix data in its GPGGA NMEA sentences AND I naively
thought that the stated accuracy of 3.0 meters was achievable with
this device - which it isn't. I can live with 3.0 m horizontal
accuracy, 1.5 m would be more ideal, and a compromize is also OK.
Yeah, I know the saying, that is why I asked for what I thought was
Somewhat expensive/moderately fast/moderately accurate/not too bulky
which I thought/(hoped) would possible.
I spoke on the phone with a GPS expert yesterday - a professor at a
nearby university, and he reported to have made GPS receivers being
capable of mesuring position with an accuracy of a fraction of a mm
for surveillance of constructions, like large bridges in areas with
seismic activity. He said we are rather at the beginning of GPS than
the end, when it comes to accuracy. Thats what made me optimistic :-)
How about adding 50% to the accuracy and price. Achievable?

Page"http://users.erols.com/dlwilson/gps.htm
I have studied that quite thoroughly already, and i would it very,
very useful.
Wormleyhttp://www.edu-observatory.org/gps/gps_accuracy.html
That one was new to me. the recommendations there correspond very well
to what I have actually done with the Holux loggers. I just used two
at the same time to better study errors due to the individual receiver
and due to the propagation of the signal from the GPS satellites.
Milberthttp://home.comcast.net/~dmilbert/handacc/accur.htm
This one is blocked "The website home.comcast.net was blocked by CSIS
Secure DNS.", so I cannot see that.

Yes. Thanks for the advice.


Well, good point. I deliberately left that out as my post was TL;DR
already and I considered my application a detail.

But since you ask, I place the GPS logger on some targets (some
airborne, some not) of different kinds with different kinematic
envelopes and maneuverabilities and my objective is to track the
target state (position, velocity and turn rate) with another sensor,
which has an accuracy of about 5 meters. To quantify the accuracy of
the tracking I need a reference with an accuracy better than my sensor
and where the maneuverabuility of the target does not hinder a
relibale estimate of the target state in between GPS sample points. By
knowing the target kinematics and the process noise in my Kalman
smoothed estimator I have calculated that a sample rate of 1 Hz is
needed as my own sensor detections happens out of sync with the GPS
log times.

Kind regards,

Kim

Slaunger wrote:

That changes the picture. Looks like you want to do things
similar to what's described in links at
www.google.com/search?q=kinematic+rtk+gps
with looser precision and decreased cost.

While searching on a related issue, I found
gpspp.sakura.ne.jp/rtklib/rtklib.htm

It may be what you need. I'm downloading it now ( over dial-up :(
At least the links should be useful.

I may have some more links in what I recovered after a system crash.

On 2011.03.15 9:18 , Richard Owlett wrote:

Check the AMOD 3080. It does log some of the satellite data.

I have a few for sale if you're interested.

Having said that, a GPS can't be used as a ground truth for less than
about 10 meters. They _do_ drift (random(ish) walk).

--
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