Nomad (with ArcPad and GPScorrect) horizontal accuracy question

[YukunXing]

It is probably off-topic, but I don't know a good place to post Trimble related questions. My email to the Trimble vendor went unanswered. Folks on this forum helped me to set up GPScorrect so someone might be able to shed some light on this. Thanks in advance!

I???ve collected some test points using ArcPad on an open parking lot and post-processed using Pathfinder. The PDOP value reported was always below 5 during the collection. The post-processing summary reported a very limited number of readings to be within 5m accuracy, the majority was worse.

ArcPad settings used:
Protocol: Trimble GPScorrect
Port: COM2:GPS Serial port
Baud: 9600

Number of positions to average: 10 (both points and vertices)

Datum: D_WGS_1984


Trimble GPScorrect setting (I guess these doesn???t matter much since I???m post-processing):
DOP Type: PDOP
Max PDOP: 20.0
Min SNR: 33.0
Min Elevation: 5 degree
Velocity Filter: Off
Use GLONASS: Auto


The feature class was created in ???NAD_1983_HARN_StatePlane_Virginia_North_FIPS_4501???, as suggested by a Trimble training I attended last year. For post-processing, the selected base station was ???CORS, STERLING (LWX1),  VIRGINIA???, which is less than 20km from where the data was collected. For Reference Position, the ???Use reference position from base provider??? was selected. The base file downloaded covered 100% of the collected points.

Is there any improper settings based on my description? If not, what???s the likely cause(s) for the bad accuracy?


On a separate note, I???ve heard that using an external antenna may increase the horizontal accuracy in addition to boosting signal reception. Is this true with the NOMAD?

[JasonTipton]

what�??s the likely cause(s) for the bad accuracy?

A:You're using a Nomad.

What Nomad are you using? 800 or 900 series? The 800 series is only accurate 2-5m, 900 1-3m.

You said very limited number under 5m. What was the actual breakdown? What do you consider worse than 5m? That is a pretty broad range.

I wouldn't waste your money on an antenna. It may help with reducing multipath and it would get the signal above your head, but you are still using a Nomad.

Everything degrades your positional accuracy. Were you around trees, buildings, any source of radio-interference? Did you wait until the points were collected before you started filling out any attributes? Were you looking down at the screen watching it collect the points, or did you have the device above your head or at least head level? Were you standing perfectly still while collecting?  How long did you have the device on before you started collecting? If you have a 900 with Windows 6.1 than it gets a "Hotfix" at first. Then, after a few minutes it gets a real fix. The "hotfix" is handy and quick, but not as accurate as a real fix. Did you have a fresh almanac?

5 isn't that great of PDOP. Try for something a little closer to 2.

[jasonblocker1]

We went through a similar test when developing a plan to deploy 15 Juno's for a state wide inventory.  Ultimately we decided that the assumed accuracy of post processing didn't warrant the cost of time and money.  We tested Arcpad/Juno without any correction against Terrasync/Juno accuracy wise and the Arcpad/Juno beat accuracy of processed data 4/5 points collected.   I later found a write up by someone from National Parks and found the excerpt in attached jpg.  All of our testing was done away from the city, wooded or pasture land.

If your only collecing code, does it really matter if it's 2-5 M accuracy as oppposed to 1-3 M? I'm also assuming that the GPS chip is similar in Juno and Nomad.

Jason

[JoshWhite]

Wow, that is bad accuracy.  Some car GPS models can do that well.  I consider a PDOP value over 3 to be unacceptable for our data collection.  We use Nomads for the field computer but have ProXH receivers mounted on a pole and can routinely get sub meter and often sub foot accuracy.  Most results I would say are sub 2 ft.

[JasonTipton]

I'm also assuming that the GPS chip is similar in Juno and Nomad.

I believe they have the same chip, but Trimble specs the Juno and the new Nomad 900's at 1-3m  where the older Nomads are at 2-5m. Both of those are assuming that you postprocess. They blame the Nomad's powerful processor chip for the interference.

Also, it is true that the elevation mask is lower, but you can throw out those calculations in Pathfinder and apply a mask on the back-end before postprocessing.

Moral of the story: Nomad is a Mapping grade receiver. It makes pretty pictures on a map, and that is all I expect from it.

[jasonblocker1]

Moral of the story: Nomad is a Mapping grade receiver. It makes pretty pictures on a map, and that is all I expect from it.

Exactly, it depends on what your using it for.  If you need the accuracy, buy the equipment and get it.  For me and our workflow, we've found that the Juno fits our general state wide inventory very very well.  Running gps correct on a receiver that isn't collecting code and carrier is a total waste of time.

Jason B.

[YukunXing]

Thank you for all your reply guys. I understand NOMAD is no match for a Geo, but that's not the point here. The Post-processed accuracy I'm getting is not as good as even the real-time accuracy from the Trimble spec sheet, that's what puzzles me and why I want to figure out if I was doing anything wrong.

To answer some of your questions.

It is a 900 G series;

My claim that the majority of my test points were worse than 5m is from the Pathfinder post-processing summary screen. Based on that out of the 20 locations collected, 2 were between 3~5m, 18 were worse than 5m;

The points were collected on an open parking lot, so there shouldn't be much effects from trees or buildings etc;

The unit had been on for a good 15 mintues (I waited until the WAAS real-time correction kicked in) before I started collecting. Most of the time during the collection, the PDOP is around 2.2, but I did notice it jumped to around 5 at times;

Yes I was standing still during the collection;


"Running gps correct on a receiver that isn't collecting code and carrier is a total waste of time."

I don't fully understand this statement. I'm using ArcPad to collect data, and the purpose of GPScorrect is so that I can post-process. Jason B., are you suggesting that post-processing is meaningless for a NOMAD?


Thanks!

[ericbock]

Can you send me your data (AXF and GPScorrect.ssf) file and I will take a look at it.  What version of GPS Analyst are you running, as well as ArcGIS and service pack.

eric_bock@neigps.com

[jasonblocker1]

xynewtry,

We had the same issue, reported accuracy did not match results for post processed data.  We did not use gps autocorrect, instead collected with terrasync (post processed) and compared against arcpad only with SBAS.  We collected over surveyed control points (6) in a vareity of environments
Results are attached, I'd guess that your picking up some multipath...somewhere. See attached image below referencing post processed sirf chip data.

I am suggesting that post-processing NOMAD data will not make or break your work.  If your comfortable with 1-3M reported, 2-5M reported isn't that big of a jump.  Our expereince with ArcPad and no correction we achieve <3M accuracy and have field verified over known control and in far worse terrain/environment than your average unit is going to come across.  If your super interested, chase down some ngs bench marks and go out- test your unit with GPS correct and without, write it up and share with the world.

Jason

[GrahamW]

Back in the days of me being a GPS salesman.... we tested Juno vs nomad and found that the juno allways had better results.. and a Trimble rep informed us that TDS put the GPS chip on the nomad under the power button(close to your body and hand!) where the juno has its chip top and centre under the Trimble m.

the test we used was to map car park lines in and out of cover at the same time to get as close as possible to the same ephemeris results...