GPS keeps jumping off position

donster22 · Occasional Visitor Joined: May 13, 2004 Posts: 18

hi

i have a holux gr230 BT connecting to an Ipaq 4150

on successive weekends i have now noticed the same phenomenon.
all of a sudden the position changes and stays changed.

last weekend it jumped about 250 feet. today it jumped c 125 feet southwest.

last week, the first co-ords were correct. this week the second co-ords were correct.

i use it for geocaching and placing a cache with this amount of error is unacceptable.

please can anyone explain why this might be happening? would changing the baud rate affect the accuracy.

i installed gps tuner after last week's problems and this morning it gave me a very accurate reading but the position was wrong. it said the same as memory map so i presume this is a hardware problem. should i swap to sirf? but then no satellites seem to show up on Memory Map although it gets a lock and sirf doesn't work with gps tuner.

any thoughts, ideas, changes etc would be gratefully received because i have gone from having 100% confidence in this device to having virtually none.

thanks
donster22 Sad

Skippy

donster22 · Occasional Visitor Joined: May 13, 2004 Posts: 18

hi

thanks for your reply.

i had a good fix at the time c6 satellites. the gps tuner software averages different readings and was giving me its averaged accuracy at sub 1 metre (it's a good bit of software).

what is DGPS by the way - how do i switch that on?

it's quite bizarre. the accuracy readings i get are very tight but it just jumps 100 feet out of the way and then gives me a very accurate reading in the "wrong" position although it says it's the right one"!! ????how????

is my GPS bust? will a cold start via winfast navigator remove all stored data and reset the thing? an error of this sort in tomtom is no big deal but as mentioned, it's a problem if my cache locations are 30m out.

cheers
for any further help from anyone.

donster22 Confused

ckuethe · Posted: Tue Feb 08, 2005 4:55 pm Post subject:

DGPS is Differential GPS. It's a technique for compensating for systemic errors by putting a receiver at a known, surveyed location, and then broadcasting deviations in the navigation solution. It's a Local Area Augmentation System, and likely to be found around airports and harbours.

The SiRFstarII chipset supports a number of dgps methods, including SBAS (Satellite Based Augmentation System). Of the SBAS systems you're likely to encounter there is WAAS (US), EGNOS (EU) and MSAT (JP). Rather than using a single location to broadcast corrections from, they use a network of sensors across their coverage area.

Depending on your exact firmware revision, you may have SBAS enabled already. If your GPGGA message has a 2 in the 6th field, you have a DGPS fix. Nonetheless, you can use SiRFdemo, GPSinfo or my toolkit to send the appropriate message to put the receiver into SBAS mode.

Some of the systemic errors you will likely encounter are delays due to humidity ionospheric and tropospheric conditions, clock error, satellite orbit measurement error, radio noise on the L1 band, multipath errors, poor satellite geometry... What sort of DOP (dilution of precision) figures were you getting? The GPGGP and GPGSA messages can tell you this. If your DOP figures are all very high, then it's quite possible to get a fix and have it be way out.

As for sub-metre accuracy: "garbage in, garbage out". GPS tuner could very well be calculating until it runs out of numbers, and handing you back some insignificant digits. I assume you mean this program: http://www.gpstuner.com/
_________________
http://www.ualberta.ca/~ckuethe/gps/

nigelp · Occasional Visitor Joined: Feb 22, 2005 Posts: 40

donster22

I went on a geocache with friends this weekend, they had a Mitac Mio 168 PDA and Old Garmin GPS.

The Garmin was spot on with it`s co-ordinates, but the Mio would sometimes vary from being within 5 ft to 50ft away !!

We gave up using the Mio, even though that is why they had bought it. They had memory map installed to get nearby, then we had to switch to using the GPS to find the cache.

We also tried using GPS Tuner, but had limited success, it said it had an accurate fix, but was nowhere near to the GPS reading.

We had excellent coverage, clear sky and multiple satellites.

When using the PDA for road navigation it is wonderful, but just doesnt seem accurate enough for geocaching, which is suprising and confusing..

Not sure what else to try, we are going on another cache this weekend.

I was going to replace my old Garmin with a PDA, the routing to actually drive there is great, but I`ve lost faith in the accuracy of its GPS.

Privateer · Posted: Mon Mar 07, 2005 12:42 pm Post subject:

Hi nigelp,

nigelp · Occasional Visitor Joined: Feb 22, 2005 Posts: 40

Hi

I`m not too sure if I can answer all your questions, I dont know the technicalities

Geocaching in west yorkshire UK

Its an old Garmin 45, all standard settings, I just add the coordinates from the cache as a waypoint and navigate to there.

The PDA has memory map latest UK version installed, when using the map, it shows the correct footpath etc and looks to be pretty accurate, the problem is when trying to find the cache, we switched to gpstuner to show the exact co-ordinates and that when we founf it was out compared to the gps.

from memory gps said xx . xx . 525 (cant remember the xx `s !!)

PDA said xx . xx . 529

gps was yy.yy.141
pda was yy.yy.148

it was that far out !

those numbers are not exact, but the 3rd decimal place being out by around 6 is what we experienced.

sorry i cant post more detailed info. I dont have the pda, was jsut caching with friends.

Nige.

Leif · Occasional Visitor Joined: Apr 04, 2005 Posts: 16

For the Mitac Mio 168 I am fairly sure about what the problem is. Units produced from about 2004.06 are delivered with GPS firmware version 2.4.12.09-XMitac-C4PROD2.0 0000003729. The good thing about this version compared to the previous is the much improved sensitivity. The bad thing is that for some reason it has the "static navigation" setting enabled by default.

I found some static navigation info in SiRF's own faq.

http://www.sirf.com/techlibrary/FAQs/FAQs.html

SiRFStarII Software

93. Explain the static navigation parameters.

Since SA was turned off, we now disable static navigation as our default, and we recommend that you do also. When it is enabled, if velocity is below 1.2 m/s for 3 seconds we will freeze the position, and leave them frozen until velocity exceeds 1.4 m/s (so there is a bit of hysteresis in the solution).

123. What criteria are used to enable and disable the static navigation filter?

Static navigation is a mode designed for motor vehicles, which causes the position to become pinned at one location when velocity is determined to be low enough. This is designed to make navigation systems operate more reasonably when the GPS Selective Availability (SA) signal degradation is turned on. When the navigation software determines that the vehicle velocity is less than 1.2 m/s for 3 seconds, the position is pinned to its current position. It remains pinned until either velocity is detected above 1.4 m/s, or position is computed to be 200 m from the pinned location.

Fortunatly there is a PocketPC program that can be used for turning off static navigation.

SP GPS VIEW
http://pocketgpsworld.com/modules.php?name=Downloads&d_op=viewdownload&cid=12

I have used it succesfully on my Yakumo delta 300 GPS (rebadged Mio 168).

Maybe the Holux has the same problem. Use SiRFDemo on PC to check and turn off if needed.

burkelchen · Occasional Visitor Joined: Apr 14, 2005 Posts: 1

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