[Loadstone] gps accuracy

Ari Moisio arimo at netsonic.fi
Tue Oct 31 06:22:25 GMT 2006


Hi!

  AFAIK waas/egnos is one type of differential gps. DGPS was originally 
developed  to compensate SA (selective availability) erros but it could 
also be used to compensate ionospheric errors.

  DGPS uses fixed reference receivers whose position is known very 
precisely. These receivers   measure their position and compare it to thei 
known position. They calculate   a correction factor for each satellite 
and this correction data is then transmitted to gps receivers for 
example via waas/egnos satellites.

  There are two main reasons for GPS error. One is ionosphere and this 
could be compensated with DGPS. Another is nearby buildings and power 
lines and they cannot be compensated easily.  Maps have also errors.

  You can check your gps accuracy by finding open area with no metallic 
objects, trees, cars and buildings.  Take some fixed point, for example 
wooden chairr and mark it as an waypoint. Now wait several hours without 
moving gps receiver and check your distance to that waypoint. Loadstone is 
an excellent tool for this.

  If you wish you can repeat this test near high buildings or towers to see 
how they affect accuracy.

  If your receiver does not report any drift you have static navigation 
turned on. Turn it of if possible.

  Because each of gps satellites is visible almost six hours you have to 
monitor your position long enough to have differrent satellites in 
differrent positions to report your position to get real accuracy of your 
receiver.

HTH
-- 
arimo

On Mon, 30 Oct 2006, Rusty Perez wrote:

> Hi!
> Thanks for the response.
> What's this about differencial GPS?
> Sometimes I press the 3 key on my phone in loadstone and I get a "gps
> fix" response. Sometimes I get a differencial fix. What does this
> mean?
> I do have the WAAS feature on, I think, so things should be good. As I
> understand it, the gps coordinates for intersections may not be super
> accurate, so this may be some of what I'm noticing as well. Often when
> I get to an intersection, loadstone says it's 20 yards away.
> I understand that it's not perfect and, truth be told, I could
> certainly find something better with this than with out it. I'm just
> trying to understand the imperfections.
>
> Thanks!
> Rusty
>
>
> On 10/30/06, Shawn Kirkpatrick <shawn at odyssey.cm.nu> wrote:
>> There will be drift with any gps receiver. Normal gps accuracy is around 15
>> meters. This can be improved if your receiver can use waas, then the
>> accuracy may be around 3 meters. These numbers will vary depending on the
>> number of satellites the receiver is using, weather conditions and other
>> factors. When you save a point the more satellites and the lower the
>> accuracy reading the better. When looking for a point it will double shift.
>> This usually doesn't work out as bad as it sounds. If you save a point with
>> a good signal and then have a good signal while looking for it again the
>> accuracy can be very good. This is just the limits of consumer grade gps. If
>> you want better you'd need a military grade gps or a very expensive gps
>> receiver that can use differential gps correction. As the technology
>> improves accuracy will improve as well. A gps will get you very close to
>> where you want to be but you'll still need a bit of skill to get to exactly
>> where you want to be.
>>
>> On Mon, 30 Oct 2006, Rusty Perez wrote:
>>
>>> But Monty,
>>> if there is this drift asociated with a star III receiver, is it
>>> possible that waypoints which I enter will be very inaccurate? If the
>>> point floats when I'm recording it, then if it floats as I look for
>>> it, doesn't that make it doubly shifted? :-)
>>>
>>> Thanks
>>> Rusty
>>>
>>>
>>> On 10/29/06, Monty Lilburn <monty at loadstone-gps.com> wrote:
>>>>
>>>> Hi Rusty,
>>>>
>>>> I can not comment on the accuracy of the US intersections as I do not
>> know
>>>> the margin of error on those points.  I would imagine they should be
>>>> within 20 metres or so if I were to guess.
>>>>
>>>> Most Serf3 receivers claim accuracy of between 5 and 15 metres 3-5 metres
>>>> if your receiver is using WAAS.  GPS receivers will get you to the
>>>> building, not necessarily to the door!  They usually work very well for
>>>> identifying an intersection, bus stop or train station!
>>>>
>>>> The drift you experience when standing still is normal with the Star 3
>>>> receivers.  Star 2 receivers do not have this drift problem but are not
>>>> nearly as sensative in terms of picking up satellites.  Most people don't
>>>> mind putting up with a bit of drift for the added sensitivity of the Star
>>>> 3.
>>>>
>>>> Monty
>>>>
>>>>
>>>> On Sun, 29 Oct 2006, Rusty Perez wrote:
>>>>
>>>>> Hi folks,
>>>>> I'm new to this, but I'd like to know more about the accuracy of my
>>>>> sirf3 gps receiver--Holux gpslim236, and accuracy of waypoints ETC.
>>>>>
>>>>> The documentation says it should be accurate to 2.5 meters or
>>>>> something like that. What I find is that when I go through an
>>>>> intersection, even in a car, the waypoint that loadstone anounces with
>>>>> this gps receiver is never all that close. If it's ten meters away,
>>>>> that's close. Does this mean that the waypoints which are on the us
>>>>> geological survey are not that accurate, or is my gps really that off?
>>>>> Also, when I'm stationary, even right on top of where I have just
>>>>> placed a waypoint, it seems to wander, sometimes one meter, sometimes
>>>>> 6 or 7 meters.
>>>>>
>>>>> Does this error correction we have in the program really help? How do
>>>>> I know if it's on; there are an on and an off sellection.
>>>>>
>>>>> Should I try and get a sirf2 receiver?
>>>>>
>>>>>
>>>>> Thanks for any enlightenment I will receive.
>>>>>
>>>>> Rusty
>>>>> _______________________________________________
>>>>> Loadstone mailing list
>>>>> Loadstone at loadstone-gps.com
>>>>> http://www.loadstone-gps.com/mailman/listinfo/loadstone
>>>>>
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