Is there a problem with accuracy of GPS at the moment (eg due to weather or
the Icelandic ash cloud)?

Over the past couple of days the GPS in my girlfriend's Nokia 5800 has
started experiencing errors of several hundred metres in its position - for
example the OVI maps application tells you to turn off at a junction just
after you have passed it, and the position on the map (or on an OS map in
Viewranger) can be wrong by several hundred metres, either ahead or behind
the real position or following the course of the road but offset 50-200
metres to one side.

Most of the time the trace is very accurate. The errors occur in the open,
not in wooded areas where there could be reception problems.

On one occasion just after I came to a halt at a junction at the end of a
slip road (after overshooting and having to do a U turn because it told me
too late to leave the main road) it told me to "observe speed limit" - so it
thought I was still moving, and faster than the limit for that road!

In message <(E-Mail Removed) >, Mortimer
<(E-Mail Removed)> writes
>Is there a problem with accuracy of GPS at the moment (eg due to
>weather or the Icelandic ash cloud)?
>
>Over the past couple of days the GPS in my girlfriend's Nokia 5800 has
>started experiencing errors of several hundred metres in its position -
>for example the OVI maps application tells you to turn off at a
>junction just after you have passed it, and the position on the map (or
>on an OS map in Viewranger) can be wrong by several hundred metres,
>either ahead or behind the real position or following the course of the
>road but offset 50-200 metres to one side.
>
>Most of the time the trace is very accurate. The errors occur in the
>open, not in wooded areas where there could be reception problems.
>
>On one occasion just after I came to a halt at a junction at the end of
>a slip road (after overshooting and having to do a U turn because it
>told me too late to leave the main road) it told me to "observe speed
>limit" - so it thought I was still moving, and faster than the limit
>for that road!

I logged a long walk in the Peak District on Thursday (2nd June) with
'Viewranger' on my Nokia 5800 and the accuracy seemed fine.
--
Chris Morriss

"Mortimer" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) o.uk...
> Is there a problem with accuracy of GPS at the moment (eg due to weather
> or the Icelandic ash cloud)?
>
> Over the past couple of days the GPS in my girlfriend's Nokia 5800 has
> started experiencing errors of several hundred metres in its position -
> for example the OVI maps application tells you to turn off at a junction
> just after you have passed it, and the position on the map (or on an OS
> map in Viewranger) can be wrong by several hundred metres, either ahead or
> behind the real position or following the course of the road but offset
> 50-200 metres to one side.
>
> Most of the time the trace is very accurate. The errors occur in the open,
> not in wooded areas where there could be reception problems.
>
> On one occasion just after I came to a halt at a junction at the end of a
> slip road (after overshooting and having to do a U turn because it told me
> too late to leave the main road) it told me to "observe speed limit" - so
> it thought I was still moving, and faster than the limit for that road!

Seems fine here (UK).

Might be [deliberately] awry in Libya or other places the US is active.

More likely poor reception by the GPS receiver so that you are not seeing
many satellites or worse relying on triangulation from GSM masts. Check
against another receiver - GPS is purely passive so receivers do not
interfere with each other.

"Mortimer" <(E-Mail Removed)> wrote:

>Is there a problem with accuracy of GPS at the moment (eg due to weather or
>the Icelandic ash cloud)?
>
>Over the past couple of days the GPS in my girlfriend's Nokia 5800 has
>started experiencing errors of several hundred metres in its position - for
>example the OVI maps application tells you to turn off at a junction just
>after you have passed it, and the position on the map (or on an OS map in
>Viewranger) can be wrong by several hundred metres, either ahead or behind
>the real position or following the course of the road but offset 50-200
>metres to one side.
>
>Most of the time the trace is very accurate. The errors occur in the open,
>not in wooded areas where there could be reception problems.
>
>On one occasion just after I came to a halt at a junction at the end of a
>slip road (after overshooting and having to do a U turn because it told me
>too late to leave the main road) it told me to "observe speed limit" - so it
>thought I was still moving, and faster than the limit for that road!

GPS Jamming Trials:
http://www.theregister.co.uk/2007/06...rial_cornwall/

How far were you from Cornwall ??

"Gilbert Smith" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> "Mortimer" <(E-Mail Removed)> wrote:
>
>>Is there a problem with accuracy of GPS at the moment (eg due to weather
>>or
>>the Icelandic ash cloud)?
>>
>>Over the past couple of days the GPS in my girlfriend's Nokia 5800 has
>>started experiencing errors of several hundred metres in its position -
>>for
>>example the OVI maps application tells you to turn off at a junction just
>>after you have passed it, and the position on the map (or on an OS map in
>>Viewranger) can be wrong by several hundred metres, either ahead or behind
>>the real position or following the course of the road but offset 50-200
>>metres to one side.

> GPS Jamming Trials:
> http://www.theregister.co.uk/2007/06...rial_cornwall/
>
> How far were you from Cornwall ??

A long way. We were in northern England - Lake District and North Yorkshire.
The worst inaccuracy was just after we passed the A174 junction with the
A19, near Middlesbrough, when it gave me a 500-yard warning to leave at that
junction just as I passed the turning :-(

Ovi Maps then got very confused as I came off at the next junction and did a
U turn, and the position on the map was fluctuating all over the place - the
trace recorded on Viewranger was zigzagging either side of the roads I went
on. After a mile or so it sorted itself out and followed the rest of the
route perfectly, and seemed to give directions in a timely manner indicating
that it no longer thought I was front of or behind my true position.

We saw several cases previously where it was a long way off my true
position. And that's ignoring the various cases where it's a few tens of
metres wrong and has me driving through adjacent rivers or fields - I take
that as "normal"!

I could understand errors if I was in a wooded or built-up area (though I've
been impressed with how well it has tracked my route through woodland in the
past), but in the open, I'd be surprised if the phone couldn't see the
required number of satellites.

"Mortimer" <(E-Mail Removed)> wrote in message
news:-9Cdndp7-(E-Mail Removed)...
SNIP
>> GPS Jamming Trials:
>> http://www.theregister.co.uk/2007/06...rial_cornwall/
>>
>> How far were you from Cornwall ??
>
> A long way. We were in northern England - Lake District and North
> Yorkshire. The worst inaccuracy was just after we passed the A174 junction
> with the A19, near Middlesbrough, when it gave me a 500-yard warning to
> leave at that junction just as I passed the turning :-(
>
> Ovi Maps then got very confused as I came off at the next junction and did
> a U turn, and the position on the map was fluctuating all over the place -
> the trace recorded on Viewranger was zigzagging either side of the roads I
> went on. After a mile or so it sorted itself out and followed the rest of
> the route perfectly, and seemed to give directions in a timely manner
> indicating that it no longer thought I was front of or behind my true
> position.
>
> We saw several cases previously where it was a long way off my true
> position. And that's ignoring the various cases where it's a few tens of
> metres wrong and has me driving through adjacent rivers or fields - I take
> that as "normal"!
>
> I could understand errors if I was in a wooded or built-up area (though
> I've been impressed with how well it has tracked my route through woodland
> in the past), but in the open, I'd be surprised if the phone couldn't see
> the required number of satellites.

Let me see - you are in a car or similar metal vehicle and so radio waves do
not pass through metal. Try mounting the phone on the dash at least.

"R. Mark Clayton" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>
> "Mortimer" <(E-Mail Removed)> wrote in message
> news:-9Cdndp7-(E-Mail Removed)...
>> I could understand errors if I was in a wooded or built-up area (though
>> I've been impressed with how well it has tracked my route through
>> woodland in the past), but in the open, I'd be surprised if the phone
>> couldn't see the required number of satellites.
>
> Let me see - you are in a car or similar metal vehicle and so radio waves
> do not pass through metal. Try mounting the phone on the dash at least.

Agreed - but why should the results be generally worse over the past few
days than they have been previously, with the phone in roughly the same
position?

Actually, the miracle is that GPS receivers work at all inside metal-framed
vehicles. The VHF radio in my iPod struggles to get a signal inside the car
when it's fine outside (OK - I was just testing it!) and satnavs use even
higher frequencies which will probably be attenuated even more.

In message <(E-Mail Removed) >
"Mortimer" <(E-Mail Removed)> wrote:

> "R. Mark Clayton" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> >
> > "Mortimer" <(E-Mail Removed)> wrote in message
> > news:-9Cdndp7-(E-Mail Removed)...
> >> I could understand errors if I was in a wooded or built-up area (though
> >> I've been impressed with how well it has tracked my route through
> >> woodland in the past), but in the open, I'd be surprised if the phone
> >> couldn't see the required number of satellites.
> >
> > Let me see - you are in a car or similar metal vehicle and so radio
> > waves do not pass through metal. Try mounting the phone on the
> > dash at least.
>
> Agreed - but why should the results be generally worse over the past
> few days than they have been previously, with the phone in roughly
> the same position?
>
> Actually, the miracle is that GPS receivers work at all inside
> metal-framed vehicles. The VHF radio in my iPod struggles to get a
> signal inside the car when it's fine outside (OK - I was just testing
> it!) and satnavs use even higher frequencies which will probably be
> attenuated even more.
>

All sorts of factors can affect the accuracy of a GPS receiver. Don't
forget that the GPS satellites are in continuous moving orbits so at any
given time the number and orientation of the satellites visible will be
different from another time even at the same location. If your GPS has a
satellite screen you may be able to determine the number of satellites,
their relative positions, and the strength of signals. That will at
least give you an indication of possible problems. For example if you
see that the satellites available aren't positioned for good
triangulation i.e. they're clustered in one quadrant rather than evenly
spaced, then you can't expect to get as accurate a location. In a car
even the direction of travel may have an impact on the signal strength.
For example in the UK there tend to be more satellites available towards
the south, than the north. Thus if your GPS receiver has a better view
of the sky towards the front of the car, than towards the back of the
car, you're likely to find it performs better when travelling south,
than when travelling north. Other factors of course include local
geography, and again you frequently find in the UK that you lose
reception more often when obstructions are to the south, rather than to
the north of your position.

Mike
--
o/ \\ // |\ ,_ o Mike Clark
<\__,\\ // __o | \ / /\, "A mountain climbing, cycling, skiing,
"> || _`\<,_ |__\ \> | caving, antibody engineer and
` || (_)/ (_) | \corn computer user" http://www.antibody.me.uk/

Gilbert Smith wrote:
> "Mortimer" <(E-Mail Removed)> wrote:
>
>> Is there a problem with accuracy of GPS at the moment (eg due to
>> weather or the Icelandic ash cloud)?
>>
>> Over the past couple of days the GPS in my girlfriend's Nokia 5800
>> has started experiencing errors of several hundred metres in its
>> position - for example the OVI maps application tells you to turn
>> off at a junction just after you have passed it, and the position on
>> the map (or on an OS map in Viewranger) can be wrong by several
>> hundred metres, either ahead or behind the real position or
>> following the course of the road but offset 50-200 metres to one
>> side.
>>
>> Most of the time the trace is very accurate. The errors occur in the
>> open, not in wooded areas where there could be reception problems.
>>
>> On one occasion just after I came to a halt at a junction at the end
>> of a slip road (after overshooting and having to do a U turn because
>> it told me too late to leave the main road) it told me to "observe
>> speed limit" - so it thought I was still moving, and faster than the
>> limit for that road!
>
> GPS Jamming Trials:
> http://www.theregister.co.uk/2007/06...rial_cornwall/
>
> How far were you from Cornwall ??

That article is from 2007!

"Mike Clark" <mrc7--(E-Mail Removed)> wrote in message
news:(E-Mail Removed) .uk...
> In message <(E-Mail Removed) >
> "Mortimer" <(E-Mail Removed)> wrote:
>
>> "R. Mark Clayton" <(E-Mail Removed)> wrote in message
>> news:(E-Mail Removed)...
>> >
>> > "Mortimer" <(E-Mail Removed)> wrote in message
>> > news:-9Cdndp7-(E-Mail Removed)...
>> >> I could understand errors if I was in a wooded or built-up area
>> >> (though
>> >> I've been impressed with how well it has tracked my route through
>> >> woodland in the past), but in the open, I'd be surprised if the phone
>> >> couldn't see the required number of satellites.
>> >
>> > Let me see - you are in a car or similar metal vehicle and so radio
>> > waves do not pass through metal. Try mounting the phone on the
>> > dash at least.
>>
>> Agreed - but why should the results be generally worse over the past
>> few days than they have been previously, with the phone in roughly
>> the same position?
>>
>> Actually, the miracle is that GPS receivers work at all inside
>> metal-framed vehicles. The VHF radio in my iPod struggles to get a
>> signal inside the car when it's fine outside (OK - I was just testing
>> it!) and satnavs use even higher frequencies which will probably be
>> attenuated even more.
>>
>
> All sorts of factors can affect the accuracy of a GPS receiver. Don't
> forget that the GPS satellites are in continuous moving orbits so at any
> given time the number and orientation of the satellites visible will be
> different from another time even at the same location. If your GPS has a
> satellite screen you may be able to determine the number of satellites,
> their relative positions, and the strength of signals. That will at
> least give you an indication of possible problems. For example if you
> see that the satellites available aren't positioned for good
> triangulation i.e. they're clustered in one quadrant rather than evenly
> spaced, then you can't expect to get as accurate a location. In a car
> even the direction of travel may have an impact on the signal strength.
> For example in the UK there tend to be more satellites available towards
> the south, than the north. Thus if your GPS receiver has a better view
> of the sky towards the front of the car, than towards the back of the
> car, you're likely to find it performs better when travelling south,
> than when travelling north. Other factors of course include local
> geography, and again you frequently find in the UK that you lose
> reception more often when obstructions are to the south, rather than to
> the north of your position.
>
> Mike
> --
Well his problem is going to be that assuming his car has a [metal] roof
then only satellites at low elevation will be visible to the phone, creating
errors, especially if the sun is also low rendering satellite(s) in that
direction invisible.