Snow and GPS

Reid said:

In what passes for snow down here this morning, my car GPS could not get a fix, would same happen
with walking GPS in a blizzard?

Are you sure it wasn't just bad satellite geometry in conjunction with all those building thingies
that you have down your way?

Can't recall using a GPS in a full on blizzard though.

Paul
--
Calendars for 2004
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In article <[email hidden]>, Paul Saunders <[email hidden]> writes

Quoted message said:

Reid said:

In what passes for snow down here this morning, my car GPS could not get a fix, would same happen
with walking GPS in a blizzard?

Are you sure it wasn't just bad satellite geometry in conjunction with all those building thingies
that you have down your way?

Or even the windscreen wipers if the antenna is beneath them. They can be sufficient to
interrupt the 30 second of (almost) continuous data the receiver need from each satellite before
it can use it.

Quoted message said:

Can't recall using a GPS in a full on blizzard though.

I have just taken an eTrex, eMap and GPS 12 out in a snow shower (not a huge blizzard but pretty
heavy) to have a play and none of them took longer than usual to get going.

The eTrex had previously been on under 5 inches of snow with no problems earlier this morning -
again no noticeable delay in getting a fix. The other two had not been used for a few days.

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

Quoted message said:

Or even the windscreen wipers if the antenna is beneath them. They can be sufficient to
interrupt the 30 second of (almost) continuous data the receiver need from each satellite before
it can use it.

Its in the same place it always is but took about 20 minutes to get started, hope its not failing.
--
Mike Reid "Art is the lie that reveals the truth" P.Picasso Walking, Wasdale, Thames path, London
etc "fellwalk.co.ukfellwalk.co.ukOpen ↗" <-- you can email us@ this site Spain, food and walking "fell-fell-Open ↗
walker.co.uk" <-- dontuse@ all, it's a spamtrap

In article <[email hidden]>, Reid <[email hidden]> writes

Quoted message said:

Quoted message said:

Or even the windscreen wipers if the antenna is beneath them.

Meant to say when the wipers are in use.

Quoted message said:

Quoted message said:

They can be sufficient to interrupt the 30 second of (almost) continuous data the receiver need
from each satellite before it can use it.

Its in the same place it always is but took about 20 minutes to get started, hope its not failing.

If it is a manufacturer installed system hopefully they will have sighted the antenna in a
sensible place.

If it is a portable system and the antenna view is swept by the wipers when they are in use it may
find it difficult to gather the necessary data. The constellation changes a little from day to day.
A combination of constellation geometry exacerbated by wipers in use and moving past obstructions to
the signals may just be enough to have caused the problem.

If you can start the system with the wipers off and vehicle stationary in a place with a reasonable
view of the sky it should be up and running in a minute or so. Thereafter it should be able to keep
using the satellites even as they become obscured and subsequently exposed.

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

Quoted message said:

If it is a manufacturer installed system hopefully they will have sighted the antenna in a
sensible place.

No, some wally installed it :-) Its antenna is internal so i'm not sure exactly where it is. I'll
look in the manual if I have more trouble.

Quoted message said:

If it is a portable system and the antenna view is swept by the wipers when they are in use it may
find it difficult to gather the necessary data. The constellation changes a little from day to day.
A combination of constellation geometry exacerbated by wipers in use and moving past obstructions
to the signals may just be enough to have caused the problem.

It been a bit slow last few days so perhaps its a bad time at the moment. I though the equipment was
geo-stationary or whatever its called?

Quoted message said:

If you can start the system with the wipers off and vehicle stationary in a place with a reasonable
view of the sky it should be up and running in a minute or so. Thereafter it should be able to keep
using the satellites even as they become obscured and subsequently exposed.

It just got a fix in about a minute so I reckon you could be right. There was possibly snow on the
bottom edge of the screen before I moved off this morning and of course the wipers were on once I
did. What obstructs the line of sight, is it just metal?

(Had the car serviced yesterday and they washed and valeted it, waste of time that was, black car +
salted road = instant grey car. *And* they charged *£50* to reprogramme the keys so the doors lock
as you drive off and SWMBOs key only opens the drivers door.. £49 pounds profit? Or perhaps theres
more to it than I think)
--
Mike Reid "Art is the lie that reveals the truth" P.Picasso Walking, Wasdale, Thames path, London
etc "fellwalk.co.ukfellwalk.co.ukOpen ↗" <-- you can email us@ this site Spain, food and walking "fell-fell-Open ↗
walker.co.uk" <-- dontuse@ all, it's a spamtrap

"Reid" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

In what passes for snow down here this morning, my car GPS could not get a fix, would same happen
with walking GPS in a blizzard?
--

I used my 12XL up Kinder in a blizzard on new years eve/day. It worked ok.

Tim Webber

Reid said:

It been a bit slow last few days so perhaps its a bad time at the moment. I though the equipment
was geo-stationary or whatever its called?

Nope, wouldn't be much use if it was geo-stationary. Satellites can only be geo-stationary above the
equator (think about it) but GPS needs satellites all over the sky. The latest WAAS and EGNOS
satellites are GS, for extra accuracy but users this far north will often have trouble picking them
up, especially on north facing slopes.

Paul
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Reid said:

In what passes for snow down here this morning, my car GPS could not get a fix, would same happen
with walking GPS in a blizzard?

Possibly if it's a real heavy snow shower (GPS theroetically doesn't like heavy rain) but I've run
one inside a tent with a good few inches of snow over the top with no problems.

In article <[email hidden]>, Reid <[email hidden]> writes

Quoted message said:

Following up to Chris Street

Quoted message said:

Possibly if it's a real heavy snow shower (GPS theroetically doesn't like heavy rain) but I've run
one inside a tent with a good few inches of snow over the top with no problems.

One thing strikes me from what's been said, it might be harder work for a GPS at 60 mph in a car
than at walking pace or in a tent.

The Earth is spinning fairly quick and the satellites even quicker (orbiting 20000km up twice a day)
so our terrestrial speeds make little difference.

However movement can lead to intermittent blocking / exposure of signals and reflections which can
make life harder for the receiver. Coupled with the fact that antennas in vehicles often do not have
a full view of the sky the receiver may not do as well as out in the open.

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

In article <[email hidden]>, Chris Street
<[email hidden]> writes

Quoted message said:

GPS theroetically doesn't like heavy rain

?

Care to point to the theory behind that?

A layer of water, or wet fabric, over the antenna may degrade or completely mask the signals but the
rain itself does not degrade them.

gpsinformation.netgpsclouds.htmOpen ↗

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

Following up to Tim Webber

Quoted message said:

"Reid" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

In what passes for snow down here this morning, my car GPS could not get a fix, would same happen
with walking GPS in a blizzard?
--

I used my 12XL up Kinder in a blizzard on new years eve/day. It worked ok.

Tim Webber

It worked last night in a blizzard/thunder storm so it seems it isn't an issue.
--
Mike Reid "Art is the lie that reveals the truth" P.Picasso Walking, Wasdale, Thames path, London
etc "fellwalk.co.ukfellwalk.co.ukOpen ↗" <-- you can email us@ this site Spain, food and walking "fell-fell-Open ↗
walker.co.uk" <-- dontuse@ all, it's a spamtrap

Following up to Chris Street

Quoted message said:

Possibly if it's a real heavy snow shower (GPS theroetically doesn't like heavy rain) but I've run
one inside a tent with a good few inches of snow over the top with no problems.

One thing strikes me from what's been said, it might be harder work for a GPS at 60 mph in a car
than at walking pace or in a tent.
--
Mike Reid "Art is the lie that reveals the truth" P.Picasso Walking, Wasdale, Thames path, London
etc "fellwalk.co.ukfellwalk.co.ukOpen ↗" <-- you can email us@ this site Spain, food and walking "fell-fell-Open ↗
walker.co.uk" <-- dontuse@ all, it's a spamtrap

Quoted message said:

gpsinformation.netgpsclouds.htmOpen ↗

2) I notice that when it rains and I turn on my windshield wipers my GPS has trouble locking.
Doesn't this mean that rain affects my GPS reception? Answer: No. What is affecting your GPS
signal reception is the fact that the wipers running back and forth across your windshield
is intermittently blocking reception and making it difficult for your GPS to get a complete
navigation data string. Turning off your wipers until the signal locks will speed the
initial lock.

I think they should have mentioned stopping the car first! :-)
--
Mike Reid "Art is the lie that reveals the truth" P.Picasso Walking, Wasdale, Thames path, London
etc "fellwalk.co.ukfellwalk.co.ukOpen ↗" <-- you can email us@ this site Spain, food and walking "fell-fell-Open ↗
walker.co.uk" <-- dontuse@ all, it's a spamtrap

Reid <[email hidden]> wrote

Quoted message said:

Quoted message said:

gpsinformation.netgpsclouds.htmOpen ↗

2) I notice that when it rains and I turn on my windshield wipers my GPS has trouble locking.
Doesn't this mean that rain affects my GPS reception? Answer: No. What is affecting your GPS
signal reception is the fact that the wipers running back and forth across your windshield is
intermittently blocking reception and making it difficult for your GPS to get a complete
navigation data string. Turning off your wipers until the signal locks will speed the initial
lock.

I have another theory, based on 'hearing' the sound of windscreen wipers on a sensitive ham radio
receiver I used to have in the car.

The noise from the wiper motor could perhaps cause some interference to the signal, although I'm not
sure whether it would affect UHF frequencies.
--
Gordon

In article <[email hidden]>, Gordon <[email hidden]> writes

Quoted message said:

Reid <[email hidden]> wrote

Quoted message said:

Quoted message said:

Quoted message said:

gpsinformation.netgpsclouds.htmOpen ↗

2) I notice that when it rains and I turn on my windshield wipers my GPS has trouble locking.
Doesn't this mean that rain affects my GPS reception? Answer: No. What is affecting your GPS
signal reception is the fact that the wipers running back and forth across your windshield
is intermittently blocking reception and making it difficult for your GPS to get a complete
navigation data string. Turning off your wipers until the signal locks will speed the
initial lock.

I have another theory, based on 'hearing' the sound of windscreen wipers on a sensitive ham radio
receiver I used to have in the car.

The noise from the wiper motor could perhaps cause some interference to the signal, although I'm
not sure whether it would affect UHF frequencies.

It would be interesting to try a quick cold-start test with your Summit on the outside near the
motor and your Vista on the inside under the running wipers. You might need to wait for a rainy day
- like today.

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

Quoted message said:

Quoted message said:

Quoted message said:

I have another theory, based on 'hearing' the sound of windscreen wipers on a sensitive ham radio
receiver I used to have in the car.

The noise from the wiper motor could perhaps cause some interference to the signal, although I'm
not sure whether it would affect UHF frequencies.

It would be interesting to try a quick cold-start test with your Summit on the outside near the
motor and your Vista on the inside under the running wipers. You might need to wait for a rainy day
- like today.

It stopped raining before we went out, and I had no time to mess about so I stuck the Vista and
Summit on the dash and waited about one minute before driving off. This was sufficient to acquire
lock with the wipers running.

I used the screen washers and wipers for several sections of the trip to simulate rain (!) but a
quick look at the Vista track showed NO breaks in the track and no real deviations from the roads on
the map, except that when leaving the venue, partly screened by a building and trees, it took 2
minutes to acquire.

Next time there is a real downpour you would perhaps like to try the test with a couple of
your toys! ;-)
--
Gordon

<[email hidden]> writes

Quoted message said:

I have just taken an eTrex, eMap and GPS 12 out in a snow shower (not a huge blizzard but pretty
heavy) to have a play and none of them took longer than usual to get going.

Twice recently at the start of a short walk, my first position was Waymarked and the Gps 12 left on
to record the track. On Playing the track in OZI I found that right at at the start I got one
spurious reading shooting far away before the track proper commenced. It was easy to delete this odd
trackpoint in Ozi so that the track record made sense.

What causes this renegade reading at the start?
--
Bill Grey billboy.co.ukbillboy.co.ukOpen ↗

In article <[email hidden]>, W. D. Grey <[email hidden]> writes

Quoted message said:

<[email hidden]> writes

Quoted message said:

I have just taken an eTrex, eMap and GPS 12 out in a snow shower (not a huge blizzard but pretty
heavy) to have a play and none of them took longer than usual to get going.

Twice recently at the start of a short walk, my first position was Waymarked and the Gps 12 left on
to record the track. On Playing the track in OZI I found that right at at the start I got one
spurious reading shooting far away before the track proper commenced. It was easy to delete this
odd trackpoint in Ozi so that the track record made sense.

What causes this renegade reading at the start?

If there has been a change of elevation since the receiver was last used it may be fooled a little
by using the previous elevation as part of the initial position solution. It soon corrects for this
as long as it gets enough satellites to go straight to 3d. If is stays in 2d for a little while it
can take a few minutes to recover.

--

dscs.demon.co.ukdscs.demon.co.ukOpen ↗

W. D. Grey said:

<[email hidden]> writes

Quoted message said:

I have just taken an eTrex, eMap and GPS 12 out in a snow shower (not a huge blizzard but pretty
heavy) to have a play and none of them took longer than usual to get going.

Quoted message said:

Twice recently at the start of a short walk, my first position was Waymarked and the Gps 12 left on
to record the track. On Playing the track in OZI I found that right at at the start I got one
spurious reading shooting far away before the track proper commenced. It was easy to delete this
odd trackpoint in Ozi so that the track record made sense.

Quoted message said:

What causes this renegade reading at the start?

You mean a renegade reading at switch on? The unit picks up satellites in sequence. As soon as it
has just enough to calculate position it does. Then it acquires a few more, and is able to calculate
position with much lower error. It may also be the case that one of the early satellites was
reflected from a nearby cliff or building, feeding a spurious distance into the calculation. This
gets discarded when a few more sats are acquired and vote against it.

It would be a most sensible improvement to the software to junk any early readings if there is no
measure of movement and later readings use more satellites. This can be done because velocity is not
derived from position differences but from Doppler frequency changes in the signals, i.e., it's
directly measured at the same time as position. In this way position settling due to improving
signals can be distinguished from position moving.

I've not heard of any unit which actually does this, however.

--
Chris Malcolm [email hidden] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
["]http://www.dai.ed.ac.uk/homes/cam/]