GPS Vs COMPASS

Quoted message said:

If you feel happier with a GPS, take a GPS. But that's not the same as
if you feel happier with a GPS, everyone should feel the same.

Agreed, it is a personal decision but one that COULD effect the lives
of others???????

David.

Quoted message said:

Agreed, it is a personal decision but one that COULD effect the lives
of others???????

Possibly, but possibly not. As could the decision to carry just about
anything else.

The above construction suggests to me a fallback point of "I don't want
to undermine your personal freedom, but I do think you're being
foolish". In the cold hard light of reality the Real World (TM) is
rather too complex for such simple conclusions, IMHO and IME. There's
all sorts of stuff that can improve your safety in certain
circumstances, but if you carry them all then you'll be far more likely
to get into trouble to start with! Looking at it from further away you
see that, for all the potential hazards, recreational walking is a
pretty safe way to pass your time if approached with very basic levels
of cluefullness and even rudimentary equipment.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Peter Clinch wrote:
"As could the decision to carry just about anything else."

Perhaps we differ on this point.

The most fundamental point of walking is going from one point to
another. On a scale of "what if" my judgement is that any device that
assists directly with that process must be rated very highly.

<[email hidden]> wrote

Quoted message said:

Sorry, fail to see the "connection" between safety and failing to
connect with the environment?

If you're always safe and cosy you'll only ever connect with anything on
a very superficial level. To go back to old Chinese proverbs, I expect
it was that clever Confucius bloke who first said, "No pain no gain".
Either him or Bruce Lee. You need to get outside your comfort zone,
feel nervous, feel doubt, feel discomfort, feel that you've only got
yourself to rely on. It is only then that you will truly find yourself
my son.

AndyP wrote " You need to get outside your comfort zone,
feel nervous, feel doubt, feel discomfort, feel that you've only got
yourself to rely on. It is only then that you will truly find yourself
my son. "

What?????

You have to commit suicide to get closer to find yourself, yeah, right!

AndyP said:

It is only then that you will truly find yourself
my son.

But will the MRT be able to find you?
--
R
o
o
n
e
y

"I always knew the entire Green party were nutters" - Ken Livingstone

<[email hidden]> wrote

Quoted message said:

The most fundamental point of walking is going from one point to
another. On a scale of "what if" my judgement is that any device that
assists directly with that process must be rated very highly.

A car?

Quoted message said:

Perhaps we differ on this point.

The most fundamental point of walking is going from one point to
another.

Perhaps what we differ on is what constitutes "fundamental"...

I don't think anyone has any particular right to decide a universal
fundamental reason for recreational walking. Some people, after all, do
it on a treadmill which won't get them anywhere, so your definition
certainly won't fit everyone. I've often just wandered about, seeing
what I came across, rather than going anywhere in particular, and I've
been doing that since I could crawl. I doubt I'm the only one.

What works for you doesn't necessarily work for me. What works for me
doesn't necessarily work for Mr. J. Public and his friend A.N.Other.
And so on.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

<[email hidden]> wrote

Quoted message said:

AndyP wrote " You need to get outside your comfort zone,
feel nervous, feel doubt, feel discomfort, feel that you've only got
yourself to rely on. It is only then that you will truly find

yourself

Quoted message said:

my son. "

Quoted message said:

What?????

You have to commit suicide to get closer to find yourself, yeah,

right!

I look upon my few near death experiences as good things. Anyway, as
far as comfort zones go, some people are more fortunate than others in
that respect. Some people (like uh, my mum) can scare themselves
[censored] by just going out of sight of the car on Dartmoor for an hour
or so. Others like Reinhold Messner need to do big solo climbs in the
harshest environments on earth. Not sure my mum gets anything spiritual
out of it though, she needs to stop the hysterical whinging and chill
out a bit first.

"Paul Rooney" <[email hidden]> wrote

Quoted message said:

But will the MRT be able to find you?

Only if you're one of those wimps who tells people where you're going or
one of those even bigger wimps who takes a phone and would be too lazy
to be arsed to drag yourself a few miles with hypothermia and a broken
leg.

AndyP said:

"Paul Rooney" <[email hidden]> wrote

Quoted message said:

But will the MRT be able to find you?

Only if you're one of those wimps who tells people where you're going or
one of those even bigger wimps who takes a phone and would be too lazy
to be arsed to drag yourself a few miles with hypothermia and a broken
leg.

Ha ha! I can't tell people where I'm going because I often don't know
till I get there. And when I do get there there's usually no mobile
signal anyway. My biggest fear is running out of tobacco in the wilds.
--
R
o
o
n
e
y

"I always knew the entire Green party were nutters" - Ken Livingstone

"Paul Rooney" <[email hidden]> wrote

Quoted message said:

Ha ha! I can't tell people where I'm going because I often don't know
till I get there. And when I do get there there's usually no mobile
signal anyway. My biggest fear is running out of tobacco in the wilds.

My biggest fear last time I went to the Lakes was finding somewhere safe
to leave the car for a few days. That and waking up to find myself
surrounded by a dozen other tents.

AndyP said:

"Paul Rooney" <[email hidden]> wrote

Quoted message said:

Ha ha! I can't tell people where I'm going because I often don't know
till I get there. And when I do get there there's usually no mobile
signal anyway. My biggest fear is running out of tobacco in the wilds.

My biggest fear last time I went to the Lakes was finding somewhere safe
to leave the car for a few days. That and waking up to find myself
surrounded by a dozen other tents.

I was in the highlands at the weekend. Pull off the road for more than
a few minutes to check out a sleeping spot and before you know it
you've got company. What is it with some people - can't they bear to
be on their own?
--
R
o
o
n
e
y

"I always knew the entire Green party were nutters" - Ken Livingstone

The message <[email hidden]>
from Norman <[email hidden]> contains these words:

Quoted message said:

Quoted message said:

Quoted message said:

I would think that in another 10 years no-one will be using the
magnetic compass and it will go the same way as the slide rule and
logarithms when the pocket calculator became available.

Quoted message said:

Quoted message said:

Think again. If I am still walking in 10 years time I will still be
using my compass to orient my map and to take bearings when it is not
immediately obvious from the lie of the land which way to go. Why?
Because I find it much simpler and much more flexible than faffing
around with routes and waypoints for every possible eventuality. A GPS
(I am on my second) is a wonderful instrument for showing where you have
been and, in extremis, where you are, but they are a waste of effort for
fair weather navigation and only of marginally greater assistance in the
clag.

Quoted message said:

Clearly you must be using your GPS merely as an emergency device which
is a shame considering the cost and benefits it has to offer.

Wrong again. I carry a GPS to tell me where I have been. I have
sufficient confidence in my own ability not to (usually) need the
comfort of absolute assurance of where I currently am.

Having once been a production engineer I am always on the lookout for
the easiest and simplest solution to any problem. As far as hill walking
navigation in clear weather is concerned map and compass wins on both
counts. It also wins on flexibility which is an issue with those who
don't stick to a rigidly planned itinerary.

Quoted message said:

There is generally no need to bother with routes unless you want to
keep to a clearly defined path which in the UK is nearly always
visually discernable anyway.

Which begs the question about the nature of a "clearly defined path"
which is not "visually discernable".

Quoted message said:

My technique with a GPS is to mark the position of my intended
destination waypoint by saving the coordinates or transferring the
position via my PC. Then do the same for the start point, eg car in
car park at starting point at the beginning of a trek by pressing the
appropriate buttons. This can be done quickly either at home or when
you arrive at the start.

And if you happen to have chosen a route that does not lead almost
directly to your target how does go-to really help you to navigate
there?

Quoted message said:

Normally, with a map, if you are walking up a hill etc it is usual to
start at the bottom and gradually follow the path to the summit
checking your route is correct by consulting your map in combination
with compass triangulation fixes with a compass and noting/comparing
localised terrain features with what you see around you and what is
printed on your map. Most of the time there is no need to even look at
a map because in clear visibility the route would be obvious by the
errosion caused by thousands of boots churning up the ground or
polishing the rock surfaces.

I can't even remember the last time I tried to fix my position by
triangulation. It presupposes that you don't have much of a clue where
you actually are but can still recognise 2 prominent features as unique
points. OTOH a majority of my walking (stretching back more than 40
years) seems to have been on paths that either weren't visible on the
ground or didn't exist in the first place.

Quoted message said:

My method is to make use of all three ie map, compass and GPS
Using a GPS switch on and select find/goto the destination waypoint
and set off walking towards the destination by walking in the
direction the compass rose pointer indicates. If you want to use a
magnetic compass with the GPS you can set the GPS to give magnetic
variation compensated bearings and every now and then switch on your
GPS note the indicated bearing to destination and then transfer this
to your compass and use that bearing to steer with. Using this method
you can wander around exploring as you go. If you want to get back on
a path, find your position using the GPS, note coordinates, look at
map and locate present position and then work out bearing to nearest
point on path either by compass or entering coordinates of path
waypoint and getting the bearing to this new path waypoint from GPS
reading. Once on path put everything back in rucksack and plod up to
the top.

Still unnecessary faffing about. As long as you know approximately where
you are and can read a map (not always a given for some walkers) it is
easy enough to find the nearest path should you need one to get you to
your next target and just as easy to find a more direct route on the
map. Using a go-to without consulting the map is the short sighted
option. Not because you might walk off a cliff while blindly following
your GPS, not even the stupidest of individuals are likely to do that
(although it is worth bearing in mind that a German driver drove down a
slipway into a river because he failed to realise that the road he was
being guided along was interrupted by a ferry) but because the direct
route could so easily lead down a spur, not along the ridge to the next
summit.

Quoted message said:

At top use car or start position as new destination and just do same
procedure as previous or follow path down hill.

If you can see where you are going you don't need the assistance of a
GPS and with your crude usage it is difficult to see what advantage you
actually gain other than confirmation that you are there when you
eventually reach your target after following a path that is unlikely to
be anywhere near the straight line that your GPS is indicating.

Quoted message said:

Interestingly all ordnance survey maps are now corrected using GPS
derived coordinates. The triangulation points on hill tops serve as a
nostalgic reminder of the effort that went into producing the maps.

Some of the trig points have been retained unlike the Prime Meridian
that has been hijacked by the Yanks and is now fixed to some point in
North America rather than at Greenwich Observatory. I wonder what the OS
will do as the Greenwich Meridian gradually heads West.

Quoted message said:

I look forward to the day when one can buy a device that you can put
in your pocket and when you want to know where you are you can just
ask it to tell you or show you on a 1 in 25000 colour map.

Even then it will still be just as easy to use a map and compass
provided the map is of a manageable size. Maybe even easier given
current screen sizes.

--
Roger Chapman so far this year 50 summits
New - 27 (Marilyns 13, Nuttalls 5, Outlying Fells 10)
Repeats - 23 (Marilyns 7, Nuttalls 14, Wainwrights 12, Outlying Fells 0)

Much snipping made

Quoted message said:

Quoted message said:

After very extensivley using my GPS in all kinds of weather conditions and
keeping my Silva 4 in the map bag I've seen many times when my 16 channel
GPS cannot sync with enough satellites to get a good enough fix, and
sometimes locating me off cliffs. This glitches have normally been put
down
to *very* harsh weather conditions such as pea-soup's.

In practical terms weather does not affect GPS signals - it wouldn't be
much use as a military tool if it did.

I disagree with this comment. I've worked with Location Systems since the
late 80's including mililary location systems. Weather conditions are very
good at dispersing all kinds of RF signals, thunder storms and VERY dense
cloud for starters. Very dense local cloud cover containing a large amount
of water content can easily divert or impede a signal therefore giving a
smaller QoS and phase changes in signals. Thunder storms have the ability
to cause signal phase changes for land based location systems. These need to
be nearer to the transmitter than the reciever first and a not used any more
(US Omega System used by US military, retired some years ago, using VLF)

Quoted message said:

If you are having trouble receiving a good mix of satellites (so that a
good position is reported) the chances are the sky is significantly
obscured by terrain or your body. It is also a possibility that your GPS
receiver is not very healthy :-( what model is it?

The GPS I use is an entry level Magellen 16 channel Explorist 100. To
comment on the obscuring the last time I had exceptional bad fixes was on
top of Ben Nevis (May 1 this year) very little to obscur the signal I was on
the summit ridge. My location given by the GPS was some 30 m from my known
(mapped and nav'ed) location. The GPS was only able to get good signals from
2 GeoSats due to the water content in the cloud I was stood in

Quoted message said:

Quoted message said:

Take the GPS, enjoy it. But learn to nav and map read

Sound advice.

--

Dominic Sexton

I noticed that Message-ID: <[email hidden]>
from Steve Ray contained the following:

Quoted message said:

The GPS was only able to get good signals from
2 GeoSats due to the water content in the cloud I was stood in

I seem to recall it was letting go a fair bit of it...
--
Geoff Berrow (put thecat out to email)
It's only Usenet, no one dies.
My opinions, not the committee's, mine.
Simple RFDs http://www.ckdog.co.uk/rfdmaker/

Steve Ray said:

Much snipping made

Quoted message said:

Quoted message said:

After very extensivley using my GPS in all kinds of weather conditions and
keeping my Silva 4 in the map bag I've seen many times when my 16 channel
GPS cannot sync with enough satellites to get a good enough fix, and
sometimes locating me off cliffs. This glitches have normally been put
down
to *very* harsh weather conditions such as pea-soup's.

In practical terms weather does not affect GPS signals - it wouldn't be
much use as a military tool if it did.

I disagree with this comment. I've worked with Location Systems since the
late 80's including mililary location systems. Weather conditions are very
good at dispersing all kinds of RF signals, thunder storms and VERY dense
cloud for starters. Very dense local cloud cover containing a large amount
of water content can easily divert or impede a signal therefore giving a
smaller QoS and phase changes in signals. Thunder storms have the ability
to cause signal phase changes for land based location systems. These need to
be nearer to the transmitter than the reciever first and a not used any more
(US Omega System used by US military, retired some years ago, using VLF)

Quoted message said:

If you are having trouble receiving a good mix of satellites (so that a
good position is reported) the chances are the sky is significantly
obscured by terrain or your body. It is also a possibility that your GPS
receiver is not very healthy :-( what model is it?

The GPS I use is an entry level Magellen 16 channel Explorist 100. To
comment on the obscuring the last time I had exceptional bad fixes was on
top of Ben Nevis (May 1 this year) very little to obscur the signal I was on
the summit ridge. My location given by the GPS was some 30 m from my known
(mapped and nav'ed) location. The GPS was only able to get good signals from
2 GeoSats due to the water content in the cloud I was stood in

Quoted message said:

Quoted message said:

Take the GPS, enjoy it. But learn to nav and map read

Sound advice.

--

Dominic Sexton

How did you manage to navigate in cloud to your map waypoint so
accurately.
I have used my Garmin V (12 channels) in dense fog and sitting on top
of a trig point the GPS indicated I was on top of the trig point.
(Great Whernside)
A Magellan Explorist 100 has 14 channels not 16 but should be capable
of better performance than what you indicate.
Personally I prefer Garmin products which I have found to be more
accurate than a Tom Tom and an old Trimble.

Norman

On Thu, 28 Jul 2005 16:49:51 +0100, Roger <[email hidden]>

Quoted message said:

The message <[email hidden]>
from Norman <[email hidden]> contains these words:

Quoted message said:

Quoted message said:

> I would think that in another 10 years no-one will be using the
> magnetic compass and it will go the same way as the slide rule and
> logarithms when the pocket calculator became available.

Quoted message said:

Quoted message said:

Think again. If I am still walking in 10 years time I will still be
using my compass to orient my map and to take bearings when it is not
immediately obvious from the lie of the land which way to go. Why?
Because I find it much simpler and much more flexible than faffing
around with routes and waypoints for every possible eventuality. A GPS
(I am on my second) is a wonderful instrument for showing where you have
been and, in extremis, where you are, but they are a waste of effort for
fair weather navigation and only of marginally greater assistance in the
clag.

Quoted message said:

Clearly you must be using your GPS merely as an emergency device which
is a shame considering the cost and benefits it has to offer.

Wrong again. I carry a GPS to tell me where I have been. I have
sufficient confidence in my own ability not to (usually) need the
comfort of absolute assurance of where I currently am.

Having once been a production engineer I am always on the lookout for
the easiest and simplest solution to any problem. As far as hill walking
navigation in clear weather is concerned map and compass wins on both
counts. It also wins on flexibility which is an issue with those who
don't stick to a rigidly planned itinerary.

Quoted message said:

There is generally no need to bother with routes unless you want to
keep to a clearly defined path which in the UK is nearly always
visually discernable anyway.

Which begs the question about the nature of a "clearly defined path"
which is not "visually discernable".

Quoted message said:

My technique with a GPS is to mark the position of my intended
destination waypoint by saving the coordinates or transferring the
position via my PC. Then do the same for the start point, eg car in
car park at starting point at the beginning of a trek by pressing the
appropriate buttons. This can be done quickly either at home or when
you arrive at the start.

And if you happen to have chosen a route that does not lead almost
directly to your target how does go-to really help you to navigate
there?

Quoted message said:

Normally, with a map, if you are walking up a hill etc it is usual to
start at the bottom and gradually follow the path to the summit
checking your route is correct by consulting your map in combination
with compass triangulation fixes with a compass and noting/comparing
localised terrain features with what you see around you and what is
printed on your map. Most of the time there is no need to even look at
a map because in clear visibility the route would be obvious by the
errosion caused by thousands of boots churning up the ground or
polishing the rock surfaces.

I can't even remember the last time I tried to fix my position by
triangulation. It presupposes that you don't have much of a clue where
you actually are but can still recognise 2 prominent features as unique
points. OTOH a majority of my walking (stretching back more than 40
years) seems to have been on paths that either weren't visible on the
ground or didn't exist in the first place.

Quoted message said:

My method is to make use of all three ie map, compass and GPS
Using a GPS switch on and select find/goto the destination waypoint
and set off walking towards the destination by walking in the
direction the compass rose pointer indicates. If you want to use a
magnetic compass with the GPS you can set the GPS to give magnetic
variation compensated bearings and every now and then switch on your
GPS note the indicated bearing to destination and then transfer this
to your compass and use that bearing to steer with. Using this method
you can wander around exploring as you go. If you want to get back on
a path, find your position using the GPS, note coordinates, look at
map and locate present position and then work out bearing to nearest
point on path either by compass or entering coordinates of path
waypoint and getting the bearing to this new path waypoint from GPS
reading. Once on path put everything back in rucksack and plod up to
the top.

Still unnecessary faffing about. As long as you know approximately where
you are and can read a map (not always a given for some walkers) it is
easy enough to find the nearest path should you need one to get you to
your next target and just as easy to find a more direct route on the
map. Using a go-to without consulting the map is the short sighted
option. Not because you might walk off a cliff while blindly following
your GPS, not even the stupidest of individuals are likely to do that
(although it is worth bearing in mind that a German driver drove down a
slipway into a river because he failed to realise that the road he was
being guided along was interrupted by a ferry) but because the direct
route could so easily lead down a spur, not along the ridge to the next
summit.

Quoted message said:

At top use car or start position as new destination and just do same
procedure as previous or follow path down hill.

If you can see where you are going you don't need the assistance of a
GPS and with your crude usage it is difficult to see what advantage you
actually gain other than confirmation that you are there when you
eventually reach your target after following a path that is unlikely to
be anywhere near the straight line that your GPS is indicating.

Quoted message said:

Interestingly all ordnance survey maps are now corrected using GPS
derived coordinates. The triangulation points on hill tops serve as a
nostalgic reminder of the effort that went into producing the maps.

Some of the trig points have been retained unlike the Prime Meridian
that has been hijacked by the Yanks and is now fixed to some point in
North America rather than at Greenwich Observatory. I wonder what the OS
will do as the Greenwich Meridian gradually heads West.

Quoted message said:

I look forward to the day when one can buy a device that you can put
in your pocket and when you want to know where you are you can just
ask it to tell you or show you on a 1 in 25000 colour map.

Even then it will still be just as easy to use a map and compass
provided the map is of a manageable size. Maybe even easier given
current screen sizes.

Roger
I am surprised when you indicate repeatedly in your comments how easy
it is to use a map and compass compared to using a GPS. Having walked
for a similar period as yourself mainly in Scotland and Yorkshire and
used maps and a compass for the majority of the time I find a GPS much
simpler to use. I dont have any fixed views about GPS receivers but
write in support of them because I am fed up with people in this news
group slating the things. It reminds me of when I used to go caving
and had to endure criticism for using SRT gear instead of clambering
up and down electron rope ladders and that I prefered wellies to
leather boots.
If you are quite happy using your system then there is no need for you
to change to another system and I support you in your choice.

Norman

Paul Rooney <[email hidden]> writes

Quoted message said:

AndyP said:

It is only then that you will truly find yourself
my son.

But will the MRT be able to find you?

<Corpse>

(No pun intended).
--
Gordon Harris

In article <[email hidden]>, Steve Ray
<[email hidden]> writes

Quoted message said:

Much snipping made

Quoted message said:

Quoted message said:

After very extensivley using my GPS in all kinds of weather conditions and
keeping my Silva 4 in the map bag I've seen many times when my 16 channel
GPS cannot sync with enough satellites to get a good enough fix, and
sometimes locating me off cliffs. This glitches have normally been put
down
to *very* harsh weather conditions such as pea-soup's.

In practical terms weather does not affect GPS signals - it wouldn't be
much use as a military tool if it did.

I disagree with this comment. I've worked with Location Systems since the
late 80's including mililary location systems. Weather conditions are very
good at dispersing all kinds of RF signals, thunder storms and VERY dense
cloud for starters. Very dense local cloud cover containing a large amount
of water content can easily divert or impede a signal therefore giving a
smaller QoS and phase changes in signals.

The effect is dependant upon the frequency of the signal. Ground based
systems have a limited frequency band if they are to be of use for a
greater distance than line of sight. They also have to contend with
other ground based radio systems and avoid interfering or being
interfered with.

Space based signals do not have the distance restriction for line of
sight frequencies so have a far greater range of frequencies to choose
from.

Some links say "all weather" etc.
http://www.af.mil/factsheets/factsheet.asp?fsID=119
Bullet point 3

http://www.usace.army.mil/usace-docs/eng-manuals/em1110-1-1003/c-2.pdf
Para 2

http://gpsinformation.net/gpsclouds.htm

http://www.garmin.com/aboutGPS/
1st para 3rd sentence

http://www.thalesnavigation.com/en/products/aboutgps/howworks.asp
Fifth para.

Water vapour and droplets do not pose a problem to the signals. A thin
continuous film will begin to degrade the signals and a few mm of
continuous water is enough to stop them.

Quoted message said:

Thunder storms have the ability
to cause signal phase changes for land based location systems. These need to
be nearer to the transmitter than the reciever first and a not used any more
(US Omega System used by US military, retired some years ago, using VLF)

Quoted message said:

If you are having trouble receiving a good mix of satellites (so that a
good position is reported) the chances are the sky is significantly
obscured by terrain or your body. It is also a possibility that your GPS
receiver is not very healthy :-( what model is it?

The GPS I use is an entry level Magellen 16 channel Explorist 100.

I think that is a 14 channel receiver - allowing 2 for WAAS/EGNOS and
the usual 12 for the GPS sats.

Quoted message said:

To
comment on the obscuring the last time I had exceptional bad fixes was on
top of Ben Nevis (May 1 this year) very little to obscur the signal I was on
the summit ridge. My location given by the GPS was some 30 m from my known
(mapped and nav'ed) location. The GPS was only able to get good signals from
2 GeoSats due to the water content in the cloud I was stood in

A GPS receiver cannot compute a position from the signals from only two
satellites without making assumptions as to the altitude and the time
(sub microsecond). Any error in those assumptions will lead to position
error. For a good 3d position solution four satellites spread out in the
sky are required.

The GPS satellites are not geo-stationary but the EGNOS test signal
satellites are. Perhaps it was reporting the use of two of those in
which case they could have been contributing to the error. In the
current test mode the signals sometimes make the position less accurate
rather than more accurate. I do not know if you can switch off WAAS /
EGNOS use with that unit but if you can't you may well experience a
reduction of accuracy at times.

Unless the receiver had just been switched on or was under wet material
or was significantly obscured by your body / rocks / wall / backpack it
sounds very much like it is faulty.

If the receiver had not been switched on for a while (the time is
dependant upon the software but likely to be between one and four hours)
and is next used a considerable distance (hundreds of miles) from the
last position or with a difference in altitude of a few hundred metres
it may suffer some confusion with the initial position solution. This
may take up to a couple of minutes of good signals to recover from due
to the fact that there is some moving averaging going on in the position
solution to smooth out short term errors.

--

Dominic Sexton