[pct-l] Follow-up to the earlier GPS discussion

Wed May 19 20:09:47 CDT 2010

Good afternoon, Gary,

 A GPS receiver doesn’t have an atomic clock, but as soon as it identifies
nav-sat signals it begins a process of correcting it’s own internal clock
upwards of 1,000 times per second.  That correction process is one of the
real geniuses of GPS without which the system could not function at a price
any of us could afford – that plus the use of pseudo-random transmission
codes.  To correct the clock it is necessary for the GPS to lock onto at
least four nav-sats rather than the three that are theoretically necessary
to isolate a point on the earth.

Steel-Eye

Hiking the Pct since before it was the PCT – 1965

http://www.trailjournals.com/steel-eye
http://www.trailjournals.com/SteelEye09

On Wed, May 19, 2010 at 2:44 PM, <gwschenk at socal.rr.com> wrote:

> The main error in GPS readings nowadays, in addition to multi-path, is the
> time. The satellites are basically moving survey monuments. They broadcast
> their exact location at an exact time. The receiver then notes the time that
> it receives the signal. Subtracting the two times it derives the distance.
>
> The satellite has an atomic clock. The Garmin has a Swatch. There's the
> rub.
>
> Vertical position is based on mapping of the earth's gravity. This mapping
> is densified in more urban areas where it is economically justified. In
> remote mountain regions it is not considered as important. And so, every so
> often you will read where someone will complain that the elevation of
> Whitney is wrong, because his Magellan says so!
>
> My geek post of the week.
>
> Gary
>
> ---- CHUCK CHELIN <steeleye at wildblue.net> wrote:
> > Good afternoon, GPS users,
> >
> > Whenever a GPS signal bounces off some object prior to reaching a
> receiver
> > the result is known as a multipath perturbation -- meaning the GPS
> receives
> > a direct, straight-line signal, plus a signal(s) that has bounced off
> > something making it’s distance slightly longer.  This happens quite
> often,
> > but the receiver’s logics are supposed to automatically accept only the
> > shortest of otherwise identical signals.  A problem only becomes apparent
> > when the direct, straight-line signal is somehow shaded and the receiver
> > doesn’t have two identical signals – only an incorrect multipath
> signal(s).
> > Then it has no choice but to use the bad signal.  The normal cure for
> this
> > is to move around, even if only a few steps, to reacquire the true
> signal.
> >
> >
> >
> > There are a number of other perturbations – who but a GPS geek would say
> > that a signal has been perturbed? – but most of the others are
> atmospheric
> > and beyond the concern of a casual GPS user.
> >
> >
> >
> > Many times the “can’t get a signal” problem can be avoided or overcome.
>  If
> > a GPS is operating and locked-on before you enter some bad spot the odds
> are
> > good that it will retain its lock, particularly if you are moving to give
> it
> > an opportunity to interrogate all the available nav-sats.  Conversely,
> some
> > users wait until they’re in a nasty location before they turn on the GPS
> to
> > find out where they are, and they usually stand still why they are
> waiting.
> > Under those conditions it’s a tough chore for the GPS to start cold, lock
> > on, and determine a position.
> >
> >
> >
> >  Sometimes a “loose” location – one known to be off by hundreds of feet,
> or
> > where a large probable error is displayed – is good news.  Instead of
> just
> > displaying, “No signal”, what the GPS is saying is, “I’m doing the best I
> > can boss, but I just don’t have good signals.”    In that case keep
> moving
> > around to try to get some clear sky, at which point the location will
> > tighten up.
> >
> >
> >
> > Steel-Eye
> >
> > Hiking the Pct since before it was the PCT – 1965
> >
> > http://www.trailjournals.com/steel-eye
> > http://www.trailjournals.com/SteelEye09
> >
> > On Wed, May 19, 2010 at 9:25 AM, Jim Keener ( J J ) <
> > pct2010 at ridgetrailhiker.com> wrote:
> >
> > > Dropping in a creek? That doesn't hold water. Smart phones would be
> > > affected if they're not protected, but most dedicated GPS devices are
> > > water resistant for a long time. Any. Any navigation equipment or
> > > device can be lost or damaged.
> > >
> > > Knowing how to use a GPS device and having one is especially smart
> > > this year.
> > >
> > > Walk well,
> > > Jim Keener ( J J )
> > > http://postholer.com/jj
> > >
> > > On May 19, 2010, at 9:14 AM, "Eric Lee" <saintgimp at hotmail.com> wrote:
> > >
> > > > I guess you meant to send this to the whole list.
> > > >
> > > >
> > > >
> > > > Unfortunately I'm not really current on GPS technology right now so
> > > > I'm not
> > > > a good source of recommendations.  I have a several-year-old one
> > > > that still
> > > > works so that's what I use when I anticipate a lot of snow.
> > > > Otherwise I
> > > > don't bother.
> > > >
> > > >
> > > >
> > > > Just to reiterate the point I was trying to make: a GPS is really
> > > > nice to
> > > > have when snow covers the trail, but it's not mandatory equipment.
> > > > Bring
> > > > one if it'll make you feel more comfortable, but ask yourself what
> > > > you'll do
> > > > if you drop it in a creek or forget to turn it off and run out the
> > > > batteries, or whatever.  Be sure to be proficient with a map and
> > > > compass,
> > > > too.
> > > >
> > > >
> > > >
> > > > Eric
> > > >
> > > >
> > > >
> > > > From: Melanie Clarke [mailto:melaniekclarke at gmail.com]
> > > > Sent: Wednesday, May 19, 2010 5:53 AM
> > > > To: Eric Lee
> > > > Subject: Re: [pct-l] Follow-up to the earlier GPS discussion
> > > >
> > > >
> > > >
> > > > Dear Eric or anyone,
> > > >
> > > > I just join this e-list.  Which GPS do you recommend?  I am heading
> > > > out on
> > > > the JMT June 20th and I'm concerned about all the snow covering the
> > > > trails
> > > > to the high passes and peaks.
> > > >
> > > > Melanie
> > > >
> > > > On Tue, May 18, 2010 at 11:03 PM, Eric Lee <saintgimp at hotmail.com>
> > > > wrote:
> > > >
> > > > Interesting postscript to the GPS discussion we had back in March:
> > > > a lot of
> > > > the trail journals are describing situations where non-GPS-carrying
> > > > thrus
> > > > chose to latch on to the GPS-carrying thrus in order to navigate
> > > > through the
> > > > San Jacintos.  As Granite and Terrapin Flyer wrote, "Not a single
> > > > hiker we
> > > > met with just a map and compass wasn't thrilled to check our GPS
> > > > data."
> > > >
> > > > Empirical evidence seems to show that while a GPS isn't *necessary*
> in
> > > > snow-bound conditions (and shouldn't be relied on as the sole form of
> > > > navigational aid), it's pretty darn useful and when people are
> > > > actually out
> > > > there they're more than happy to follow the folks who have them.
>  Just
> > > > interesting data . . .
> > > >
> > > > Examples:
> > > >
> > > >
> > >
> http://postholer.com/journal/viewJournal.php?sid=d06471f65621deba49961e93977
> > > > <
> > >
> http://postholer.com/journal/viewJournal.php?sid=d06471f65621deba49961e9397
> > > > 7%0A35ed3&entry_id=14895>
> > > > 35ed3&entry_id=14895
> > > > http://www.trailjournals.com/entry.cfm?id=310733
> > > >
> > > > Eric
> > > >
> > > > _______________________________________________
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