[Wylug-discuss] Re: [Wylug-announce] WYLUG Social and micro-mapping

[ROBERTS, Mike]

> Keep in mind that at 54 degrees north, the GPS satellites are 
> largely towards the southern sky.  There will be times where 
> they are straight overhead or slightly to the northeast and 
> northwest depending on your longitude.  These satellites will 
> never be to the north from this latitude.  Therefore, it will 
> be helpful to avoid blockage from southern sky.  I am not 
> sure how much blockage the human body will add.  It might be 
> good to try some experiments ahead of time to determine how 
> much your body blocks the signals.  These satellites are in 
> 12 hour orbits, so it would be good to try it several times 
> throughout the day as the geometries are constantly changing.

Hi Roy 

I am afraid that isnt quite right. There are a constellation of GPS
satellite in 6 different orbits at different angles in MEO (Medium Earth
Orbits) These criss cross the planet meaning that there are always a
number close to being overhead. GPS satellihes are not geostationary and
do not occupy the Clarke belt. So they are not predominantly to the
South as is the case for Sky and other telecomms satellites. 

Check out the wikipedia article here http://en.wikipedia.org/wiki/GPS to
see an animation of the orbits and for a much more detailed explanation
of how GPS works

The human body will attenuate the signal as it is full of quite a lot of
water. The GPS signals are in the  >1Ghz frequency range and people
arent tansparent at this end of the radio spectrum.

The geometries are indeed continually changing. Your GPS receiver has a
thing called an "almanac" to help it figure out where they will be at
what time and it also down loads an "ephemeris" from the satellites when
it is turned on which allow it to figure out where on the planet you
are.

You will have more problems from reflections off tall buildings in built
up areas and from trees in leafy suburban streets attenuating the
signals if you have older recievers I suspect. Modern devices are much
much better at getting a lock and can even work in extremely shielded
loctatios such as might be found on a system used to track stolen
vehicles which needs to be deeply hidden in the bodywork of a car.

I work for Astrium who will be building the next generation Gallileo GPS
contellation.

Cheers Mike 8-{>

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Mike Roberts     -=*=- Design Engineer -=*=-      Astrium Limited
             -=FREQUENCY CONTROL PRODUCT GROUP=-

> 
> roym
> 
> 
> -----Original Message-----
> From: wylug-discuss-bounces at wylug.org.uk 
> [mailto:wylug-discuss-bounces at wylug.org.uk] On Behalf Of Anne Wilson
> Sent: Thursday, June 19, 2008 7:10 PM
> To: wylug-discuss at wylug.org.uk
> Subject: Re: [Wylug-discuss] Re: [Wylug-announce] WYLUG 
> Social and micro-mapping
> 
> On Thursday 19 June 2008 18:54:35 Stephen Patterson wrote:
> > On 19 Jun 08, Dave Fisher (wylug-discuss at davefisher.co.uk) wrote:
> > > On Thu, Jun 19, 2008 at 02:44:13PM +0100, Stephen Patterson wrote:
> > > > I'll be along on a pushbike, hopefully I can borrow a 
> GPS from someone
> > > > there or team up with another cyclist.
> > >
> > > I was also thinking of going by bike (for greater 
> coverage), but am
> > > short of a GPS tracker and bike mount for it. I can 
> probably buy one of
> > > the former before Sunday.  That latter may be trickier.
> >
> > Pocket it? I guess the gps just needs to be turned on unless you're
> > using it for navigation too.
> 
> Or what about one of those see-through map pockets that hang 
> round your neck?
> 
> Anne
> 
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