[mdlug] GPS suggestions

[Robert Meier]

Ray,

[redirected to mdlug-discuss]

>  I'm looking for a good GPS unit, and hoping for suggestions or pointers
> for good, current information.

The most popular GPS receiver for MiGO geocachers is the Garmin ETrex.

In 2004, the Metropark Geocaching Adventure picnic included a closest
to pin contest (i.e. who placed a flag closest to precisely measured
point.)  After their experience in prior years, they had two categories
Garmin users, and Magellan users.  The Garmins were all clumped within
about 20ft.  The Magellans were spread over about 60ft.

>  Must-haves:
> 
>   - Bluetooth connectivity so it can talk to my Palm Treo

I understand several Etrex models are bluetooth enabled.

>   - Fast signal acquisition (time-to-fix), ideally from both cold
>     and hot start (the old GPS unit I had for my old Palm could take
>     up to *twenty minutes* if it was started cold)

There should be three levels of acquisition:
(2 should be the most common.)
      1. The GPS unit has not been significantly moved from where it
           last locked on the satellites, and the time since it was last
	   locked was short enough.
	 The GPS unit estimates state sufficiently to immediately
	   acquire the signals.
	 The Kalman filter (i.e. generalized phase-lock-loop) simply
	   converges normally.
	 This should take only 15-120 seconds depending on satellite
	   configuration, atmospheric distortion, and local environment
	   distortion.

      2. The GPS unit cannot immediately acquire the signals.
         The GPS memory still holds the correct ephemeride
	   (i.e. table of satellite orbital parameters).
	 The GPS receiver uses hueristic sampling to lock onto
	   the signals and then begins the Kalman filter.
	 This should take about 3-10 minutes depending on luck
	   of hueristics.

      3. The Kalman filter fails to converge or the ephemeride mismatches.
	 GPS unit's ephemeride is obsolete.
         The GPS must wait until the satellites transmit the ephemeride,
	   then start the Kalman filter.
	 This should take ~30 minutes since the ephemeride is only
	   transmitted about every ~30 minutes.

>   - Good battery life

    - error estimation (ideally RAIM-compliant)
      
      The GPS signals are easy to interfer with, but are designed
        so that such interference is easy to detect.
      Knowing that there is only a 37% chance that you are within
        x ft of the coordinates measured is important since x
	typically varies from 6 (open field, clear sky (to radio)) to
	500 (near fence, in forest, ...) makes a practical difference.
      When the desired site has poor reception, you can move to
        acquire a more precise signal, and then offset to precisely
	find the desired site.

> using an external antenna, mounted on a vehicle, with a
> fixed, appropriate (horizontal) orientation.

As satellites orbit, their horizon and the horizon of receivers scattered
over wide areas of the Earth change rotate through about 150 degrees
as the satellite rises and sets.

So without an horizontal orientation, non-geostationary satellite
antennas including GPS should be ideally hemi-isotropic or nearly
always isotropic.  Practical antennae are thus helical.

Good luck,
-- 
Dr. Robert J. Meier

    "Warble, warble, little sat,
     How I wonder where I'm at.
     Twice around the world each day,
     Four and twenty birds at play.

     Clear and ever steady beat,
     Pseudo Random Code repeat.
     An Ephemeris, your lore,
     Tells me where your flock will soar."
		-- extract from Dr. Robert J. Meier 2000