GPS queries [long]

[Thayer]

        After two months' recent experience using a GPS unit in Australia, I
have some comments to add.  As Steve Shattuck mentioned, many such units are
available at marine supply stores; many (most?) are designed especially for
boating use and may not be the most suitable for land use.
      We used a Magellan NAV1000PRO, which is a survey model specifically
designed for on-land use.  Some other manufacturers also have survey models
which I assume are more-or-less comparable.  We didn't use the many naviga-
tional features of the unit (e.g., what course & how far to waypoint X?),
which I think are standard in most marine units as well.  As far as I know, the
survey models are only available from their manufacturers, not through retail
outlets.
Some features of the NAV1000PRO are:
    -it saves altitude as well as coordinates for each waypoint (the Trimble
Ensign, at least, does not; but see below about altitudes),
    -it allows repeated readings at a site to be averaged (and reports a std
dev. for coordinates, though not altitude), and
    -it can provide satellite schedules for any time, any place (within a
few hundred miles of last reading, I think) so that you know in advance
whether/when there will be enough sats available to take a reading (3 for a 2-D
reading if you know the altitude, 4 for a 3-D if you don't).
    -you can get extremely accurate coordinate fixes by using 2 units,
downloading the data and doing post-processing of it ("differential GPS").
(For our purposes, such accuracy would be spurious as well as unnecessary,
since our collecting "sites" cover an area up to a hectare or so.)
        We didn't have too much trouble with satellite unavailability, but used
and were glad to have the plan-ahead capability.  One cost of such features (I
think) is that you have to "collect an almanac" to use it more than about 300
miles from its last use/almanac collect.  Although almanac-collecting only
requires one satellite, we had trouble with this and ate up a couple sets of (6)
AA batteries getting almanacs twice.  This unit is a few years old and has been
superseded by the Magellan NAV5000 PRO, which is a 5-channel instead of
1-channel receiver.  I don't know if the 5000 also requires almanac acquistion.
It should do better at "holding onto" satellites (though that's probably more
important for navigation than for point fixes) and should probably get its
fixes faster.  Late last year, the NAV5000PRO was being sold for the same
price the NAV1000PRO had been a few years earlier.
        Both of these units allow direct downloading to a computer and include
software for manipulating and analyzing the downloaded data.  Unfortunately,
because of foul-ups at home base, we had neither the cable for doing so nor an
AC or car-battery adapter (either of which would have speeded our almanac
collecting).  The NAV1000PRO is a robust unit, though somewhat heavier than
one might like, and our only complaint about its physical design is that the
power switch can easily be turned on by accidenti (including through its
carrying case), leading to dead batteries, which means loss of waypoints and
almanac!
        About altitude:  One major disappointment we had is that you can't
really count on getting usable altitude fixes because of the "Selective
Availability" signal degradation built into the satellite system for security
reasons (by the US govt).  This degradation, when turned on, still leaves you
with reading accurate to about 100m for coordinates, but it affects the altitude
readings far more significantly.  (This is a problem well-known to the technical
people at Magellan.)  We found that if we watched repeated fixes (at about 5 sec
intervals), the coordinates would fluctuate by perhaps 0.02 minutes, but the
altitude often by 100m or more.  Since the altitude fixes don't get averaged if
you select averaging, there's no easy way to get a mean reading or to tell if
the readings are even fluctuating around the correct value.
        I have phone numbers for at least Magellan, Trimble, and Garmin
but can't get my hands on them right now.  I should be able to
post them tomorrow.  Trimble has a ca. 80 page book about GPS (theory & opera-
tion *without* regard to their products) that is a nice introduction to the
subject.  As of last year they were distributing it free on request.  I'll
give the name of that with their phone number.


        After I wrote, but before I sent the above message, I saw Julian
Humphries' query & will add a few responses here:

        4/5/6 channel refers (in effect) to how many satellites the unit can
keep track of at once.  It needs 4 for a 3-D fix; if it can keep contact with
more than that, you're less likely to lose positioning info when one of the
four goes below the horizon or behind an obstruction.  My guess is that this
is more important for navigation than point fixes, as mentioned above.
        NMEA interfaces (I think it's National Marine .... maybe Electronic
Association?)  are used in piloting equipment on boats, etc.  I don't know
if it's possible to connect them to more familiar things like serial ports.
Given a choice, I'd go for something that's directly PC-connectable (see above).
        Presumably any unit that can take 3-D fixes shows elevation.  Not all
save elevation when you save a fix, however.  See discussion of elevation above!
        Number of lines of LED display: basically a question of how the
displayed info is formatted and whether you have to page through multiple
screens for all the info potentially associated with a fix.  I think that at
least lat, long, and elev would always been shown on a single screenful.
        Units may differ in how long they take to get a fix under the same
conditions, but a given unit may also vary from site to site because of
obstructions, etc.  Whether a unit stores the equivalent of an almanac would
also affect the time, since if it has an almanac, it knows what should be
available in the general area of the last fix.  I don't know, however, if
all units do store an almanac or if they search anew for sats each time they're
turned on.
        Most units made for marine use (especially hand-held types) should be
waterproof.  The survey model I used also is, but I don't know about others.
I suspect this is a fairly high design priority for all.
        I don't know how the prices you mentioned are (the market has changed
tremendously since last fall, when I was looking around).  One good source
(mail-order) is West Marine in California, which I heard of from an entomology
list.  I'll post info to reach them tomorrow with the other promised phone #'s)

Hope this helps!

Margaret Thayer, Zoology-Insects, Field Museum of Natural History
Chicago IL 60605, USA  Phone:312-922-9410 x404  Email:thayer at fmnh785.fmnh.org