From: STU@mwvm.mitre.org
Subject: Comments and Answers on GPS usage
Organization: The MITRE Corporation, McLean VA 22102
Date: Thu, 19 Nov 1992 13:58:18 GMT

Silas Wild Writes:
 
Saw your name mentioned by the only GPS item on [rec.boats] recently.  I'm
looking for a portable GPS to use on a Patagonia Icecap (land) traverse.  What
would you suggest; . . .
 
Trimble looks the lightest.  Are there other OK models that are as light and
less expensive.  Thanks.  Silas Wild - Seattle.
 
Thought any of you with information might want to give him a call at (206) 685-
0785.
 
I replied with the following - that may be of general interest:
  ----------- included material follows---------------------------
 I'd recommend you keep looking for now - three problems:
 
1.  As you go closer to the poles, Lat/Long becomes a less efficient method of
determining position.  While GPS is capable of the same precision (at least in
theory) in any location, the algorithms (according to Magellen) don't work as
well at the poles as they do nearer the equator - where most of us sail.
Unlike LORAN (probably not an alternative for you, given there are probably no
stations near the poles), the current round of GPS equipment won't (to my
knowledge) give the underlying lines of position (LOP) in a manner similar to
LORAN's time differences (TD's).  Thus, while the GPS may "know" where you are
with a precision of 100 meters (more or less), the conversion to Lat/Long
significantly degrades the presentation of the information.
 
Since this loss of conversion efficiency wasn't a major issue for me, I chose
the Magellen for ease of user interface.
 
2.  As noted in my original posting, my Magellen (and apparently several other
units of unknown manufacturer) gave an incorrect reading on several occasions
with no indication of failure.  The unit told me I was traveling at a high
velocity (70++ knots) in an unusual direction - and gave several other
indications of failure.  As other posters on the net have noted, I can't tell
if this is a software failure of the on board software or an intentional
degradation of the signal by the military - but, I suspect it was the military
since the problem was confirmed on two occasions by 4-5 other GPS users who
were seeing the same problem.
 
At sea, the temporary loss of navigation information is of no consequence -
assuming you are maintaining an accurage dead reconing log and not using the
GPS for piloting.  [let me digress, I think of navigation as long term
direction/speed setting and position estimating, while piloting is short term
computations for, for example, entering a harbor.  Piloting must be precise to
high accuracy - while navigation happily tolerates outages of several hours
and distance errors of several miles.]
 
If your application requires continuous position knowledge - as for example,
returning to a safe haven in a blinding snowstorm - you just can't trust GPS
in its current state.  If you are trying to fly an aircraft and not
maintaining some other form of position indicator, you can't trust GPS.  Any
dream (if you need it for your application) of landing an aircraft with GPS
must be catch as catch can - since the outages are fairly frequent (I say two
major, extended outages in about days - total of about 5 hours).
 
3.  The weight of the unit itself is insignifcant compared with the weight of
the batteries required.  My Magellen uses 6 AA batteries every few hours -
maybe 3-5 hours.  In continuous use, I suppose I'd consume the weight of the
unit every few days.  The Sony is far lighter - but eats batteries at a faster
rate - according to Practical Sailor.  The Garman is a little lighter and
(according to sailors at the Beaufort bay - my ultimate source of the truth)
it uses batteries at the rate of the Magellen.  So, unless you have an
independent source of 12V power and can plug the unit in, forget it.  The
batteries will eat your lunch - unless you use the unit for occasional fixes
to replace a sextant.
 
In summary, as a sextant replacement, my Magellin (and I suspect all the
others) is excellent.  It gives me a reliable, loss of sleep independent, fix
several times a day with very low battery consumption.  As a piloting device
required to be operational on a continuous basis, it isn't mature enough yet
to be trusted for marine, aircraft or any other navigaion that may result in
loss of life or mission if the pilot (walking, sailing or flying) becomes
disoriented at a critical moment for more than a few minutes.
 
Good luck on your adventure.  /Stu