GPS, or "Global Positioning System", is a satellite-based navigation
system in which timing data and emphemerides are downloaded from up to 12
satellites to a hand-held computer, on which longitude, lattitude,
elevation, bearings, track, speed and time can be recorded and displayed.
The potential accuracy is 5-10 meters, when used in conjunction with a
DGPS (Differential-GPS) unit, but accuracy for civilian use is currently
degraded by the DOD "Selective Availability Program" to 100 m (2-sigma
rms). 

87 responses total.

I've entered this item to see if there are other GPS users here, who
could share information on the use of GPS units. I have purchased the
Garmin GPS-40, which is very compact (9 oz), and has good battery life.
I have a number of immediate questions about GPS:

1. Do GPS units differ significantly? The GPS-40 can acquire up to 8
satellites (I've had it up to 7), and can record 250 waypoints on 20
reversible routes. Are there any significant differences between units
in what counts - sensitivity and navigational accuracy?

2. By how much do "remote" antennas improve sensitivity (in addition
to allowing the GPS unit itself to be used within a vehicle or building)?

3. I have been observing the "dither" - the "Selective Availability
Program" variations. What is the pattern on this? If regular sampling
is done of instantaneous position, do the means converge to an unbiased
estimate? What is the effect of the DGPS accessory on the RMS error?

4. The GPS-40 will take battery power, and has a data-in/data-out port
(one wire each, plus gnd and 5-9 v power). The unit supports numerous NMEA
interface formats. What are these? (The manual writes of "approved
sentences" such as GPBWC, GPGLL, etc, and "proprietary sentences" (PGRME,
etc).) There is a "PC Kit" available, for interfacing to a PC, but I do
not yet know what capabilities this can offer (perhaps the averaging
I spoke of above?).
  
Well, that's enough for now - especially considering that there may not
be many other GPS "micros" users here.

Louise Hose was working with GPS and PC links in Mexico the year before
last.  I saw her give a talk at the 1993 NSS convention.  I don't have my
program at hand to give more particulars.  Someone else handled the
technical talk, and she did the discussion of the field work.

  Computer software used with the GPS can do a lot of filtering of data. 
By taking a series of readings simultaneously at two locations, one known,
you can create a plot of your variance as well as a "bombing pattern" of
all your readings which can be displayed on a graph.  This graph can show
if you have a tight pattern, and the data can be averaged.  The computer
software can also be used to determine optimum times for taking readings,
by calculating when various satellites will be visible from your
location and when the angle of various birds is best for reading.  A flat
angle between two birds and your unit will create a larger overlap area on
the ground.

  Trees of course confuse the unit.  Her experience was that so did
people.  In one reading taken in a school yard with a number of children
standing around, the dispersal pattern splattered all over the graph.

  What impressed me most was the use of computers to crunch the numbers
and provide a more exact idea of all the various correlations than ever
possible by just reading the GPS display.

Get a GPS, Mike, and we can correlate. I should probably get to the
bookstore and find _GPS for Dummies_ - the GPS-40 Owner's Manual,
like most, leaves a lot to be desired, and cites no other sources.

Ms. Hose had 2 or 3 units, with computer..on loan from a mfg. or
distributor.  Without a computer to massage all that nice data being
transmitted by the various satellites, one is really missing the boat. I'm
surprised the manual doesn't mke  that clear.  i];)
     

The Manual doesn't make a lot of things clear. I think I better get to the
library, and look for better explanations of theory and practice. Sounds
like I should contact Ms. Hose, too. Say, isn't anyone else here on Grex
into GPS? 

Gee, Rane, the closest I come to needing GPS is for walks in the woods.  I
find 7.5' Topographic maps work fine.

(A nice map store is at the corner of 12-mile and mound; they have every
Michigan topo map in stock, for $4 each though.)
 

The GPS lets you find yourself *on* the topo map. After bashing through
the north woods for a few miles of cedar swamp (no relief), its nice to be
able to locate yourself on the map (especially if you are looking for a
particular lake, or other feature). It would also be useful when driving
the forest two-tracks. I'll be using it as part of a geographic-feature
inventory for one of the National Forests in MIchigan.

Border's Books also has an assortment of Michigan topos, though they don't
seem to maintain it too well. 

Both my father in law and my step father in law are avid soaring pilots.
When flying contests, in days slipping past, they use to photograph
designated landmarks during the competition.  Once complete, the sealed
camers are opened and processed on site before the winner is announced.
Even recording barographs are used to insure no one landed and got another
tow during the flight.
GPS systems are starting to take over those functions now.
I'll get one when they get to around $100.

re 7: Cedar swamps, eh?  Counting deer in winter bedding areas?

 There is also a store called Geography Inc, located on Winewood St west
of staduim dr. Thousands of maps--- one could get lost ;)

Re #9: no, I'd be reconnoitering holes in the ground. I stay out of the
woods during deer season! 

I took a turbogopher trip and did a veronica on gps, and found all sorts
of dregs of old gps exchanges. There is a newsgroup on the topic.  There
is obviously a lot to this, with a lot of professional use of DGPS hooked
to computers. So, here I am, a newbie again! 

Thanks for the tidbit, omni: I'll navigate there ASAP. Can you give me its
Lat/Long? 

Winewood off Stadium?  I know Winewood off Washtenaw but not Stadium.
I'd like to check this place out too :-)

 It is a *very* short street and this is why you may not have seen it before.

I don't have the lat/long, sorry rane ;)

Winewood is 1 block long, from Stadium to Maple.  It's approximately across
Stadium from McDonald's, with a gas station on the south corner and Stadium
Opticians (I think still) on the north corner.  At the Maple end, it's across
from what was until recently a large furniture store.  It's the first street
going south (on that side of Stadium, & on Maple) from where Stadium &
Maple split.  Cottage Inn's parking lot exits on Winewood.

(I used to work on Winewood, but haven't been there for several years.)

I have a gut feeling that map store closed last winter or this spring.

which store?  The one in Warren (or Centerline, or whatever) was open a few
weeks ago.

I telephoned Geography on Winewood. They are open 12-6, Tuesday to
Saturday. They carry maps. They don't have anything about GPS (didn't
know the acronym). They were friendly, but didn't seem to know how
to answer my question about what they carried in addition to topos 
(MI only, by the way)...maps, they said.

I went to Geography Ltd, on Winewood, today. The place is a jumble. There
may have been some order at one time, but now every bookshelf is
overflowing, not very much is arranged by topics, there are boxes full of
maps and books everywhere, piled up, sagging and bulging, and it is hard
to thread your way into the several small rooms. Most of the books are
relatively new, and they are almost all priced at more than the published
prices. They are having a 50% off sale, however, so you might get good
prices on some travel/geography books. I was going to suggest that they
carry navigation and GPS books too, but after seeing the place, I didn't
bother to suggest anything. 

I have found the GPS "Mother Lode". It is the CANSPACE anonymous ftp
archive at unbmvs1.csd.unb.ca (in directory pub.canspace). The following
is the $README file from the archive: (warning! long). 

                        *********************

This directory is intended as an archive for discussions on the CANSPACE
list.  Created for Prof. Richard Langley on March 4, 1991. 
 
                          CANADIAN SPACE GEODESY FORUM
 
The Canadian Space Geodesy Forum (CANSPACE) is a computer-communications-
based information dissemination and discussion group that was established
several years ago as an aid to improving communication among scientists
and engineers working with the techniques of space geodesy.  Messages
consisting of news, comments, questions, and answers are exchanged among
forum participants by means of a LISTSERV list.  LISTSERV is an electronic
mail utility running on a BITNET computer that allows a message to be
"exploded" (sent simultaneously) to a list of individuals simply by
sending the message to the list address.  The address of the Canadian
Space Geodesy Forum is canspace@unb.ca (or canspace@unbvm1.bitnet). 
 
Any topic related to the space geodetic techniques (Navstar Global
Positioning System (GPS), Glonass, Transit, very long baseline
interferometry, satellite laser ranging, satellite altimetry, etc.) may be
discussed.  Questions are particularly encouraged.  Daily postings of
information bulletins include GPS satellite constellation status reports
(Department of Defense Notice Advisories to Navstar Users) and reports of
solar and geomagnetic field activity. Information concerning satellite
launches and orbital elements is posted regularly.  Although initially
intended to link Canadian geodesists and geophysicists together, CANSPACE
now has a wide international subscriber list and is open to all. 
 
CANSPACE is directly accessible through computers on the Internet/BITNET/
NetNorth/EARN networks.  CANSPACE is also accessible from other networks
such as CSNET, UUCP, SPAN, etc. through various gateways.  Individuals may
also communicate through commercial telecommunication and information
services such as MCI Mail, CompuServe, and the American Institute of
Physics' PINET, or through the various bulletin board systems on the
FidoNet network. 
 
How to Subscribe
----------------
To subscribe to CANSPACE it is necessary for you to send an e-mail message
to listserv@unb.ca (or listserv@unbvm1.bitnet but NOT to the CANSPACE
address itself) with the following text in the message body: 
 
SUB CANSPACE your name
 
Once you are a subscriber, you may post a message to CANSPACE simply by
sending the message to canspace@unb.ca.  The message will be automatically
distributed. 
 
Setting the Topics Option
------------------------- 
CANSPACE makes use of the LISTSERV "topics" option.  The subject headers
of postings optionally contain a three-letter designator.  These
designators (identifiying the organization responsible for the information
in the posting) are "topics" identifiers that allow you to specify which
kinds of CANSPACE postings you would like to receive.  You do not have to
receive all postings from CANSPACE -- you can select which groups you
would like to receive. 
 
At the moment, there are four topics:
 
DOD:   for the GPS Constellation Daily Status Report from USNO (DoD NANUs)
GSC:   for the Daily Geomagnetic Forecast and Review from GSC
STD:   for the postings from the Solar Terrestrial Dispatch
OTHER: for everything else that is posted to the CANSPACE list.
 
Initially, CANSPACE subscribers are automatically registered for all four
topics.  If you wish to change the topics to which you are subscribed, you
will have to send an e-mail message to LISTSERV@unb.ca (DO NOT SEND IT TO
CANSPACE!). The form of the message is as follows: 
 
SET CANSPACE TOPICS: xxxxx
 
xxxxx may be the list of all topics you wish to receive or one or more
topics preceded by a plus sign (+) to add a topic to or a negative sign
(-) to drop a topic from your current list.  For example, to drop the
GSC's Daily Geomagnetic Forecast and Review and keep the other topics,
just send the following: 
 
SET CANSPACE TOPICS -GSC
 
Signing Off from CANSPACE
-------------------------
To remove yourself from the list, send the following message to
listserv@unb.ca: 
 
SIGNOFF CANSPACE
 
Further Information
-------------------

All CANSPACE messages are archived in monthly notebooks that may be
retrieved or searched using the LISTSERV database function.  An archive of
related information files is also maintained on a computer at UNB (not the
computer running LISTSERV).  Among the many files included are a brief
description of GPS, tables showing the present status of the GPS and
Glonass satellite constellations, and a 1,000 item GPS bibliography. 
Files may be retrieved via anonymous FTP.  Connect to unbmvs1.csd.unb.ca
(131.202.16.2) and change the working directory to PUB.CANSPACE.  E-mail
to FTP gateways are available for those who are not able to do
computer-to-computer file transfers directly.  The files are also
available via the Gopher server on unbmvs1.csd.unb.ca.  See the file
ARCHIVES.GOPHER.ACCESS for further information. 
 
CANSPACE is maintained at the University of New Brunswick as a free
service by Richard Langley and Terry Arsenault.  They can be reached
directly by e-mail addressed to se@unb.ca or by phone at (506) 453-4698 or
by fax at (506) 453-4943. 
 
If you have any difficulties in accessing CANSPACE or want more
information on the use of LISTSERV facilities, consult your local computer
communications resource person or contact the Department of Geodesy and
Geomatics Engineering at UNB. 
 
Richard B. Langley (lang@unb.ca)
Geodetic Research Laboratory
Dept. of Geodesy and Geomatics Engineering
University of New Brunswick
Fredericton, N.B.
Canada  E3B 5A3
                        **********************

The file GPS.INFO.SOURCES is readable in /u/rcurl/GPS.INFO.SOURCES
This lists current books, periodicals, special publications, 
internet sources, etc, with GPS information.

Per Rane's request, this is now linked between micros and travel.

Hello, travellers! Do you want to know where you are? Which way to
go? Where you've been? Install GPS on your boat, car, bike, skiis -
and never be lost again (this message for new readers in  travel  ).

I finally took GPS on the road (with my wife driving, I could run the
Garmin-40 through its tricks). Just held out under the windshield it
quickly acquired 6 satellites, and reported ground speed and direction
with a lag of ca. 4 seconds. The speed was reported to 0.1 mph, much more
accurate than the speedometer. I had set a waypoint at where am staying,
and "navigated" home with the GPS. It continually reported which way to
head, the (beeline) distance, and the ETE (estimated time enroute). Of
course, in a car, one can't just head cross country (!), but even though
we followed the roads, the ete counted down so well that the last second
was counted as we turned into the driveway (it also showed that turn). I
can see why it is taking over boating and flying navigation. I understand
some cities have equipped all their buses with GPS, and radio links, so
the dispatcher knows where every bus is and its direction and speed,
essentially instantaneously. 

To quote an ATT commercial "That's cheating!".  Heh.  One of the things
that I quickly found while doing my cross country flights for a private
pilot's license was that those navigational gizmos are really really
cool, and accurate, but they tend to quit on you just when you need
them.  I think I'd still keep a map in my lap even with GPS, in other
words.  Still, it's interesting to see GPS working its way toward
affordability and use by the average person.

So when can we expect gps sytems to be offered as manufacturer's options in
mainstream production cars in the US?

rcurl - that's really neat! And reinforcing what kentn said, i'll
still keep a map and a compass around. Magnetic poles seem to
have a greater reliability although less precision.

I tried the GPS at the airplane window, but it acquired only one
(ideally located) satellite: doesn't work well in what amounts to
a Faraday cage. I was hoping to measure the groundspeed. Now, if
the plane had had a Vista Dome...

The warnings about not depending *solely* upon GPS others have given
above are well taken, and often repeated in the manual. Electronics
fail and batteries run down. So...get two ;->.

  
        customer: Hello, i'd like to order an extra please.
        sales: an extra what?
        cust: well, one of those gadgets i guess, gps?
        sales: we don't have jeeps, jeeps are vehicles
        cust: this is for a vehicle
        sales:for a vehicle? or is a vehicle.
        cust: gps is a locator
        sales: you can't find your car?
        cust: no,no, i can find it alright, but i want to
              know where i am.
        sales: you're standing right there, i think.
        cust: well, of course i am, where else would I be?
        sales: you wanted to be located, in a jeep.
        cust: no - i wanted the jeep to be located
        sales: you just said you knew where it was ...
        cust: yes, i +do+ know where it is, the gps is
               supposed to locate me on the planet
        sales: would you like a map and a compass perhaps?
        cust: no, gps is better, i want another gps, an extra one.
        sales: yeh - i'd like an extra jeep too.
        cust: i want to by an extra locator, is that ok with you?
        sales: oh, sure, the maps and compasses are over here
        cust: maps and compass's aren't accurate enough, the gps is better
        sales: not accurate enough? we have the best maps in the world.
        cust: i'm sure you do, but the gps uses satellite stuff, not 
              the north and south poles.        
        sales: those are the only poles i know, north and south.
        cust: right, me too, but that's not accurate enough
        sales: it's not? just how accurate do you want, anyway?
        cust: pinpoint accuracy
        sales: i think you need a seamstress, they use pinpoints.
        cust: i think you need a shrink
        sales: they won't shrink, these maps are waterproof!
        cust: yeh - i'm sure you are right, they won't shrink but
              they need a compass, and a compass uses the north
              and south poles but that's not very accurate.
        sales: the compass won't shrink either. it's all sealed
                up with a hermit, see, it says hermit-likely sealed right
                there.
        cust: uhhh, ok, sure, i see that. you won't let the
               hermit out will you.
        sales: oh, no, can't do that. there would be no one inside
               to work the compass then. the hermit has to stay sealed.
        cust: right ---- uh, look, i just want to buy another one
               one of these gadgets. 
        sales: what is that gadget?
        cust: it's a gps.
        sales: are you alright mister?  that doesn't even have a
                steering wheel.
        cust: it doesn't use a steering wheel, it doesn't need one.
               it uses satellites.
        sales: were are the satellites, i don't see them
        cust: satellites are way high up in the air, you can't see them.
        sales: but you can? wow, you must have great eyes.
        cust: no, not really, they're just so-so, but the gps uses
                the satellites to see where you are.
        sales: at night too?
        cust: yes, all the time, night included.
        sales: to see at night your jeep needs headlights, they
                are over here, brightest ever made, see right
                were you are, just like daytime, want a couple?
        cust: no, no, no, i don't want any headlights, i already
                have two.
        sales: you only have one in your hand and you said you
                wanted an extra, that makes two in my book.
        cust: maybe your book is missing a couple of pages. 
        sales: what?
        cust: nothing, nothing . . . that's not what i meant.
        sales: well, what did you mean then? if you have one
                and you want an extra, isn't that two?
        cust: yes, it's two - but not headlights.
        sales: every jeep i know about has two headlights. what's the
                matter with yours?
        cust: nothing's the matter with mine.
        sales: that's good, i was worried there for a minute. are
                you alright?
        cust: yes, i'm ok, you're ok, we're all ok, i just want
                to buy another one of these, for a spare.
        sales: you said that before, what is is?
        cust: it's a gps, i said that before too.
        sales: yeh, and not only is it missing a steering wheel ...
                it's missing tires too, including the spare.
        cust: i don't want a spare, i want an extra.
        sales: that's what a spare is, it's an extra one, and you
                just said you wanted an extra, that's a spare.
        cust: i think i'm getting tired.
        sales: me too, this is work.
        cust: yeh - something like that  - look i'll just come back
                some other time.
        sales: sure, and get some rest too, it helps when you need
                to figure out what you want. 
        cust: right, ok, bye now.
-----curtain down----

ROTFL!
Actually, it went a little like that when I called Geography Inc, and
asked if they had any books or other literature on jeeps.

I've been trying to obtain information on the accessories for external
power, computer interfacing, and external antennas, for the Garmin-40.
I telephone the company, but got voice mail, and left my inquiry there.
Not hearing anything for a couple of weeks, I faxed my inquiry - heavy
on asking for technical information on the computer interface type and
protocol (see also the connectors item #81 in the hardware cf). Both
targets have now sent the identical package of advert sheets and price
lists for accessories, but NO technical information. In addition, the
computer interface accessory and software is only for PC compatibles.
I want it for a Mac. I haven't found any user sources of interface
information, and no shareware (PC or Mac) for it yet. The search goes on.

Almost more than you ever thought you would like to know about GPS
is now available on

http://wwwhost.cc.utexas.edu/ftp/pub/grg/gcraft/notes/gps/gps.html

butit's interesting to know that the military crippled the system
for non-military users.

"Crippled" is too strong. 100 meter 95% Confidence Region isn't bad for
many uses. You can get 15 meters with Differential signals, which are
available widely. There is a beginning "movement" in the GPS community
to have SA (selective availability - which add the error) turned off,
so everyone would get 15 m 95% C.R. 

I suspect the military will object strenously. 15 meters is about
the burst range for an artillery shell, 100 meters just scares
you a bit. No use providing "the enemy" with that 15 meters of
accuracy stuff when "we" can keep them 100 meters away .....

Anyone with enough know-how to build a GPS guided gizmo should be
able to correct for the dither too.  I also think others are putting
up birds sans the drunken output.

More Travels with GPS:

I took the Garmin 40 (G40) skiing at Winter Park, CO, and marked waypoints
at the Sunspot (Elev. 11,000 feet) and Berthoud Pass (11,315). Did not use
it for navigation on the slopes, however - accuracy is not sufficient to
mark trees. There is a very noticeable lag in the G40 calculating the
correct elevation. It appears to use some previous calculated elevation to
simplify the initial calculation of Lat/Long, and then calculates the
elevation based on that, alternating the calculations. This causes a
relative slow (10-15 minutes) convergence to the correct elevation (+/-
SA).

Next trip with G40 was west by car on VA/WV route 9, crossing the
Appalachian Trail. Somewhat to my surprise it acquired and held lock on
the requisite 4 satellites, while traveling on a winding mountain road
through a forest. I expected the shielding of mountain side and trees to
cause it to lose locks. 

I've built a cable and junction box to interface the G40 and my Powerbook.
It also lets the G40 run from the power adapter for the computer (or
another 5-8 volt supply, if not used with the computer), using MacGPS45, a
freeware program for displaying and up and down loading data. (Hmm..the
software won't let the G40 use the printer port, so I can't demonstrate
the data download on line.)

At the moment I am at 42d 14.955m N, and 83d 43.161m W; Elev. reads 645
+/- 492 feet (right!). I am again surprised that it has acquired 4
satellites, as the G40 is sitting on my bookshelf above my desk. SA is
dithering my location, so I only know where I am within ca. 100 feet. Do
you know where you are as well? G40 says distance and bearing to Winter
Park CO is 1160 miles at 276d (and sunset there was at 6:34:47). Sigh. 

Sounds as though you have made significant headway Rane!  Have you come
up with a reasonable external antenna system yet?  (DC Electronics now
has MAR-6 [11 dB/Typ., 2GHz] amplifiers in their catalog.  $2.50 ea  ;-)

That's the next challenge! I think an microwave antenna guru is needed,
though.

GPS users are  integrating map datbases and GPS positioning for both
business and pleasure. The following brief account came from the
newsgroup sci.geo.satellite-nav:

"I've had superb results from Delorme's MAPEXPERT (CRDOM... enhanced
version of their Street Atlas USA) + their GPS Link II software. Last
October I drove from Alexandria, VA to San Francisco with my GPS 45
strapped to the steering column of my p/u w/external antenna, connected
to my Toshiba T-2400CS (connected to my NEC 3Xp CD drive) and 
"breadcrumbed" my way across the U.S. I used road changes and 
approximate coords (Lat/Long) of my planned enroute stops as waypoints.
The whole trip I never plotted more than a few meters (less than 10)
off the road on the moving plot. Moreover, I found that as I neared a
destination, I could use the 45's course (steering) display to take
me to my stopover motel without referring to a map or any other
display."       (jsart@ix.netcom.com)

- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss