Garmin GPS 45/40/38 Frequently Asked Questions
==============================================
V 1.07 April 23, 1996
Compiled by Lane A. Holdcroft
Garmin 45/40/38 FAQ Copyright 1995, 1996 by Lane A. Holdcroft.
Original contributors retain their copyright for all of their contributions.
You may copy this in its entirety, but not for monetary benefit.
If a portion of this document is quoted, please give the original author
credit.
After many, many delays, here is the new version of the Garmin FAQ which now
includes the new GPS 38 and the GPS 120. I am still reviewing the many
suggestions and postings that I have received for inclusion in future
versions.
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This document is not in any way endorsed by Garmin International.
I am not affiliated with Garmin or any of their retailers.
This FAQ is targeted primarily at the GPS 45, GPS 40, and GPS 38.
The GPS 90 is very similar to the GPS 45 but has different software and much
more non-volatile memory for its aero navigation database.
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The GPS 38 (released late Feb 1996), is very similar to the GPS 40. However,
it has improved software, no external antenna connector, no beeper, no video
tape, and a lower price. See the new section "GPS 38 reports".
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The GPS 120 (released late 1995) has almost identical software to the GPS 45
but a mobile mount, external antenna and a much larger display .
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There are many other Garmin instruments that have many internal similarities
(and some software compatibility) to the GPS 45/40/38 such as the GPS 50, 65,
and 75. There is also a table at the end of the FAQ with most Garmin
products.
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TABLE OF CONTENTS:
==================
0.00
ABOUT THIS FAQ:
-General FAQ info?
-Attribution?
-FAQ revision history?
-Future features?
-Suggestions for improvement?
1.00
GENERAL FEATURES:
-What are some key features of the Garmin GPS 45/40/38?
-What are the disadvantages of the GPS 45/40/38?
-What are the differences between the GPS 45 and GPS 40?
-What is the accuracy of the GPS 45/40/38/90?
-What is the Garmin GBR 21 Differential Beacon Receiver?
-Are there any other differential receivers available? |
-How can I improve the accuracy of a GPS unit w/ averaging? |
-How Accurate is the clock on the GPS 45/40/38/90? |
-What is the MOB function? |
-What is a waypoint? |
2.00
SPECIFICATIONS/REPORTS:
-GPS 45/40 specifications?
-GPS 45/40 reports/description?
-GPS 90 press release/specs?
-GPS 30/31 description?
-GPS 38 reports? |
-GPS 120 description? |
3.00
FIRMWARE (GPS 45/40/38 INTERNAL SOFTWARE):
-90 knot speed limit?
-GPS 45/40 firmware versions?
-What are the undocumented modes?
-Why is internal temperature available?
4.00
BATTERY/POWER:
-How is power selected between the internal batteries and the external supply?
-What voltages do the fuel gauge correspond to?
-Current consumption of a GPS 45 at different external supply input voltages?
-How does BatterySave mode work?
-Rechargable batteries?
-Other battery/external power options?
-How long is memory conserved without batteries?
-Does the serial I/O consume much more power when it is enabled?
5.00
SERIAL/POWER INTERFACE:
-External serial/power connector alternatives?
-How do I interface the GPS 45/40/38 to an IBM-PC compatible computer?
-How do I interface the GPS 45/40/38 to a Macintosh?
6.00
APPLICATION SOFTWARE:
-What Macintosh software is available for Garmin GPS units?
-What MS-DOS/Win software is available specifically for Garmin GPS units?
-What MS-DOS/Win software is available that communicates using NMEA 183 format?
-Other MS-DOS/Win software? |
-GPS ftp/www sites?
-CD-ROM maps with lat/lon?
7.00
ANTENNAS:
-What is the GPS 40/38 internal antenna?
-How is the GPS 45/90 antenna internally constructed?
-How well does the internal antenna work inside a car?
-What are the remote antenna options?
-GPS 45/90 antenna extension cables?
-Can I make my own external antenna?
-What is the loss for different coaxial cables?
-What is the GPS 40 antenna connector?
8.00
MISCELLANEOUS:
-GPS mounting options?
-Waterproofness?
-Cases?
-Garmin 45/40/38 wish list?
-Can I use my GPS unit in a plane?
-Performance in trees?
-Internal electrical hardware?
-Why is the firmware not in a socket?
-Can I receive GPS signals through my house?
-For how long will a particular track interval interval collect data?
9.00
GARMIN PRODUCTS:
-Garmin GPS models?
-Garmin accessories?
-Where can I purchase a unit?
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QUESTIONS AND ANSWERS:
======================
0.00
ABOUT THIS FAQ:
---------------
-General FAQ info?
The Usenet newsgroup "sci.geo.satellite-nav" has a high proportion of
questions dealing with the very popular Garmin GPS 45, GPS 40, and GPS 38.
I hope this will answer most of them.
Because of the large number of questions that are still posted to the
newsgroup that have been answered in this FAQ, I usually post this every
couple of weeks to the newsgroup "sci.geo.satellite-nav".
The newsgroup was originally intended to be oriented more towards
professionals in the GPS business rather than recreational users that are
more inclined to use the GPS 45/40/38 because of their low prices. However,
there is a lot of discussion about Garmin units.
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This FAQ is based on my experience with my GPS 45 and others' postings
to the GPS newsgroup since the GPS 45 was first introduced about March 1994.
I have a particular interest in the electrical aspects of these units.
The latest version of this FAQ and some links to other sites can be found
at my www or ftp sites (or Peter's site in the next section):
http://www.eskimo.com/~klah/index.html (preferred)
or ftp to "ftp.eskimo.com", cd to "/u/k/klah"
or ftp://ftp.eskimo.com/u/k/klah/index.html
For more general information not specific to the Garmin units, read the new
general GPS FAQ maintained by Peter Bennett. It is available on his
excellent ftp site. I also send the latest version of this Garmin 45/40/38
FAQ there.
ftp://sundae.triumf.ca/pub/peter/index.html
or ftp to "sundae.triumf.ca", cd to "/pub/peter"
A fair amount of the information posted about the Garmin units in the GPS
newsgroup is wrong. I have spent a lot of time trying to make this document
error free, but there is no guarantee to the accuracy of this information!
Minor errors have been found in the past.
When the term "GPS receiver" or "GPS unit" is used in this FAQ, this is meant
to refer to a handheld unit that uses the Global Positioning System.
Technically, the "receiver" is the radio portion of the handheld device only
and the term "GPS" refers to the complete navigation system and not the unit
itself.
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-Attribution?
Thanks to the many people who have contributed to this document! I tried to
give credit to the original author at the end of each contribution. If an
answer does not have an attribution, I usually answered it based on my
recollection of the consensus of previous postings and/or my personal
experience or the contributor did not want credit. If I felt clarification
within another person's quoted material would be beneficial, my comments
are contained within the brackets "[" and "]".
-FAQ revision history?
Ver Date Major changes
--- ---- -------------
1.00 Aug 4, 1995 Initial release
1.01 Aug 7, 1995 Minor corrections
1.02 Aug 20, 1995 Added detailed speed limit info
Added GPS 90 info
Added more detailed list of specs
Added Garmin accessory list
1.03 Aug 22, 1995 Minor corrections
1.04 Aug 29, 1995 Added Garmin 45/40 wish list
Added section head numbers
Added GPS 45 external current consumption
Added FAQ revision history
1.05 Sep 18, 1995 Added info about "Diag mode"
Added info about Accuracy
Added info about Garmin differential receivers
Added "|" indicating major changes in line or paragraph
Reordered several sections
Split out application software into its own heading
Many minor corrections
1.06 Oct 10, 1995 Added section on coax cable losses
Added section on CD-ROM map programs
Added section on houses blocking signals
Added GPS 40 mode from a GPS 45
Added GPS 30 info
Expanded ftp/www sites considerably
Expanded Garmin DGPS info
Split application software to Garmin & NMEA 183
Split out intro into a separate section
Minor corrections
1.07 April 23, 1996 Added sections on GPS 38 & 120
Added info on BatterySave mode
Added info on memory backup
Added Eagle Accunav info (8.00, Performance in trees)
Added other PC software section
Added more info on Macintosh software
Added improving the accuracy section
Expanded Garmin GPS models section
Reduced the Garmin software price
Updated Garmin's address and phone numbers
Updated several E-mail addresses
PLEASE DESTROY ALL PREVIOUS VERSIONS, ESPECIALLY BEFORE VERSION V1.06
because they had at least several errors.
The "|" character is used to mark paragraphs or a line for significant
changes in content from the previous version of the FAQ.
-Future features?
Simple explanation of GPS, time, S/A, and differential receivers and how
accuracy is affected.
-Suggestions for improvement?
You can help improve this document by sending E-mail to me about errors or
suggestions for improvement. It would be very useful if you could give your
sources of information. Also, please include the version of the FAQ and the
firmware version of your GPS 45/40/38. There is often conflicting information
and your detailed references are valuable for resolving those discrepancies.
I receive lots of feedback but it takes time to process and verify
all of the information. Because of the large volume of E-mail I receive,
I can not always acknowledge all mail. I also can not answer individual
questions not pertaining to the FAQ. However, I do read all feedback and
do take note of it. I read the newsgroup very regularly but occasionally
I do miss something Garmin related.
Please send CONSTRUCTIVE suggestions to Lane A. Holdcroft
(klah@eskimo.com)
1.00
GENERAL FEATURES:
-----------------
-What are some key features of the Garmin GPS 45/40/38?
Garmin GPS 45/40/38 specific:
Relatively inexpensive (list < $500 US, discounted < $300 US).
Very feature rich.
Long (10 hour) battery life w/ 4 AA alkaline batteries.
Very compact size.
Serial port allowing transfer of data to/from a PC/Mac.
Neat 768 point track log feature which can be graphically displayed.
Ability to download the above track log data to a PC/Mac.
Easy to use due to a simple menu system.
Water resistant.
64 x 100 pixel graphical display.
Over 100 mapping datums used around the world.
-What are the disadvantages of the GPS 45/40/38?
1) There is a firmware speed limit of 90 knots / 104 MPH. See "90 knot speed
limit" question elsewhere in the FAQ for more details. The GPS 90 does
NOT have this limitation.
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2) The list prices on some options are more expensive than some competitors.
For example, a replacement 45 antenna is $100 list and external power
and/or data cables are between about $25 and $50. However, some dealers
offer substantial discounts.
3) Because of the GPS 45/40/38/90's compact size, there is not sufficient space
for number buttons. It has a 4 position rocking button to enter numbers
and names. This is not as fast as using a keypad. The Garmin GPS 75 is
somewhat larger and does have a full numeric keypad.
4) The display size is very small. This is a very compact GPS unit.
The character size is also very small on the GPS 45/40/90. However the
the new GPS 38 uses a wider font to be more readable. If you have good
corrected vision, it should not be a problem up close. If not, then the
GPS 45/40 may not be a good choice for you.
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5) The GPS 45/40/38 (like all other sequential receiver GPS units) is not
able to track as well under heavy tree cover as parallel receiver designs.
See "Performance in trees" section for details.
-What are the differences between the GPS 45 and GPS 40?
1) The GPS 45 has an external quad-helix swivel antenna (about 3/4" x 3/4"
x 3") whereas the 40/38 has compact "patch" antenna built into the main
case. A GPS 40/38 looks like a GPS 45 without the swivel antenna.
The GPS 45 uses a standard BNC connector for the antenna, whereas the GPS
40 has a "MCX" type connector for the optional external antenna.
The GPS 45's external antenna hangs out the side of the instrument and
could be broken if it were snagged or handled roughly ($100 replacement
cost!). This is not a problem with the GPS 40/38.
The GPS 45's BNC connector is preferable if you want to add an extension
cable since BNC extensions are available from many more sources. This is
an advantage because you can avoid the cost of an additional antenna if
you want to temporarily mount it remotely from the GPS unit.
The GPS 45's BNC connectors are more subject to corrosion from salt water
and the 45's antenna could leak if it was not well sealed.
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The GPS 45 can be held at different viewing angles as long as the
antenna is swivelled vertically whereas the GPS 40/38 must be held at a
specific angle for optimum reception.
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There is a lot of debate about the exact differences in antenna
sensitivity, however the GPS 45 is apparently slightly more sensitive for
satellites that are directly overhead.
2) The GPS 45 takes a much wider range of input voltages (up to 40 Volts).
This is because the 45 has an internal switching power supply whereas
the GPS 40/38 do not and the GPS 40/38 are only rated for 5 to 8 Volts for
external power. Connecting the GPS 40/38 directly to a higher voltage
supply would be potentially damaging.
Therefore, the mobile power cigarette adapter is $20 more expensive for
the GPS 40/38 (because it must include a regulator). Also, the GPS 40/38
has an additional center pin in the center of the power/data connector to
prevent the use of the GPS 45 cigarette lighter cord which would probably
damage the GPS 40/38 due to overvoltage.
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3) The GPS 40 includes a video tape on its operation.
4) The GPS 40/38 has a dark gray case, whereas the GPS 45 has a white front
case and a black rear case.
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5) The GPS 40 is about $20-$40 US cheaper.
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-What is the accuracy of the GPS 45/40/38/90?
Note that the error tolerance displayed on the GPS does NOT include errors
due to SA. |
Note that all position accuracy specs for the GPS are based on 95% DDRMS
and assume the best satellite coverage possible.
Horizontal accuracy:
100 meters Official accuracy (Selective Availability ON)
~45 meters Actual accuracy (Selective Availability ON) |
15-20 meters Theoretical accuracy (Selective Availability OFF) |
8-10 meters Differential accuracy (regardless of Selective Availability)
Vertical accuracy (altitude):
Approximately 1.5 times worse than horizontal accuracy depending on the
geometry of acquired satellites. In fact, you can get better vertical
accuracy with an altimeter (but it must be regularly adjusted to compensate
for changes in barometric pressure). The errors in altitude due to SA are
especially obvious when you are on the ocean!
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-What is the Garmin GBR 21 Differential Beacon Receiver?
The US and Canadian governments are setting up Differential GPS (DGPS)
stations to transmit correction signals. They are usually broadcast on
existing marine beacon frequencies around 300 KHz. They are already
operational in many areas along the coasts.
In addition to compensating for errors in the postion, these correction
signals also undo the deliberate degradation of the accuracy by the U.S.
Department of Defense.
Look at the US Coast Guard's web site (under "ftp/www sites" in this FAQ)
for locations and status of their differential stations.
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When connected to your Garmin GPS, the GBR 21 will increase its accuracy to
under 10 meters when within receiving range of Differential Beacon stations
on the coasts. This also makes your speed indications accurate to about 1/10
knot, which is great for sailboat racers. Works with DGPS-ready GPS 40, 45,
50, 65, 75, and GPSMAP 210/220 models. Coupler measures 1.6"D x 4.8"H and
requires a 4' whip antenna. 30' RG-58 coax included. Receiver box measures
5.2"x3.6"x1.5". Draws 130 mA. One year parts and labor includes water damage.
List $799
[West Marine Catalog]
-Are there any other differential receivers available? |
Some other manufacturers' differential units that pick up the Coast Guard
broadcasts can be used with a Garmin GPS unit but they can not automatically
tune the frequency of the differential receiver as Garmin's can. I don't
know how much of an advantage this is. There are probably some other
differences also. I would appreciate a detailed review by an user of this
option.
There are services that transmit correction signals on FM broadcast band
subcarriers but you must buy their differential receivers and pay a monthly
charge.
There are also services that transmit correction signals via satellite and
again you must buy their differential receiver and pay a monthly charge.
-How can I improve the accuracy of a GPS unit w/ averaging? |
Because the Garmin 45/40/38 units do not save the individual satellites used
and their timing information for each fix, you can not postprocess the data.
Another problem is that the displayed information is averaged rather than
being real-time.
However, you can average your position. Although the official accuracy of
the GPS system for non-military users is 100 meters 95% of the time, in fact
when you look at the data plots (such as available from MIT elsewhere in the
FAQ), you can see that the accuracy is more like 45 meters.
This can be improved much further by averaging. You can turn on the track log
feature, and leave the GPS on for a long time, and then look at the map screen
and then just visually average the postion. Of course this can be done more
precisely by downloading the data and averaging it on a computer.
It has been suggested by several people that if you want maximum accuracy,
you could take 1 reading every 15 minutes. This is believed to be the minimum
amount of time to get nearly independent readings without the affects of SA.
Of course to get a reasonable number of readings you should collect data for
many, many hours and you would need an external power supply so that you
don't wipe out your batteries before your data collection is done.
The concept of averaging is based on the assumption that the errors
(especially due to SA) are centered about zero. In fact the specification
for SA does not require zero offset, but this is usually the case. It is
felt by some in the newsgroup that that the maximum accuracy with extensive
averaging is about +/- 10 meters.
-How Accurate is the clock on the GPS 45/40/38?
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First of all, there is a clock IC in the unit to keep the approximate time.
To get accurate time, you must be locked on several satellites.
Internally the GPS unit knows the time more accurately than 1 uS,
however the display and serial port are lower priority tasks and will not
be updated immediately. We don't know the maximum error but expect that
it should be less than several seconds. When I have crudely compared the
displayed time to the international time standards (received by shortwave
radio), the difference has usually been well under 1 second.
To get extremely accurate time, there are other GPS's that have a precise
timing output.
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-What is the MOB function? |
The Man Over Board function is a common feature for marine GPS units. If a
person were to fall overboard, the skipper uses this function to immediately
mark the current location and the GPS goes into a "Go To" mode where it
directs the skipper back to the point where the button was pressed. It is
not trivial to find a person at sea with strong winds and large waves.
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-What is a waypoint? |
A waypoint is a nautical term referring to a label used to mark a specific
location. For a GPS unit, a waypoint can be any location you want to mark
and label in memory for future reference.
2.00
SPECIFICATIONS/REPORTS:
-----------------------
-GPS 45/40 specifications?
Here are the specs out of the manual:
Physical
Case: Waterproof, dry nitrogen-filled
Size: 6.15"H x 2"W x 1.23"D
(15.6 x 5.1 x [3.12] cm) |
Weight: 10 ounces [0.284 kg] w/ batteries.
Temp Range: 5 to 158 degrees F [-15 to +70 degrees C]
Performance
Receiver: Differential-ready MultiTrac8
Acquisition Time: Approx 20 seconds (warm)
Approx 2 minutes (cold)
Approx 7.5 minutes (autolocate)
Update rate: 1/sec, continuous
Position Accuracy 5-10 meters (16-33 ft) with DGPS correction
15 meters (49 ft) RMS*
*subject to accuracy degradation to 100m [95% of the time]
under the DOD imposed Selective Availability Program
(note: I average 150')
Velocity Accuracy 0.1 knot RMS steady state
[w/o SA, several knots error w/ SA]
Dynamics: Performs to specs to 3g's
Power
Input: 4 AA batteries or 5-40 vDC
Usage: .75 watts
Battery life: 10 hours (normal mode)
w/alk. batt. Up to 20 hours in battery saver mode
Kai Schumann Schumann_Kai@Lilly.com
----------------------------------------------------------------------
-GPS 45/40 reports/description?
Here are 2 good reviews that were posted to the newsgroup. One is from a
45 owner, the other from a 40 owner. Keep in mind that other than the
antenna and input voltage range, both units are IDENTICAL and most comments
apply to both. I ha