Shareware Overload Trio 2/Shareware Overload Trio Volume 2 (Chestnut CD-ROM).ISO/dir24/aprs308.zip/README.GPS

Text File | 1993-12-04 | 35KB | 531 lines

                    GPS or LORAN INTERFACED TO APRS

    This file has evolved radically as this APRS project has developed.  We
first began doing Packet Radio tracking of GPS units when the Magellan GPS
circuit card became available for $445 in September 92 (down from over $1000).
Later the Motorola GPS card came down to the same price range, and these two
devices were the only ones that we could find that were cheap AND which had
USER programmability so that they could be set up to operate stand-alone with
only a TNC and radio as a tracking device.  Several HAMS began to build these
autonomous tracking devices.

    Next, in version 2.0, I added an optional GPS serial interface to APRS so
that a laptop user could see himself tracked on the map.  Since I could
put my own parsing and timing routines in APRS, this interface did not require
any unique programming of the GPS device and so it was compatible with ANY GPS
or LORAN device using the NMEA-0183 interface.  In version 3.05 this capability
was enhanced for single port laptops to permit simultaneous operation of the
GPS and TNC on the same serial port.  See Single Port Mode (SPM) below.

    Next, PACCOM added a universal GPS interface into all of its product line
of TNC's.  This capability reversed the previous situation, by now permitting
ANY GPS to be used with only PACCOM TNC's instead of ONLY MAGELLAN/MOTOROLA
GPS units with ANY TNC.  Due to a small difference in PACCOMM's implementation,
APRS was not compatible with PACCOMM TNC's until APRS version 3.02.  It seems
that Howie Goldstein, who writes the software for many TAPR-2 Clone TNC's is
considering improving this situation by adding code to the TNC that will
permit ANY TNC to work with ANY GPS/LORAN.  After Howie writes this code, we
do not know how long it will take before it is available through the TAPR-2
clone manufacturers (PACCOMM, DRSI, MFJ, etc).

    Finally, just this weekend I visited the boat store and found that the
typical hand-held GPS is now selling below $500!  This means two things:
First, it is hard now to choose between a $500 circuit card GPS, which you can
program to work with ANY TNC, or a complete hand-held unit WITH display for
the same price (but only compatible with PACCOM and no course and speed);
Secondly, this means that the price of the circuit card will drop below $300
by April 94!

    To respond to this evolving technology, as of version 2.12+, this file
began to contain three major subsections.  The first describes the new PACCOM
direct TNC interface.  Second is the direct APRS software interface of GPS to
your PC, and third is the direct TNC/GPS interface for building stand alone
GPS trackers using the MAGELLAN and Motorola OEM cards.

PACCOM GPS INTERFACE:  All PACCOM TNC's with firmware 3.1 or later have a GPS
ON command which allows you to hook up ANY NMEA-0183 GPS device to the serial
port and the GGA position report will automatically be inserted into your
BEACON text (after stripping off the $GPGGA header.  Thats why APRS did not
recognize this stripped down GGA data until version 3.02).  You then set your
beacon period and away you go!  This obviates almost the entire remainder of
this file, since  1) You will NOT need a special GPS card,  2) you will not
need modified TNC code.  HOWEVER, since it only sends the GGA sentence, you
will only get position, NOT course and speed.  PACCOMM found another bug in
APRS that would not work with GPS devices outputting three decimal points of
precision in the LAT/LONG fields.  This was fixed in APRS version 3.04.
Please read the section below on NMEA interfacing.

    We would prefer that PACCOM and other TNC's considering this option to
place the GPS fix in something other than BText.  For future universal
applications, I am suggesting a new UI frame called LOCATION TEXT.  This LText
is just like a Beacon Text, except it is a separate entity with its own
timing.  Then you would also have an L E N command for setting how often you
want the LText to be transmitted (default is 1 hour).  By placing position
reports in the LText, this would keep the BText free for other applications.
(particularly, for announcing what your mobile is doing, and what symbol to
use, etc....)  This maintains the same distincton between BTEXT and POSITS that
APRS already handles easily.  Similaraly, an LText command would allow you to
manually enter your LAT/LONG or grid square in your TNC, even without a GPS,
so that all TNC's in all networks will send their locations periodically.  The
LText should be a free text format so that it is compatible with any
future specific formats (currently APRS parses GGA, RMC, VTG, APRS L/L, now
PACCOMM and grid squares and a future 8 character compressed L/L format) and
there will probably be others too.

    The minimum L period (L E 1) 1 should result in once every 10 seconds if
raw GPS reports are being automatically inserted (like PACCOMM).  But should
be 1 minute for manual entries.  Further, an LText UI frame should be sent out
everytime a new manual entry is made.  This is so that a new location report is
immediately transmitted.  The best implementation for MANUAL LTexts, (and for
BTexts as well) is to use the APRS timing algorithms.  This could be called the
DECAY option.  The TNC would have an L D N or B D N option.  The D stands for
DECAY and the N is the final beacon period.  With the DECAY option, each new
manual entry of BText or LText will force a UI frame immediately, 15 sec later,
30 after that, 60 after that, 2 mins, then 4 mins, then 8 mins and so on to N
minutes, and stay at N minutes forever.  This way, new UI information is
transmitted immediately to all stations on the net, but old beacons soon fade
away.  With this algorithm, I would expect the minimum value of N for the
DECAY option would be 1 minute, but a default value of 60 would be appropriate.
This way, stations that have unchanging information only beacon once an hour.
One other addition to complete the APRS philosophy, is to have the TNC respond
with both its LText and BText randomly within one minute of seeing an APRS
query (UI frame to the address of APRS with text = ?APRS?)  This way, stations
could drop back to a decayed beacon rate of once every 4 hours or so, but still
would pop up on an APRS map if requested.


DIRECT APRS GPS/LORAN INTERFACE OPTION:  As of version 2.0, APRS contains an
optional ($9) NMEA-0183 software routine that parses the output of any standard
GPS/LORAN device plugged directly (almost) into a serial port of your APRS
computer.  APRS will not only plot the position of the attached station and its
movements, but will also transmit those position reports into the APRS net.
This feature was added by popular demand from all the James Bond guys out there
that wanted to see themselves driving around on their laptop.  In version 3.04
I improved on this option by adding a moving map display (select TRACK on the
P-list) to keep your mobile always on the map.  The problem with most Laptops,
however, is the availability of only one external serial port, so you probably
have to give up the APRS packet position reporting if you want to see yourself.
One advantage of this mode of operation, though is that it gives you something
to play with if you already have a GPS and no one to play with on APRS yet.  It
is also ideal for boats and RV's that have room for a full size PC with two
serial ports.  With version 2.00 and later there are four possible operational
configurations:


     TNC only  - 1 Serial - Normal APRS for tracking other stations
     TNC/GPS   - 2 Serial - Normal APRS but your position update is automatic
     GPS only  - 1 Serial - Tracking yourself (no other stations appear)
     TNC/GPS   - 1 Serial - Possible only with programmable MAGELLAN and
                            Motorola cards (see Single Port Mode below)

NMEA INTERFACING NOTES:  Operation of a GPS with the optional $9 APRS software
routine is automatic.  But first you must interface the NMEA output of your GPS
or LORAN to your RS-232 input.  NOTE that NMEA and RS-232 are not exactly
compatible.  The NMEA specification is actually EIA-422, which means an
isolated differential receive circuit is recommended.  An opto-isolator is the
receommended interface to RS-232.  But it should also work by simply connecting
the NMEA pin A to RXD and pin B to ground.  Both standards are the same sense,
with NMEA a 0 and +5 volt signal, and RS-232 a + and - 3 volt signal.  The
direct connection may not work with many serial interfaces without a - voltage
pulldown resistor.  Often a 5k resistor tied to your unused TXD data line will
suffice to provide the - voltage.  Do NOT connect the APRS serial output to
the NMEA input, since APRS does not send anything to the NMEA device.  N6LGC
in California reports that the TRAXAR GPS devices will lock up solid if you
connect anything to the NMEA input!   You must then remove the battery and do
a hard reset to get it back!  Of course, if you are using one of the program-
mable GPS devices (Motorola/Magellan), then you WILL make this connection.
Next, set your serial port to the NMEA-0183 baudrate.   APRS scans the
interface data looking for a GLL/GGA/RMC and VTG data format to extract
position information.  The data on the NMEA interface is continuous and
refreshed every second.  (If you have been using a Magellan or a Motorola and
have programmed a very slow data period, you may want to reset this to a more
normal few second rate.)  In order not to saturate an APRS net or to overload
your disk storage or to slow down your other APRS keyboard processing, APRS
only samples the data at slower rate.  This is called the REFRESH rate and is
set during intitialization of APRS for GPS or by using the alt-S command.  This
period determines how often your screen is updated from your own GPS.  Usually
this is still too rapid for transmitting at 1200 baud on a shared packet
channel, so APRS also has another period called PACKET PERIOD which is usually
set for 1 to 10 minutes.  We have found that 30 seconds updates are OK for
special events when there are only one or two mobile APRS stations.  As more
and more stations go mobile with GPS/APRS, 1 minute or 2 minute updates are
more appropriate.  To further reduce channel loading, APRS will decay the
period when the station is not moving.

APRS OPTIONAL NMEA-0183 INTERFACE:  The optional COMM port routines to APRS
for direct connection of any NMEA-0183 device (GPS or LORAN), is available to
registered APRS users from the author for $9.  It can be purchased at the time
of registration or as an option later on.

NOTE:  A version of APRS is available that is plug compatible with the MAGNAVOX
1105 SATNAV system.  This is a 1970's vintage TRANSIT SATNAV system which has
both LORAN and SATNAV integrated together in the same box.  This unit is found
on many US NAVY ships.  It sends position updates in a very verbose protocol
once every minute.  This is the system used on the Naval Academy boats.  If
you have use for this module, please contact me.


NOTES ON MOBILE GPS OPERATION:  After over a year of operating GPS mobile, for
other to track me, I finally borrowed an old 8088 Laptop and went James Bond
mobile to the inlaws over Thanksgiving 93.  It worked beautifully.  I actually
never thought it would be anything more than a toy, but when you are stuck in
holiday weekend traffic for hours, and you are on an unfamiliar interstate,
crawling at 10 MPH or less, with no signs in sight, there is nothing that will
tell you where you are other than GPS!  LESSONS LEARNED!  1) we were 5 miles
out when I finally got everything going and then had to turn around and drive
back home to get my GPS Validation number!!!  Write it down!  2) Recommend
making a trimmed down disk with only the maps you will need on the disk.
3) when you QUIT APRS, your TRACK HISTORY is NOT saved UNLESS you sepcify a
file name OTHER than BACKUP.BK (or do a ctrl-S save).  4) In version 3.04 you
could select TRACK mode for any station so that APRS would recenter the map if
that station moves to the edge of the screen.  I have now improved this moving
map display so that new map is drawn not with you at center, but so that the
new map anticipates your direction and centers ahead of your movement.
5) GPS fixes indicate GGA/NUL as course and speed if no VTG data is available,
or "Last GPS fix" if the GPS device is reporting GPS not available and the fix
is older than a few seconds.  6) set your refresh rate to a long enough time
period so that APRS is not always processing GPS and has time to service the
keyboard.  I set to 20 seconds or more usually.  7) make notes of any map
errors or disagreements with GPS, with the latest MAPFIX.bas, you can now fix
any map easily on-screen.  8) for most highway maps and 1 minute reporting at
60 MPH, zooming in below 8 miles is usually a waste of time.  For this reason
don't waste your time making maps with every little twist and turn in the roaad;
it just takes time and memory and makes no difference.  In all applications of
APRS so far, you just want to know what road the mobile is on, and how far
along he is between point A and B.  A straight line between A and B is not as
pretty, but shows the road as well as 20 points showing all the curves.  If
you do save any RAW GPS data outside of the APRS environment, the following
two programs may be useful in reconstructing GPS data.  They are provided
as-is, I just made them for some quick and dirty file management that I had
to do in the past, but thought they might be useful to others as a basis for
writing your own routines.

FILTRHST.bas:  APRS automatically builds a track history for all moving
stations.  To avoid saving redundant position reports, a filter was added in
APRS version 2.0 to filter all reports and to only save positions that are
changing.  The default value of the filter is wide enough to include the
variations in position due to GPS selective avaiability. (+/- 0.03 minutes)
This value can be changed with the alt-F Filter command.  I wrote a QBasic
utility called FILTRHST.bas that can be used to re-filter a track history file
to remove additional points.  Since the source code is provided, this program
makes a good starting point for writing other routines for manipulating APRS
track history files.  In addition to filtering, this program can be used to
combine a number of separate track history files into one file.

GPStoHST.bas:  Since the simplest GPS interface is to just plug the output of
a GPS receiver into a laptop computer and save a text file.  This program will
take such a text file and generate an APRS track history file.  Actually, it
only looks for the GGA and VTG NMEA-0183 sentences and combines them into the
one line APRS format.



          TNC INTERFACE TO GPS or LORAN-C FOR MOBILES WITHOUT PC's

     This section describes an alternate method to the PACCOM interface
described above for interfacing navigation devices directly to a TNC for
building small autonomous mobile position reporting devices without requiring
a PC computer to do format conversions.  This method has the advantage of
transmitting any of the NMEA-0183 sentences (to include course, speed and
altitude) but requires the use of special programmable GPS/LORAN devices.
Although almost all GPS/LORAN devices have an NMEA-0183 serial data output
(except for the Rockwell engine and the SONY Pixis), most of them do not give
the user the ability to modify the periodicity of the data reported via the
interface.

     In most devices, navigation data is continually updated about every two
seconds at 4800 baud.  This is far too much data to transmit over a shared
1200 baud AX.25 packet link.  Fortunately some devices do permit the operator
to specify not only the reporting rate, but also what data formats are included
in the reports.  I have seen some LORAN devices that have a separate
"printer" port which can be configured by the user to output a report once
every N minutes or even hours.  Unfortunately, most users manuals I have
peruised in my local boat store do not make it immediately obvious what the
user configuration options are.  We have found two GPS engines which are
designed for the experimenter.

1.    The MAGELLAN OEM 5000 circuit board that I use is a GPS engine on a
3.5 by 7 inch circuit card that costs about $445 and produces RS-232 output
in NMEA format and requires only a GPS antenna and 12 volts at 250 MA input;
it also includes the RTCM-104 differential correction.  Call Emiel Yakoub at
MAGELLAN 960 Overland Ct, San Dimas, CA 91733, phone [phone].  Since it
is an OEM card, it has full user programmability.  It can be set to output any
of the dozens of NMEA standard formats at any periodicity between 1 second up
to 5 minutes.  My APRS software recognizes four of the NMEA-0183 formats:

   $GPGGA - for position and height  (no loran equivalent)  ] Use only one
   $GPGLL - for position only        ($LCGLL for LORAN)     ] of these two
   $GPVTG - for velocity and course  ($LCVTG for LORAN)
   $GPRMC - Posn, Course and speed   (Has all but height) (not in MAGELLAN)

2.  Tom Clark (W3IWI) has found that the Motorola OEM prototype card also has
user programmability of the NMEA outputs and can be slowed down to APRS
application rates for direct connection to a TNC without the need for a
computer in between.  This card includes the RMC message which contains
everything for land mobile in one NMEA sentence.  Call Jennifer Spitzen at
MOTOROLA, 708 480-5699 and ask for the OEM circuit board.  Here is what
I have learned so far for quantities of 1-99.  Card runs on 12 volts, has NMEA
0183 output and RTCM-104 (differential correction) standard and comes with an
active patch antenna for $484.  Without antenna is $435, but they dont know
if it will run without an active antenna.  The 1 pulse per second timing
option is an additional $100.  The combined Rcvr/Ant pricing is  better
than the MAGELLAN card/antenna combination, is smaller, and it outputs the
RMC message which is all you need for Position and velocity combined into one
packet.  It also outputs altitudes to 56,000 feet in the GGA message.
CAUTION:  I DO NOT HAVE ONE OF THESE YET.  BUT TOM's WAS ON THE AIR AND WAS
COMPATIBLE WITH APRS.  Oh yes, the full MOTOROLA evaluation kit was $1200
which includes everything you would ever want.  The $484 kit does NOT include
the support software and documentation for full control of the card.  But if
we can get Tom CLark or someone else that has the documentation to just tell
us the command string for setting the card into the right mode, then we can
develop our own HAM RADIO documentation and save big bucks.

     An automatic vehicle tracking system can be assembled by simply
connecting the RS-232 output from the GPS directly into the TNC and setting
the periodicity to 1 minute or more.  The TNC must be placed in UNPROTO
CONVERSE, and from then on, every minute a GPS position report will be
transmitted.  The APRS software will decode the raw NMEA position reports
above and plot the station on the map!

MAGELLAN CARD OPTIONS:   Since this card was designed for the OEM market,
for the individual purchaser it is a good idea to pay the additional $60 for
their  development kit consisting of some excellent PC software, the
technical manual, a wall power supply, RS-232 cable, Power switch, and short
SMB to TNC adapter cable.  Then all you need is an antenna.  They sell a $130
external "egg" antenna with built-in LNA for operation through either 18 or
25 feet of cable.  This antenna cable is terminated with a TNC connector, and
that is why the development kit includes the SMB/TNC pigtail.  OR you can
purchase their $60 passive antenna which has its own 6 inch SMB pigtail for
direct connection to the circuit board.  This antenna is a 1x1x3 inch weather
proof antenna like you see on their handheld GPS units.  Using this antenna (or
a 1.8 inch paperclip stuffed into the SMB antenna jack) obviates the need for
the SMB/TNC pigtail ($25 separately) so that you might be able to do without
the development kit if you really want to be cheap.  Since the circuit card
has no display, it can actually be mounted in a weather proof container right
at the antenna (or paperclip).  Only 12 volts and RS-232 need to come down
inside your vehicle.  Yes their $60 passive antenna is quadrifilar helix
antenna with true hemispherical coverage, but a 1.8 inch ground plane antenna
is just as good as long as you are not interested in good 3D altitude fixes.
Overhead satellites are not used for 2D fixes, but are used for 3D fixes.  I
leave my card on 2D all the time since 3D requires 4 vice 3 satellites, and
the 2D fix is not as good while running 3D.  Also, altitude is measured
against DATUM which is not necessarily SeaLevel.  So unless you are carrying
DATUM charts, (or flying) the altitude figure is of little value.  The
MAGELLAN cannot output an altitude above 999 meters except in a propriatery
NMEA sentence which I have not yet included in APRS.

GPS ENGINE SET UP:  Follow all MAGELLAN instructions for initializing your
GPS engine using your PC and their NAV program.  After the system is running
and producing fixes, send commands to turn off all outputs one at a time and
change the periodicity for the position and velocity reports from once a
second to a slower rate as shown below.  An alternate startup procedure is to
simply apply power, attach an antenna, and wait an hour.  The GPS will
automatically aquire satellites and be operational without any external
initialization after being exposed to full sky.  (this precludes needing a
laptop if you can spare the time).  Using this method, there will be no
outputs until you send the GPS card the following commands to set up the
reporting rates for position and velocity.  These commands may be sent from
any dumb terminal as follows:

$PMGLI,00,B00,7,A  (for GGA GPS position only)       Where 6 = 30 Secs
$PMGLI,00,B01,7,A  (for GLL LORAN position only)           7 = 1 Minute
$PMGLI,00,EOO,7,A  (for course and speed with either)      8 = 2 Minutes
                                                           9 = 5 minutes

     Each line must end with a carriage return-linefeed.  The GPS engine
gives no responses to commands, other than doing what it is commanded.  You
might try a value of 5 which is once every 10 seconds as a test to be sure
the GPS card is recognizing your commands.

BATTERY BACKUP:  Be sure to add the battery back up supply so that the card
can be turned off without having to re-initialize every time.  I use a simple
9 volt battery, diode isolated from the main supply rather than bothering
with the special 3.6 volt lithium memory cell suggested.  The GPS card has a
12 volt input and a separate ON/OFF line.  With the diode isolation of the 9
volt battery, the on/of line detects the loss of the 12 volt supply, and
powers down the GPS engine.  Current drain drops to microamps, and the 9 volt
supply through the regulator keeps all memory backed up.  An Alkaline battery
lasts about 6 months with the GPS off 99% of the time; longer if the GPS is
powered up longer.

TNC SETUP DETAILS:  Unfortunately the simple direct connection from the
MAGELLAN GPS card to the TNC is slightly more complicated because they do
not output the RMC sentence which contains everything needed for APRS in one
line.  To see Course and Speed from a MAGELLAN or a LORAN, you must enable
both GGA (or GLL) and the VTG sentence.  These two sentences are separated a
few milliseconds and force the TNC to generate two packets, one right after
the other.  This is a problem if a digipeater path is used, because the
digipeater will begin digipeating the first position fix packet and cover up
the trailing velocity packet.  To solve this problem, since most applications
require a digipeater path for longer ranges, the sending TNC needs to be
instructed to send packets not on receipt of every carriage return, but on a
timing function.  Set CPACTIME ON and change the SENDPACK character from $0D
to anything else (say $01).  This way, both the position fix and velocity
lines will be sent together in the same packet one second after the last
character is received from the GPS.  This packet, containing two frames, will
then be digipeated all together by the digipeater with no break in between.
If you use the Motorola which inplements the RMC sentence, this double packet
problem does not exist. (Even if you also turn on GGA so that you can get
altitude for a balloon, the problem is not significant, since you will not
need a digipeater for a balloon!).

LINEFEEDS and FLOW CONTROL:  Since the GPS is sending each line with a CR/LF
on the end, your TNC will always end up placing the superfluous linefeed at
the beginnning of the next packet.  To defeat linefeeds, set LFIGNORE on.
(for the non-standard Kantronics products, use Linefeed Supress, LFS ON)
Similarly, your terminal program must send CR-LF on each command to the GPS
card.  When you try to talk to your TNC with CR-LF, you will experience a
lockup condition since the extra LF will look to the TNC like the beginning
of a new command line and will hold off all TNC output.  To overcome  this
problem, set FLOW OFF.  Here are the commands which must be changed from
factory defaults for most TAPR-2 TNC's:

  ECHO OFF,  FLOW OFF,  LFIGNORE ON,  CPACTIME ON, SENDPAC $01

UNPROTO-CONVERSE-MODE:   And now for the last problem; keeping the TNC in
converse mode.  TNC's always default to command mode when turned on.  Until
the manufacturers put an UNSTART command in their TNC to cause it to power up
in Unproto-Converse, you must either keep the TNC permanently turned on after
setting converse mode, carry along a terminal to issue the CONV command, or
try to make a firmware patch to the TNC code.  Transparent mode could be
used, but the monitor function does not work in transparent mode and the TNC
can not then be used for receiving APRS packets.  Fortunately, Howie
Goldstein who wrote the original TAPR-2 code, identified a software patch to
the DRSI version of the ROM that will power up in UNPROTO converse.  This ROM
should work in most TAPR-2 clones.  I have used it in the MFJ-1274, and it
should easily work in the PACCOM Tiny-2.  I have asked DRSI to make this ROM
available to amateurs at a nominal cost.  Their price is $27.

DUMB TERMINAL SETUP:  So I can see the command that I am typing into the GPS
card, I configure my terminal device as half duplex.  The GPS also needs the
CR/LF sequence at the end of each command, so I set the terminal to translate
CR to the CR/LF sequence.  In order to use the same terminal with the TNC,
then, that is why I turn ECHO and FLOW off in the TNC.  My GPS/TNC box has
one DB-9 serial connector and two switches to select whether the terminal is
talking to the GPS or the TNC, and the second switch to enable the data
output from the GPS to go into the TNC after all configuration is complete.

SYMBOLS:  Starting with version 1.17, APRS now has 28 or more different
symbols for packet stations or objects placed on the map.  Since a simple
TNC/GPS tracking combo does not have the advantage of a PC running APRS to
format the APRS position report, I had to make two kludges to permit the TNC
alone to designate the desired display symbol.  First, APRS will assume that
all stations outputting direct NMEA data that have a -7, -8 or -9 SSID are Air,
Marine, or Mobile platforms.  Secondly, any of the APRS symbol designation
characters can be placed at the beginning of the TNC BText surrounded by {}
braces.  Once the BText with that symbol is received, the station will then
appear with the proper display symbol.  See the README.SYM file for details.

OPERATION:  With the special UNPROTO start-up ROM, and after initialiation
the other TNC parameters once, all future tracking evolutions are initiated
by simly applying power to the GPS/TNC/Radio.  In over 6 months of daily
operation, I have never had to re-initialize the GPS engine. (The seventh
month the 9 volt battery died!).  Without the special ROM, every tracking
evolution requires applying power, turning on a dumb terminal, and sending
the TNC the CONVERSE command.  Then the terminal can be removed or turned off
until the next activation.  If you do not have the UNSTART ROMS, be careful
if you use a battery supply of C or D cells with spring loaded battery
holder!  A bicycle equipped with this system reset the TNC after hitting the
first bump, and there was never time to stop and reset the TNC until the race
was over.  This shows the problem of the TNC not having a power up CONVERSE
mode in it!

We have assembled a nmumber of these GPS/PACKET tracking devices.  In fact,
the 7 inch by 3 inch MAGELLAN card fits nicely against the inside cover of
the MFJ 1270 or 1274 TNC.  The only evidence that the TNC is GPS equipped is
the kludge on the backpanel to hold the GPS antenna connector and the
presence of the two switches added to the front panel to select whether the
external terminal device is talking to the GPS or TNC, and to enable or
disable GPS packet reporting.  Other smaller packages have been made using
the PACCOM and DRSI TNC's and the TTL only model of the MAGELLAN GPS card
which is only about 5 inches by 3 inches.  I shy away from this card for the
casual experimenter because of the absence of any data or power supply
buffering.  One wiring error or static charge and you have blown a $395 card!

The $445 model with onboard 12 volt regulators and RS-232 buffers is much
more forgiving.



      GPS MOBILE AND TNC WITH ONLY ONE SERIAL PORT (Single Port Mode)

     Since most laptops only have a single external serial port, it would seem
to be impossible to both run GPS and operate APRS packet at the same time.  But
by using the programmble MAGELLAN or Motorola, to reduce the GPS data rate, it
is possible to wire-OR the TNC and GPS data outputs together going into the
single serial port.  This requires APRS to be looking for packet headers on
everything comming from the TNC, but then to also recognize raw GPS data too.
The only problems with this arrangement are data collisions and ambiguity on
incomming VTG packets.  At a once-a-minute GPS rate and a 100% saturated 1200
baud packet channel, the 9600 baud TNC data will experience a collision less
than 1% of the time and 85% of all GPS reports will be collision free.  The
ambiguity problem is caused by VTG packets transmitted as a second frame from
a tracking device (see above).  These off-the-air VTG packets also have no
packet header and cannot be distinguished from VTG data from the local GPS.
Since GGA sentences off-the-air preceed the VTG and are always received with
an attached packet header, they can be uniquely identified.  APRS will only
process a VTG sentence if it has been received within 1 second of a GGA
and it will assume that the VTG came from the same station.

     Again, this configuration will only work if you are using the programmable
MAGELLAN or Motorola which can be told to send GPS data only once every minute
or so.  To activate this Single Port Mode (SPM), bring up APRS in one-port TNC
mode and then hit shift-F2 to activate SPM.  Remember to use the V command to
validate your GPS interface.  Since the GPS data is wire-ORed with the TNC,
they must both be operating at the same baud rate.  Since APRS is receving the
GPS data, it will transmit the resulting APRS position report for the station
and so the GPS should NOT also be sending data to the TNC (as is normally the
case with the standalone MAGELLAN or Motorola GPS).

     Another way to do it, is to keep the GPS output going to the TNC input
for normal stand-alone operations, but then turning on ECHO in the TNC, so
that the outgoing GPS reports are also echoed back to the APRS program.  This
works fine, but makes your 25th line busy with an echoed copy of all of your
outgoing packets.  The other problem is that APRS always turns ECHO off every
time you go out of TALK mode.  To defeat this, I go to talk mode, issue the
ECHO ON command, then unplug the cable long enough to hit ESC and return to
APRS.  Then plug it back in.  Another problem is that APRS will also send out
a copy of the position report too.  I will fix both of these in the next APRS
version... after I am sure that the ECHO off will not generate other bugs!
P.S.  First bug found is that the ECHO on prevents the Alt-V command from
properly interpreting the UNPROTO path from the TNC.  You can still set  it
manually using TALK mode.  Obviously, there still is work to do here.  But,
Be sure, I will support a single port mode... somehow...


     In anticipation of this SPM mode of operation, I improved the GPS
interface in version 3.03, so there is less time spent out on the GPS port
waiting for data.  With version 3.03, you can keep your MAGELLAN or Motorola
at a one minute rate and still use it as a direct connection to APRS.  In the
past, if you did this, you would be stuck for a whole minute until the next
fix came along.  In version 3.04 I added the moving map display to keep your
mobile always on the map.  Select the TRACK option on the P-list display.


CONCLUSION:  With the cost of the new MAGELLAN GPS card falling below $250
this spring (94) and the size approaching a match box, there is every reason
to begin considering GPS applications in Amateur Radio.  At your next club
budget meeting, instead of throwing another $800 at the repeater monster, buy
the components to build a GPS/TNC tracking device in to a cigar box size
package.  Then at all future public service events, you have a package with
whip antenna on top that can be duck-taped to the top of any vehicle for
automatic vehicle tracking.  Let your imagination roam!


OTHER EXPERTS OR APRS USERS THAT HAVE GPS INTERFACES RUNNING:

WB4APR Bob Bruninga. Annapolis. Built 4 GPS/TNC devices and 22 SATNAV/TRANSIT
W3IWI  Tom Clark. AMSAT GURU working on GPS for Satellites. Uses Motorola
N3MNT  Bob Boles. Annapolis MD. Has MAGELLAN GPS/TNC mobile
W8RIK  Joe Hussy. Columbus OH is GPS/TNC mobile
N6JSX  Dale Kubicheck Works at MAGELLAN! San Dimas, CA
W9UWE  Julius Breit. Chicago. Has MAGELLAN GPS
W1BEL  Gwyn Reedy. Tampa. PACCOM makes commercial TNC/GPS automatic combo!
W9LZQ  Kent Helman, Onalaska WI. Built GPS-TNC interface & Did map of WI!
W1KRU  Jim Warakouis. WestRoxbury MA. Built GPS-TNC interface
WB6LPG Bill Bliss. HalfMoonBay CA. Has LORAN (and now GPS mobile)
N5SSY  Ross Mocklin.  NewOrelans LA. Has MAGELLAN cards on order...
KD1E   John Moore. GPS mobile at GM Proving Ground. @ N8NNN.#SEMI.MI