DP Tool Club 8/CDASC08.ISO/VRAC/APRS114.ZIP/README.GPS

Text File | 1993-07-07 | 12KB | 176 lines

                    TNC INTERFACE TO GPS or LORAN-C


     APRS is compatible with almost all naviagtion devices that have an NMEA-
0183 serial data output.  The only GPS that I have seen that does not have
this national standard is the Rockwell engine and the SONY Pixis.  The
problem is that many GPS or LORAN-C devices do not give the user the ability
to modify the periodicity of the data reported via the interface.  In most
devices all navigation data is continually updated about every second 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.  Fortunately I found a GPS engine which was designed for the experimenter.

     The Magellian OEM 5000 circuit board 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.  Call Emiel Yakoub
at Magellian 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 three 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)

     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!

MAGELLIAN 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 cable is terminated with a TNC connector, 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 1.5 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 1.5 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.

GPS ENGINE SET UP:  Follow all Magellian 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,B02,7,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 GPS card
to the TNC is slightly more complicated if the Course and Speed format is also
included.  This is because the course and speed report is in a different NMEA
format line than the position fix and therefore generates 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.

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

     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 helped me
make a modified ROM that will power up in UNPROTO converse.

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.

OPERATION:  With the special 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 Magellian 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 Magellian 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.

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!