Libris Britannia 4/science library(b).zip/science library(b)/HAMRADIO/APRS400.ZIP/README/README.DF

Text File | 1994-02-07 | 17KB | 265 lines

                      USING APRS FOR DIRECTION FINDING

VERSION 3.13:  In order to permit operating APRS mobile with GPS as part of a
DF exercise, I had to change the DF reporting format to separate the DF bearing
from the vehicle HEADING.  This change is NOT backward compatible and DF'ers
should upgrade to 3.13 ASAP.  With this change, mobile DF stations can simply
enter thier RELATIVE bearing to the FOX and APRS will add that information to
the vehicle's TRUE HEADING so that a TRUE DF report results!  When the N7LUE
auto DF interface to doppler units is complete, then I will make one more
change so that BOTH GPS and AUTO-DF can run simultaneously!

VERSION 3.09, 3.10, 3.11:  With 309, a small bug was fixed which had always
reported a single line of bearing on the very first sample of the LEDs as soon
as a signal broke squelch.  THis line was almost always wrong.  The next and
all subsequent lines were well averaged and much more accurate.  In 3.09 this
first report is delayed 1 second to allow accumulation of more hits.  In ver
3.10 I changed the ALT-P filter so that you could also save DF bearings to the
track history file.  This way you can REPLAY DF exercises conveniently!  To
show you what the Doppler DF interface looks like, I added an AUTODF.HST file
in verison 3.11 to the distro disk to allow you to replay an auto-df
simulation.  In this version, I also fixed a small bug which prevented auto-DF
bearings from showing up each time without re-drawing the screen.

   As of version 3.00, APRS included the option to support an automatic
serial interface to Doppler DF units so that DF bearings are automatically
determined and transmitted without operator intervention!  See the later
sections below.  Note, this required some mods to the DF format and makes
3.00 DF reports NOT compatible with older APRS.  Older APRS versions will try
to DeadReckon the new DF report, instead of drawing the DF bearing.  The
direction of the DR indication, of course, does show the correct DF bearing.
CAUTION:  APRS does not do spherical geometry, it assumes a flat earth.  This
will not be noticable unless you attempt to use DF bearings beyond a few
hundred miles.  Even tracking balloons over 200 miles, this error will probably
be less than the typical innaccuracies of the average HAM beam antenna.

INTRODUCTION:  APRS is an ideal tool for rapidly triangulating fixes to
locate a hidden transmitter, an interfering signal, or for tracking balloon
payloads.  APRS instantly plots vectors from all stations making a signal
report and the intersection of these lines of bearing indicates the location
of the target signal.  To see a sample, load the file named FOXDF.BK into
APRS and see the results of our DF on a fox hunt in OCT 93.  We nailed the
guy to within about a half mile, and notice that none of our stations were
any closer than 15 miles away and more than half of our DF stations were more
than 25 miles away!  Notice too, that none of these stations were
particularly calibrated and only two stations were actual APRS stations.  The
others just reported their position and bearing by voice and we put them on
the map.  MAKE SURE you know how to convert from magnetic to true bearings.
We did it the wrong way and were 10 miles off the first time!

MANUAL APRS STATION DF REPORT:  To accomplish this triangulation, each
participating station simply enters his bearing to the target using the
Ctrl-B key.  This sets his position report to include the DF bearing as his
station's course, sets his station symbol to a Triangle, and sets the speed
field to indicate a quality factor (1 to 8).  To make the DF bearing more
visible without requiring the X key, and to indicate that this is a manual
report, 900 is added to the Quality factor.   The Triangle symbol alerts all
APRS stations that the position report should be interpreted as a DF report
and that it should NOT be dead-reckoned. (earlier versions do not recognize
this and will DR the station as a moving contact!)

     Since APRS would continue to report this manually entered DF bearing
indefinately, it will normally time out after 2 hours to eliminate any
confusion caused by old/stale reports.  The operator can optionally select
Permanent to overide this timeout feature.  As soon as two or more stations
are reporting a line of bearing to the target, everyone can predict the
intersection of the lines by pressing the X key for a cross fiX.  A solid
yellow line indicates an excellent line of bearing, and a more dotted line
indicates less and less quality.  As a further aid, the R key can be used to
superimpose a set of range rings on the screen around any one station for
estimating distances for subjective analysis of signal strnegths.  If you are
running the WX station option, then, use of the ctrl-B key will overide the
WX station report with the Beam Heading report.

NON PACKET DF REPORTS:  Even for stations not running packet or APRS, their
lines of bearings can be quickly entered by any APRS station using the alt-A
key which adds them to everyone's map in real time.  In this case, simply
select the triangle symbol, include their DF bearing in the CSE field and set
the SPD field to a quality number between 1 and 8, where 8 is best.  A Direct
entry feature is also available (using the alt-D key) for placing a reporting
station on the map by entering his numeric LAT/LONG.  This makes plotting of
distant stations on the map easier without a lot of cursor movement.  This
feature is useful when taking reports by voice over HF for example during a
balloon tracking event.

DUMB PACKET TERMINAL DF REPORTS:  Non APRS packet stations can also
automatically report their lines of bearing into the system by simply
entering a beacon text in the APRS format with their line of bearing.  The
format for an APRS position report is included in the README.DUM file, and is
reporduced here:

 BText     !DDMM.xxN/DDDMM.xxW\BRG/N0Q/DF report...

 Where:     DDMM.xxN is Latitude, DDDMM.xxW is Longitude
            \ (Backslash indicates a Triangle symbol for DFing)
            BRG is the DF bearing in degrees True
            N0Q is a Quality indicator where Q is a quality value (1-8) and
                N is an optional Number of HITS indicator. If N is 0, then it
                means nothing.  Values from 1 to 8 give an indication of the
                number of hits per period relative to the length of the time
                period.  So 8 means 100% of all samples possible, got a hit.
                The N is not processed, but is just another indicator from
                the automatic DF units.


SIGNAL REPORTS:   Even stations without BEAMS can at least report whether
they hear the signal or not.  An APRS operator can place these stations on
the map using the big Z symbol for strong signals, the medium sized dot (no
symbol character) for weak signals, and the (little x) L symbol for no
signal.  Their indication of where the signal CAN and CANNOT be heard are
also very valuable.  Maybe in the future I will add circular contours around
these stations so that the intersection of these circles can be used to
localize a signal!  If the size of the circle was inversly proportional to
signal strength, then the intersecting circles could be very valuable!


AUTOMATIC DOPPLER DF UNIT INTERFACE:

     Since APRS is a map based packet system which can not only show the
location of all stations on frequency but also any object or symbol placed on
the map by any other station in the network, it is ideal for displaying the
data from networked direction finding stations.  Two stations in Arizona,
Randy KA7UUS and Bob N7LUE @ K7BUC have developed a serial interface
to the popular ROANOKE Doppler DF unit (or any other DF unit that drives an
8 to 16 LED display).  They have added a divide by N counter and a UART to
reduce the sampling rate from 500 samples per second and to output an ASCII
character only 8 times a second or so.  Each character is a letter from
@,A,B,.. ,O indicating the azimuth of the 16 LEDS.  Since some DF units
rotate clockwise and others counterclockwise, the board will optionally output
the lower case letters for the opposite rotation.  One manufacturer of the
boards, Douglas RF devices in Sacramento, CA is interested in incorporating
the UART into his next production run of boards.  At this time (and without
the serial interface), the Board and all parts (less antenna components) is
under $90.  Their address is Douglas RF devices PO Box 246925, Sacramento, CA
95824, Phone number 916 688-5647.

     With the N7LUE serial interface, any station with the DF unit connected to
one COMM port and a TNC to the other COMM port will automatically transmit an
averaged DF BEARING azimuth in his periodic position report.  All other APRS
stations on frequency will see a yellow DF line plotted in real time from his
station.  The line is dotted in proportion to the Quality of the sampled
data.  Even APRS stations without the automatic interface can simply manually
enter their beam heading and they too will appear on the map with their beam
headings plotted.  With three or more manual or automatic stations on the
net, an instant triangulation is plotted on everyone's map every few seconds.
Big Brother, move over! Here we come!

REMOTE DF SITE:  The real advantage of an automatic DF network can only be
realized if the PC can be eliminated.  The ultimate remote DF site should
require only a programmable scanner, a TNC and a Doppler DF unit.  With the
modification to the ROANOKE DF unit and the APRS software, such a network can
now be assembled.

NOTE: VERSION 3.00 HAS THE LOCAL DF INTERFACE WORKING, BUT IS NOT YET READY
TO PARSE THE RAW DATA COMMING FROM A TNC/DF-ONLY COMBO.  WILL BE IN 3.0X!

     Ignoring for the moment the means which might be used to tune the DF
receiver and to enable or disable DF reporting, an automatic DF site can be
assembled by just plugging the radio, TNC and DF unit together. By setting
the TNC in the UNPROTO CONVERSE mode, it will simply packetize the data out
of the DF unit periodically for display on all APRS stations on the network!
It is simple to configure the TNC to do this as follows:

  A.  The character rate out ot the DF unit is strapped to 7.5 characters per
second by using a divide by 8 circuit to strobe the UART from the Q7 output
of U9.  This is the mod being developed by N7LUE.

  B.  Set the TNC packet length PACLEN to 75.  On a continuous signal, then,
the TNC will transmit once every 10 seconds after it has accumulated a full
packet of 75 characters.  Each transmission will contain the last 75 samples
from the DF unit!

  C.  So that APRS knows where the remote DF unit is located and so that it
knows that the characters from that station are to be treated as bearing
samples, the BText of the DF TNC must contain the TNC LAT/LONG in the
standard APRS format and the station symbol must be the character (\) for a
remote DF site.  The BText format would be BT !3856.55N/07629.11W\DF
station...

  D.  APRS software will receive the packet and compute the average direction
for all the characters in the packet and plot it on the map.  In addition
APRS will compute the quality of the result based on the deviation of the
samples and will also note the total number of samples in each packet.  It
will use the quality factors to modify the 'dottedness' of the bearing line.
A good quality line will be solid.

  E.  Since the FOX will probably not transmit in 10 second increments, the
TNC is also set to automatically send all bearing samples accumulated at the
end of the fox transmission.  This is done by setting PACTIME to AFTER 10 (1
sec) and CPACTIME to ON.  The PACTIME setting was chosen relatively short so
that a packet is transmitted at the end of each FOX transmission, but before
another station keys up.

  F.  To prevent all DF sites from keying up at once at the end of the FOX
transmission, each automatic DF site must have a differnet value of DWait.
Each additional site should have an additional 100 ms.

   With the design noted above, each DF site will transmit a maximum of one
packet every 10 seconds, or one packet for every short transmission of the
fox.  With the parameters chosen above for 5 stations, the network would be
pretty well saturated just handling the data from all sites.  This is fine
for intensive operations in search of a FOX or jammer, but a more routine
level of operation could be realized by reducing the data rate from the the
DF unit from 7 to 3.5 characters per second or less.  This would result in
only one packet report every 20 seconds or more which might be more suitable.
At these high data rates, and since a good DF site should have good altitude,
digipeater paths for routing the data should be avoided if possible.


DOPPLER DF HARDWARE MODS:  The hardware interface is very simple and can
actually be added to almost any DF unit which drives an 8 or 16 LED display.
A simple CD4024 divider chip is added to the DF timing circuits to obtain a
signal near 10 Hz to establish the character reporting rate (strobe to the
UART).  For the ROANOKE board, this chip should be connected to the Q7 output
of U9, to give character rates of 32, 16, 8, 4, 2, 1, and .5.  There should
be jumper pads for the user to select the rate appropriate for the
application (probably 8 is best).  Next there should be a DATA valid signal
to enable or disable the UART from sending data.  In the ROANOKE board, this
is the LOW-DETECT signal which is present whenever the the data is not of
sufficient quality.  (N7LUE is not happy with the LOW-DETECT circuit, and is
adding his own squelch circuit)  The only other connections to the ROANOKE
board are made to the 4 bit lines feeding the 1-of-16 driver for the LEDS.
For an 8 LED circuit only 3 bits are used and the least significant bit is
strapped to 0.  The other 3 bits of the UART are strapped so that a 0 input
results in the ASCII character (@) being transmitted.  The result will be the
letters @,A,B,C....O.  Since different DF units may rotate the LEDS the
opposite direction, the 6th data bit can be strapped to 1 to produce the lower
case alphabet.  This will indicate a CCW rotation.


AUTOMATIC REMOTE SITE DF NETWORK CONTROL:

Since the automatic DF interface between a TNC and a DF unit will generate a
lot of packets, there has to be some means for remotely turning it on and
off.  I consider that beyond the realm of APRS, since for a remote DF site,
there must already be some kind of control link in place in order to command
the DF receiver what frequency to listen to.  If such a link already exists,
then the capability is probably also there for enabling or diasabling the
DF/TNC interface.

     In the absence of such a control link, however, a very simple remote
control and receiver command link can be derived from the TNC itself!  First,
take the voltage from the CONECTED LED and use it to enable the DF unit
output to the TNC input (some TNC's bring this signal out on one of the RS-
232 pins).  This way, the automatic reporting will begin as soon as the DF
Net Control station connects to the TNC.  This means of control also has the
advantage of using the serial data channel from the DF Net Control SYSOP up
to the site for setting the frequency of the receiver!  Since APRS software
only checks the TO address for valid APRS data, and does not care whether the
packet is connected or not, it will still be able to monitor all data from
the remote site.  To facilitate this process, APRS now also accepts packets
addressed to DFNET which should be used as the callsign of the NET CONTROL
station.  This is legal, as long as the NET CONTROL station also places his
true call in his BText and sends his beacon once every 10 minutes.

DF NET CONTROL OPERATION:    The scenario for this kind of operation, would
be for the network SYSOP to use a dumb terminal in the multi-stream connect
mode to connect in turn to each of the remote sites.  Once each of these
connections is established, each DF station begins sending DF data as long as
the connection is in place.  To disable a site, the SYSOP simply disconnects
from that station.  The only disadvantage of this means of control is the
additional QRM on frequency from all the ACKs required from the SYSOP TNC for
every DF packet transmitted.   Having an alternate means of control, avoids
this CONNECTED environment but adds complexity.