DF.TXT 7.1a          USING APRS FOR DIRECTION FINDING

New in 7.d:  APRS now supports a DF single port mode for sharing a
single comm port between the TNC and DF unit.  See DF SINGLE PORT MODE!
Also see some OPERATIONS tips at the end of this file. (I finally went
mobile with a Doppler DF unit!)

OVERVIEW:  APRS NOT ONLY PLOTS BEAM HEADINGS (Both Manual and DOPPLER)
BUT IT ALSO HAS TWO METHODS FOR TRANSMITTER LOCATION USING ONLY OMNI
DIRECTIONAL SIGNAL STRENGTH CONTOURS!  Since ANYONE can use the OMNI
techniques, they are presented first, followed by the classical BEAM
heading triangulation, followed by details of the automatic serial
interfaces to the Doppler equipment.

     The first omni technique displays overlaping circular signal strength
contours on the map based on signal reports from a number of reporting
stations.  The second omni technique plots lines of bearing based on a
single moving omni station (Aircraft or vehicle) plotting three or more
FADE points on his map.  All fade-points on a map where the transmitter
signal fades out should characterize a circle with the transmitter
at the center.  APRS computes this circle and therefore the location
of the transmitter based on these three or more points.


OMNI-DIRECTION FINDING:

APRS incorporates a whole new aspect to direction finding by permitting
the plotting of signal strength contours.  THIS PERMITS STATIONS WITH ONLY
OMNI ANTENNAS TO PARTICIPATE AND PROVIDE VALUABLE INFO!  This is possible
since APRS has a line-of-sight Power-Height-Gain (PHG) reporting and
display format.  This format adds a station's power, antenna height above
average terain, and his antenna gain in his APRS position report.  APRS
can then draw range circles around each station showing his relative
communication range.  For stations not reporting the PHG format they are
assumed to have the default parameters of 10 Watts, 20 feet HAAT, and a
3 dB antenna.

     If each station accurately includes these parameters in his posit, then
APRS plots a map of circles around all stations.  Where two circles inter-
sect or overlap, direct communications are possible.  This PHG plot is an
ideal tool for setting up ANY radio network WHETHER OR NOT APRS or PACKET
is being used!  Note that these circles represent transmitting range based
on your Power and antenna relative to a nominal 10 Watt station at ground
level.  Your ability to hear him, depends on his transmitter relative to
10 Watts.

DFING WITH OMNI SIGNAL STRENGTH REPORTS:  By modifying these PHG equations
for plotting received signal strengths, a weak signal is drawn as a larger
circle of probability than a very strong circle.   I chose a scale of 0 to
9 for relative signal strength indication.  These signal strength numbers
replace the transmitter power in the PHG reporting format and are preceeded
with DFS to represent DF Signal strength.  APRS uses these signal strengths
to plot receive contours as follows.  The numbers 1 to 9 will be plotted
as circles from a dark gray up to a bright red, with the radius of the
circle decreasing with stronger signals.  A signal strength of 0 represents
NOTHING HEARD and is plotted last as dark gray on top of everything else. 
They clearly show where the transmitting station is NOT.

     Since the PC can only overlap circles, the user should visualize all
the overlapping colors and not just the brightest ones on top.  The probable
location of the transmiter will be in the area of the most concentrated
intersections of circles.  Do not be fooled by the brighter circles nor
the CENTERS of any circles.  The location of the hidden transmitter will
never be at the center of a circle.  THE LOCATION OF THE HIDDEN TRANSMITTER
IS ALWAYS NEAR THE EDGES.  If it was near the center, then that station
would have reported a stronger signal, and the circle would be brighter
and smaller!  Load the DF-OMNI.BK file to see our first omni-df attempt
and see the section below about what you will see in that file.


OMNI-DF COMMAND SUMMARY:  The following list sumarizes all of the commands
used in performing direction finding both OMNI and with BEAMS.  Please note
that stations with BEAMS should NOT input OMNI signal strength readings,
since their gain will upset the consistency of the OMNI plots.  Beam stations
should always enter their BEAM HEADINGS.


INPUT-DF - Used to enter either a beam heading or a signal report.  If a beam
      heading of (0) is entered, then APRS assumes the entry is for an OMNI
      signal strength report.   Remember that 360 means North, not 0.

INPUT-ADDobj - Used to add non-APRS station's DF reports to the map.
      By selecting the DF symbol, you will be prompted for the appropriate
      BeamHeading information.  If this is an OMNI report, then enter a beam
      heading of 0 to continue with the OMNI report.

INPUT-HEADING - Used to enter the heading of your vehicle.  Useful when you
      are stopped and your GPS heading is useless.

INPUT-PwrHtGain - Not necessarily used for DF, but is used for showing
      your station Power, Height and Antenna gain parameters.

INPUT-SAVE - Used by mobiles to save a copy of your current position (with
      DF bearing) as an object so that the fix remains on the map for
      future reference.

INPUT-UPLINK - Same as SAVE, but the FIX object is marked for uplinking.

MAPS-PLOT-DF - This command is used to plot the OMNI-DF profiles.

MAPS-PLOT-HEARD - Plots only the DF rings for stations that have HEARD the
      fox.  (for monochrome displays to separate the NOT-HEARD circles)

MAPS-PLOT-NOTheard - Plots only the DF rings for stations that have NOT
      HEARD the fox.  This is to eliminate confusion on monochrome screens.

MAPS-PLOT-PwrHtGain - Plots Power-Height-Gain range rings around all stns.

MAPS-PLOT-RngRngs - Draws range circles at the selected map scale. AND shows
                    the range and Bearing of the cursor from the center.
                    This is in great-circle coordinates.


DESCRIPTION OF EVENTs STORED IN DF-OMNI.BK:  Although APRS can plot the
circles of signal strength, it will take some time to develop the skills to
interpret the result.  On this first attempt, there were NO other APRS
operators at home that I could raise that Sunday afternoon, I scoured several
voice repeaters and got a few RF signal reports on the FOX. I then added
these stations to the map using the INPUT-ADD command.  FILES-LOAD the file
named DF-OMNI.BK and hit the MAPS-PLOTS-DF command.

   First, notice that APRS does a good job with the dark gray circles of
showing you where the FOX is NOT!  Actually, since the DF mobiles (not
aware of APRS and NOT in communications with me) took more than an hour
just to get close enough to hear the FOX, APRS users could have
immediately begun to drive to North Baltimore and cut at least an hour
off of their search times.  Second, notice the offset circle of KA3DZZ.
IF he had not added as an afterthought that he had a ridge blocking his
East view, the gray null circle from him would have misslead us for a
while.  Notice, that most of the stations had never participated in a
fox hunt before, and had no talent in estimating signal strength and
some were even using HT's with rubber ducks!

    The most interesting report was from W3PWF who said it was a very
strong signal and he was much further than either of the nearby mobiles that
reported weak signals.  ALthough he was in his driveway, he had almost 200
feet of height above average terain, but could not quantify it at the time.
This points out how tricky it will be to use the OMNI-DF plots.  Do NOT
rely on any one report.  You must visually take it all in.  His report is
correct, and although he has a large horizon, APRS draws his pink circle
smaller to show that the FOX could be closer to him.  Remember to look at the
edge of his circle, not the center.

    APRS draws stronger reports smaller for two important reasons.  First
a stronger signal means the FOX is closer to to the reporting station.
Second, since PC screens cannot MIX colors and only the last one drawn
is visible, APRS draws all OMNI-DF reports on the screen starting with
the weakest (largest) going up to the strongest and smallest.  After all
of these colored reports are plotted, then APRS goes back and plots all
of the 0 or NULL reports.  They are drawn on top, since they are a
POSITIVE report that the FOX is NOT within their range.  If we could have
gotten a NULL report from a station to the northeast of the pink circle,
then it could have overlapped the NE section of the PINK circle and told
us that the signal was clearly coming from the southwest of W3PF.

YOU MUST REMEMBER TO LOOK AT THE EDGES OF ALL CIRCLES, NOT THE CENTERS!
THE FOX SHOULD BE NEAR THE LOCATION WHERE THE EDGES OF MOST CIRCLES
INTERSECT OR OVERLAP.

     This was just my first test, and unplanned.  Notice that with all of
the stations that we rounded up, only 4 of 13 even heard the FOX at all.
For serious work, each station reporting should have a very good idea of
his Height above average terrain and general geographic horizon.  If each
of those stations was also watching the APRS plots unfold, they could have
modified their reports to be more meaningful!


RECOMMENDED OMNI-DF PROCEDURE:  When the APRS net is alerted to a FOX
or hidden transmitter, each APRS station should listen on the reported
frequency and enter his signal strength.  Next each APRS operators
should go onto the local voice repeaters and ask for OMNI-SIGNAL strengths
from any other fixed or mobile stations.  The APRS operators use the INPUT-
ADD command to add these stations to the map.  NEGATIVE reports are VERY
valuable too!  By having one APRS operator listening on EACH local voice
repeater, and solliciting reports, the maximum number of reports can be
gathered with a minimum amount of chatter.  Be sure to get the station's
reported signal strength, location, Antenna height-ABOVE-AVERAGE-
TERRAIN (not sea level or above ground) and any offset in his horizon.  It
is amazing how many people do NOT understand ANTENNA HEIGHT.  Be  conser-
vative.  Use the following scale:

   0   No signal detected what-so-ever
   1   Detectible signal (Maybe)
   2   Detectible signal (certain but not copyable)
   3   Weak signal marginally readable
   4   Noisy but copyable
   5   Some noise but easy to copy
   6   Good signal with detectible noise
   7   Near Full-quieting signal
   8   Dead Full-quieting signal no noise detectible
   9   Extremely strong signal "pins the meter"

Remember, stations DO NOT NEED TO BE APRS stations to participate!
Any voice report can be entered on the map by any other APRS station using
the INPUT-ADD command and selecting the DF symbol type.  Enter a beam
heading of 0, to be prompted for an omni-signal strength report.  For more
information on the Power-Height-Gain formats, see the DIGIs.txt and
PROTOCOL.txt files.

PLOTTING DETAILS FOR OMNI-DF CIRCELS;  I used the radio horizon forumla
for the radius of the circles, modified by the signal strength value. 
Here is the equation for the four DFSshgd or PHGphgd characters.

   P = 10 / s        For Power plots, P = p;  For DFS, P is INVERSLY
                     proportional to signal strength s.
   H = 10 * 2 ^ h    Convert character to power in Watts
   G = 10 ^ (g / 10) Convert from dB
   D = 45 * VAL(d)   Convert to degrees.  If D is not zero, then the circle
                     is offset in the indicated direction by 1/3rd radius
  
   R = SQR(2 * H * SQR((P / 10) * (G / 2)))   range modified by adding
                     SQR(P/10 *G/2) to make it unity at 10 watts and 3 dB
    
   R = R * .85  Present fudge factor


EQUAL FADE CIRCLE TECHNIQUE FOR MOBILE OMNI DFING:

     This method has been used for years by Airborne search and rescue teams
to locate downed aircraft based on the location of points where the signal is
just detectable.  The advantage of this technique is that NO BEARING info
and NO SIGNAL STRENGTH info is required.  The key factor, is that ALL points
where the signal fades to zero are located on the edge of a large circle with
the hidden transmitter at the center.  By simply flying (driving) through the
area of the hidden transmitter and plotting at least three points where the
signal fades out, you can identify the circle and therefore the location of
the transmitter.  For aircraft searches, this technique can be repeated at
lower and lower altitudes to repeatedly reduce the size of the circle and
therefore increase the accuracy.  For ground based searches, an attenuator
or tighter squelch can be used to reduce the size of the circle for successive
runs.

     The only assumption in this process, is that the radiation pattern
from the transmitter is relatively omnidirectional.  An advantage of this
technique is that the aircraft does NOT have to fly over the transmitter
to find a signal peak (which is very ambiguous, considering that there is
often a NULL directly overhead of an OMNI transmitter).   See the following
plot to see how the data is plotted.  Between each pair of fade points, a
line is computed and then a line of bearing is drawn midway between the points
and perpendicular.  The intersection of these lines-of-bearing give the
location of the transmitter.  The sketch below is symetrical due to the
limitations of the angle of the slash characters used in drawing it, but the
technique does work no matter where the flight paths intersect the circle!

       Entry               .   .   .  Fade Circle
     Flight path      .                 .
             \     .                    *  .
               \ .       *           *       .  / Exit flight path
               A.\          *     *          D/
                .  \           T            / .
                .    \      *     *       /   .
                 .     \ *           *  /    .
                   .  *  \            / *  .
                   *  .    \        /    . *
                *          . \ . C/.          * Perpendicular
                             B \/               lines of bearing
                              /  \
                             |    |
                              \__/ oops, nothing heard,
                                   turn the other way!

     APRS implements this algorithm.  No matter what pattern you
drive (or fly), simply drive until you first aquire the signal and hit the
F5 key.  Then continue driving in the same general direction until you just
lose the signal.  At this point hit F5 again.  APRS will then compute a line
of bearing perpendicular to the line connecting those two points and bisecting
the distance.  This perpendicular line of bearing is represented by the
asterixed lines above.  Turn and choose a new line to drive until you
re-aquire the signal and do the same process again.  Hit F5 on aquisition
and hit F5 again when the signal fades.  When APRS plots this second line of
bearing,  you will have two intersecting lines of bearing that roughly
indicate the location of the hidden transmitter.  Drive directly to that
point and insert enough attenuation in your antenna to make the signal weak
enough to do the whole process again but with a much smaller FADE circle.
This added attenuation is similar to aircraft reducing altitude to reduce the
fade circle for each additional run.

    Note that each time you press the F5 key to mark a fade point on the map,
APRS asks you if this is a NEW CONFIGURATION or not.  This is important,
because APRS should use only the points made by the same station and in the
same configuration for each plot.  To keep track of these, APRS labels each
new fade point with your callsign suffix in parentheses and then a letter
for the given configuration and then a sequential number.  Whenever the
MAPS-PLOTS-FADE commmand is given, APRS only computes bisectors and bearing
lines from each group of points from the same station, and from the same
configuration group (letter).  So, for any given configuration (antenna and
attenuation combination) just hit return at the configuration prompt.  When
either the antenna or attenuation are changed, then answer Yes for the first
point in the new configuration.

NOTE!   It is very important to understand that this is just a technique.
The operator MUST have experience in DFing and must thoroughly appreciate
the vagaries of propogation and antenna height-gain.  Just pressing F5 does
NOT find the FOX!  Give me a violin and it will NOT make music!  Garbage in
implies garbage out! ETC.  What I am saying, is to make sure that each time
you are ready to mark a new fade point, consider the average terrain and be
sure you are in a comparable propogation position.  Obviously, if you have
some kind of S-meter, you do NOT have to drive all the way to a fade
condition, but just to a measureable and repeatable signal strength level.
As long as you press F5 at multiple points of equal signal strength, the
fade technique will work.


FURTHER DETAILS:  When you press the F5 key for manual reports, APRS creates
a Fade marker at the location of the cursor.  If you are GPS equipped, you
can instantly move the cursor to your current location by simply pressing
the Go key.  For each press of F5, a new fade spot is created.  Once APRS
has two or more of these locations, it can plot the lines of bearing.  Use
the MAPS-PLOT-FADE command to display the plot of all of the lines of
bearing.


PLEASE NOTE!  The difference between this technique and the OMNI-DF function
in APRS, is that the FADE technique takes advantage of a SINGLE MOBILE
to locate the edge of the FADE circle.  FIXED stations can NOT provide ANY
useful information for the FADE circle technique.  The chances that they are
exactly on the FADE circle edge is a chance in a million.  Yes, they can
induce attenuation to cause the signal to just fade, but that is for their
antenna, height, radio and opinion only which can NOT be reproduced any
where else, by anyone else, to find other comparable points.  To summarize,
the FADE circle is for single mobile OMNI fox hunters using the SAME station
at MULTIPLE locations, wheras the OMNI-DF capability plots signal strength
contours for MULTIPLE NON comparable stations.


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


CLASSICAL APRS DIRECTION FINDING WITH BEAM HEADINGS AND/OR DOPPLER DF UNITS


     APRS is an excellent tool for instantly plotting and diseminating DF
bearing information using a variety of techniques:

     MANUAL APRS    - Any APRS station simply selects the INPUT-DF command
                      and enters his beam heading

     MANUAL OTHER   - Any APRS station can take voice reports from other
                      stations, and place them as DF reporting OBJECTS on
                      his APRS map

     AUTODF D.S.Inc - Connecting a second COM port to the 300 baud serial
                      data output of a Doppler Systems Inc system will
                      automatically plot and transmit DF bearings.
          
     AUTODF N7LUE   - An APRS compatible Doppler interface to permit
                      connecting ANY doppler DF unit to APRS.

DF DEMONSTRATIONS:  To see the results of manual DF bearings in a Baltimore
foxhunt, FILE-LOAD the FOXDF.BK file.  You will see the multiple lines of
bearing all converging to within 1/2 mile of the final location of the Fox.
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!

     To see what the AUTOmatic Doppler DF interface looks like, zoom into
Phoenix, Arizona and FILE-REPLAY the AUTODF.HST file.  You will see N7LUE's
DF unit make multiple hits on three local repeaters in the area.  If you are
doing a DF exercise from a fixed location, you can enable APRS to save all
DF reports in a track history file by setting the CONTROLS-POSFIL to off.
With the Position Filter off, APRS will save every DF posit to the track
history file.  If you are moving, APRS saves all posits anyway.  To see
my first Doppler mobile event, replay DOPPLER.DF.

CAUTION:  APRS does not do spherical geometry, it assumes a flat earth.
For this reason, APRS will not draw a DF bearing line beyond 8 times the
current map scale or 256 miles.  This is contrary to the MAPS-PLOTS-RNGRNGS
command which does use great circle calculations.


MANUAL APRS STATION DF REPORT:  Each APRS station can include a beam heading
in his position report by using the INPUT-DF command.  Unless the station
indicates Permanent, this bearing will normally time out after 2 hours to
eliminate any confusion caused by old/stale reports.  A solid yellow line
indicates an excellent line of bearing, and a more dotted line indicates
less and less quality.  You can use the MAPS-PLOTS-RINGS command to
superimpose range rings on the screen around any station for estimating
distances.  If you are running the WX station option, then the DF report
will override your WX station report with the Beam Heading report.

NON PACKET DF REPORTS:  Even for non APRS stations, their lines of bearing
can be quickly entered by any APRS station using the INPUT-ADD command. 
In this case, simply select the DF symbol, enter a beam heading, and enter
a quality between 1 and 8, where 8 is best.


AUTOMATIC INTERFACE TO DOPPLER SYSTEMS INC UNITS:

    These units have an optional serial data output that outputs a three
digit azimuth once per second at 300 baud.  Just hook it up to APRS and
watch the DF bearings plotted on the map and transmitted to other users
on frequency.  BE SURE TO SELECT 300 BAUD to match the DSI output. 
Include an on/off push button so that garbage reports are not sent to the
APRS computer.


AUTOMATIC DOPPLER DF UNIT INTERFACE FOR ALL MULTI-LED DF UNITS:

     Randy, KA7UUS and Bob N7LUE developed a 2400 baud serial interface to
the popular ROANOKE Doppler DF unit (or any other DF unit that drives an
LED display).  They added a divide by N counter and UART to produce a single
ASCII character 8 times a second or so.  Each character is a letter from
@,A,B,.. ,O indicating the azimuth of the 16 LEDS.  For some DF units that
rotate counterclockwise, the board will optionally use lower case letters
for the opposite rotation.  A VOX circuit disables data output when there
is no DF signal, and an optional PTT circuit can be used to disable the DF
unit when ever a co-located TNC transmits the resulting DF data.  This last
circuit was necessary to prevent the DF unit from generating false bearings
whenever the packet TNC transmitted!

     APRS accumulates, averages and calculates the deviation of these samples.
It then plots a bearing line in the average direction and shows the variance
of the data by the "dottedness" of the line.  A solid line is a solid
non-varying signal, whereas a very dotted line, had a lot of variance in the
reports.  Since APRS averages the data and computes the deviation and
average to 1 degree, the fact that the DF unit is only reporting in 16ths
of the compass is averaged out.  Anyone who has watched a doppler DF unit in
action, understands that the signal bounces everywhere due to reflections
and the distribution of the data is broad enough that the quantization of the
raw data to 4 bits is insignificant.  The add-on N7LUE universal APRS serial
interface is available from N7LUE at the address shown at the end of this 
file.


REMOTE DF SITE:   ALthough any APRS site with the DF interface can be an
automatic DF station, a remote DF station only needs a remote controllable
scanner, the DF unit with serial interface, and a TNC and packet radio. 
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!  A suggested arrangement is as follows:

  A.  Take the 8 characters per second data from the DF unit and connect
them to the serial data input of the TNC.  Take the PTT output of the TNC
and connect it to the optional PTT-SUPPRESS input of the N7LUE interface
to prevent the DF unit from generating erroneous data when the TNC transmits
(and overloads the DF unit).

  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 the location of the remote DF unit and that it is
a DF station, the BText of the DF TNC must contain the LAT/LONG and the APRS
DF symbol character (\):     BT !3856.55N/07629.11W\DF station...

  D.  APRS will then plot a new bearing line for each DF packet received.

  E.  For short FOX transmissions, the TNC should have PACTIME set 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 8 to 4 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.


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 station can also then send
tuning commands via the TNC to the radio serial port!  Even tho there is a
connected link between the control operator and the DF station, APRS will
still monitor all data from the remote site as long as CONTROLS-OTHER is
selected.  Or the DF control station can temporarily make his TNC callsign
be DFNET which APRS will always monitor.  This is legal, as long as he
also places his true call in his BText 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.  If you are ready to
implement automatic remote site DF stations contact me so we can both make
sure it works right.
----------------------------------------------------------------------------


MOBILE APRS DIRECTION FINDING

    APRS is the ideal tool for integrating together all of the DF equipment
in modern DFing, the Doppler DF, the GPS, and the TNC packet link.  If you
have a dual serial port computer, the one port is connected to the TNC (if
used), and a GPS, using the Hardware Single Port (HSP) switch.  The second
COMM port is dedicated to the DF unit.  With this arrangement, the GPS
provides continuous data on the location of the vehicle and the TNC provides
the communication links to the APRS DFing network.  The DF unit provides the
DF data whenever the FOX transmits.  Using the GPS heading, APRS will do an
automatic conversion from the relative bearings from the DF unit to TRUE.
With this arrangement, the mobile DF unit will be seen in the APRS network,
moving along and providing constant bearings to the hidden transmitter.
In practicality, however, there are problems in this plug-and-play scenario.

  1)  First, The heading information from the GPS is ONLY ACCURATE, IF
  THE VEHICLE IS MOVING!   Therefore, APRS only uses the LAST Heading for
  which the vehicle velocity was over 5 MPH.  To help, the F8 key will
  override the normal HSP timer and let you force an immediate GPS fix.
  If you are stopped, and have a compass, enter your heading using the
  INPUT-HEADING command.  (if using magnetic, be sure to set the alt-SETUP
  OTHER-MAG-VAR to the variation in your area and save your CONFIG file.)

  2)  Sometimes the GPS is not putting out good and, more often, the DF
  unit is putting out GARBAGE!  We are beginning to conclude that only
  a Human operator can really figure out when the data is good and
  when to ignore it!  To avoid processing and transmitting BAD data, you
  should place a Push button in the DF data line and only press it when
  data is valid.  See the DFSP DF Single Port Mode description.

  3)  Whenever the TNC transmits APRS DF or position data, it totally
  garbles the DF unit!   There are two solutions:

      a) Same as item 2 above!
     
      b) For fully automatic sites, the DF unit must have a MUTE circuit
      connected to the TNC PTT line so the the DF is DISABLED whenever
      the TNC transmits.  Diode ORing of the PTT leads of every transmitter
      at the site should be conisdered.  The next N7LUE interface will have
      this MUTE included.

  4)  Most laptop computers only have one usable COMM port!


DIRECTION FINDING IN DF SINGLE PORT MODE!

   Since we have just about concluded that you need a manual push button
for the operator anway (thanks Joe Moel, K0OV), it becomes trivial for
us to add the DF serial data to the existing shared GPS/TNC/HSP serial
port.  The following schematic shows how the serial data from all three
devices is switched and how a second pair of contacts is used to alert
APRS, when the DF data is connected.

                                 
                                      DF PUSH           LAPTOP
            ----------                BUTTON            SERIAL
TNC DATA >--|  HSP   |                                   PORT
            |        |--------------------*
GPS DATA >--| SWITCH |                    |
            --*-------                   ----------*----------> RXD
              |                  
              |                           |
DF DATA  >--------------------------------*
              _
              ^ D1                       ----------*
              |       4.7 k                        |
TNC DSR  >-----------/\/\/\/--*           |      -----
              |               |           |      /////
              |               |           |                    
TNC DTR  >----*---->|---------*-----------*-------------------> DSR
    RTS            D2
         
   
     The second pole of the DPDT push button not only grounds the DSR
input to the PC so that it knows to begin parsing DF data, but also
grounds the TNC DTR (RTS) input so that the TNC holds off any packet
data until after the DF switch is released.  Notice that two diodes,
D1 and D2 isolate the TNC DTR (RTS) line so that either the HSP or
the DFSP can pull the line to ground without affecting the other.  Some 
TNC's (PACCOMM) use the RTS line instead of the DTR line for holding off
TNC data,  so check your TNC manual.  The TNC DSR line is only used to 
provide a source of +V.  APRS distinguishes between the TNC and GPS data 
using the normal HSP logic.  Remember to provide the DSR or other source 
of +V for the HSP circuit.

     This DFSP circuit can be built within its own back-to-back DB-9
connectors with a pigtail to the hand-held push button.  This circuit
can then be inserted between the HSP and the Laptop at any time and
the HSP can still be used with or without the DF circuit.  Or, since
it is so trivial, just wire it permanently to your existing HSP device.

NOTE:  THE N7LUE DOPPLER INTERFACE MUST BE MODIFIED FOR 4800 BAUD vice   
THE PRESENT 2400 BAUD IN ORDER FOR DFSP to WORK at the same baud rate
as the GPS and TNC.   This is easy to do by cutting a trace and adding
a jumper so that pin 14 of the baud rate chip U3 is held high, and pin
15 is grounded.  Similarly, you must modify the DSI unit from 300 baud 
to 4800 baud.

CONFIGURATIOIN:  APRS should be in the normal HSP mode.  Then select the
alt-SETUP-OTHER-DFSP mode.  This enables the sensing of the DSR line to
indicate that APRS should begin DF processing.  You can tell that DFSP
is enabled on the CONTROLS panel (tab key) by seeing DFS in the left
window.  the DFSP command toggles the mode on and off.  It can be saved
in a config file.

CAUTION:  DO NOT SELECT DFSP mode without the above interface or without
having the PC serial port DSR pin held high.  If the pin is not held
high and DFSP mode is selected (and validated), then APRS will only look
for DF data and will NOT SEE ANY TNC packets NOR ANY GPS/HSP DATA!  It
will appear to be in a total lock up condition.

OPERATION:  When you press the DF button, APRS begins a 5 second dead time
during which it is collecting DF data for averaging.  At the end of this
5 second period, a DF fix is computed and displayed.  Therefore you should
hold the button for at least 5 seconds to get good data, and you can hold
it longer, if you like.  These DF reports are added to your current posit.
This means that there can be an ambiguity between the time of the last
POSIT and the DF data.  For this reason, the operator should be driving
on a straight course from the time of the last GPS fix, until he completes
the release of the push button.  To minimize this problem, I have added
the F8 key so the operator can force an instant HSP update at any time.

If your vehicle has a heading of 000 or there is no heading information,
then APRS will ignore any DOPPLER inputs or RELATIVE entires using the
INPUT-DF command.  Fix this by using INPUT-HEADING to enter 360 if you 
are really pointed due north, or enter any other heading if you have a 
compass.  If you have turned since your last fix or your speed has dropped 
below 5 MPH, then you will also need to use the INPUT-HEADING command.
To help you visualize your heading, the normal velocity leader on your
vehicle symbol is expanded by a factor of 4 while in DF mode.

MARKING FIXES:  I have also added the INPUT-SAVE-my and INPUT-UPLINK-my
commands.  These permit the operator to save a DF FIX or special position
on their map for future reference.  Pressing these commands makes a copy
of your current position report (with DF bearing if available) as an
OBJECT.  They are named with a serial number and the last three digits
of your call.  These objects will remain on the map at that location
to serve as reference points.  The INPUT-UPLINK-my is the same, except
that the saved OBJect is marked for uplinking to the net.

OPERATIONS:  Start driving.  As long as you are getting good DF data,
periodically press the DF button.  If you want to mark one of these
fixes for future reference, hit INPUT-SAVE.  If you want to share it with
others, hit INPUT-UPLINK.  If you are maneuvering, hit the F8 key before
taking a DF bearing to get a current heading.  If you are stopped, or
your speed has dropped below 5 MPH then you must use INPUT-HEADING to 
update your heading with a magnetic compass or a map.  If you simply  
pull over and maintain your last heading, then your heading will still
be good.  If you do not have a DF interface, use the INPUT-DF command 
to manually enter your DF bearing, either true or relative.

------------------------------------------------------------------------
SOURCES:  Products not tested nor endorsed by APRS:

N7LUE Universal DOPPLER APRS serial INTERFACE:     
     
     Robert Swain, N7LUE        410-766-2494 eves
     360 Phirne Rd              * He's military and travels alot, so
     Glen Burnie, MD 21061        be patient
     cmpk59d@prodigy.com

ROANOAK BOARDS:  Marty Mitchell, N6ZAV at 340? Otero St, Costa Mesa, CA 92626
                 His number was 714 760-6060.

AB5CK Doppler UNIT:  Based on the Roanoke Doppler shown in "Transmitter 
Hunting - Radio Direction Finding Simplified" by Joseph D. Moell - K0OV.  

     AB5CK Doppler PC board ................................ $49.95
     AB5CK Doppler with parts .............................. $99.95
     Machine DIP socket set for ICs (recommended) .......... $14.95
     Antenna parts kit (chokes/PIN diodes/resistors/caps) .. $14.95
     Fully assembled and tested AB5CK Doppler PC board .... $199.95
     Fully operational AB5CK Doppler in cabinet ........... $399.95
     Shipping free for most US orders.   Texans add 7.75% sales tax
 
Thomas M. Lewis - AB5CK
6721 Rolling Hills Drive,  
North Richland Hills, Tx.  76180
Phone     817-428-8200
Internet  tlewis@dfw.net


