Using APRS For Direction Finding

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 MULTIPLE 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.


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 which it can use to 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.

OMNI-DF EXAMPLE: The file DF-OMNI.BK is a snapshot of my first Omni-DF attempt on a Sunday afternoon foxhunt. I simply called around on various 2m repeaters in the area and asked stations to listen for the fox. It turns out that all but one other report were from mobiles. I added these stations to the map using the INPUT-ADD command. To see the result, do a FILES-LOAD of DF-OMNI.BK and hit the MAPS-PLOTS-DF command.

First, notice that APRS blacks out the areas where the fox is NOT based on the null reports. You will always get far more NULL reports than HEARD reports. These are very VALUABLE!!! because there are more of them and they instantly eliminate most of the surrounding area! With these first 6 null reports, I knew instantly that the fox was not to the west, south, or east of Baltimore. It took the mobile fox-hunters another 45 minutes to figure this out!

Second, notice the offset circle of KA3DZZ. This is because he reported that he was west of a ridge and didn't hear as well to the east. 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 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. This overlay blocks out some segments of colored circles to eliminate areas where the FOX is not.


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 operator 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 soliciting 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.HTM and PROTOCOL.HTM 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

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. See the following figure 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 perpendicular between the two points. 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 midway and perpendicular to those two points. This line of bearing is represented by the asterixed lines above. Turn and choose a new direction 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 transmitter. Drive to that indicated 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.

NUMBER OF POINTS: You need three points before APRS can compute and display the 3 intersecting lines. Four points will generate SIX lines and the map will be quite messy. Five points will genereate TWELVE lines and it will be impossible to make ANY sense out of the mess. You are best off limiting to THREE Fade points. Then go to the probable area, reduce antenna gain by 30 db and do it again with a 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 because their stations are not identical and its a one in a million chance that their fixed location is on the fade circle anyway. In summary, 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 UNITS - Connecting a second COM port to the serial output of these units:

Doppler Systems Inc (300 baud) N7LUE interface (2400 baud) KA4IIA Doppler unit (4800 baud) Agrelo DFjr (4800 baud)

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!

To see what the AUTOmatic Doppler DF interface looks like, zoom into Phoenix, Arizona and FILE-REPLAY the DF-AUTO.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 DF-FOX2.DF.

CAUTION: APRS does not do spherical geometry, it assumes a flat earth. For this reason, APRS should not be used for HF DFing beyond about 250 miles. This is contrary to the MAPS-PLOTS-RNGRNGS command which does use great circle calculations. Also to minimize out-of-area confusion, APRS marks the LENGTH of the bearing lines to be only equal to the current MAP scale. If you are DFing on a 4 mile map, then the bearing lines will only be 4 miles long. etc...

MANUAL APRS STATION DF REPORT: Each APRS station can include a beamheading in his position report by using the INPUT-DF command. 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 the map center for estimating distances. If you are running the WX station option, however, your DF bearing report will be overwritten as soon as your next WX report comes in.

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. These reports will NOT timeout, however, and should be killed after use.

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.

Designed from the ground up to be APRS compatible, it includes data processing so that erroneous readings are significantly reduced from other doppler designs. Has both automatic and manual modes. Operates at 4800 baud for ease of combining with GPS data using their multi-port adapter. In AUTO mode, the DFjr outputs a GPS posit and DF bearing at a fixed rate. In manual, data is only passed to APRS when the user presses a button. The multi-port adapter combines your TNC, DFjr and GPS data all into the same PC COMM port. Using this adapter, configure APRS for one port TNC operation (even if you are not using packet), then use alt- SETUP-DF menu to select the DFjr. This tells APRS to watch for DF data on the TNC comm port. THen select the GPS-SPM mode under the alt-S-GPS- MODES menu so that APRS will also parse GPS data on the TNC port. Even if you normally run HSP mode with other TNC's, your HSP adapter is not being used. The DFjr is doing the switching so SPM mode should be selected. (If you have the dual port Pico-TNC, however, then you must select HSP.) The multi-port adapter normally passes TNC data straight through to the PC. But when the DFjr outputs a report, it sends out a POSIT followed by a DF report which are combined with the TNC data at the COMM port.

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 interfce is no longer available but the new KA4IIA unit is. See below.

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:

  1. 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).

  2. 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.

  3. 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/07629.11WDF station...

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

  5. 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.

  6. 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.

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.


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:

    1. Same as item 2 above!
    2. 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!

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 tell APRS to process DF data on the serial port 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.

CAUTION: DO not enter DFSP mode with N7LUE enabled if you do not have the DSR pin held high, or the system will appear to lock-up while waiting for DF data...

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.

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-MY-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-MY-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: The INPUT-SAVEpos and INPUT-UPLINKpos commands 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-UPLINKpos 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-SAVEpos. If you want to share it with others, hit INPUT-UPLINKpos. 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 4 MPH then you must use INPUT-MY-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:

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