Shareware Overload Trio 2

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Text File | 1994-09-27 | 32KB | 533 lines

DF.TXT USING APRS FOR DIRECTION FINDING OVERVIEW: APRS NOT ONLY PLOTS BEAM HEADINGS BUT IT HAS TWO METHODS FOR TRANSMITTER LOCATION USING ONLY OMNI DIRECTIONAL SIGNAL STRENGTH CONTOURS! The first omni technique displays overlaping circular signal strength contours over 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 now computes this circle and therefore the location of the transmitter by computing the FADE circle based on these three or more points. This FADE circle technique was added in APRS version 5.8d and has its own section below. Since the use of omni directional signal strengths for locating a hidden transmitter is all new, it is presented first, followed by the FADE circle technique, followed by the more classical BEARING TRIANGULATION method. OMNI-DIRECTION FINDING: VERSION 5.7c: This version has incorporated 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 became possible with APRS version 5.7A which added a line-of-sight Power- Height-Gain (PHG) reporting and display format. This format allows a station to enter his transmitter power, antenna height above average terain, and his antenna gain which will be included in his APRS position report. APRS can then draw range circles around each station showing his relative communication range. For backward compatibility, all stations not reporting the PHG format are plotted assuming the default parameters of 10 Watts, 20 feet HAAT, and a 3 dB antenna. If each station includes these parameters in his position report, 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! Initially, my equations are straight from the textbook and may need to be increased or decreased based on experience. Please 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: To use these PHG equations on receive, obviously, a very weak signal would imply a much larger circle of probability than a very strong circle. In the absence of any precise measurable signal strength indication on most VHF radios, I simply chose a scale of 0 to 9 as a relative receive 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 modify the normal PHG display circles 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 the stronger the signal is reported. The number 0 represents a NULL or NEGATIVE report, meaning that NOTHING within that stations horizon was detected. These NEGATIVE reports are plotted last as dark gray on top of everything else, since they identify circles where the transmitting station is NOT. Since the PC computer does not ADD colors together, but only overlaps colors, the user should visualize all the overlapping colors and not just the brightest ones on top. The probable location of the jammer will be in the area of the most concentrated overlaps. Also do not be fooled by the brighter circles. Almost by definition, 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 EDGE OF THESE CIRCLES. If it was near the center, then the signal would have been much stronger, and the circle would be a brighter color and smaller! Please load the DF-OMNI.BK backup file to see our first omni-df attempt. See the section below that describes what you will see in the DF-OMNI.BK 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 - This command is used by an APRS station to enter either a beam heading or a signal report. He will be prompted for station parameters. If a beam heading of (0) is entered, then APRS assumes the entry is for an OMNI signal strength report. Remember that 360 degrees means North. INPUT-ADDobj - This command can be used to add a voice reporting or non-APRS station to the map. If the DF station symbol is selected, then 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-PwrHtGain - This command is not used for DF, but is used for showing your station Power, Height and Antenna gain parameters. MAPS-PLOT-DF - This command is used to plot the OMNI-DF profiles. MAPS-PLOT-HEARD - Plots only the DF rings around stations that have heard the fox. This is for monochrome displays to separate the NOT-HEARD circles. MAPS-PLOT-NOTheard - Plots only the DF rings that bound the stations that have not heard the fix. This is to eliminate confusion on monochrome screens. MAPS-PLOT-OVERLAY - Plots the DF rings on the existing map so that roads can be seen. MAPS-PLOT-PwrHtGain - This command will plot the Power-Height-Gain range rings around all stations. MAPS-PLOT-Rings - This command is the old Range Rings command which is used to draw a circle of 1 and 0.5 of the selected map scale. DESCRIPTION OF ACTUAL EVENT 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. Two days after finishing the DF capability, I learned of a FOX hunt in Baltimore. Although there were NO other APRS operators at home that I could raise that sunday afternoon, I scoured several voice repeaters in the area and got a few RF signal reports on the FOX. I then added these stations to the map using the INPUT-ADD command. Please LOAD the backup FILE named DF-OMNI.BK and hit the MAPS-PLOTS-DF command. First, you will 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 ever 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 thing is the report 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. If the FOX was closer to him, then his signal strength would have been even stronger, and the circle even smaller! APRS draws stronger reports smaller for two very important reasons. First it recognizes that a stronger signal means the FOX is closer to to the reporting station. Secondly, 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 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. If we would have had only a few more stations hearing the FOX, imagine how nicely we could have eliminated a lot of the ambiguity. 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! I can't wait to get a lot of APRS stations doing this! RECOMMENDED OMNI-DF PROCEDURE: As soon as the APRS net is alerted of a FOX or a hidden transmitter, each APRS station should first listen on the reported frequency and enter his signal strength. Next each of the APRS operators should go onto the local voice repeaters and ask for OMNI-SIGNAL strengths from mobiles and any other fixed stations. The APRS operators use the INPUT- ADD command to add these stations to the map. 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. Having random APRS statios randomly soliciting reports on a random number of voice repeaters causes a lot of duplication and repeats. Be sure to get the stations reported signal strength, location, Antenna height-ABOVE-AVERAGE- TERRAIN (not sea level or above ground) and any offset in his horizon. My interpretation of the signal strength scale is as folllows: 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" I hope there are some fox hunting groups out there that can give this new feature a good test. Don't forget that 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-ADDobj command. If the object is given the DF symbol type, and a beam heading of 0 is indicated, then the user will be prompted to enter the proper DF signal strength and station information. For more information on the Power-Height-Gain formats, see the DIGIs.txt and PROTOCOL.txt files. 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 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 has now implemented 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, APRS creates a Fade marker. In order to be compatible with both GPS equipped and manual stations, APRS puts the marker at the location of the cursor. This is not a limitation since a GPS station can simply press the Go key to instantly place the cursor at his present location befor pressing F5. This also permits him to use the cursor in the usual manner for placing manual reports also. 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. Although this FADE circle technique is one of the neat optional features provided to registered DF users of APRS, I have also made it available in the basic package as well for up to three fade points so that everyone can try it out. Registered DF users, of course, can plot any number of points. PLEASE NOTE! ******************************************************* The difference between this technique and the OMNI-DF function in APRS, is that the FADE circle technique takes advantage of mobile direction finding stations 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 their exact sensitivity, antenna gain, and antenna height above average terrain CANNOT be reproduced anywhere else, by anyone else, to find a second or even third comparable point. So that is the difference between the two techniques. The FADE circle is for mobile OMNI fox hunters, and the OMNI-DF capability which plots signal strength contours is for fixed OMNI stations. APRS DIRECTION FINDING WITH BEAM HEADINGS AND DOPPLER DF UNITS APRS is an excellent tool for instantly plotting and diseminating DF bearing information. APRS has several methods of obtaining lines of bearing for plotting: MANUAL APRS - Any APRS station simply selects the INPUT-DF command and types in 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 your second COM port to the serial data out- put of a Doppler Systems Inc system will automatically plot and transmit the bearing of the FOX. AUTODF N7LUE - An innexpensive APRS compatible interface to permit connecting ANY doppler DF unit to the APRS serial port. 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, 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. 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. For this reason, APRS will not draw a DF bearing line beyond 256 miles. MANUAL APRS STATION DF REPORT: Each APRS station can include a beam heading in his position report by entering 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. As a further aid, the MAPS-PLOTS-RINGS command 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 the DF report will override your 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 INPUT- ADD command which adds them to everyone's map in real time. In this case, simply select the DF symbol, enter a beam heading, and enter a quality number between 1 and 8, where 8 is best. 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 PROTOCOL.txt file, and is reporduced here: BText !DDMM.xxN/DDDMM.xxW\CSE/SPD/BRG/N0Q/DF report... Where: DDMM.xxN is Latitude, DDDMM.xxW is Longitude \ (Backslash indicates a Triangle symbol for DFing) CSE is course (000 for fixed station) SPD is speed (000 for fixedstation) 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. AUTOMATIC DOPPLER DF UNIT INTERFACE: To capitalize on the excellent map features of APRS, 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 LED display). They have added a divide by N counter and a UART to produce a single ASCII character report 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. 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! When the interface is connected to the serial port, APRS accumulates, averages and calculates the deviation of these samples. It uses this info to plot a bearing line in the average direction and also indicates 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 following address: Robert Swain, N7LUE 820 38th St West Bradenton, FL 34205 Marty Mitchell, N6ZAV at 340? Otero St, Costa Mesa, CA 92626 is selling an improved version of the ROANOAK DF unit. His phone number is 714 760-6060. REMOTE DF SITE: ALthough any APRS site with the DF interface can be an automatic DF station, the APRS PC computer can be eliminated for remote site operations. All that is needed is a DF receiver, the DF unit and 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! It is simple to configure the TNC to do this as follows: A. Take the 7.5 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 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. 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.