OMNI-DFING with APRSdos 1994 =========================================================================== 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. APRS plots a map of range contours around all stations which represents the possible range of that station based on his antenna gain and height over average terrain. It also includes up to 10 dB of directivity to account for any favored or blocked direction. Where two coverage areas intersect or overlap, direct communications are generally possible. Knowledge of local topography is important in making this assessment. This PHG plot is an ideal tool for visualizing ANY radio network WHETHER OR NOT APRS or PACKET is being used! DFING WITH OMNI SIGNAL STRENGTH REPORTS: By modifying these PHG equations for plotting received signal strengths, a weak signal is drawn as a larger area of probability than a very strong smaller area. The signal reports are made on an S0 to S9 scale for relative signal strength indication. These signal strengths will be plotted as circles from a dark gray up to a bright red, with the radius of the circle decreasing with stronger s ignals. A signal strength of 0 represents NOTHING HEARD and is plotted last as dark gray on top of everything else. This way they clearly show where the transmitting station is NOT. Since the PC can only overlap circles, the user should visualize only the perimeter of the overlapping colors and ignore the centers of the circles. The probable location of the transmiter will be in the area of the most concentrated perimeters of brighter circles. The location of the DF signal will never be at the center of a circle. THE LOCATION OF THE SIGNAL 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. PHG CIRCLE OF SIGNAL OR JAMMER: Also do not be fooled if the signal or jammer has a high position or power. Then you must not think of him as a point on the map, but a PHG circle that covers his range area! In this case, you are looking for where his imaginary PHG circle intersects the perimeter of all the DF SIgnal Report contours on the map. See the example on the web page of Omni-DFing a signal over 100 miles away at 2800'. The Omni Signal Plot made no sense until we realized we had to look at the signal-sources own PHG circle too. OMNI-DF COMMAND SUMMARY: The following list sumarizes all of the commands used in APRSdos for performing direction finding with the OMNI technique. 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 - Enter either a beam heading or a signal report. Enter "0" for an OMNI signal strength report. (360 means North, not 0.) INPUT-ADDobj - Add voice DF reports to the map. Select the DF station symbol and you will be prompted for the BeamHeading or "0" for OMNI. INPUT-PwrHtGain - For your station to properly know its DFing listening range, it needs to know your PHG data. MAPS-PLOT-DF-OMNI - This command is used to plot the OMNI-DF profiles. MAPS-PLOT-HEARD - Plots Signal strength circles from only stations that have HEARD the fox. (for monochrome displays) MAPS-PLOT-NOTheard - Plots only signal strength circles from stations that have NOT HEARD the fox. (for monichrome screens) MAPS-PLOT-PHG - Plots Power-Height-Gain range rings around all stations. MAPS-PLOT-RngRngs - Plots range/Bearing to the cursor. OMNI-DF EXAMPLE: The image on the WEB page shows the result of my first 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 (showing you the value of this come-as-you-are technique). I added these stations to the map using the cursor and INPUT-ADD 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. Thus, his PHG DF circle is offset to the west. 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 terrain, 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 but the aggregate. 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 the NOT-HEARD or Black "0" plots last since they are POSITIVE report that the FOX is NOT within their range. 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 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 (be sure to be on right freq!) 1 Detectible signal (Maybe) 2 Detectible signal (certain but not copyable) 3 Weak signal marginally readable 4 Noisy but readable 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. 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. Due to fading and multipath, mobiles have HALF the range of Fixed stations. So new in APRS852, all PHG circles are shown at HALF their original value to better represent the effective range of the APRS network to mobile users. For purely PHG circles between fixed stations, then the original PHG circles can be optionally called up. 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))) The modifier of SQR(P/10 *G/2)is unity at 10 watts and 3 dB R = R * .85 Present fudge factor 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 blindly looking at circles 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 you understand that APRS is only a tool to help YOU visualize the reports you get. They dont help improve the quality of them. And you must take many reports with a grain of salt. Does the person have a clue about height gain? Does he know really where he is. Does he really know which way is north (if he reports a directional offset) is he really hearing the fox or something else, is his radio stone-deaf, etc? FADE CIRCLE TECHNIQUE: Once you know the area, now you can use the other APRS omni technique in your mobile and just drive right/left/past/and back to localize the signal even down to the driveway. See the Fade-Circle Technique in README\DF.TXT de WB4APR, Bob