VICINITY.TXT APRS Vicinity Tracking (tm) =========================================================================== Document version: 8.4.3 Document dated: 25 Feb 99 Author(s): Bob Bruninga, WB4APR ABSTRACT VICINITY.TXT Tracking by closest Digipeater (new since ver 829) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ APRSdos implements Vicinity Tracking (tm) which gives a first order approximation of position based on the nearest digipeater to any new station. This has advantages in many situations as follows: 1) On first packet receipt from a new station. No matter what the packet, APRSdos will plot him using an ambiguous plot in the vicinity of his closest Digi. 2) Plots WinAPRS "positionless WX" so you don't have to wait an hour to find out where it is... 3) Plots stations who are transmitting 0000/0000 posits 4) Can crudely track THD7's, Mic-E's and MIM's without GPS if they set a 0000/0000 posit. 5) Can track indoors by using nearly-deaf digipeaters. See following section. This last function is experimental and not yet fully agreed among the APRS authors. It is included for experimentation. WIDEn-n IMPLEMENTATIONS: For now, the path of RELAY,WIDEN-n works well in a WIDEn-n network, because the first digi will be identified. But home stations must also use a similar path such as either WIDE,WIDEn-n or DIGI,WIDEn-n to also make it work. Finally all WIDEn-n digis must have the UIFLOOD "ID" disabled. This is because ID inserts the LAST digi heard in the path, so that the FIRST digi is lost. I have asked Kantronics to add an option to the WIDEn-N algorithm so that in all cases the FIRST digipeater will do callsign insertion, so that the packet will always arrive as a DIGI1,WIDEn path. This adds the 7 bytes to all packets, but saves having to do TRACEn-N digipeating which would add 7*N packets! In the mean time, it works fine if stations just use WIDE,WIDEn-n. INTERIOR TRACKING APRS may be used for crude tracking indoors such as in convention centers and other large distributed structures. This is accomplished by installing nearly-deaf digipeaters called BOXn digipeaters in each major room area. A good example is the HARA Arena at the Dayton Hamvention which consists of approximately seven large interior spaces. The APRS software simply plots the location of interior users in the vicinity of each of about a dozen-or-so BOXn digipeaters that last heard the users packet. THis process is called VICINITY Tracking and can also be used to approximate the location of new stations near any digipeater until a valid posit is received. The Interior application of Vicinity Tracking can actually be more accurate than GPS (under 50 meters), and can be adjusted to the size of the rooms in any given application. A receive range of about 50 m (150') is the nominal design range. As long as all users set their transmitters to approximately 100 mw, then the threshold of these ranges is more accurate than one might think. THis is because with the BOXn digi's antenna in the clear in the room, then the path to any walking user is almost line-of-sight and follows the 1/r squared law quite well. Since it is a square law, the detection difference between 50 and 100 meters, for example is over 6 dB. These BOXn digipeaters may even be used outdoors so that users in a confined area, such as the flea market at Dayton, can be localized without them having to carry a GPS. Since each BOXn digi is able to have its own hearing range and it conveys that to the system in its BText, these BOXn digis may use a longer range. Even though the reliability of this system is based on an assumed 100 mw transmitter power from the users, the threshold range for a 1 watt user in a 50 meter system would be only 3 times greater or about 150 meters. If most BOXn digis are confined to physical rooms, the loss through adjacent walls will probably reduce, if not eliminate this problem. BOXn DIGIS: These are standard APRS digipeaters that use either the PacComm or KPC-3 TNC's. These TNC's are given the MYCALL of BOXn and the UIDIGI of RELAY and a local application name such as HARA (at Dayton). The local interior path name (HARA) is the preferred via PATH for users indoors to prevent bedlam on the 144.39 frequency from the hundred vehicles in the parking lot who may also digipeat RELAY packets. But RELAY is included for those few people who forget. Any packet heard in the vicinity of any of these BOX'es will be digipeated with MYCALL substituted so that all APRS software can tell which digi is closest to the user. At Dayton, the old frequency of 145.785 may be used for interior tracking to help split the load on 144.39 and to allow just RELAY to be used. RANGE REDUCTION: To make interior tracking work, the range of the BOXn receiver must be reduced by many orders of magnitude. One way to reduce receive sensitivity while maintaining normal antenna operation for the BOXn digi tramsmitter is a whip antenna with back- | to-back diodes and a partial dummy load at the base. THe | diodes in series with the antenna connection, effectively | isolate the receiver but allow the antenna to still be used | by the transmitter. By adding two 100 ohm resistors to the | whip, a 50 ohm load is presented and the length then of the | whip can be trimmed to adjust final sensitivity. In this | drawing the Diodes are shown as "D" and the resistors as "R". | *-***-* The letter "V" is trying to show the center pin of a BNC R D D R and the two bottom *'s show the ground ring of the BNC. | *** | The transmitter should be operated at about 100mw or less. | | | The transmitter will see the antenna and only needs enough * V * power to be heard in the general area of operations. Note, however that diodes on antennas generate Intermod for everyone, so these kinds of antennas should not be used in a rich RF environment such as Dayton. MFJ 8621 DATA RADIO. AN IDEAL BOXn DIGI APPLICATION A BETTER solution is to use a transceiver that can have additional atten- uation added to the receiver only. At Dayton, a receive range of about 50 meters requires about 85 dB of attenuation to a nominally good receiver. Unfortunately, this cannot easily be done at the antenna as shown above or the Transmitter is also similarly attenuated making the BOXn digi to weak to be heard. Usually for these short ranges it is necessary to do minor surgery on the receiver to add separate attenuation. This is very easy to do in the low cost MFJ 8621 data radio. Simply lift one end of the T/R Receive switching PIN diode D8. It is the diode that looks just like a plastic transistor closest to the antenna connection. Lift the end closest to the Antenna. This adds more than 60 dB of attenuation and makes the radio perfect in this application. If the range is too short, then try inserting a 5K or less resistor in series until the proper range is reached. Probably good to set this for about a 200 meter range with full 1/4 wave antenna and then you can adjust the final distance per application by trimming the antenna. If you use the MFJ data radio as modified here, then there is NO NEED for the diodes as shown. In fact, they should NOT be used at Dayton where they would generate horendus QRM. But do keep the 100 ohm resistors to serve as a good load as you trim the antenna as needed for the final 50 meter range. SOFTWARE: APRS scans incoming packets for the presence of the key path "BOX" in the first digipeater field. These packets will be given a position in the vicinity of that specific BOXn digipeater. CALLSIGNS only will be displayed, probably as a list in an orderly fashion around the location of the BOXn so it is clear that they are in that vicinity. BOXn ICON: The BOXn digipeaters are given a special ICON of \i or #i. On APRS maps, these ICONS are overlayed with the number # and so there is no need for the callsign to clutter the local area. Thus these \i or #i icons never display their own calls. Further the BText of these digis must include their exact location and also their RANGE. Their range is indicated in braces, for example {50} would mean a nominal range of 50 meters. The APRS software uses these values to know how closely to display the nearby callsigns of users... APRS SOFTWARE SPEC FOR VICINITY TRACKING: BOXn ICON #i. These ICONS are boxes with the # overlay character STN ICON /_ where "_" here represents the SPACE character. This is the APRS null ICON which is nothing. All stations heard via a BOXn digi will be assigned this ICON so that no ICONS are plotted to clutter the map. BOXn BText These digis use the standard ! position format and must include the approximate listening range in braces in meters, such as {50} for example. Values up to 999m are permitted. These listening ranges are defined to be for 100mw users. A system based on one watt users would imply a factor of THREE times further. BOXn DISPLAY Only the BOXn ICON is shown on the map. The "BOXn" callsign is redundant and not shown to leave more room for Vicinity station's callsigns. PROCESSING: Any incoming packet with the occurrence of the string "BOX" in the digipeater header is assumed to have been digipeated via one of the BOXn's with MYCall substitution. The exact call plus its number is extracted, and a STATION search is conducted to find the location of that BOXn digi. The position of the reporting station is then assumed to be in the vicinity of that BOXn and is displayed accordingly. In APRSdos, THe BOXn digi location is copied as the assumed position of the reporting station plus/minus randomly in both LAT and LONG the hearing-distance as reported in {xx} within the BOXn BText. These calls are plotted in the vicinity of the BOXn with no ICONS, thus showing that their location is not known precisely. COINCIDENTAL LARGE SCALE VICINITY TRACKING APPLICATION: APRS can use this technique on any station which has no POSIT on record. In this case, it is treated just like a BOXn packet except that the "vicinity" display is larger (about 1 mile). This display enhancement is advantageous in the following situations: 1) New station with no posit on record 2) Receipt of the WinAPRS Positionless WX format 3) Receipt of a 0000/0000 position. Although all APRS software is not supposed to transmit any 0000/0000 posits to save air-time, a THD7 or Mic-E can use this null posit to advantage to trigger the VICINITY plotting function.