APRS In Search And Rescue

NOTE: I had to reconstruct this page from a defunct Wiki page and a couple of other pages it refers to here were snagged from the Wayback Machine. Please notify me if there's something missing or incorrect on this new page: Curt.WE7U at GMail dot com

APRS Clients Useful for SAR (topo map capability)

Programs which connect APRS clients with external mapping software

List of APRS Features Useful for SAR


Summary of Symbols to use on SAR Status Maps

Canada ICS Map Symbols: SAR Status Map Symbols (no longer available?)

Note that FEMA (U.S.) uses Green/White for the ICP symbol. There may be other differences between the Canadian and U.S. map symbologies.

Search and Rescue, APRS and Xastir

''Having a mapping system could provide additional information that a basic mapping program could not cover. In addition, not all SAR teams can afford mapping programs.''

Our county's SAR group is all volunteer, except for a few county sheriff's running the show. Money is a major issue and one of the main reasons I've been adding SAR and topo features to Xastir. I prefer having choices, particularly inexpensive/free choices. I'd also like some features that the lesser-cost mapping programs just don't have. Features that can be used to speed up a search, reduce a search area, save lives.

You don't have to have money to buy the latest laptops, OS, or mapping programs. Xastir on Linux or Windows is a viable alternative, and the maps are free or almost so. You'll need a bit more horsepower/memory to run it under Windows as opposed to one of the Unix variants, but it does work. If you want the most bang for your buck, run old laptops using Linux/Xastir. You can probably pull together a free mapping station for SAR using donated laptops, free OS (Linux), free Xastir, and free maps. You can also duplicate the system without fear of anyone coming after you.

Curt, WE7U Snohomish County Volunteer SAR, Washington.

Search & Rescue Features in the Xastir APRS Client, on Wetnet.net


Note that using GpsBabel, a free program, you can convert various GPS formats into other formats that may be useful with mapping programs.

One use is to convert waypoints from National Geographic Topo! into NMEA format or GPX format files. Gpsbabel now has support for reading/decoding Topo version 2.x/3.x/4.x save-files. With a simple command you can convert a TPO or TPG file to a GPX file, then in the Xastir/scripts directory is "gpx2shape", a script that will convert the GPX data to a Shapefile map. Use that Shapefile map with Xastir. This is a great way to get hiking trails for your area to appear on your maps! National Geographic's mapXchange server is where to download these waypoint files contributed by other Topo! users, or create your own using your GPS and Topo. Note that NatGeo has now hidden this info behind a license page: You must agree to the license before continuing on to the user-contributed map data.

GpsBabel can convert between a wide variety of formats and is an open-source program (free!). Check it out to see if it may be the "glue" you've been looking for to go between various GPS and mapping formats.

"All Topo Maps" plus APRS2ATM and an APRS Client

This can be used with UI-View, WinAPRS, APRS+SA, or Xastir. It has been reported that connecting APRS2ATM directly to a TNC has not been reliable in the past (requires manual intervention when the packets stop flowing). UI-View/WinAPRS/APRS+SA/Xastir can be connected to the TNC, then APRS2ATM can get its data cleanly from them.

Start up the APRS client. Set up the APRS program so that it provides a listening socket for another program to connect to in order to get APRS packets.

Start up "All Topo Maps" executable.

Start up "aprs2atm.exe". Set it to connect to the server socket provided by the APRS client program. APRS2ATM will connect to both programs then start drawing objects and tracks on All Topo Maps. There is an optional config file which specifies the callsigns you wish to track.

You can bring up other APRS maps using the APRS client program at the same time.

OziExplorer plus OziAPRS and an APRS Client

Start the APRS Client. Ensure that the server function is enabled.

Start Oziexplorer.

Start OziAPRS. If UI-View is the client, connections will occur automatically to both UI-View and Oziexplorer.

If you start OziAPRS before Oziexplorer, select the OziAPRS function to connect to Oziexplorer.

If you start OziAPRS before UI-View, or are running a different client, select the OziAPRS function to connect to the server.

Garmin RINO Radio/GPS plus Xastir APRS Client

Xastir, combined with GPSMan, can perodically download waypoints from an attached RINO radio. Any waypoint beginning with "APRS" will have that portion of the string stripped, then the waypoint will be turned into an APRS object which will appear on the map and may be retransmitted out over APRS as well. Useful for those SAR missions where some people are equipped with RINO's and others with TH-D7A's or TM-D700A's. The RINO users will see each other, the APRS users will see the other APRS users plus the RINO users. The Xastir screen will show all of them.

Garmin RINO Radio/GPS plus OziAPRS Client

OziAPRS has the capability of decoding Garmin Rhino positions. You must have a Garmin Rhino connected to the OziAPRS serial port for this to work. This will allow you to plot the positions of other Rhino units on the Oziexplorer map screen, but it won't feed the data to your APRS client. Some SAR units are experimenting with this method rather than using APRS. I don't believe position reports are allowed on GMRS, but they are on FRS.

Why use APRS Compressed Objects/Items for SAR?

Note: It says "Objects/Items" above, not GPS trackers.

Simple: The relative precision for placement on a map is only to dd mm.mm significant digits in "normal" APRS position packets. By going to "compressed" posits for transmitting stations and object/item positions, you gain at least an extra digit of precision.

I'll speak in terms of Xastir here: There's a checkbox in File->Configure->Defaults that lets you send "Compressed" objects and items. If you don't check this box then any object/item you place will jump to the regular APRS grid the first time it is transmitted. This new position that the object jumps to is what everybody else on RF will see. Downside: This precision isn't adequate for some SAR uses. You can only determine positions down to dd mm.mm resolution. If you're trying to place an object/item relative to another object, or relative to a map feature, you often need higher resolution placement of the object.

By going to "Compressed" posits, you can represent positions with higher resolution, and any APRS client that implements decoding for this format (it's in the spec, so why shouldn't they support it?) will also show the more accurate position. Downside: Many APRS clients don't support the compressed format. Kenwood APRS radios (TH-D7A and TM-D700A) don't support it either. You can get to at least dd mm.mmm significant digits using "compressed" posits.

Now, in terms of trackers, most can only send normal APRS posits or Mic-E posits, which are not all that precise. Then again, your chances of getting GPS accuracy in the field high enough to warrant more precision in the posits is slim. Portable trackers with the highest position resolution are the latest TinyTrak-III variants (which incorporate more digits using their NMEA-output transmit format), and the Opentracker devices. Opentrackers can put out NMEA, Standard APRS, Base-91 compression, or OpenTrac protocol (which is not APRS at all, but a new protocol).

NMEA sentences are particularly long. Avoid transmitting them over RF if possible.

For transmitting posits with more precision from trackers, Opentrackers with Base-91 compression are the way to go due to the short sentences transmitted and the extra precision. Short sentences equate to more posits received on the other end.

Hint: To get a very clear picture of how the various sentences affect the precision of the transmitted packet, the latest Xastir (in CVS only right now) will display white "precision rectangles" at zoom levels 1 through 16 for any station. Zoom in to a Mic-E station and you'll see a box approximately 40' by 60'. Zoom in to a station transmitting NMEA packets and you'll see a box that represents the precision of their particular NMEA packets (varies depending upon how many digits after the decimal they're transmitting). Zoom in to a station sending Base-91 compressed packets and you may have to turn off the display of symbols to see the box, as it is quite small, something like 2' by 3'.

With good satellite geometry plus many satellites locked, you'll have a good HDOP figure. In that case you can expect roughly a 6 meter GPS error ellipse (your position will generally be good to within 6 meters). Base-91 compressed or NMEA packets are capable of representing that level of accuracy due to their higher precision, standard APRS and Mic-E packets are not.


Current List of Way-Cool Trackers for SAR

Network Infrastructure for SAR

One of the main problems to deploying APRS in the field is the lack of digipeaters for supporting such operations. As soon as a team goes over a ridge, base may lose contact with them. At least one of the new wave of trackers will alleviate this problem (see the list of way-cool trackers in the previous section). If each of the SAR mobiles and each of the SAR portable trackers is also a preemptive WIDEn-N digipeater, you end up carrying your entire network along with you on each team's back.

For instance, with the Tracker2 model you can set it to digipeat preemptively on "SAR7-7". Set each tracker to send to "WIDE1-1,WIDE2-1,SAR7-7". The trackers will then use the standard APRS digipeaters if available but here's the cool part: If the trackers are within range of another SAR tracker they'll get digipeated using the "SAR7-7" part of the path. You get the best of both worlds! EVERY TEAM THEN BECOMES A DIGIPEATER FOR EVERY OTHER TEAM, each team sees where all other teams are (the anti-tracking waypoint stuff), plus more positions make it back to base. If a Tracker2 is also used in the SAR mobiles, they also may function as digipeaters.

Also the Tracker2 scans the attached GPS for new waypoints, any found get transmitted as APRS Objects. This can be used as a limited form of messaging between teams or to/from base, or as a method to mark boundaries, clues, etc.

Example of digipeating through standard digipeaters:

 WIDE1-1,WIDE2-1,SAR7-7 (as-transmitted packet)
 WIDE1-1*,WIDE2-1,SAR7-7 (first digipeat)
 WIDE1-1,WIDE2-1*,SAR7-7 (second and last digipeat)
Example of digipeating through preemptive SAR digipeaters:
 WIDE1-1,WIDE2-1,SAR7-7 (as-transmitted packet)
 WIDE1-1,WIDE2-1,SAR7-6 (first digipeat)
 WIDE1-1,WIDE2-1,SAR7-5 (second digipeat)
 WIDE1-1,WIDE1-1,SAR7* (7th and last digipeat)
The key here is that the preemptive digipeaters will skip the first part of the path, the "standard" digipeaters will ignore the SAR7-7 portion. The path works in the SAR digipeater system or the standard system.

Portable Trackers

Portable Digipeaters

Mobile Trackers

Related Links