Document dated: 16 Apr 97
Author(s): Bob Bruninga, WB4APR
ABSTRACT: CAVES.TXT Using APRS for cave communications. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The August 96 issue of QST carried an article by Mike Bedford, G4AEE about the difficulty of radio/emergency communications underground. The only thing that works is induction methods using VLF frequencies. Ranges are limited to about 600 feet or so... VHF and UHF are useless except for line of sight... OR IS IT? I wonder if they have considered the advantages of packet radio as a means to extend the range of VHF/UHF radio via multiple hops?

Although APRS is normally based on GPS position reporting, its map display could be ideal for tracking underground teams in the cave! (A sketch of the cave passages can be easily prepared in just a few minutes using a mouse since absolute lat/long accuracy is not required because a GPS could never be used underground anyway). As long as the sender places himself on the map approximately by cursor movement, all receving stations will see him at that location! Even positions in uncharted passages can be reported by estimating your location relative to the known sketch.

The advantage of using packet is based on one fact.... That you can EXTEND VHF/UHF radio ranges by simply dropping packet digipeaters along the cave route as you pregress inward. These digipeaters can relay VHF or UHF communicatinos over and over again (up to 8 times)!

DIGIPEATERS can be built with a single HT and tiny packet TNC. Running on AA cells each digipeater could last for a few days and be packaged into less than a 1/2 liter volume. The effecient APRS connectionless protocol and GENERIC callsign ALIASES make it easy since the cave units do NOT have to remember the callsigns of the digipeaters or how to construct the path! Simply enter the path of HOP7-7 for a maximum seven hop path. And set all TH-D72 radios to UITRACE HOP to enable the NOPn-N digipeating.

If you were able to get even 1/4 mile between digipeaters, then you could use this system for up to two miles inward! Here is how I would do it:

  • Set a digipeater in the mouth of the cave which couples well, both into the main cave passage and well out to the above ground team.
  • Proceed into the cave. You can "ping" the digi at any time. When you begin to see the signal strength get low and when you make a major path change, drop a digi or set it on a convenient rock shelf.
  • Repeat step 2 as you proceed.
  • Add a digipeater on a nearby hilltop to extend the comms link back to town for arm-chair tracking by loved ones back home..

    THis APRS generic digipeating is ideal in caves because not more than a few digis will hear each other at any time. Every APRS'er knows that a packet launched to WIDE7-7 could bounce around dozens of times in a conventional above ground network. This would practically kill a network trying to support dozens of users. But since the cave application will only have a few stations underground, and they will only use the HOPn-N path, then any leaks will not decimate the normal APRS network.

    The BIG advantage of APRS and digipeating is for the long straight paths and the ability to couple the communications over long straight distances underground to automatic links above ground. This enables you to establish solid APRS packet COMMs from anywhere above ground well down into a "forward outpost" deep in the cave. Those last 600 feet can still use the bulky LF loops for crawlers as needed.

    Areas for research:

  • How far does VHF/UHF go in a cave? Is UHF or VHF better?
  • Is there a waveguide effect or is it reflections?
  • What is the best way to couple energy between passages?
  • Does a long wire couple VHF/UHF deep into passages?
  • What is the ideal portable APRS terminal? The HP palmtops? (update 2010: The D72 HT!)
  • Is there a digital control capability to avoid the infinite loop that would result from the use of DIgital voice record /playback chips via multiple simplex repeaters?

    The advantage of straight APRS packet, though, is that NO development is required!

    Bob Bruninga, WB4APR
    WB4APR at AMSAT dot ORG