I did a little article on the Summits Base site 12 months ago about I get round the problem with coverage in mountainous areas.
Here are the relevant bits
There are a number of factors which limit the usefulness of portable VHF APRS for position information these include: lack of IGates in a given area; low power levels (typically 5W from handhelds); fairly inefficient antennas; and signals usually have to be line of sight (being the “wrong” side of a hill tends to block the signals). Using portable HF APRS equipment on 30m is not that realistic given the size of the antennas and the RF power required for reliable beacons to be seen; but from a mobile it is not too difficult to get reliable tracks, especially if you already have a reasonable HF setup. So the thought crossed my mind, is there a practical way of utilising the European HF IGate network to provide tracks which can be viewed on the APRS mapping website (http://aprs.fi/) while walking with a simple system?
So far I had built (and learnt how to set-up) my own APRS beacon for mobile use (APRS - TinyTrak), began to understand how the VHF and HF APRS networks functioned, acquired a Byonics portable beacon (the All In One) (Byonics - MicroTrak) and had built and programmed a Tiny Trak 4 (Byonics - TinyTrak4) to act as a beacon/digipeater (but as yet only on the bench to prove the principle). I was now slowly working my way to having a more reliable tracking system for those places where there was very little coverage on VHF.
There is a caveat to the following: the Amateur licence is very vague regarding some aspects of digital/remote operation so please check your licence conditions before experimenting, especially if you hold a foundation or intermediate licence. Receive-only IGating to the internet can be done by anyone but almost anything to do with gating data from the internet or digipeating requires a full license and, for some aspects, a NoV (my system is not acting as a traditional digipeater). The grey area stems from the lack of control (and content) when retransmitting other people’s signals. In this case the license requires that a prompt shutdown can be performed if the system is being used as a repeater.
My configuration of the hardware I use will only respond to my data stream from the portable unit and will not repeat any other signals it sees, additionally if there is no valid GPS data (from a GPSr when it is switched off or disconnected) the remote transceiver will not transmit its position.
The key to the system is the TT4 as it can be configured to operate in various modes such as APRS beacon, smart digipeater or as a KISS TNC. In digipeater mode it will accept 1200bps packets and can then be programmed to repeat them at 300bps; this is the function I have exploited.
The TT4 can also be set up in two independent configurations controlled by a switch; one a normal VHF mobile APRS beacon (with smart beaconing) the second as a dual function HF beacon/VHF to HF digipeater. The HF beacon/VHF to HF digipeater function is controlled by it accepting either “GPS data” or an “alias” within the received data from a remote source. The TT4 will not initiate a transmission unless it receives a valid NMEA signal (in which case it will transmit its location periodically) or sees the correct alias (which will trigger a digipeat on HF).
The AIO also has the ability to be programmed with 2 separate configurations. One is a normal beacon set to transmit at preset intervals, the other contains the information to which the TT4 will respond (both settings can be seen by any VHF digipeater without issue).
I now had the various units to begin real world experiments, the AIO, a TT4 connected to a multi-band multi-mode mobile transceiver and an antenna for 2m receive and 30m transmit (my actual system is a Yaesu FT-857 and ATAS 120A but any multimode/multiband HF/VHF transceiver and suitable antennas will do)
Basic configurations of the various components are as follows:
• Within the VHF beacon’s transmitted data an “alias” should be included in the Digi Path. The data should be in the format: CALLSIGN, WIDE1-1, WIDE2-1 (where CALLSIGN is your own callsign, the “alias”). A VHF IGate or digipeater will ignore the CALLSIGN in the path and just act on the WIDE commands. Keeping the WIDE hops small prevents potentially clogging both VHF and HF networks.
• To set the TT4 to look for the alias, the unit’s configuration program is used. In the Beacon/Digipeater section, Digipeater Settings, ALIAS1 should be set to the CALLSIGN used in the data stream from the remote unit. When the data is received the path is checked and the beacon will only be repeated if the CALLSIGN corresponds to the first instruction in the path; this is the key to controlling what is retransmitted and filters out any other packet data that might be seen. In the Packet section the Transit 300 Baud (P300) should be ticked (the received data will still be at 1200bps). The TT4 is the only self-contained module that can perform this translation function as far as I am aware.
• The transceiver is set up to receive 2m packet data and then switch to 30m to transmit, which is easily accomplished by programming a memory in split-frequency mode.
A few things to remember is that all transmissions should be kept as short as possible and less frequently than VHF to avoid overloading and blocking the HF APRS system, the lower baud rate of the 300bps HF means that transmissions are significantly longer than on VHF. On the VHF beacon disable smart beaconing, use MIC-E compression, transmit every 3-5 minutes minimum and only add a very brief APRS message if you want.
There are limitations to VHF to HF digipeating. If you happen to be on the opposite side of the hill to your mobile the link maybe unreliable, 30m is generally a daylight band and is subject to the whims of propagation, the European IGate network can be very busy, which means your beacon may not even be seen, using limited WIDE paths could also mean no gating to the internet through lack of hops. But even with these potential problems I have been getting very good tracks while walking in Wales.
All in all, if you know there will be limited VHF IGate coverage near to where you are going, the VHF to HF digipeater option described above could provide a method of getting your position on to the APRS network. Have fun experimenting.
Carolyn (G6WRW)
