The The Trackers transmit a 500 Milliseconds data packet,
seconds on the VHF band. These can be received directly on a laptop
computer, located at a nearby forward field base. The laptop has a hand
held radio and radio Modem attached to it to receive the signal.
But obviously, the Trackers may quickly get out of range when they
disappear in the woods, or between the hills. So, for the same reason
as voice repeaters are set up at a nearby hilltop for voice
communication, a Digital Repeater (or Digipeater) must be set up to
repeat the digital tracker packets (Often at the same location as the
Voice repeater). Also, as we have done in
Central Otago, we now have a couple of permanent Digipeaters on high
mountain tops, which pass on the received packets from the Trackers
direct, or via other Digipeaters in range. This means a whole Network
of Digipeaters pass traffic on, which can then be received by any
station in the large covered area.
However, Digipeaters work in a very different way then Voice repeaters.
The Digipeater is a 'store-and-forward' repeater. This means, the
repeater receives a radio 'packet' from a Tracker. It decodes this
packet and calculates the checksum. If the checksum checks :), the
packet is accepted. If not, it is discarded.
But before the Digipeater decides to re-transmit the packet, several
To prevent multiple Digipeaters re-transmitting packets over and over
again, they all keep a record of all received packets over the last 30
seconds. If they get an identical packet in that time, they will not
repeat the packet again.
But there is more. All Trackers are programmed with a 'Path' which
tells the Digipeater how many Hops the packet may travel, from one
Digipeater to another. It is therefore possible to program all Trackers
to only pass through 2 or 3 Digipeaters, after it will stop.
Lets say, we have a search in a gully in Balclutha. The search party
starts by installing a Voice and a Digital repeater on a hill looking
down into the gully. The Digipeater is also in range of the Blue
Mountain Digipeater. And the Blue Mountain is in range of the Old Man
The Tracker is programmed to Hop three times maximum. So, every time it
transmits a packet, the local Digipeater will re-transmit it. But it
will decrease the 'Hop-Count' by one. The Blue Mountains Digipeater
does the same. And then the Old Man range Digipeater does it as well.
But, by then the 'Hop-Count' is zero, and the Remarkables repeater (Uh,
this does not exist yet, but still) will NOT repeat the packet anymore,
despite the fact it did receive it.
This is very important, as it limits how far a packet can travel. The
Digipeaters themselves can also be programmed to limit the number of
Hops. If another search is happening at the same time say in
Queenstown, it is possible to remotely program the Old man Range
repeater not to repeat any packets from the Balclutha search in either
direction to free up the available bandwidth at both locations.
We now have a system where a search in a local gully is re-transmitted
over a large area, and it can be followed (and participated in) by base
stations, like the Balclutha Police station, or the Alexandra police
station. And all this without cellular coverage, or even voice
connectivity. Text messages, including Map objects can be shared
between all stations in range of the Digipeater network.
Even better: if one of the stations has Internet access, it can pass
all traffic on to an Igate Server (of which we have one running in an
Auckland Telehouse area), and at that point, all stations with Internet
access can participate in exactly the same way as if they could receive
the packets by radio. So even the Rescue Co-ordination Centre in
Wellington could participate and communicate with the forward field
base in Balclutha, without voice or cellphone connections. They only
need the Software package.
But despite all these possibilities, the primary object of the APRS
system stands: It will work in remote areas, and is independent from
cellular coverage or satellite coverage (except for the GPS satellites,
of which there are many).
We use the TNC-X board with the X-Digi daughter board. This combination
has proven to be extremely reliable, and has nearly all the features we
need. It also draws only a small amount of current, which is critical
for the battery life of the Digipeater. A Vertex VX-160 hand held radio
is installed in the waterproof box, together with a 7 Amp H battery,
which gives the Digipeater about 48 our battery life under busy
conditions. An extra power source like a car battery or a solar panel
can be connected via the water proof connector on the side. The unit
can also be programmed by connecting a laptop to the same connector.
It is switched on and off with a key, which is removed when the unit
has been installed at location.
The complete unit weighs about 3.5 KG and can easily be carried in a
backpack, together with the half-wave antenna included.
However, in most cases you will need a small mast of 4 to 5 meters, to
get the antenna above local obstacles. We build a few of these masts
from aluminum pipes, together with ropes and pins in a 1.2 meter bag.
The Digipeater can also be used from a car, with a magnetic antenna on
the roof. This has already been proven very useful as a mobile
The power of the Digipeaters, as opposed to Voice repeaters, is that
you can simply put one on one hill, another on the opposite hill,
switch them on, and they all communicate with each other automatically,
and transfer data between them, without any input from the field teams
except to turn the key.
But the important final stage of all this radio traffic, is where it
shows up on the screen, with our purpose build software