ALE
in Ham Radio
For the past 7 years,
a group of
Amateur Radio operators has joined together for
communications
using ALE and Selective Calling. The number of hams has grown from just
a handful active in 2001, to the thousands of enthusiasts in it today.
Some
are following the traditional ham curiosity to explore interesting
aspects
of communications; others are developing dependable HF nets, or just
using
it to keep in touch with a circle of ham friends. The need to call up
emergency
nets or inter-operability and liason with government HF systems has led
many hams to adopt the government ALE standard, called FED-STD-1045
or MIL-STD 188-141. This standard caught on slowly in the ham
community,
initiated by a few operators with limited government surplus gear and
some
with expensive commercial equipment having embedded ALE or hardware controllers.
Recently, the cost of embedded ALE transceivers has been reduced, and
they are now available at similar to the cost of a medium priced ham
radio.
Also, with ALE software, a ham HF transceiver, a PC computer as the
controller, and an
appropriate
antenna system, hams can harness the power of ALE.
How
ALE Works
Each ham radio ALE
station uses the
operator's callsign as an address in the ALE controller.When
not
actively in a QSO with another station, each HF SSB transceiver
constantly
scans through a list of frequencies (called channels in ALE jargon), listening for its callsign. To
reach
a specific station, the caller simply enters the callsign just like
dialing
a phone number. The ALE controller selects the best available frequency
and sends out brief selective calling signals containing the
callsigns.
When the distant scanning station detects the first few characters of
its
callsign, it stops scanning and stays on that frequency. The two
stations'
ALE controllers automatically
handshake to confirm that a link is
established and they are ready to communicate. The receiving station,
which
was muted up until now, will typically emit an audible alarm and visual
alert for the receiving operator of the incoming call. It also
indicates
the callsign of the linked station. The operators then can talk in a
regular
QSO. At the conclusion of the QSO, one of the stations sends a
disconnect
signal to the other station, and they each return their ALE stations to
the scanning mode. Some military / commercial HF transceivers are
available
with ALE options. Amateurs commonly use G4GUO's PCALE soundcard
software
interfaced to a ham transceiver via RS-232 CAT port, multi-frequency
antenna.
ALE
Using a Computer
as the Transceiver Controller Unit
In 2001, Charles
Brain G4GUO, wrote and released a soundcard PC program for ALE
appropriately named PCALE.
Charles is to be commended for his wonderful generosity and technical
achievement.
By doing this, he effectively opened up ALE to the non-professional
user
at low cost. The availability of PCALE in various
improving versions made it possible for more amateur radio
operators
to start experimenting with ALE.
Ham
Group for Coordinating
ALE
During 2001, several
hams in USA
(including this author) started using PCALE, at the suggestion of Elaine WA6UBE. Elaine's
already-operational
Motorola Micom 2R transceiver (with embedded ALE) became one of the
center
points for testing and linking. In December 2001, from this core of
early
ALE operators,
HFLINK
was founded by the author. The purpose of HFLINK would be to
further the development
of methods and coordination for use of ALE and Selective Calling in HF
Amateur Radio. HFLINK worked out guidelines for use and developed the digital programming protocols and standards for
ham-friendly ALE. The scanning frequencies essential for
effective
ALE operation evolved and were coordinated with all IARU Regions of the world to become the International ALE Channel List.
Link
Quality Analysis
For operation as an
ALE system, the
HF communications system normally has a number of
frequencies throughout the HF spectrum. The system works much like a
telephone
in
that each radio in a net has an address (callsign or ID). When not in
use,
each radio receiver constantly scans through its various frequencies
(channels),
listening for calls addressed to its own callsign. To reach a specific
station, the
caller
simply enters the desired callsign just like dialing a phone number. It
takes a lot of
time for the radio to go through the sequence of calling a station on
every possible frequency. But there are several ways the ALE system
can be programmed to decrease the time it takes, over simple random
scanning
and sequential channel calling. Methods for decreasing the time by
using
a "smarter" way of predictive or synchronized linking can be applied.
An
ALE system utilizing (Link Quality Analysis) capability uses periodic
sounding
and linking signals between other stations in the network, to stay in
touch, and to predict
which
channel is best to call a particular station on at any given time.
Various stations may be operating on different channels, and this
enables the stations to find and use a common channel that is clear,
that they both have.
Here’s
how it works
in an adaptive
system using LQA. Once every hour or so, each station in a network
will
attempt to "sound" out each channel by sending a short transmission to
all the other stations in the net, the transmission is
its own callsign, and it may be sent on each of the channels it is
operating on. All stations in the net who are scanning that channel may
receive the
sounding
transmission and measure the signal quality on each channel for each
other
station IDs it receives. These signal quality "scores" are stored in a
complex matrix of:
-
STATION ID
-
TIME DATE STAMP
-
CHANNEL NUMBER
-
SIGNAL QUALITY LEVEL
When a call is initiated
to a station,
the radio automatically checks its LQA matrix “memory” to make a
determination
of the most probable and best quality channel for the call to the desired
station,
based upon its record of recent Link Quality data it has logged on that
station. It then makes its first attempted linking call on that most
probable
channel. If the link cannot be established, it will try again on the
next
best frequency in the matrix, and so on, until a link is established. Typical ALE systems using LQA
make
use of recently measured soundings or stations received within the past few hours.
---Bonnie Crystal, KQ6XA
HFLINK Founder
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