ALE, or Automatic Link Establishment, is a system used to establish quick voice or data links on HF radio nets. ALE is primarily used by military & government stations worldwide, although it is not limited to those users. Any professional users of HF spectrum can benefit from ALE by using it to set up quick HF communications.
The propagation characteristics of the HF band require different frequency ranges to be used at different times of day. Generally, higher frequencies are used during the day, and lower frequencies are used at night. Users of the HF spectrum may have several frequencies spread out all over the band. Professional HF spectrum users, unlike amateur radio operators, may not understand the properties of HF propagation, so therefore they may not know which frequencies will be best to communicate with other stations. They may not even know what frequencies they use. Without a way to check each channel to find the best one that supports communication, setting up a radio link may be time consuming. Regular check-ins to radio nets are also required to ensure that the control station can communicate with the outstations.
ALE solves these problems by keeping all stations in a net aware of what frequencies best support communications between any two stations in the net at any time. Each station in the net periodically transmits a "sounding" signal on each frequency used by the net, and each station receiving the signal records the signal quality. When one station in an ALE net needs to contact another station, the initiating station selects a frequency based on the best signal reports for the other station. The time required to set up a successful communications link is greatly reduced.
We can monitor ALE nets by using an excellent program written by Charles Brain, G4GUO, called PC-ALE. PC-ALE is a fully-functional ALE controller designed for use with either a receiver or transceiver, and can be used to control a station in an ALE net. It can also be used to just monitor other ALE stations, which is what we will focus on.
PC-ALE can be downloaded at http://www.chbrain.dircon.co.uk/pcale.html. It is a 1.5 MB file which easily loads onto a computer. I won't go into installation & setup details here since the PC-ALE manual has enough information to set up the program. The main requirement for the program to work is a soundcard with an input from the radio. This can be done by using a patch cord with 3.5" mono plugs on both ends. It should be noted that a 1:1 audio transfomer should be used between the radio & sound card to prevent static voltages from the radio from damaging the sound card.
Once the program is set up, there are two ways we can monitor ALE. If our radio is not computer controlled, we can leave it on one frequency and monitor all traffic on that channel. If our radio is computer controlled and supported by the PC-ALE software, we can set up the program to scan through 100 channels at a time to monitor all frequencies in a net. The channels can be manually entered into the program and then dumped to a .qrg file, or a .qrg file can be written from a text editor. Once our .qrg files have been written, we can load them to monitor traffic in different nets.
PC-ALE
screen shot
The above screen shot shows the main PC-ALE screen. The
program has a second, smaller window for transmit information, which I have
reduced to the point of not being visible. Since I am not transmitting and only
monitoring, I don't need that window and like to maximize the area on my receive
window.
The PC-ALE program displays all the decoded information in bracketed text. Once we have monitored ALE for a while, we will see something which looks like the following on our screen:
[14:31:05][FRQ 3137000][SND][ ][TWS][AF9 ][AL0] BER 28 SN 17
[14:31:04][FRQ 2805000][SND][ ][TWS][AF9 ][AL0] BER 21 SN 22
[14:30:06][FRQ 6721000][SND][ ][TWS][OFF ][AL0] BER 30 SN 16
[14:30:04][FRQ 5708000][SND][ ][TWS][OFF ][AL0] BER 26 SN 18
[14:30:02][FRQ 2805000][SND][ ][TWS][OFF ][AL0] BER 28 SN 22
[14:30:01][FRQ 23337000][SND][ ][TWS][OFF ][AL0] BER 30 SN 27
[14:29:59][FRQ 18003000][SND][ ][TWS][OFF ][AL0] BER 27 SN 24
[14:29:04][FRQ 27870000][SND][ ][TWS][440189 ][AL0] BER 30 SN 19
[14:29:00][FRQ 18003000][SND][ ][TWS][440189 ][AL0] BER 27 SN 19
To the untrained eye
this appears to be gibberish. But it is very easy to understand what the data
says. The first item in brackets is the time. The second item is the frequency in Hz the program heard the signal on during a scan. The third item, in this case [SND], indicates that the station was transmitting a sounding signal. The next item is the TO field, in this case they are blank due to the sounding signals being sent. The next field shows [TWS], which means "that was." Alternatively it will show [TIS] for "this is." After that we have the "from" field, and this is where the callsign of the transmitting station will be. The first two lines show AF9, which is the callsign used by SAM 29000, or Air Force 2. OFF is Offutt AFB, and 440189 is a USAF aircraft. The next field, [AL0], is the application level indicator. This indicates if any encryption is used by the transmitting station. AL0 means no encryption, and is probably the only thing you'll ever see in this field. The last field is the signal quality field, with bit error ratio & signal-to-noise reading. This shows how well your setup heard the transmitting station, and the higher the numbers the better the signal.
When two stations link up, you will see something like the following:
[14:26:18][FRQ 11226000][TO ][? ][TIS][280050 ][AL0] BER 27 SN 22
[AMD]CCCCPCHSNNNN [[CMD AMD] CCCCPCHSNNNN ]
[CMD LQA KA1 = NO RESPONSE REQUIRED,MULTIPATH = UNKNOWN,SINAD = 9 dB,BER = 0.0]
[14:26:06][FRQ 11226000][TO ][280050 ][TIS][ADW ][AL0] BER 30 SN 30
[CMD LQA KA1 = RESPONSE REQUIRED,MULTIPATH = UNKNOWN,SINAD = 16 dB,BER = 0.0]
[14:26:02][FRQ 11226000][TO ][ADW ][TIS][280050 ][AL0] BER 28 SN 22
This shows an exchange between ADW, or Andrews AFB, and a USAF aircraft. When the PC-ALE program detects a link being made like this, the program will automatically stop scanning and will record the audio on the channel to a .wav file if the record function is enabled. I have the recorder set up to record 300 seconds, or five minutes, of audio when two stations link up. The program continues scanning when either the stations drop the link or five minutes have passed. This allows unattended monitoring of ALE nets and automatic recording of stations when they link for communication. I can leave the computer & radio running all day or night and come back later to see what was heard. In the case of USAF Scope Command frequencies, it is usually in the form of interesting phone patches.
A text file of various ALE stations & frequencies can be downloaded by clicking here.