Customer Solutions

HF Radio Control

Author(s):

Allan Pakett, AGP Engineering

Industry:

Telecommunications

Product:

LabVIEW, LabVIEW Real-Time

The Challenge:

To develop a Graphic User Interface (GUI) for a Datron World Communication Automatic Link Establishment (ALE) HF transceiver. The transceiver contains minimum front panel features and requires computer control through a serial port.

The Solution:

LabVIEW is the obvious choice for creating a complex user interface. LabVIEW also offers powerful serial port tools for real-time control and communication with the Datron radio.

Radio Control Program
Radio GUI gives an operator complete control of the transceiver ALE features. When in manual mode the operator types in a channel. When in Scan mode channel control changes to an indicator and displays each channel number as the radio is scanned. A means is also provided to send and display messages.

Datron ALE Radio Background
HF Communications was the core of long distance communications during the middle years of the 20 th century. From the time radios could be built that could reliably be set at a specific frequency, there was always some area of the 3-30 MHz band that could establish communications with any part of the world. Skilled operators familiar with the vagaries of propagation and using Morse code could send messages quickly and accurately.

Propagation was the fundamental obstacle for HF communications. Which frequency to use depends on the distance to the receiving station, the type and placement of the antenna, the local time of day for both the transmitting and receiving stations, and the sunspot cycle. When satellite communications became available, HF usage started to decline. In recent years however, the quantity of data sent by satellite has increased dramatically, along with the cost of operating satellites and the ground stations. People sending routine priority messages are finding it harder to get time on the satellite links for their messages. Therefore, HF communications has been rediscovered.

With an ideal antenna, installation propagation can be accurately modeled. Nevertheless, quite often, the antenna is whatever piece of wire could be hung in the trees and poles. Even skilled operators have trouble predicting the propagation under these circumstances, and skilled operators are difficult to find and retain.
Several years ago, a few manufacturers and the military formed a group to address these issues. The results of the series of meetings became the Automatic Link Establishment (ALE) standards. MIL-STD-188-141A covers radio systems that include ALE and FED-STD-1045A for ALE systems that are subject to less stringent operational requirements. ALE bypasses the issues of trying to predict propagation by continuously measuring propagation. Since the radio with the ALE feature already knows which frequency will work the best over the required path, the operator sets the address of the desired station and makes the call. When the link is established, the ALE notifies the operator that he may use voice or data communications to do whatever is required.

The bookkeeping involved in this measurement requires a considerable amount of data to be stored and manipulated. The ALE standards state that the radios will be capable of 100 channels and 100 other addresses. It must keep track of 20 different addresses for itself as well as 20 different net names. It must rate each of these combinations according to the path strength. Part of the protocol allows the radio to request the receiving radio to evaluate the transmitters strength, then return the value. This means a local evaluation, plus the remote evaluation for each channel and each "other address" - up to 20000 measurements.
To make these measurements, each radio "sounds" at a preset interval, usually between ½ hour and 4 hours. The sounding transmission length is based on the size of the channel group to insure that the receiving radios scan through that particular channel. The sounding is a digital transmission of the selected "self address" of that radio. As each radio sounds, all the radios that can receive the signal and have the transmitting radio in its "other address" list, will record the signal strength for that address. When a new radio comes on line, this process can be speeded up by "polling". The new radio calls another radio on each of the channels in the group and requests a return evaluation. This is especially useful for a portable radio, since all the other radios in the network will fill their Link Quality Analysis (LQA) table based on information from the polling. When a call is requested, the controller will automatically base its order of calls on the ratings in the LQA table. Figure 2, LQA Matrix shows relative signal integrity for all stations listed. Note that station "DA1TR" has problems with all channels except channel 5. "BIL" on the other hand can be received on all channels.

If a call is successful, the two radios make a link, and release he radio for the transfer of information. This may be voice, digital by the ALE modem, or digital by some other modem. Once the information is passed, the link may be broken manually, by operator request, or it may be broken automatically by the activity timer. There are timers for both transmitter and receiver activity.

Program Architecture
The Radio Control program contains two main loops. The first is a user interface loop which monitors front panel controls and acts on value changes or button clicks. A state machine is used to provide proper action. Because the program must react quickly to radio broadcast messages, serial communication processing is handled in an independent loop. The serial communications loop queues incoming messages, buffers and sends outgoing commands, and sets flags depicting radio status. Communication between the two main loops and other parts of the program is through global variables, Notifications, and Queues.

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