Computer Models Ensure Open Navy Communication Lines
Tool suite helps avoid systems conflict in a crowded environment.
Communication decision aids are enabling U.S. Navy shipboard-system developers to improve system designs and on-station communicators to prepare better communications plans by predicting performance. The tools help designers take into account the variables of the entire communications environment, including a sea of antennas or other obstacles that could block communications. Perhaps more importantly, the tool set helps commanders answer the quandary, “I have the systems, but can I communicate?”
This question would have a simple answer if every antenna installed aboard a ship had unobstructed, omnidirectional communications coverage and no transmitter-emitted interference. But the rapid proliferation of topside antennas on naval ships has resulted in antenna blockage from other antennas, the main mast and other components of the ship’s superstructure, which degrades communications in certain areas.
The Surface Communications and Information Systems Division of the Naval Air Systems Command (NAVAIR), St. Inigoes, Maryland, and BAE Systems, California, Maryland, have developed computer models to assist engineers in evaluating Arleigh Burke-class destroyer radio communication system design and address these issues. Three of the tools help predict the effect of blockages as well as other forms of degradation. A fourth communication decision aid (CDA) addresses an additional problem. High frequency transmitters radiating at the same time result in hull-generated intermodulation products (IMPs). When IMPs or interference generated from broadband transmitters fall within the communication receivers’ bandwidths, the ability to receive communications can be reduced or even disabled. The fourth CDA in the tool set predicts which receivers the IMPs will affect and what combination of transmitters causes the IMPs.
In addition to assisting in system design, CDAs are useful for shipboard communications planning and training. Communicators and electronics technicians on board destroyers can use these tools to plan circuit configuration, evaluate communication circuits’ performance and train others on radio frequency propagation.
The CDAs include a very high frequency/ultrahigh frequency line-of-sight communications range prediction computer model, a sea-based ship-to-ship high frequency groundwave communications range prediction computer model, a worldwide high frequency groundwave communications area coverage prediction computer model, and a high frequency intermodulation interference (IMI) prediction computer model. All of the computer models are programmed in Microsoft Excel.
The very high frequency/ultrahigh frequency line-of-sight communications range prediction computer model has both surface-to-air and surface-to-surface applications. Both applications use radio circuit system parameters, Ship Electronic System Evaluation Facility (SESEF) shipboard antenna radiation patterns, and three path-loss algorithms to provide a visual, 360-degree azimuth plot of predicted communication range in nautical miles. When used to evaluate a communication system design, any number of the variable system parameters can be changed, and the resultant effects to the communication range prediction plot are displayed instantaneously on the computer screen.
The variable transmitter system parameters include frequency, power output, coupler loss, coaxial cable type, coaxial cable length, antenna gain, antenna voltage standing wave ratio and antenna height. The variable receiver system parameters include frequency, sensitivity, coupler loss, coaxial cable type, coaxial cable length, antenna gain, antenna voltage standing wave ratio and antenna height.
Tim Hickey, chief engineer at NAVAIR, explains that this capability enables his division to evaluate proposed changes to Arleigh Burke-class radio communication systems before installing them on board ships. For example, he cites a simple coaxial cable change that was first evaluated with the computer model and significantly improved a radio circuit’s performance.
Early in the ship’s design process, software-generated antenna radiation patterns can be substituted for the at-sea, SESEF-generated patterns. Prior to getting underway, the ship’s crew can input system parameters from their communications plan and determine which antenna exhibits the best radiation pattern for the intended use. False trouble calls, precipitated by communication outages caused by antenna blockage, can be prevented when they have been anticipated through communication decision aid prediction.
Electronics Technician 1st Class Matthew Paul, USN, aboard the USS Mason, relates that the tools offer additional benefits. “CDAs are useful as a troubleshooting tool when equipment appears to be operating normally but still does not work,” he offers. “CDAs can provide answers to questions such as ‘I can see that ship on the horizon. Why can’t I communicate with it?’” he explains. The range prediction computer model can help anticipate just such a communication blockage.
The sea-based high frequency groundwave communication range prediction computer model predicts low very high frequency—below 50 megahertz—and high frequency—2 to 30 megahertz—communication ranges in nautical miles. This groundwave application predicts propagation of the surface waves of a high frequency signal that follows the curvature of the Earth across the water between ships.
The design version of this computer model uses radio circuit system parameters, SESEF shipboard antenna radiation patterns, and loss data for high frequency propagation over the sea to provide a 360-degree azimuth plot of a predicted communication range. This model also contains sea-state-loss data from a study by Donald E. Barrick, a recognized authority on radio frequency propagation, and radio-noise data from an International Telecommunication Union (ITU) recommendation.
Shawn Jackson, NAVAIR project engineer for CDAs, states that the ITU noise data is very useful at low high frequencies. The noise level can change by more than 20 decibels in a 24-hour period, which can cause up to a 100-nautical-mile reduction in communications range. At the upper range of high frequencies, elevated sea states cause similar reductions in communications range, he explains. In this application, as when the CDA is used to evaluate system design, any number of variable system parameters can be changed, and the resultant changes to the communication range prediction plot will be displayed instantaneously.
The variable transmitter system parameters include frequency, power output, mode, coupler loss, coaxial cable type, coaxial cable length, antenna gain and antenna voltage standing wave ratio. The variable receiver system parameters include frequency, sensitivity, coupler loss, coaxial cable type, coaxial cable length, antenna gain and antenna voltage standing wave ratio. Early in the ship design process, brass model or software-generated antenna radiation patterns may be substituted for the at-sea SESEF patterns. The communicator’s version of this application has been streamlined to display and accept only those system parameters that realistically could be changed aboard ship. As with the very high frequency/ultrahigh frequency line-of-sight model, this application can be used for communication planning and training.
The newest CDA is the worldwide high frequency groundwave communications area coverage prediction computer model. Many high frequency skywave communication prediction computer models are available to assess the design of high frequency radio communication systems, but few high frequency groundwave communication prediction computer models exist. This computer model was developed using ITU conductivity and propagation loss curves. With this innovation, 64,008 point-to-point computations across varied surface conductivities can be computed automatically on an Excel spreadsheet. Additionally, a color map is generated for a visual representation of the computation results.
As with the other CDAs, this computer model can be used as a high frequency communication assessment tool for both system design and military operations. It can be equally useful to both sea and ground forces and platforms in all geographic locations. The operator can input the transmitter site’s longitude and latitude, power, frequency and antenna gain and can either input an antenna radiation pattern or use an omnidirectional pattern. The operator also can input the target field strength, select colors for areas within the target field strength and areas outside this parameter, and select longitudes and latitudes for two receiver sites. After running the computer model, a representation of all three sites will be displayed on a map depicting the area of communication coverage. The calculated field strength also will be displayed on any area of the map selected.
The high frequency IMI prediction computer model uses the shipboard communication plan and the intermodulation product orders measured during shipboard testing then predicts IMI victim receive frequencies and corresponding high frequency IMI source transmitter frequencies. Jackson says that IMI is perhaps the most insidious form of electromagnetic interference because it is impossible to eradicate, and its products are not as easily predictable as harmonic interference. Considering just the high frequency, very high frequency and ultrahigh frequency communication receivers typically on board a naval ship, there are 1,179 potential victim frequencies. The high frequency IMI prediction computer model matches these potential victim frequencies with the potential 518,616 IMI source frequencies and alerts the ship’s communicators to possible electromagnetic compatibility conflicts before they become operational problems. With this information and some flexibility in frequency selection, a communications planner can generate a communications plan that results in the least number of IMI source-victim conflicts.
On newly commissioned Arleigh Burke-class destroyers, the CDAs are installed on the AN/USQ-147A(V)1 Quality Monitoring Control Set computer and on other selected PCs in the radio room and electronics technician spaces. Future plans could include integration of the communication decision aid models into a “smart,” automated communications system, where equipment and antenna selection is based on the predicted performance and priority of the communications circuit. Hickey says that this is the direction that communications system designs are headed: automated, intelligent systems that require little or no operator intervention.
Lawrence Nosek is the branch manager for special projects in the Aegis Engineering Department, communications and electronic systems operation, BAE Electronic Systems.