A drawback to standard VHF radio communication is that it is limited in range to little more than line of sight. Unlike HF and lower frequencies, VHF's transmitted waves propagate minimally around the Earth's surface. When, on occasion, VHF transmissions do reach well beyond the horizon, they often are regarded as anomalous and a nuisance.
A number of systems, however, have capitalized on anomalous propagation modes and made them the basis for viable over-the-horizon (OTH) VHF communication. Such extended-range systems can be a useful adjunct to normal air traffic control (ATC) ground/air communications, particularly in locations having high levels of air traffic, where early contact with aircraft before they enter a core region would be useful.
East Asian Use
One such location is in the People's Republic of China, where the country's Air Traffic Management Bureau (ATMB) has implemented over-the-horizon VHF communications for earlier contact with traffic over the South China Sea. Typically, air traffic using southern routes is transiting from Bangkok, Kuala Lumpur and Singapore to destinations such as Hong Kong and Japan.
Aircraft that are perhaps 400 miles away and at altitudes of 20,000 feet or more are outside the conventional VHF radio range, but there is a need to communicate with them. More than two years ago, a Northrop Grumman subsidiary, Park Air Systems, supplied ATMB with a turnkey system that meets this need and, indeed, has exceeded expectations. It is located at the southern coastal city of Sanya, on Hainan island, just off the Chinese mainland. By augmenting communications capacity, the Park Air System has proven to be a key enabler. The number of air routes crossing this region is now almost double the pre-installation level.
Over-the-horizon VHF implementation followed an agreement between the aviation administrations of China and neighboring Vietnam to establish a joint area of responsibility (AOR) over the South China Sea. This led to the first use of China's rapidly expanding ATC infrastructure to take control of an area of international airspace.
Advantages Over HF and Satcom
David Chandler, marketing and business development manager for Park Air's UK-based radio communications arm, says appropriately engineered systems can usefully extend the range of VHF, avoiding both the well-known vagaries of HF communication and the need for more expensive satellite-based or networked solutions.
"Our system will not cross oceans," Chandler concedes. "But for extending VHF reach for an aircraft flying at 24,000 feet, say, from around 250 miles at present to nearer 400 miles, over-the-horizon VHF can be a highly cost-effective solution."
Park Air has implemented some 20 OTH VHF systems around the world, including in Greenland, Iceland and Singapore, as well as China. China was able to consider the risk of adopting this "non-mainstream" system after examining its use in Singapore and Hong Kong (the latter being a non-Park Air installation).
As is well known, HF radio transmissions can be made to follow the Earth's curvature by ionospheric refraction. But success depends on the state of the ionosphere, which varies according to the time of day or night, season of the year, sunspot activity, and atmospheric conditions.
In contrast, shorter-wave VHF transmissions tend to penetrate the ionosphere and radiate to space (hence their use by orbiting astronauts). Most VHF operators are aware, however, that VHF can propagate over long distances–thousands of miles in extreme cases–around the Earth. In such instances of anomalous propagation, the transmission follows the Earth's curvature by being reflected repeatedly between the Earth's surface and upper reflective layers in the atmosphere, or by being "ducted" between various layers of the atmosphere. Both of these mechanisms are transitory, induced by unusual atmospheric conditions, typically involving temperature inversions. They cannot, therefore, be relied upon for prolonged communications.
However, there exists a far more stable, but low-energy, phenomenon that can extend the reach of this affordable and reliable communications medium well beyond the horizon. The idea that tropospheric backscattering could be harnessed as the basis for extended-range VHF is decades old. But it has taken more recent advances in transceiver technology and digital signal processing to make it viable for air traffic control.
No Mods to Airborne Radios
Essentially, due to the irregularities in the refractive qualities of the upper atmosphere (troposphere) layers, some of the energy within a directed VHF transmission is scattered (in effect, randomly reflected) back towards Earth, along paths tangential to the Earth's surface. Because only tiny amounts of the original energy will arrive at any given point on or above the Earth's surface, the technique requires powerful transmitters and highly sensitive receivers. However, for ATC application, standard VHF communication sets must remain usable in the air, restricting the more demanding power and sensitivity requirements to the ground equipment.ï¿½
Park Air's solution requires no modification to airborne radios meeting International Civil Aviation Organization (ICAO) standards. Recommended field strengths for extended-range VHF communications are defined within ICAO Annex 10 for both the aircraft and the ground installation. Field strength has to be significantly higher at the aircraft than at the ground station because of the aircraft's more compact, lower-gain antenna, which can suffer performance impairment due to airframe shielding, aircraft-generated electrical noise, and other system factors. The ground station operates in a more benign electromagnetic environment, and a more ideal installation can be achieved.
Park Air uses a ground transmitter of 250-watt carrier power to achieve the desired field strength for aircraft at long range. At the Sanya installation, the transmitter is connected to a pair of directional antennas aligned to project a powerful beam in the required direction. The antennas operate in theï¿½ 112-138-MHz frequency range and are rated for a power level of some 750 watts, continuous wave. They are mounted in a slightly elevated position on a dome tower/platform.
Each antenna is a high-gain, stacked Yagi array, consisting of six elements arranged vertically on a mast and fed via a distribution harness. Each element comprises four dipole radiators mounted on a horizontal support. Total vertical aperture is 47.2 feet (14.4 meters).
Directional performance is assisted by phasing the outputs from the individual elements to maximize gain in the direction in which coverage is required. The resulting horizontal beam width is some 90 degrees, and the vertical beam width is approximately 60 degrees. This beam is broad enough to provide coverage for all aircraft flying within the area of responsibility, but is concentrated enough to afford sufficient forward gain to reach aircraft out to the maximum range and altitude.
When one factors in the 250-watt transmitter power, system losses and aerial gain, an effective isotropic radiated power (EIRP) of more than 10 kilowatts (kW) is achieved in front of the antenna. This translates to about 15 microvolts per meter in the space occupied by an aircraft at long range–sufficient to exceed the "mute," or sensing, threshold of the onboard receiver.
A separate directional receive antenna is employed for ground reception of the aircraft's transmissions, which are likely to be extremely weak by the time they have been backscattered from the troposphere. This antenna, like its transmit equivalent, is a high-gain, stacked Yagi array. In certain other installations, the same directional antenna combination serves for both transmit and receive, given appropriate switching between them. But the separate-antenna option was considered optimum for the Sanya installation. The highly sensitive receive antenna, used in combination with advanced digital signal processing (DSP), accounts for the system's ability to detect and process ultra-weak, backscattered signals.
With high antenna gain, the ground receiver is presented with both the wanted, low-level audio signal and much unwanted atmospheric noise. Highly evolved DSP in Park Air's PAE T6 Series transceivers, however, incorporates algorithms which can discriminate between coherent speech and spurious signals that could degrade normal audio communication. The software looks for a carrier signal and band-limited content that together signify wanted audio, and it rejects random noise.
Park Air claims that this proprietary, digital coherent mute (DCM) technology results in exceptionally high signal capture. Speech remains both audible and intelligible even at maximum range. Park Air developed DCM in response to requests from airlines, such as British Airways, who experience much on-channel interference because their VHF facilities are near large airports. China experiences significant intermodulation and other channel blocking interference due to the presence of emitters outside the control of the ATC authorities.
The transmit antennas at Hainan island are fed by four high-power, model T6T transmitter racks, operating at 122.60, 123.65, 130.20 and 134.40 MHz. The racks are housed in a self-contained transmitter building. A separate receiver building houses a T6R (receive) rack that accommodates the receiver and associated DSP circuitry. Each of the two buildings also houses another rack, for transmit and receive, respectively, associated with a standard 50-watt VHF communications system.
With VHF and other short-range radio, coverage depends not only on atmospheric/ionospheric effects, but also on terrain factors. In recent years, advances in software modeling, combined with satellite-generated terrain data mapping, have enabled system design engineers to predict accurately the coverage achievable by a potential over-the-horizon VHF installation.
Sophisticated software packages enable variations in antenna design, ground station location, aircraft altitude/attitude, and other factors to be analyzed. By comparing the results, engineers can optimize system design and performance on a desktop workstation, rather than by expensive trial and error in the field.
Park Air technicians used such software simulations to develop their extended-range VHF solution for China. Inputs of system data and terrain/topology data led to predictions of regional radio coverage. Flight trials subsequently undertaken by the ATMB confirmed the accuracy of these predictions, which provided a major contribution to an efficient communications environment.
For data link, Park Air already has implemented VDL-2 and has teamed with ITT Industries in the United States to deliver VDL-3 for the FAA's NEXCOM (Next-generation air/ground Communications system) program. The company can migrate these capabilities to any of its installed systems through software downloads.
The Chinese are using the Park Air technology in the voice mode and haven't declared plans to implement VDL. But they are watching controller pilot data link communications (CPDLC) and similar developments closely and could implement the technology at any time.
Over-the-horizon VHF communications systems are preconfigured and transportable. They are built and tested at Park Air's facility in Peterborough, UK, before being sent for installation and commissioning.
The ATC staff at Sanya has welcomed the extended-range system, and as a result, Park Air is negotiating with Chinese officials for similar installations in other regions of the country. Chandler points out that over-the-horizon VHF systems can be equally effective in oceanic and land environments, including deserts and other uninhabited terrain, where locating and maintaining radio ground stations may prove to be difficult.
Park Air PAE T6
Park Air's PAE T6 Series multimode digital radios provide both analog and digital capability. They support conventional voice communications and, if required, VHF digital link (VDL) operations.
In standard VHF mode, the radio recognizes frequencies entered in ICAO format and automatically adjusts to the correct 8.33-KHz or 25-KHz channel spacing. Any combination of these spacings can be programmed for multichannel operation. A 100-channel memory permits immediate recall of stored frequencies. Front panel operation can be locked to prevent unauthorized adjustment of radio parameters.
Operating within the 118-137-MHz frequency band, the transmitter provides frequency offset capability, as defined by ICAO, for wide area coverage. The standard master oscillator supports operation with two, three or four offset carriers. An optional, high-stability oscillator allows five-carrier operation.
The system will automatically inhibit five-offset-carrier operation if the high-stability option is not fitted. Frequencies can be selected locally at the control panel, or remotely. An internal control provides for selection of any carrier power between 125 and 250 watts. This enables power to be matched to the requirements of an overall system in which, for example, other radio communications equipment may be located close by.
To minimize hardware component count, modulation and demodulation processes are software-intensive. They are carried out using digital signal processing (DSP) algorithms that provide consistent long-term performance, enhancing equipment reliability. Built-in test data is presented on a liquid crystal display, along with selected frequencies.