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Efficient Design and Analysis of Airborne Radomes
A mm-wave, Wireless Local Area Network for Use inside an Aircraft
Erwin A. Ulbrich, Jr. and Dan Bryden
McDonnell Douglas Aerospace,
Transport Aircraft Division
Long Beach, CA
Existing aircraft contain many transmitters and receivers in their avionics suites. It is a common experience now for aircraft operators to limit the use of electronics by passengers in their aircraft during part or all of a given flight to protect the operation. In passenger aircraft, many operators are installing two-way, fiber-optic feeds for each seat to allow passengers to access a variety of network revenue services. In military transports or quick-change aircraft, these same problems exist, but they are not addressed easily because of continuing changes in aircraft loads and placements. In addition, communication may be required with paratroopers, dumb loads or smart loads that leave the aircraft during flight after the aircraft Faraday shield is opened (cargo ramp lowered in flight). These factors indicate that some sort of wireless system for communication to digital terminals throughout the aircraft may be necessary. Many older aircraft have cable control systems that run long distances near or through aircraft structures. More modern aircraft have switched to fly-by-wire control systems that also run long distances through the aircraft. Future aircraft appear to be moving toward the use of fly-by-light star coupler systems that operate all over the aircraft. All three of these technologies pose significant risk in the event of structural damage caused by decompression, fire, an explosion, shrapnel, bullets, chemicals or catastrophic fatigue. Similarly, the systems may be impacted by lightning, high energy RF power, nuclear electromagnetic pulses, high energy particles, magnetic storms and precipitation static. Considerable efforts have gone into reducing these risks using special low friction bearings and enclosures, duplicate systems, shielding, error checking and surface bonding. A typical flight-control failure specification using these techniques is five failures per 10 million flights with a goal of only one failure per 10 million flights during routine operations. Clearly, nonroutine or military flights do not experience a level of operation this safe. It may be desirable then to have a backup system that is not affected by any (or most) of the problems mentioned previously. A short list might include large holes shot through the cable runs, smoke and fire, decompression and jamming with elimination of the fuselage Faraday shield.
Initially, it was decided that the system must pass an electrical signal, which is probably multiplexed. The ability to pass such a signal suggests the availability of electrical power and implies prime power for control actuators, which is commonly hydraulic. However, the system should work in the event that all engines are out (no hydraulics) and with failures in the auxiliary power unit and dropout generator. This requirement suggests DC operation from one of the essential buses, the same as the standby instruments, and assumes that power is available at both ends of the aircraft or from batteries in the case of paratroopers, dumb loads or smart loads. In general, it was concluded that only one of these links would be available, which strongly indicates multiplex operation. A signal bandwidth of 2 Mbps would qualify the system for Mil-Spec-1553B, and ARINC 429 and 629 operation. This signal bandwidth was selected with an emphasis on the 1553B utilization for military operations. All three of these specifications were full duplex in a sense but half-duplex in effect since only one receiver or transmitter can communicate at any given time.
As network elements, a pair of these transceivers are required to replace any piece of twisted-pair cable in a 1553B system. Thus, 31 remote terminals and one bus control terminal can be distributed throughout the aircraft, communicating over these transceivers. For this work, a specific transceiver is required to switch from transmit to receive in one microsecond. Two logical frequency ranges exist within the electromagnetic spectrum: mm-waves or infrared waves, which are propagated from one end of the fuselage to the other. Infrared signals were eliminated based on the possible exposure to indeterminate smoke and fire. Submm-waves were considered attractive, but hardware is not mature enough for development. Thus, mm-waves were selected.
Three possible frequencies were considered, including 33, 60 and 95 GHz. Considerable effort was directed at 33 GHz, and a simple transponder pair was built and evaluated. The transponder pair operated successfully across a distance of 250 m through smoke and fire, and in transports with large loads onboard. The receivers were quite sensitive and tests finally showed that they could be jammed. The 95 GHz frequency was rejected out of hand based on known experimental radars being flown in this band. This process of elimination left 60 GHz to be considered, specifically on the first line of atmospheric oxygen. The attenuation of 10 + dB/km for a 60 GHz signal at sea level promised jamming and interference immunity, and minimized use of the frequency by other devices. For transmission outside the aircraft, the attenuation also guaranteed a low probability of interception, limiting such interception to a spherical radius of approximately 300 m.
Physics research revealed that the center frequency of a transmitter must be relatively stable because of the occurrences of resonant oxygen frequencies approximately every 513 MHz from 50 to 70 GHz. The fact that absorption varies largely with temperature and pressure was also noted. It was concluded that stable operation at 60 GHz ±250 MHz would be acceptable for normal cabin temperatures up to 20,000 feet, therefore development could proceed. One set of 60 GHz transceivers capable of transmitting Mil-Spec-1553B-type data back and forth in an aircraft fuselage was developed. A 60 GHz flush antenna for use with the units was also built. It was hoped that EIA RS-170 video also could be transmitted on the link, but not simultaneously with the digital data. Power consumption was constrained to be less than 100 W. Maximum range was specified to be 45 m. A power density of less than 10 mW/cm2 was specified at one foot from the antennas. Harmonics were 40 dB down and stability was one part in 10,000 after a one-hour warmup. This figure started out much higher but was not realizable economically without an oven and its relatively higher power cost. These specifications were intended to produce a system that could be used inside an aircraft full of people and equipment with a fallback position of using the aircraft air conditioning ducts as the propagation path, if required.
The design was not straightforward and required considerable revisions over approximately two years. What resulted, the V-band transceiver, is shown in Figure 1. Each unit measures 20 cm ¥ 9 cm ¥ 9 cm, not including the antenna/horn on one end or the cooling fan on the other end. The transceivers are powered by ±20 V and consume 25 W. Each transceiver is composed of five parts: the mm-wave front-end assembly, X-band crystal reference, phase-locked loop and power regulator, 1553B processing circuit board and IF circuit board. The transceivers operate in pairs; one at 60.60 GHz and the other at 60.54 GHz. The IF is 60 MHz. The units track each other in frequency drift in the laboratory with fan cooling from –30° to +45°C, but have not been tested at widely different temperatures. Each unit weighs 1.81 kg. The transceivers are half duplex and each receiver incorporates circuitry to enable transmission only if the other unit is not transmitting. The output power of each unit is approximately 40 dBm with the horn antenna; the minimum usable receiver power is approximately –90 dBm with the horn antenna. This power level results in a maximum operating range of approximately 150 m. Each unit is designed to be mounted on a camera tripod. Figure 2 shows two units on tripods being tested in a wide-body aircraft mockup. The transmitter uses a V-band Gunn diode VCO. The VCO must be frequency locked for stability, so a small amount of power is coupled to a harmonic mixer where it is mixed with the sixth harmonic of the X-band LO. The 200 MHz output from the harmonic mixer IF is phase compared with a 200 MHz crystal reference, which closes the loop on the VCO at 60.54 or 60.60 MHz. The stability of the X-band oscillator is critical to the correct operation of t he system. Figure 3 shows a block diagram of this arrangement. The system has an effective signal bandwidth from approximately 200 kHz to approximately 2 MHz and input amplitudes of 1 to 10 V. It can switch from receive to transmit mode in less than a microsecond and thus has no trouble with this transition in a Mil-Spec-1553B-mode transmission. Video transmission was not successful due to noise and a lack of low and high frequency responses. The original specified path length can be met with much lower gains than are available with the horns.
The units worked in the laboratory immediately upon receipt and have continued to work. In a typical enclosed aircraft mockup environment, they are relatively insensitive to pointing, personnel movement, metallic objects and ceiling reflections. The devices work well in long conduits, such as the overhead air conditioning ducts in a wide body, and near the floor of a military transport aircraft and bulkheads. Wavelength variations such as standing and reflected waves have not been detected with either horn or flat-plate antennas, which was expected to be a significant problem. When pointed out of the window of the aircraft, the units are very directional and the signal attenuates rapidly. No immediate plans exist for their use in any future aircraft, although they are so new that no consideration has been given yet to certification. Still to be performed are smoke and fire absorption, differential temperature, altitude, condensing humidity and jamming tests. The potential weight savings of this approach over a mechanical cable and bellcrank backup system can be hundreds of pounds, which equates to many times the unit's cost.
A 60 GHz wireless data transmission system has been developed for use inside an aircraft. The system is potentially usable under extreme crash situations that would destroy conventional data transmission systems and thus is being thought of as survivable. Initial tests show that it is reliable and dependable. The unit could be used for a variety of digital data transmissions from the loadmaster or pilot flight stations to a variety of computer terminals, paratrooper data units, and dumb and smart load data units, as well as aircraft remote terminals.
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