The result of the measurement is displayed on a four-digit liquid-crystal-display (LCD) screen: hundreds, tens, and units of feet and one decimal indicating one-quarter-ft. increments. The microprocessor also removes unnecessary information from the LCD: the decimal is blanked above 10 ft. and both the units and decimal are blanked above 100 ft. The PIC16F84's internal electronically erasable programmable read-only memory (EEPROM) stores the offset of the aircraft-installation delay that has to be subtracted from the measured result.
A numerical display is of limited use during the quick and critical flare of a small aircraft. Therefore, the radio altimeter is equipped with a voice synthesizer built around the ISD2560P voice-recorder chip (analog EEPORM storage) from Winbond Electronics Corp. America (San Jose, CA). The ISD2560P contains 21 prerecorded voice messages actually using less than one-half of the chip's total storage area. The actual message telling the current altitude in feet is selected by the PIC16F84 and played back by the ISD2560P through a loudspeaker or through the aircraft's intercom system.
The electronics package of the radio-altimeter prototype is 140-mm long, 66-mm wide and 42-mm high (Fig. 6). The package cross-section is mainly defined by the size of the front-panel LCD. The modulator, transmitter, and receiver RF stages are located on the bottom side in Fig. 6 (bottom). SMA rather than TNC connectors are used for both antennas to keep the weight and size as small as possible.
The dual-channel intermediate-frequency (IF) signal amplification and processing, microprocessor, and voice synthesizer are located on the top side in Fig. 6 (previous page). Since both the PIC16F84 and ISD2560P had to be reprogrammed several times during the experiments, dual-in-line-packaged (DIP) versions of both chips were installed in sockets in the prototype. Of course, most of the remaining components are surface-mount devices (SMDs) placed on the copper side of the single-sided PCBs.
The offset to compensate for the aircraft-installation delay is set by a single pushbutton on the rear panel. A quick depression of this pushbutton just shows the value stored in the EEPROM on the LCD. A long depression (more than 6 s) actually writes the new value into the EEPROM, setting the display to zero. This operation is therefore conveniently performed on ground before the actual flight.
Radio altimeters are usually installed on large aircraft with fuselages made from conducting materials like aluminum or carbon-fiber composites. In such a case, it is relatively easy to obtain a good isolation between the transmit and receive antennas. Further, the results are quite predictable regardless of the actual type of aircraft. Unfortunately, many small aircraft have nonconducting fuselages made of fabric, wood, or glass-fiber-epoxy. Additionally, wood and glass-fiber composites are part of the structure of small aircraft, therefore large holes for radio-altimeter antennas can not be cut in the fuselage without compromising the structural strength of the aircraft. On the other hand, internal antennas are a viable solution on aircraft with transparent fuselages for microwave transmission and reception.
Figure 7 shows an effective internal radio-altimeter antenna design. The antenna includes a linearly-polarized patch with air dielectric installed inside a cavity. Choosing the correct size of the cavity significantly decreases the unwanted coupling between the two radio-altimeter antennas. A quarter-wavelength choke around the cavity further decreases the coupling. The cavity and choke walls also act as a practical spacer when the antenna is placed on the glass-fiber-composite floor of the fuselage. This antenna is relatively simple to manufacture from thin brass sheet. It achieves more than 11-dBi gain and about 15 dB impedance matching across the full 4.2-to-4.4-GHz radio-altimeter band. Of course, the antenna must be tuned to the actual glass-fiber fuselage thickness.
The prototype radio altimeter prototype, including antennas, was installed on a motorglider aircraft and tested in several hundred landings on different runways and in different weather conditions (different soil moisture) over two years. Of course, the performance of any radio altimeter depends heavily on the soil reflectivity and roughness. The prototype was found to operate to about 1000 ft. over average (grass) surfaces. The range is much higher on well-reflecting surfaces like still water while it may drop to 500 ft. under unfavorable conditions (trees). The range of this design is mainly limited by the relatively high noise figure of the receiver and low transmitter power. Both could be improved using better components. Since this radio altimeter was mainly intended as a landing aid operating during the final approach and flare of a small aircraft, a range of 500 ft. was considered sufficient.
The accuracy of the radio altimeter mainly depends on the reflecting target. Reproducible results were always obtained over smooth, paved (concrete) runways. Over grass runways the result of the measurement typically deviates by ±0.5 ft. This means that dithering is not strictly necessary over grass runways, since the changes of the grass reflectivity produce a similar averaging. Without dithering the counter increments are around one foot in a quadrature design in the standard 4.2-to-4.4-GHz band. Unfortunately, on some rare occasions the reflectivity of grass runways was found so low that the radio altimeter could not provide any meaningful results.
Since a digital altitude display was difficult to read and therefore of limited use during challenging landings, a simple acoustic interface (Fig. 8) was tested during the experiments with good success. The radio altimeter is switched on at the beginning of the final approach, when the acoustic signal is an intermittent tone of constant pitch and variable period. As the aircraft descends down to the runway, the beep period increases. At the beginning of the flare, the beep period increases to infinity, the sound therefore becomes a continuous tone. The altitude information is thereafter relayed by the decreasing tone pitch. The tone pitch decreases down to zero when the landing gear touches the runway.
Although the acoustic interface was found to be quick and reliable, supporting the final approach and flare in the most critical configuration of the aircraft with the flaps and/or spoilers fully deployed, it required additional training for the pilot. An additional disadvantage is that the simple acoustic signal can easily be confused with beeps originating in other instruments on-board an aircraft. As a result, it was necessary to develop a simple-to-understand voice synthesizer. It was found sufficient to describe the altitude of the aircraft in feet with 21 different words: minus, zero, half, one, two, three, four, five, seven, ten, fifteen, twenty, thirty, fifty, seventy, one-hundred, one-hundred-fifty, two-hundred, three-hundred, four-hundred, and five-hundred. The resolution was intentionally made coarse at higher altitudes to avoid overloading the pilot with unnecessary information.
The success of the prototype radio altimeter demonstrates that an effective design can be implemented with low-cost electronic devices. The design could be improved with a better RF front end to improve the range and reliability over poorly reflecting surfaces. In addition, a digital signal processor (DSP) could be used to remove crosstalk between the two antennas. Finally, if both quadrature beat signals are fed to a signal processor to measure the phase, the dithering becomes unnecessary.
This research was supported by the Ministry of Higher Education, Science, and Technology of Republic Slovenia under the research program P2-0246.
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