A Landing Radio Altimeter for Small Aircraft

Matjaž Vidmar, S53MV

An accurate radio altimeter with a dual-channel (quadrature) homodyne receiver is presented in this article. This new design improves the accuracy while operating in the standard 4.3GHz radio-altimeter allocation. The new radio altimeter is intended as a landing aid for small general-aviation aircraft, experimental aircraft and UAVs and is equipped with a simple synthesized-voice human interface.

Introduction

Aviation radio altimeters are usually designed as short-range FM radars operating in the 4.2-4.4GHz frequency band [1]. Their main application are instrumented approaches and landings of large commercial aircraft. The accuracy and resolution of aviation altimeters is usually limited to a few feet due to the limited available bandwidth of 200MHz in the 4.3GHz frequency band. This accuracy is considered sufficient even for the flare during an autoland manoeuvre of a large commercial jet aircraft.

Most of the design efforts went into developing highly-reliable radio altimeters that allow a parallel operation of two or three instruments on the same aircraft. Most aviation radio altimeters have separate transmit and receive antennas although considerable efforts were invested into developing a single-antenna radio altimeter [2].

Radio altimeters are not considered of much use on small aircraft, since the latter usually operate in visual meteorological conditions. Since the approach and landing speeds of small aircraft are three to four times smaller than those of large jet aircraft, an autoland of a small aircraft requires a comparably more accurate altimeter. Current aviation radio altimeters are not accurate enough to provide useful information during the flare of a small aircraft.

Nevertheless, bad landings are certainly the number one cause for accidents of small aircraft. The skills of their “weekend” pilots are hardly comparable to those of commercial airline professionals. Night-time landings, poor visibility in bad weather conditions, strong and gusty winds, local dangerous obstacles like high-voltage power lines distracting the pilot or simply a tired pilot and sick passengers after a long trip may all add up to a bad landing. Emergency landings of both glider aircraft or after an engine failure are even more difficult. In the latter case it is particularly difficult for the pilot to estimate the altitude of his aircraft above a water (sea) surface or any other landscape without known reference points.

New composite materials allow aircraft designers to push the aerodynamics to the limits, maximizing the glide ratio, fuel efficiency and/or range. These new aircraft are no longer easy to land, like the example shown on figure 1. In all these cases an accurate radio altimeter could solve many problems, effectively preventing both landing accidents and/or unnecessary wear of the landing gear.

Figure 1 – A high-performance, but difficult-to-land motorglider.

Yet another application for accurate radio altimeters are UAVs. These unmanned aircraft are even smaller and slower. While GPS and/or pressure altimeters are accurate enough for en-route navigation, a high-accuracy radio altimeter is required for an automated landing. It therefore makes sense to investigate the design of a short-range, but very accurate radio altimeter. An improved FM radar operating in the standard 4.2-4.4GHz radio-altimeter allocation will be presented in this article.

Radio-altimeter operation

The principle of operation of a radio altimeter is explained on figure 2. Aviation radio altimeters are short-range, low-power, continuous-wave radars and generally require two separate transmit and receive antennas. The radio-wave propagation delay is usually too short to switch a single antenna between the transmitter and the receiver.

Figure 2 – Principle of operation of a radio altimeter.

In order to operate correctly, the receiving antenna of a radio altimeter should only receive the reflected signal from the runway and avoid the radio signal coming directly from the transmitting antenna. The radio-altimeter antennas have to be widely separated to avoid unwanted crosstalk between the two antennas. Although electronic filtering of the crosstalk allows the design of single-antenna radio altimeters, the operation of the latter is usually limited above a specified minimum altitude.

Most aviation radio altimeters are frequency-modulated radars. The carrier frequency of the transmitter is swept continuously in a given frequency range. Since the received signal is delayed, the receive frequency differs from the transmitter. If the rate-of-change of the transmitter frequency is constant, the delay and therefore altitude are directly proportional to the measured frequency difference between the transmitter and receiver.

The design of a conventional FM radio-altimeter is shown on figure 3. The sweep waveform is triangular and both slopes are usually used for the altitude measurement to compensate for the Doppler shift due to the vertical speed of the aircraft. The sweep frequency is usually between 50Hz and 300Hz. The higher limit is imposed by the receiver thermal noise, the lower limit is the ability of the radio altimeter to eliminate the Doppler shift in the case of a descending or climbing aircraft.

Figure 3 – Conventional FM radio-altimeter design.

Most aviation radio altimeters operate in the 4.2-4.4GHz frequency band. Out of the 200MHz available only the central part of about 150MHz is usually used. The 4.3GHz (7cm) frequency band is a compromise between the bandwidth available (accuracy of the measurement) and the surface roughness of the runway or other reflecting target.

The transmitter power ranges from 10mW (+10dBm) up to 500mW (+27dBm). The directivity of both transmit and receive antennas is limited to about 10dBi to allow the operation of the radio altimeter at moderate pitch and bank angles of the aircraft.

The receiver is a homodyne design using a mixer to derive the difference between the transmit and receive frequencies. The beat frequency is usually less than 1MHz. Part of the transmitter signal is also used as the local oscillator for the receiver. Some radio-altimeter designs may simply use the crosstalk between the transmit and receive antennas to feed the LO signal in the receive mixer.

The beat signal is filtered first, then amplified and limited. A frequency counter drives the altitude indicator and various altitude alarms if required. Of course, the aircraft-installation delay (mainly the cables connecting the antennas to the electronics of the radio altimeter) has to be subtracted from the measured altitude.

The accuracy and resolution of a radio altimeter are limited by the RF bandwidth available as shown on figure 4. For a given frequency sweep, the electronics produces a certain beat frequency with a limited number of transitions that can be counted. As the measured altitude changes, the beat pattern shifts and the counter result actually makes several oscillations between two adjacent values.

Figure 4 – Accuracy and resolution of a radio altimeter.

There are different ways to improve the accuracy and resolution of a radio altimeter. The simplest solution is to increase the frequency sweep up to 400MHz as suggested in [3]. A better solution is to add a low-frequency (around 10Hz) triangular dither waveform to the main triangular sweep. In this way the oscillations between two adjacent values are averaged out during several measurements, however some additional bandwidth is required for the dither!

The solution suggested in [4] and described in this article is to add a second receiving channel in quadrature. In this way the number of available transitions is doubled and the accuracy and resolution are improved by a factor of two. Further, the low-frequency dither amplitude can be halved so that less bandwidth is wasted for the dither. Finally, a quadrature design of a homodyne receiver is required anyway to extract all of the available information out of the received signal!

Radio-altimeter design

An accurate radio altimeter with a dual-channel (quadrature) homodyne receiver was developed and built. The main application of this radio altimeter was to help the pilot during the landing of a small aircraft. The block diagram of this new design is presented on figure 5.

Figure 5 – Block diagram of the landing radio altimeter.

The transmitter modulator includes two triangular oscillators: the main sweep at 150Hz and the dither at 15Hz. The dither amplitude is set to about 10% of the main sweep amplitude. The sum of both waveforms is applied to the microwave VCO operating directly at 4.3GHz. The VCO includes a BFP420 amplifier and an interdigital filter in the feedback. Due to the relatively narrow sweep, only the central microstrip resonator is tuned with a single BBY51 varactor [5].

The VCO is followed by two amplifier-buffer stages using another BFP420 bipolar transistor and a MGF4918 HEMT. The latter produces a RF power of about 40mW (+16dBm) at 4.3GHz. Most of this signal is fed to the transmit antenna, while a small fraction (about a milliwatt) is coupled and sent through a lowpass filter to provide the homodyne local oscillator.

The receiver RF front end includes a single-stage LNA with another MGF4918 HEMT and two IAM81008 balanced mixers in quadrature. The RF and LO signals are split with two Wilkinson hybrids. Different length microstrip lines are used to obtain the required phase shifts. The (now obsolete) IAM81008 mixers are used beyond their designed frequency range in this application, therefore the overall noise figure of the receiver is in the 15...20dB range.

The RF section of the radio altimeter is built on two printed-circuit boards (transmitter and receiver) in microstrip technology. Each board is 80mm long and 20mm wide. Both boards are etched on 19 mils thick “Ultralam 2000” teflon laminate (Rogers)with a dielectric constant of 2.43. The upper side of both boards is shown on figure 6, while the bottom side is not etched to act as the microstrip groundplane. Both boards are soldered in a frame made of thin brass sheet for shielding purposes.

Figure 6 – Microstrip printed-circuit boards of the RF front end.

Both in-phase and quadrature beat signals are filtered and amplified. The dual-channel amplifier has a common AGC. The AGC time constant has to be chosen carefully to minimize the effects of signal dropouts due to poor reflections. Noise is removed by two Schmitt-trigger stages driving a pulse-former circuit that produces one output pulse for every zero crossing of any of the two input signals.

The pulses are fed to a frequency counter implemented inside a 8-bit PIC 16F84 microprocessor. The gate of the counter is not synchronized to the main sweep nor to the triangular dither. The microprocessor however performs digital averaging (filtering) of the measured result. Due to the relatively low frequencies involved, a clock frequency of 20MHz is more than sufficient for the PIC 16F84.

The result of the measurement is displayed on a 4-digit LCD: hundreds, tens and units of feet and one decimal indicating quarters of feet. The microprocessor also removes unnecessary information from the LCD: the decimal is blanked above 10 feet and both the units and decimal are blanked above 100 feet. The internal EEPROM of the PIC 16F84 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). The ISD2560P contains 21 pre-recorded voice messages actually using less than half of the total storage area of the chip. The actual message telling the current altitude in feet is selected by the PIC 16F84 and played back by the ISD2560P into a loudspeaker or better into the intercom installation on-board the aircraft.

The electronics package of the radio-altimeter prototype is 140mm long, 66mm wide and 42mm high and is shown on figures 7 and 8. The cross-section of the package is mainly defined by the size of the front-panel LCD. The modulator, transmitter and receiver RF stages are located on the bottom side as shown on figure 7. SMA connectors are used for both antennas to keep the weight and size as small as practical in place of the commonly used TNCs.

Figure 7 – Bottom view: modulator, transmitter and receiver.

The dual-channel IF signal amplification and processing, microprocessor and voice synthesizer are located on the top side as shown on figure 8. Since both the PIC 16F84 and ISD2560P had to be reprogrammed several times during the experiments, dual-in-line packaged versions of both chips were installed in sockets in the prototype. Of course, most of the remaining components are SMDs placed on the copper side of the single-sided printed-circuit boards.

Figure 8 – Top view: IF processing, microprocessor and voice chip.

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 seconds) 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 this case 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 non-conducting 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 microwaves.

A successful internal radio-altimeter antenna design is shown on figure 9. 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.

Figure 9 – Radio-altimeter antenna for transparent fuselages.

The antenna shown on figure 9 is relatively simple to manufacture from thin brass sheet. It achieves a gain of more than 11dBi and an impedance matching of about 15dB across the whole 4.2-4.4GHz radio-altimeter band. Of course the antenna has to be tuned to the actual glass-fiber fuselage thickness.

Practical results

The described radio-altimeter prototype including the antennas was installed on a motorglider aircraft (figure 1) where it was practically tested in several hundred landings on different runways and in different weather conditions (different soil moisture) during almost two years. Of course, the performance of any radio altimeter depends heavily on the soil reflectivity and roughness.

The described radio-altimeter was found to operate up to about 1000 feet over average (grass) surfaces. The range is much higher on well-reflecting surfaces like still water while it may drop to 500 feet under unfavourable 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 feet 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 feet. 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 4.2-4.4GHz 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.

In addition to a digital altitude display, that is hard to read and is therefore of limited use during difficult landings, a simple acoustic interface was successfully tested during practical experiments as shown on figure 10.

Figure 10 – Simple acoustic human interface.

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 described acoustic interface was found very 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 also required some 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.

Therefore a simple-to-understand voice synthesizer was found necessary. Practically 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.

Conclusions

A practical landing radio altimeter was developed, built and tested in several hundred landings. The results of the practical testing show that cheap electronics can replace expensive maintenance and/or repairs of the landing gear and/or other damage on a small aircraft.

The radio-altimeter circuit presented in this article can be improved in several ways. A better RF front end could improve the range and reliability over poorly-reflecting surfaces. A digital signal processor could perfectly remove the crosstalk between the two antennas, again improving the reliability of the instrument over poorly-reflecting surfaces. Finally, if both quadrature beat signals are fed to a signal processor to measure the phase, the dithering becomes unnecessary.

References

[1] Werner Mansfeld: “Funkortungs- und Funknavigationsanlagen”, 1994 Hüthig Buch Verlag GmbH, Heidelberg, ISBN 3-7785-2202-7, pages 337-342.

[2] Leo G. Maloratsky: “An Aircraft Single-antenna FM Radio Altimeter”, Microwave Journal, May 2003, ISSN 0192-6225.

[3] Giorgos E. Stratakos, Paul Bougas and Kostas Gotsis: “A Low Cost, High Accuracy Radar Altimeter”, Microwave Journal, February 2000, ISSN 0192-6225.

[4] Matjaž Vidmar: "Design Improves 4.3-GHz Radio Altimeter Accuracy", Microwaves&RF, June 2005, ISSN 0745-2993.

[5] Matjaž Vidmar: “A Wideband, Varactor-tuned Microstrip VCO”, Microwave Journal, June 1999, ISSN 0192-6225.