Short-range radio altimeters are important safety and navigational tools in small aircraft. Usually designed as short-range frequency-modulated (FM) radars in the 4.2-to-4.4-GHz band,1 their main applications are for instrument-based approaches and landings for larger commercial aircraft, although they are also suitable for smaller aircraft and even unmanned air vehicles (UAVs). The accuracy and resolution of aviation altimeters is usually limited to a few feet due to the limited availability of bandwidth (200 MHz) in the 4.3-GHz range. Fortunately, by adding a second receiver channel in quadrature, it may be possible to dramatically improve the resolution and accuracy of these short-range radio altimeters.
The basic operation of a radio altimeter is shown in Fig. 1. It is a low-power, continuous-wave (CW) radar system that generally requires separate transmit and receiver antennas. The radio-wave propagation delay is usually too short to switch a single antenna between the transmitter and the receiver. Most attempts were directed toward improving the radio-altimeter reliability involving 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
For correct operation, the receiving antenna should detect only the reflected signal from the runway and not the radio signal coming directly from the transmitting antenna. The two antennas must be widely separate to avoid crosstalk. 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 (FM) CW 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.
Figure 2 shows the design of a conventional FM radio altimeter. The sweep waveform is triangular and both slopes are typically 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 50 and 300 Hz. 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.
Most aviation radio altimeters operate in the 4.2-to-4.4-GHz frequency band. Of the 200 MHz available, only about 150 MHz in midband is typically used. The 4.3-GHz (7-cm-wavelength) frequency band is a compromise between the bandwidth available (accuracy of the measurement) and the surface roughness of the runway or other reflecting target. Transmitter power ranges from 10 mW (+10 dBm) to 500 mW (+27 dBm). The directivity of both transmit and receive antennas is limited to about 10 dBi 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 1 MHz. Part of the transmitter signal is also used as the local oscillator (LO) 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, transmission delays (due mainly to the cables connecting the antennas to the electronics of the radio altimeter) must be subtracted from the measured altitude.
Figure 3 shows that the accuracy and resolution of a radio altimeter are limited by the RF bandwidth. 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.
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 400 MHz as suggested in ref. 3. A better solution is to add a low-frequency (around 10-Hz) 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 approach 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 2. 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.
To demonstrate the author's approach, an accurate radio altimeter with a dual-channel (quadrature) homodyne receiver was developed and built (Fig. 4). The main application of this radio altimeter is to aid in small aircraft landings. The transmitter modulator includes two triangular oscillators: the main sweep at 150 Hz and the dither at 15 Hz. The dither amplitude is set to about 10 percent of the main sweep amplitude. The sum of both waveforms is applied to the microwave voltage-controlled oscillator (VCO) operating directly at 4.3 GHz. The VCO includes a BFP420 transistor amplifier from Infineon Technologies (San Jose, CA) 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 diode from Infineon.4
The VCO is followed by two amplifier-buffer stages using another BFP420 bipolar transistor and a MGF4918 high-electron-mobility transistor (HEMT) from Mitsubishi Electronics America (Sunnyvale, CA). The latter produces RF power of about 40 mW (+16 dBm) at 4.3 GHz. Most of this signal is fed to the transmit antenna, while a small fraction (about 1 mW or 0 dBm) is coupled and sent through a lowpass filter to provide the homodyne LO.
The receiver RF front end includes a single-stage low-noise amplifier (LNA) with another MGF4918 HEMT and two IAM81008 balanced mixers from Agilent Technologies (Santa Clara, CA) 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 IAM81008 mixers (formerly from Avantek and now obsolete) are used beyond their designed frequency range in this application, therefore the overall noise figure of the receiver is in the 15-to-20-dB range.
The RF section of the radio altimeter is built on two (transmitter and receiver) printed-circuit boards (PCBs) fabricated in microstrip technology (Fig. 5).
Each PCB is 80 mm long and 20 mm wide. Both boards are etched on 19-mil-thick Ultralam 2000 teflon laminate from Rogers Corp. (Rogers, CT) with a dielectric constant of 2.43. Figure 6 shows the top side of both boards; 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.
Both in-phase (I) and quadrature (Q) beat signals are filtered and amplified. The dual-channel amplifier has a common automatic-gain-control (AGC) circuit. The AGC time constant must be carefully chosen 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-b PIC16F84A microcontroller from Microchip Technology (Chandler, AZ). 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 only 4 MHz is more than sufficient for the PIC16F84.