Radiowave propagation research in the tropics using a transportable multiparameter radar system

by J. D. Eastment, M. Thurai, D. N. Ladd and 1. N. Moore

Although accurate models for ea r t h-space microwave pro pagat ion, based on extensive measured data, exist for temperate regions of the world, this i s not the case for the tropics. With the pro1 if erat ion of satel I i te communications in these areas, it has become important to test the validity of currently recommended techniques for calculating circuit ava i la bi I ity and co-c ha n ne1 interference levels due to attenuation by precipitation and scattering from precipitation, respectively. Multiparameter radar measurements, employing both polarimetric and Doppler techniques and made over a significant period of time on a statistically-valid basis, provide an ideal tool for these investigations. The paper describes the design considerations for, and construction of such a radar. A selection of its data products are presented, illustrating its potential for radio propagation and meteorological research.

’

1 Introduction and objectives of the work

Tropical precipitation, frequently more intense and prolonged than that experienced in temperate climates, is a major cause of fading and unwanted interference between systems sharing the same frequency spectrum at low latitudes. In order to make an assessment of the various attenuation and scattering effects of interest to communication system engineers, it is necessary to obtain information on the spatial distribution of precipitation

along the radio path through the atmosphere. Furthermore, it is important to distinguish between areas of water and of ice (in its various forms), as their propagation characteristics are markedly different. Identification of the melting-layer (a narrow band around the height of the 0°C isotherm where descending ice particles change phase into liquid water), when present, is particularly important because of its strongly attenuating, depolarising and scattering effects.

Although radiometer and satellite-beacon measure- ments yield total path-integrated attenuation, only radar techniques are capable of resolving the spatial structure of the precipitation and of providing useful micro-physical information, such as rain drop size distribution (DSD) and fall velocity. Of special interest are effects such as diurnal and seasonal variation of maximum rain height, which are readily derived from radar measurements.

The requirement to distinguish unambiguously the melting-layer calls for a polarimetric radar. Specifically, melting regions can be identified by their enhanced cross- polar radar reflectivities. DSD estimation is based on the measurement of the spread of raindrop fall velocities, which, ideally, requires the full Doppler spectrum for each range gate of interest.

Consequently, we have developed a vertically-pointing polarimetric S-band pulsed Doppler radar’, suitable for transportation to a tropical region. This system has been used to collect a statistically valid database of precipitation measurements, in order to evaluate existing propagation models, for example ITU-R Recommendation P.618-4’, and, ifnecessary, to improve these procedures. It has been shown in Reference 3 that a radar measurement database can be used to predict propagation effects at arbitrary frequencies, up to at least 30 GHz. So as to verify our radar measurements, we also record co-located distrometer data, allowing comparison oi radar-derived DSDs and rain rates with those from a well-characterised ground-truth instrument.

Although primarily of interest to radio systems designers, the data from this experimental campaign will also be made available to meteorologists studying tropical

4 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAI, FEBRUARY 1998

climatology, and those engaged in the joint US-Japanese Tropical Rainfall Measurement Mission (TRMM) project.

2 Definition of measured radar parameters

The four quantities measured by the radar system are defined as follows.

The co-polar reflectivity (for a horizontally polarised transmitted pulse), Z,,, is defined as:

where Ais the radarwavelength, IKI2 is a constant at agiven wavelength (related to the refractive index of water), and qR (cm3/m3) is the Rayleigh backscatter cross-section of the target per unit volume. It is assumed that the radar pulse volume is uniformly filled with scatterers, and that the dimensions of the particles are sufficiently small in comparison with the radar wavelength for Rayleigh scattering to apply.

When the antenna is pointed at the zenith, as is the case for this system, the concept of 'horizontal' and 'vertical' polarisation has little meaning, so the co-polar reflectivity will be referred to simply as 2. 2 is derived from the scattered power measured by the receiver, PR, as follows:

where r is the range to the target, and C is a calibration constant, which is a property of the radar hardware and signal processing software. 2 is usually expressed on a logarithmic scale, in units of dB2

dBZ = 10 log,, 2

2 values of interest for studies of rainfall range from around 10 dBZ for light drizzle to around 50 dBZ for very heavy rain.

The linear depolarisation ratio, LDR, is defined as:

where 2, is the cross-polar reflectivity, measured by transmitting horizontal polarisation but receiving vertically polarised returns. When observing at vertical incidence, LDR in rain is typically around -40 dB, in ice it is generally below -20 dB, but in the melting layer values as high as -10 dB are commonly encountered.

The parameters mean velocity, <v>, and spectral variance, 0,2, are the first and second moments of the full Doppler spectrum, S(v) , of the weather target (as shown in Fig. 1).

Fig. 1 Idealised Doppler spectrum of weather target

a, is referred to as the spectral width. The velocity spectrum is simply a scaled version of the frequency spectrum, which may be determined by appropriately processing the radar's intermediate frequency (IF) signal. Target radial velocity and IF Doppler shift, M are related as:

v = AAf/2

Typical values of mean velocity are 0-1 nn/s in ice, and 8-10 m/s in heavy rain. Spectral width is typically below 1 m/s in ice, but around 2 m/s in heavy rain.

100" 120" 140" 160" 20"

0"

-20"

40" 100" 120" 140" 160"

20"

0"

-20"

-40"

Fig. 2 Radar site location in PNG

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998 5

Fig. 3 Laboratory

Radar hardware undergoing soak-testing at Rutherford Appleton

Fig. 4 Unitech's electronics laboratory

The radar's 3 m diameter antenna, installed on the rooftop of

3 Choice of tropical location and implications for equipment design

The choice of location for the radar was based on appropriate climate, ease of transportation and installation of equipment, quality of infrastructure (roads, electrical power and telecommunications), availability of suitably trained personnel to assist with the installation and subsequent operation of the system, and synergy with any existing local propagation or meteorological measurement programmes.

The Papua New Guinea University of Technology (Unitech) at Lae, PNG met all these criteria. Located at latitude 7" South (see Fig. a), it has a climate which is typical of the tropics, with daytime temperatures of 30+3"C, relative humidity 285% all year round and annual rainfall of 4.5 m. h e is on the coast, so the 2 tonnes of equipment was shipped from the UK in a 'Seatainer', to the container port, and then carried by road transport for the remaining 8 km to Unitech. Staff from the University's

Electrical and Communication Engineering Department provided access to heavy engineering facilities, such as cranes, lifting- tackle and welding equipment. This greatly expedited the installation process, and several staff were trained in the operation of the radar. Unitech has been involved in an Intelsat-funded programme of radiometer and satellite-beacon measurements for several years4. This work continues and provides a valuable opportunity to compare the new radar data with these existing measurements, as well as meteorological information from Unitech's own rain gauges and distrometer.

Liaison with Unitech from the UK is straightforward via reliable telephone and fax links. Transfer of recorded data back to the UK is accomplished by courier transport of back-up copies of the files on 230 Mbyte magneto-optical disks roughly once per fortnight. These disks were the only recording media found to exhibit long-term reliability in conditions of prolonged high temperature and humidity. Conventional diskettes, floptical disks and tape-drive cassettes all suffered data corruption after only a few days exposure to the tropical atmosphere.

The aggressive ambient conditions also have an impact on the design of the electronic

packaging, if high reliability is to be maintained. Where possible, gasket-sealed enclosures incorporating dessicant, with external forced-air cooling, have been employed. The waveguide system is pressurised to 0.2 bar with dry nitrogen to prevent moisture ingress, while a small radome covering the dish-feed excludes insects from entering the hom. As rain rates often exceed 100 mm/h, it is essential that zenith-pointing dish antennas have adequately dimensioned drain-holes to prevent water collecting in the reflector bowl.

Line voltage fluctuations are a further concern for experimental systems operating in the tropics. Although nominally 240 VAC, the mains supply in Lae was observed to vary between 210 and 290 V, with frequent transients, brown-outs and occasional power cuts of several hours duration. A combination of preset variacs, line-filters and an uninterruptible power supply (UPS) was used to provide clean, regulated power to the radar system. Control software was designed to implement graceful shutdown of data recording during long power outages when UPS

6 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998

capacity became exhausted, and surge-free power-up of the radar when site mains returned.

Table 1 : characteristics

Transmitter, receiver and antenna

4 Description of radar hardware

The indoor portion of the radar installation is shown in Fig. 3, and the antenna system is pictured in Fig. 4. The transmitter, receiver and antenna characteristics are summarised in Table 1.

The key elements of the system are a dish antenna with dual-polarisation feed, a magnetron-based transmitter, a receiver front-end, an IF processor, a DSP (digital signal processing)-based data acquisition and recording system, and a timing and control unit.

The antenna comprises a 3 m diameter dish on a fixed, vertically-pointing mount. A scalar feed and orthomode transducer permit dual polarisation (linear, horizontal and vertical) operation. Antenna gain is 36.5 dBi, -3 dB beamwidth is 2.3", and far-field distance is 180 m. Integrated cross-polar isolation is around -25 dB relative to the main-lobe co-polar response. The main factors limiting the antenna cross-polar isolation are aperture blockage effects due to the feed-horn and its associated support struts.

The transmitter (Fig. 5) is a magnetron-based unit, operating at a nominal frequency of 3028 MHz, with a peak output power of some 600 kW. A pulse duration of 1 ps yields a range resolution of 150 m, while the pulse- repetition frequency (PRF) of 625 Hz implies an unambiguous velocity range of A5.5 m/s. Integrated with the transmitter are an isolator (to prevent frequency pulling due to a change in antenna voltage standing wave ratio (VSm), such as might occur due to raindrops on the radome of the dish feed), a -40 dB directional coupler (to obtain a sample of the transmitted pulse for use by the Doppler processor), a circulator (to isolate the transmitter from the co-polar receiver input while both are connected to the horizontal port of the dish feed), and a pair of receiver protection devices, each comprising a gas-filled TR (transmit-receive) cell and varactor diode limiter.

The connection between the dish feed horizontal port and the circulator is via a length of WGlO waveguide (due to the high peak power involved) while that between the vertical port and the cross-polar receiver input protection is via a low-loss flexible coaxial cable. The frlont-end (Fig. 6) consists of a PIN diode polarisation switch, a low-noise receive downconverter, a similar chain clownconverting the transmitted radar pulse, a low-phase-noise stable local oscillator (STALO), and an amplitude calibration system. The 80 dB of available dynamic range has been located such that the strongest and weakest signals of interest correspond to a 50 dBZ target at 100 m range, and a 10 dBZ target at 10 km, respectively. The large av(ai1able dynamic range obviates the requirement for automatic gain control. Considerably greater sensitivity is available, if required, for observation of clouds and clear-air effects. Sensitivity is controlled by appropriate adjustment of attenuators in both the RF and IF signal paths. The receiver is a single-

Fig. 5 Arrangement of transmitter, dulplexing and receiver protec1:ion components

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998 7

Fig. 6 Receiver front-end, including calibration unit and transmit-pulse downconverter

conversion superhet with a GaAs field effect transistor (Fm) RF stage, having an overall noise-figure of 5.5 dB, including losses in the polarisation switch and burn-out protection components. The system IF is 30 MHz, and the IF bandwidth is 4 MHz.

A parallel downconverter chain mixes a sample of the transmitter pulse down to IF for use by the Doppler processor. Both receiver and transmitter downconverters employ the same !3'l?AL,O. This oscillator is a highly stable, crystal-controlled device exhibiting very low phase-noise. The low-phase-noise properties of the STALO imply a very narrow frequency spectrum, with low levels of noise sidebands close to the carrier. This is important for accurate measurements of low-Doppler-shift meteorological targets. An amplitude calibration unit,

Fig. 7 IF amplitude and phase processor

employing a programmable PIN diode attenuator and a gated oscillator running at the nominal transmitter frequency, permits injection of known power levels (over a 64 dB range, in 0.25 dB steps) at the low-noise amplifier (LNA) input via a directional coupler. This enables logging of the overall receiver characteristic and correction of measured data to eliminate residual gain variation as a function of, for example, ambient temperature changes.

The IF processor (Fig. 7) consists of two distinct sections dealing with the amplitude (for co-polar and cross- polar reflectivity) and phase (for Doppler spectrum, mean and width) of the IF signals. These units are shown in Figs. 8 and 9. "he amplitude processing comprises alogarithmic amplifier, followed by a video lowpass filter and amplifier. A logarithmic amplifier is used to compress the receiver's

8 ELECTRONICS 8.1 COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998

80 dB measurement range into a post-detection video dynamic range suitable for sampling with an 8-bit analogue-to-digital converter (ADC). The video filter is an approximation to the optimum ‘matched filter’ for the transmitted pulse shape, and serves to improve the receiver’s detection performance. The resulting video signal (2) forms one input to the data acquisition system (DAS), representing co-polar and cross- polar target reflectivity, as determined by the state of the receiver polarisation switch.

The phase processing5 is more complicated as it involves coherent detection of both the IFS due to the receiver and transmitter. Magnetron-based radars emit pulses of random starting phase. The transmitter phase must be determined for each pulse. This is achieved by processing the IF signal obtained by downconverting the transmitter’s microwave pulse. The angle obtained from the phase detector is subtracted from the angles associated with each range gate corresponding to that particular transmitted pulse. The Doppler shift is then computed by determining the rate of change of corrected phase, at a particular gate, from pulse to pulse.

The coherent detection is implemented in a pair of I/Q

Fig. 8 Logarithmic detector, video filter and amplifier hardware

Fig. 9 COHO, I/Q detector a d video amplifier hardware

detectors fed with a 30 MHz reference signal from a crystal- controlled coherent oscillator (COHO). The receiver IF is amplitude limited prior to this detection process to achieve effective operation over a wide input dynamic range. (It is recognised that this technique results in a degree of spectral distortion; however, for monomodal spectra, the mean velocities are unaffected, while the spectral widths are broadened somewhat. The situation is more complex for bimodal spectra, but this problem will mainly arise due to clutter contamination of data in the first few range gates only. Adetailed study on a similar system addressing these issues can be found in Reference 6.)

The I and Q video signals are lowpass filtered and amplified. The pair of signals due to the transmitter IF (Zm and 9,) are fed to the DASvia a sample-and-hold unit, which is triggered at a well-defined time approximately midway through the transmitter pulse. The signals due to the receiver IF (I, and Q,) are fed directly to the DAS. A sixth input (f“), representing transmitter pulse power, is derived from the amplified output of a diode detector connected to the transmitter via a directional coupler and attenuator. The DAS itself is described in the following Section.

A timing and control unit, shown in Fig. 10, interfaces with the front-end, IF processor, transmitter and DAS to integrate the separate units into a complete radar system. It is responsible for generating the system cllock, PRF, ADC sample timing, receive polarisation state, and other signals. All time-critical signals are derived from a crystal- controlled master clock to ensure synchronous operation. This is important for optimum Doppler performance. The unit also provides control signals for IF gain setting, video- filter bandwidth selection, and DAS mode switching. The circuitry is designed to be flexible enough to enable its use with other radar systems, for example7, as most timing parameters and input/output options are selectable by means of on-board adjustments and jumpers.

5 Digital signal processing, data recording and display

The configuration of the DAS is shown in Fig. 11.The six analogue signals produced by the IF processors (Zm and QTx; Z, and QRx; Z and PTx) are first digitised by a set of three dual-channel ADC cards. Each ADC channel

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998 9

Fig. 10 System clock, timing and control electronics

consists of an 8-bit converter running at 2 MHz, with an associated 2 kilobyte FIFO (first-in first-out) data buffer to accommodate bus read-time constraints. Control signals for the ADC cards are generated by a programmable counter-timer card, which is resident on the ISA bus of the host PC. This card is supplied with the 4 MHz system master clock, and the 625 Hz TX fire’ signal from the timing and control unit (TCU). Parameters such as maximum and minimum recorded range and range-gate spacing are set-up via the PC’s keyboard. The ADC cards reside in a special-purpose interface unit, which is connected to the Hyperbus port of a commercial 40 MHz MM96 DSP card. The interface unit has additional inputs for status signals indicating receiver polarisation state and Doppler/polarimetric data logging modes. The DSP card performs high-speed data

(b)

reduction and communicates the processed data to the i-486 DX2 66 MHz-based PC via the ISA bus.

The DSP card is programmed in assembly language, and implements the following algorithms to extract the parameters 2, LDR, U and 0,:

(a) 2 is computed as the sum over 512 samples of the co- polar IF power at a particular range-gate, normalised by the number of samples, and corrected for the inverse square-law range dependence and radar system calibration constant:

1023 1 Z=Z,,(z] =-CX P(z] CP,(i ,k) 512 k-1.3.5 .

where i is the range-gate number and k is the pulse- number.

LDR is computed as the ratio between the sum of 512 co-polar IF power samples and 512 cross-polar IF power samples for a particular range gate:

where

Fig. 11 Arrangement of the data acquisition, processing and recording system

10 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998

Table 2: Polarisation and Doppler parameters Table 3: Data recording and dynamic range issues

(c) <U> is computed by firstly correcting the phase angle of each set of range-gates associated with a particular transmit pulse, so as to remove the random pulse-to-pulse phase associated with the incoherent magnetron transmitter. This is done by performing a multiplication of the complex number representing each range-gate phase angle by the complex conjugate of the number representing the phase of the particular transmit pulse:

The series of values arg[V(i,k)] is the result of subtractions of the phase angles, as required for a 'coherent-on-receive' radar. The mean velocities for each range gate are then determined from the normalised auto-covariance function, Rl(z], of the resulting time-series of, in this case, 256 phase angles at a single interpulse period time-lag:

<U(+ = -(A/4xT) argR,(z]

where

and the interpulse period T= l/(PRF). This is the so- called 'pulse-pair processing' algorithm, described in greater detail in Reference 8.

(a?) 0" is computed from a similar normalised auto- covariance function, &($, but this time with a lag of two interpulse periods:

where

Table 2 summarises the parameters measured and the accuracies achieved. Table 3 details dynamic range, detection and sampling issues.

0 45

Time. minut-

Fig. 12 Typical PNG stratiform precipitatiion event

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998 11

Fig. 13 (c) Doppler mean velocity, and (d) spectral width for minute 15 of the event of Fig. 12

Vertical profiles of (a) co-polar reflectivity, (b) linear depolarisation ratio,

0 c. 0 90

Time, mnnm

Fig. 14 Typical PNG convective precipitation event

Note that in (a) and (b) , we have to linearly sum IF powers, but that a logarithmic amplifier has been used in the IF processor, in order to obtain the necessary high dynamic range. This means that a look-up table representing an exponential function has to be implemented by the DSP code and applied to each ADC value representing the IF power level prior to summation. In (c) and (4, in order to apply the pulse-pair processing method, Gaussian Doppler spectra are assumed for the meteorological targets. In practice, this is usually found to be a good approximation. The radar has the ability to record full spectra (examples of which are shown later), rather than just the spectral moments <U> and o,, though this results in very large volumes of time-series data. A single full-spectrum data set involves recording 2, Im, QTx, IRy and QRyfor every range- gate, for each of 256 transmitted pulses. Consequently, the normal mode of operation is to

record the mean and width of the spectra, while one full spectrum set is recorded per file, that is once every 47 minutes.

The fixed radar PRF of 625 Hz has some consequences for the data collection strategy. At this PRF, the maximum unambiguous Doppler velocity is around 15.5 m/s. If receiver polarisation switching is employed on alternate transmitted pulses, this velocity is effectively halved for CO-

polar returns to only 7.75 m/s. This would be insufficient, as large raindrops falling in still air may reach a terminal velocity of around 8-10 m/s, and there may also be downdraft effects to consider. Therefore, in order to make polarimetric and unambiguous Doppler velocity measurements near-simultaneously, a switching scheme is employed which toggles the received polarisation of successive pulses while 2 and LDR are measured, for a total of 1024 transmit pulses, then holds the receiver in the co-polar state for another 1024 pulses, during which the Doppler measurements are made, before repeating the cycle indefinitely. Note that although only 256 pulses are used out of the 1024 potentially available for the Doppler measurements, the remaining ‘dead-time’ is required by the DSP card for data processing and transfer, since the Doppler computations are much more numerically- intensive than those for the polarimetric variables. A complete measurement cycle, therefore, requires (2048/625) = 3.2 s, during which alarge raindrop might fall 30 m or so, as compared with a range-gate spacing of 150 m. Thus, the situation is quasi-static with respect to the

12 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998

sampling strategy chosen because most particles are still contained in the sample volume during the measurement time interval.

Data recording and archiving issues are detailed in Table 3. Data are initially recorded on the PC’s internal hard-disk drive, but are also simultaneously written to a magneto-optical disk, which is the main storage medium for data exchange between PNG and the Rutherford Appleton Laboratory (RAL). As an insurance against data loss, files on the computer’s hard disk are only deleted after the contents of the magneto-optical disk have beenverified and backed-up. The PC runs a C program which implements automatic control and output power monitoring of the radar transmitter, periodic receiver calibration, data recording, and real-time display of the four measured parameters as a function of height.

A serial port connection to the model RD-69 Joss distrometer permits simultaneous ground-truth rainfall data and surface drop-size distributions to be combined with the radar data for later analysis. The distrometer data consist of sequences of ASCII (American standard code for information interchange) characters, representing the diameter of each raindrop impinging on the unit’s microphone-like sensor during each consecutive 30 s sample period.

As internal PC clocks are often inaccurate and can accumulate large errors over long periods, a GPS (global positioning system) receiver is used to time-stamp the recorded data, permitting more meaningful temporal comp arison of the radar database with other instruments con- nected to Unitech’s indepen- dent data-logging system.

On receipt of the magneto- optical disks at RAL, data are copied to CD-ROM for archiving, and the original disks are recycled.

melting process, the large difference between the dielectric constants of ice and water, and the rotation and tumbling of the particles as they fall. The step-change in mean velocity arises because, above the melting layer, precipitation is in the form of low-density snowflakes, whereas below it typically comprises water droplets, of much higher density. Because of the differences in particle size and density, air drag causes snowflakes and rain to fall at markedly different speeds. Thus, the narrow height interval over which particle melting occurs can be identified by a rapid increase in fall velocity. Doppler spectral width also exhibits a clear change, since snowflakes tend to fall with similar velocities, and hence have a narrow spectral width, whereas raindrops fall with a larger range of velocities and so display enhanced spectral width. Vertical profiles of the parameters for minute number 15 of this event are shown in Fig. 13.

Fig. 14 is an example of a typical convective event. The polarimetric data indicate that there is no clearly defined melting region in this case. There is a considerable updraft (note the regions of negative mean velocity) and there are areas of strong turbulence (with large values of spectral width). Vertical profiles of the parameters for minute number 45 of this event are shown in Fig. 15.

An example of estimation of the full Doppler spectrumg,

6 Results and interpretation

Examples of recorded data Some examples illustrating

data acquired with the radar during its campaign in PNG are now described. Fig. 12 is an example of a colour plot for a typical stratiform precipitation event. Note the clear identification of the melting layer in all four measured parameters at 4.5 km height. The peak in 2 is due to the strong echoes from water-coated melting snow- flakes. The LDR peak results from the asymmetry of the

Fig. 15 (c) Doppler mean velocity, and (d) spectral width for minute 45 of the event of Fig. 14

Vertical profiles of (a) co-polar reflectivity, (b) linear depolarisation ratio,

ELECTRONICS 8.1 COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998 13

Fig. 16 Illustration of Doppler spectrum variation as a function of precipitation phase over a vertical profile

as opposed to just the moments <U> and o,, is shown in Fig. 16. These spectra were obtained by constmcting the complex time-series derived from the raw amplitude and phase data, followed by Fourier transformation to obtain the velocity-power spectrum for each range-gate. The spectra shown are for range-gates which correspond to heights in the ice, melting-layer and rain regions. Note the low fall velocity and narrow spectral width in the ice region, and the increase in both mean velocity and spectral width in the rain below. The spectra are roughly Gaussian in form, although ground clutter contamination is evident as a subsidiary peak at zero velocity in the spectrum for the lowest altitude.

Comparison of radar observations with distrometer and satellite beacon measurements

In order to use the radar data for propagation studies, such as the prediction of attenuation for earth-space systems operating in the 20 and 30 GHz bands, the accuracy of the radar’s calibration must first be established. Although automatic receiver, IF processor and DAS calibrations are logged twice daily, and transmitter power is continuously recorded, the full

system calibration, including actual antenna performance and miscellaneous losses in waveguides and cables, must be determined.

In the case of the transportable radar used in PNG, this was accomplished in the UK, prior to shipment, by a combination of precision laboratory measurements, and by comparative observations in several precipitation events with the well-characterised Chilbolton radar. The Chilbolton radar’s own calibration was, in turn, verified using a self-consistent comparison of measured reflectivity, 2, differential reflectivity, Z,,, and specific differential propagation phase, KDp, as described in Reference 10.

It was initially assumed that the calibration of the transportable radar, as deployed in PNG, was as determined in the UK Since the distrometer gives the size distribution and number of raindrops per unit time falling on its sensor, its data may be used to calculate the expected radar reflectivity at 3 GHz. If it is assumed that the majority of raindrops fall nearly vertically, then it is reasonable to expect good agreement between the distrometer-derived reflectivity and the measured radar reflectivity. Fig. 17 shows a comparison of measured radar reflectivity at 1 km height, and the

14 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998

reflectivity value inferred from distrometer measurements on the ground. When adjusted by a time shift corresponding to the period during which the particles fall from 1 km to ground level, excellent tracking is obtained between the two time-series, implying that the absolute calibration of the entire radar system in PNG has remained within approximately 0.5 dB of its expected value based on testing in the UK.

Finally, Fig. 18 shows that the radar and distrometer data can be used to predict slant-path attenuation accurately". Measured attenuation values were obtained from a co-located 12.75 GHz satellite beacon receiver observing the signal from the OPTUSB satellite at 73" elevation. Good agreement

Fig. 17 ground-based distrometer measurements

Comparison of measured radar reflectivity with that predicted by

is obtained, demonstrating the feasibility of using a radar and distrometer database to estimate fading effects on an earth-space path.

More detailed analysis of the examples presented here is available in the cited references.

7 Conclusion and future work

A transportable polarimetric Doppler radar for radiowave propagation research has been designed and constructed by RAL's Radiocommunications Research Unit. In autumn 1995, the system was shipped to h e , PNG for a field campaign which commenced in December of that year. The data have been analysed to determine the implications

of tropical rainfall for satellite communication schemes operating at low latitudes. Analysis of the radar database continues, and significant new results will be used to test propagation prediction methods for microwave radio system planning. Data will also be made available to the meteorological community. After the conclusion of the experiment in PNG, we hope to deploy this radar at another tropical site, in order to investigate the applicability of the original measurement database at other locations within the equatorial belt. A second radar system, which is an enhanced version of that described in this paper, featuring full azimuth and elevation scanning capability, plus improved cross-polar performance, will be installed in Singapore in late 1997.

Fig. 18 Comparison of precipitation-induced attenuation at 12.75 GHz on an earth-space path, as predicted by radar, with direct measurements from a co-located satellite-beacon receiver

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1998 15

Acknowledgments

It is a pleasure to acknowledge the following individuals for their contributions to the work reported here: Lason Watai, John McMahon, and Lolong Bonnor of Unitech, for assistance with radar installation, operations and for supplying satellite-beacon attenuation data; Kevin Morgan, Carron Wilson, Jun Tan and John Goddard of RCRU, for off-line data processing, analysis, propagation modelling and assistance with project management, respectively; and DarrylTaylor and Don Breeze of RALfor organising packing and shipment of the equipment to PNG. We thank the UK Radiocommunications Agency (RA) for funding this project as part of the National Radiowave Propagation Programme. This paper is dedicated to the memory of Keith Dunk, late of RA, who was a source of consistent support and encouragement throughout this work.

References

1 EASTMENT, J. D., THURAI, M., LADD, D. N., and MOORE, I. N.: ‘A vertically-pointing radar to measure precipitation characteristics in the tropics’, IEEE Trans. Geoscience and Remote Sensing, November 1995,33, (6), pp. 13361340

2 ITU-R recommendation P.618-4: ‘Propagation data and prediction methods required for the design of earth-space telecommunications systems’, Geneva, 1996, ISBN 92-61-

3 GODDARD, J. W. F., DAVTES, P. G., THURAI, M., and MORGAN, K. L.: ‘Radar estimates of attenuation at 30 GHz: comparisons with Olympus beacon measurements’. 7th Intl. Conf. o n h t e n n a s and propagation, York, UK, 15th-18thApril 1991, IEE Conf Publ. No. 333, Part 1, pp. 464-467

4 BRYANT, G. H.: ‘Acquisition, analysis and reporting of concurrent rainfall rate and path attenuation measurements in Papua New Guinea’. Intelsat contract report no. INTEL-870

5 LI, H., ILLINGWORTH,A. J., and EASTMENT, J. D.: ‘Asimple method of Dopplerizing apulsed magnetron radar’, Microw. J., April 1994,37, (4), pp. 226-236

6 PERSSON, L. P.: ‘Characterization of spectral distortion in a meteorological radar’. MSc dissertation, University of Kent, 28th September, 1995

7 GODDARD, J. W. F., EASTMENT, J.D., andTHURAI, M.: The Chilbolton Advanced Meteorological Radar: a tool for multidisciplinary atmospheric research‘, Electron. Commun. Eng. J., April 1994,6, (2), pp 77-86

8 DOVWS, R. J., and ZRNIC, D. S.: ‘Doppler radar and weather observations’ (Academic Press, New York, 1993), pp. 130-139

9 THURAI, M., EASTMENT, J. D., LADD, D. N., and MOORE, I. N.: ‘Measurements of fall velocity spectra in rain and ice from an Sband Doppler radar’. Proc. 7th URSI Commission F Open Symposium, Ahmedabad, India, 20th-24th November 1995, pp. 258-261

10 GODDARD, J. W. F., EASTMENT, J. D., and TAN, J.: ‘Self- consistent measurements of differential phase and differential reflectivity in rain’. Proc. IGARSS ’94, Pasadena, USA, August

11 EASTMENT, J. D.,THURAI, M., and LADD, D. N.: ‘Rain radar measurements in Papua New Guinea and their implications for slant path propagation’. IEE Colloquium on Remote sensing of the propagation environment, London, 19th November 1996, Colloquium Digest No. 1996/221, pp. 9/1-6

05961-9

1994,1, pp. 369-371

OIEE 1998 First received 18th June and in final form 29th October 1997.

16 ELECTRONICS & COMMUNICATION ENGINEERING JOURNA1; FEBRUARY 1998