Pergamon

Jw,rnal <~Armmphrru rrnd T<warrd Ph_w,cs. Vol. 56, No. 9, pp I IX- I 162. ,994

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Preliminary observations using ST mode of Indian MST Radar : detection of the signature of the tropopause

A. R. JAIN,* Y. JAVA Rao,t$ P. B. RAO,* G. VISWANATHAN,$ S. H. DAMLE,’

P. BLAMURALIDHAR~ and ANIL KULAKARNI~

*National MST Radar Facility, Post Box 123, Tirupati 517 502. A.P. India ; t Indian Institute of Technology, Kharagpur, 721 302, India ; 9: MST Radar Project, ISTRAC, Al-6, Peenya

Industrial Estate, Bangalore 560 058, India; 7 Society for Applied Microwave Electronics Engineering Research (SAMEER). IIT Campus. Powai, Bombay. 400 076, India

(Received in final f&m 25 Januq~ 1993 : accepred 25 Mq 1993)

Abstract-An MST (mesosphere/stratosphere/troposphere) radar is currently under installation at Gadanki, a low latitude station in India. As an intermediate step, a low power-aperture version of the system was operated in the ST mode. The paper gives an account on the salient features of the system and presents preliminary observations conducted on the detection of the tropopause. The observations clearly demonstrate the potential of the radar to carry out high resolution studies on the tropopause above this tropical station

1. INTRODUCTION

The specular type of echoes that are observed at ver- tical incidence by a VHF radar arise from regions of strong hydrostatic stability due to Fresnel reflec-

tion/scattering (for e.g. R~TTGER and Lru, 1978; BALSLEY and GAGE, 1980 ; R~~TTGER and LARSEN,

1990). This ability of VHF radars has been applied by many workers for detecting the height of the tropo- pause (e.g. GAGE and GREEN, 1979; LARSEN and

R~~TTGER, 1983 ; WATERMAN and CZECHOWSKY, 1983 ; GAGE et al., 1986). So far, most of these observations are available from mid- and high latitudes only. The

Indian Mesosphere Stratosphere Troposphere (MST)

Radar, currently under installation at Gadanki (13.47”N, 79.18”E), has recently been commissioned in the ST mode as an intermediate stage for pre-

liminary scientific observations. The objective of this paper is to give the salient design features of this newly established facility and to present preliminary observations of the tropical tropopause made using the ST mode system.

2. INDIAN MST RADAR IN ST MODE

2.1. MST radar system

The Indian MST radar is a highly sensitive VHF phased array radar designed to operate at 53 MHz

$ Present address : National MST Radar Facility, Post Box 123, Tirupati 5 17 502, A.P. India.

with an average power-aperture product of 7 x 10’ Wm*. Figure 1 shows a rather simplified block dia- gram of the radar system. The phased array consists

of 1024 crossed three element Yagi antennas occu- pying an area of 130 x 130 m. It generates a radiation pattern with a main beam of 3“, a gain of 36 dB and a sidelobe level of -20 dB. The main beam can, in

principle, be positioned at any look angle, but is cur- rently programmed to position at six look angles:

zenith, k 20’ off zenith in east-west and north-south directions, and 14 due north to look transverse to the

Earth’s magnetic field. A total transmitted power of 2.5 MW (peak) is provided by 32 transmitters ranging in power from 20 to 120 kW, each feeding a sub-array of 32 Yagis. Each transmitter has four stages, a solid

state amplifier (SSA), pre-driver (PDR), driver (DR) and high power amplifier (HPA). The PDR, DR and HPA stages employ Varian triodes (3CX1500, 3CPXl500 and 3CPX5000) in grounded grid oper- ation. To achieve the desired low sidelobe level to the radiation pattern, the power is tapered across the array according to a modified Taylor distribution. The required power taper is accomplished in one principle

direction by the differential powers of the transmitters and in the other direction by a series feed network as described by SARKAR et al. (1988). The transmitters can operate up to a duty cycle of 2.5%, limiting the total average power to about 60 kW. It is possible to transmit coded pulses with a PRF in the range of 62.5 Hz-l kHz and uncoded pulses with a PRF in the range of 62.5 Hz-8 kHz. The uncoded pulses can be

1157

A. R. JAIN et ul.

Pre amps. (LNAS)

I

Transmitters

8085 processor TX mixers

Phase shifters

r

ref X’tal oscil i 9 1 i

I Decoder 1 1 Decoder 1

1 I

1 t :

I Integ. 1 [ Integ. [

t t I Comp. interface

I

i Radar

L________ ,-,,_---1

controller Computer system

Fig. I. A simplified block diagram of the Indian MST Radar system.

varied in pulse width from 1 to 32 ps in multiples of

2. The coded pulses are either 16 or 32 baud bi-phase

complementary pairs with a baud length of I ps, pro- viding a range resolution of 150 m. A phase coherent receiver with quadrature channels, having an overall gain of 110 dB, a dynamic range of 70 dB and a bandwidth matched to the baud length of the coded pulse, detects the weak backscattered signal. The

switching of the antenna array between transmitter and receiver is accomplished by T/R switches using short slot hybrid couplers and PIN diodes. A fre- quency synthesizer, using a master oscillator with a stability better than one part in IO’” (short term), provides both transmitter carrier as well as receiver LO signals, rendering the system phase coherent. The quadrature outputs of the receiver are passed to a preprocessor unit consisting of two identical channels

of A/D converter (ADC), decoder and coherent inte- grator. The ADC is of 12 bit resolution to match the

dynamic range of the receiver. The analog signal in each channel is sampled and converted to digital at a rate of 1 MHz. The decoding operation essentially involves cross-correlating the incoming data from the ADC with the replica of the transmitted code. This is

implemented by means of a 16 bit 32-tap correlator/ transversal filter chip (IMS AIOO). Since the Al00 is fast enough to decode the signal pulse by pulse, it could precede the coherent integrator with some advantage. Coherent integration is a processing step introduced to effect a significant reduction in the volume of the data without, in any way, com- promising the information to be derived from the signal. It is implemented in a double buffer mode using two RAM memory banks, each 2000 words long and

Detection of the signature of the tropopause 1159

Table 1. Main specifications of the ST mode of the Indian MST Radar

Frequency Peak radiated

power-aperture* Average radiated

power-aperture Total peak power Maximum duty cycle Total number of Yagis Polarization Physical aperture Effective aperture Beam width Beam directions

53 MHz

3.00 x 10’ Wm’

4.80 x IOh Wm’ 180 kW 2.5% 16x 16 2 (EW and NS) 4018 m2 3661 rn’ 4.6 5 (Zenith, &20~ from zenith

in meridional and zonal planes)

*This takes into account the power loss in the feeder network and the power dissipated into the end loads used in the ST mode.

32 bits wide, as buffers. An interface multiplexes the integrated outputs from I and Q channels and trans- fers the data to the host computer for further process-

ing. The computer is a 32-bit super mini computer (Masscomp-MC5600). Its configuration includes a data acquisition control processor (DACP), a tightly

coupled vector accelerator and an independent graphics

processor. The data processing involves FFT, the

incoherent integration of spectra and the estimation of the three low order spectral moments which provide total signal power, radial wind velocity and its spread

for each range bin. Using the Doppler beam swinging technique, the vector wind field in height and time can

be determined with high resolution.

2.2. Operation in ST mode

The development of the Indian MST Radar was

planned to be accomplished in two phases. The first

phase was to operate in the ST mode using a partial system with one central quarter (16 x 16) of the Yagi antenna array and 16 driver units of the transmitters. each driver unit feeding one sub array of 16 Yagis. The receiver, signal processing and other related sub-

systems arc essentially the same as for the MST mode.

The radar operates under instructions from a PC-AT based radar controller which executes an experiment as per the data given in the form of an experiment specification file (ESF). The main specifications of the

system for the intermediate ST mode operation are presented in Table I. The radar has been com-

missioned recently in the ST mode with the above specifications. This has enabled us not only to validate

various subsystems and the signal processing software but also to carry out some interesting experiments on the dynamics and structure of the ST region.

3. OBSERVATIONS

Vertical soundings with 16 or 32 1~s pulses, using complementary codes with a baud length of I ps, were carried out using the ST mode system on several days

between July and December 1991. The USC of coded

pulses with a baud length of 1 ps permitted obser- vations with a height resolution of 150 m. A duty ratio of 1.6% was used for these soundings. Table 2 gives

the data, time, experimental specifications, velocity, height and time resolutions for the observations pre-

sented here. Most of the observations were carried

Table 2. Details of the experiments for detecting stable atmospheric layers reported in the present study

Date 4 July 1991 8 July 1991 I9 September I99 I

Time (IST) Start 1658:22 1610:51 1416:46 End 1714:47 1623:Ol 1529124 Waveform Bi-phase complementary Bi-phase complementary Bi-phase complementary

code code code Pulse width (ps) 32 32 I6 Baud length (ps) I I I Height resolution (m) 150 150 150 IPP (ms) 2 2 I Height range (km) 5.4-24.6 5.4-24.6 3.0 22.2

Number of FFT points (NFFT) 128 I28 128 Number of coherent Integrations

(NC) 256 256 I28 Number of incoherent Integrations

(on-line) 2 2 I Doppler range (Hz) &l *l *4 Doppler resolution (Hz) 0.015 0.015 0.06 Beam dwell time (s) 131.0 131.0 16.4

1160 A. R. JAlPi c’t ul

Indian MST radar Gadanki Date: 4ffll991 Time: 17:0:38 IST

Doppler (Hz)

(a)

SNR (dB)

(b)

Fig. 2. A vertical beam scan taken on 4 July 1991 using radar parameters listed in Table 2. (a) Isometric plot of the spectra for the heights between 13.35 and 18.75 km (power in arbitrary units). (b) Height profile of signal to noise ratio (SNR) covering all the range bins used for the scan. The solid line arrow shows the height of tropopause given by the radiosonde at Madras and the dashed line arrow shows the height of

the ‘radar tropopause’ (see text).

out near 1700 IST (i.e. 1130 UT) which is the time of

one of the regular radiosonde flights from Madras (I 3.04”N, 80.17”E). Figure 2(a) is a typical isometric plot showing the frequency spectrum of the received signal at different altitudes between 13.35 and 18.75 km for a scan taken on the afternoon of 4 July 1991. For this particular scan, Fig. 2(b) shows the height

profile of the signal to noise ratio (SNR). A sharp enhancement in the received signal near the altitude of 17.5 km can be distinctly seen from both the displays. The solid line arrow on the SNR plot indi-

cates the level of the tropopause as reported by the radiosonde measurement from Madras. The SNR is found to decrease rapidly up to an altitude of about

14 km and then starts increasing again. The sharpest cnhancemcnt of SNR above this level is identified with

the tropopause as detected by the radar and is referred as the ‘radar tropopause’ (GACZ and GREEN, 1979). For the particular case shown here, the radar tropo- pause marked by the dashed line arrow is about 1.5 km higher than that reported by the radiosonde obser- vation. The difference between radar and radiosonde observed tropopause levels has also been noted at other stations (GAGE and GREEN, 1979; GAGE et ul.,

1986).

Figures 3, 4 and 5 present average SNR (SNR)

profiles for the three days, 4,s July and 19 September 1991, respectively. The last panel of each figure repre- sents the overall average, while all the rest represent profiles averaged over approximately 5 min cor- responding to four incoherent integrations of the

Doppler spectra. A striking feature, noted in all three figures, is the remarkable similarity among the SNR profiles for each day, including the multilayer struc- ture that is observed near the tropopause. The con- sistency in the characteristic enhancement associated with the tropopause is clearly evident in all the profiles. The enhancements are strong and com- parable for 4 and 8 July, while it is relatively weaker for 19 September. For 8 July, two tropopause levels

have been reported from radiosonde observations as indicated by the two solid line arrows on Fig. 4. This bifurcation of the tropopause is not so obvious from the SNR profile, although significant structure in the SNR variation is evident close to the peak enhance- ment.

4. CONCLUSIONS

The salient aspects of the Indian MST Radar have been described. The vertical sounding observations

Detection of the signature of the tropopause 1161

Indian MST radar Gadanki Date: 4PiSl’

24 -fime: 17:0:38 ; 17:05:24 ISi

6m Si@ (dB)

(a) (b)

n

17:0:38 - 17:12:27 IST

-20 -10 s&Ydl3)

10 2

Cc)

Fig. 3. The average SNR profiles for 4 July 1991. Panels (a) and (b) show profiles averaged over approxi- mately 5 mm corresponding to four incoherent integrations, and (c) represents the overall average. The

arrow shows the height of the tropopause reported by the radiosonde at Madras.

Indian MST radar Gadanki Date: SD/91

24 Time: 16:10:51 - l&15:46 IST

221

Q-4 4L I I

IO-1 7 4 -7

A _k? 4 I d 8-1 r--

6-l

(a)

24 16:18:12 - 16:23:01 IST

22

20

18

16

14

12

10

8

6 -20 -10 0 10

Si%i (dB) -20 .lO 0 10

Sm (dBt

b) (c)

24 16:10:51 - 16:23:01 IST

0

Fig. 4. The average SNR profiles for 8 July 1991. Panels (a) and (b) show profiles averaged over approxi- mately 5 min corresponding to four incoherent integrations, and (c) represents overall average. The arrows

show the height of two tropopause levels reported by the radiosonde at Madras.

1162 A. R. JAIN et al

Indian MST radar Gadanki Date: 19/9/91

Time: 14:51:12 - l&56:58 - l&56:39 ET

24F 15:02:24 IST

24_l 22

20

2

- 16 E .F '4 I"

18 jl:

12

IO

8.

6 -20 0 -

SNR(dB) i0

22

20 1 18

16

14

12

8

6 ~

IO

-20 0 - SNR(dB)

20

22

20

18

16

14

12

10

8

6 III_ -20 0 :

SNR(dB)

22

20

18

16

14

12

IO

8

6 -20 0 ;

Sm(dB) !O

14:51:12 -

24 15:15:31 IST

22

20

18

16

14

12

IO

8

6 bI#I -20 0 : 2

Sm(dB)

(a) (b) k) (d) (e)

15:02:43 - 15:08:58 IST

24I---7--

15:09:21 - 15:15:31 IST

241

0

Fig. 5. The average SNR profiles for 19 September 1991. Panels (a), (b), (c) and (d) show profiles averaged over approximately 5 min corresponding to four incoherent integrations, and (e) represents the overall

average. The arrow shows the height of the tropopause reported by the radiosonde at Madras.

made using the ST mode of the Indian MST Radar reveal a sharp enhancement in SNR near the tropo-

pause height. The ‘radar tropopause’, or the height of

sharpest enhancement in SNR, is on the average about 0.6 km above the tropopause observed by radio- sondes. The tropopause at this tropical station often shows a multi-layer structure. This is seen in indi- vidual as well as in average SNR profiles. Our more

recent observations taken over a longer period of time confirm this. The observations presented here demon-

strate the capability of the Indian MST Radar to carry

out high resolution studies on the tropopause.

Acknouledgement.v-We are very grateful to the India Me- teorological Department for providing the simultaneous radiosonde data for Madras used in the present study. The Indian MST Radar Facility is set up jointly by the Council of Scientific and Industrial Research, the Defence Research and Development Organisation, the Departments of Elec- tronics, Environment, Science and Technology. and Space of the Government of India. with the Department of Space as the nodal agency.

REFERENCES

BALSLEY B. B. and GAGE K. S. 1980 GAGE K. S., ECKLUND W. L. and RIIIDLE A. C. 1986 GAGE K. S. and GKEEN J. L. 1979 LARSEN M. F. and R~TTGER J. 1983 R~~TTGER J. and LARSEN M. F. 1990

R&TGER J. and Llu C. H. 1978 SARKAR B. K., TOLE P. B. and AGARWAL A. 1988

WATERMAN A. T. and CZECHOWSKY P. 1983

Pure uppl. Geophys. 118,452. J. atmos. Oceun Tech. 3,248. Science 203, 1238. Geophys. Res. Lett. 10, 325. UHF/VHF Radar technique for Atmospheric Research

and wind profiler application. In Rudar in Meteor- o/og?, D. ATLAS (ed.). American Meteorological Society, Boston, U.S.A.

Geophys. Rex Lett. 5, 357. Feeder Network for the Indian MST Radar. In MAP

Handbook 14, 523. C. H. Llu and BELVA EDWARDS (eds). SCOSTEP Secretariat, University of Illinois, Illinois 61801, U.S.A.

Proceedings of the URSI Commision F Symposium, Louvain, Belgium, June 1983.