Indian Journal of Radio & Space Physics Vol. 29, August 2000, pp. 149- 171

Indian MST radar-An overview of the scientific programmes and results A R Jainl, D Narayana Rao2 & P B Raol

)National MST Radar Facility, Post Box No 1 23, Tirupati 5 1 7 502 2Department of Physics, S V University, Tirupati 5 1 7 502

Indian MST radar facility has completed five years of successful operations in MST mode. The objective of this paper is to summarize recent technical developments and various scientific programmes at the National MST Radar Facility (NMRF). An attempt is also made to highlight the scientific results obtained during first few years of operations in MST mode. Future programmes of NMRF are also summarized.

1 Introduction Woodman and Guillean 1 demonstrated that it is

possible to explore atmospheric dynamics up to a height of about 1 00 km by means of a high power VHF backscatter radar. It led to the concept of MST (Mesosphere-Stratosphere-Troposphere) radar. An MST radar is a highly sensitive, high-resolution pulse coded phase coherent radar, operating in the lower VHF band, typically and around 50 MHz, with an average power aperture product exceeding about

5x 1 07 Wm2• Radars, operating at higher frequencies or smaller average power aperture products

. are

termed ST radars. A number of STIMST radars have been established all over the world and this class of radars have come to dominate the atmospheric scene over the past 25 years . An overview on these radar systems is given by Rottger and Larsen

2. A major MST radar system has been established as

a national facility at Gadanki near Tirupati ( l3 .47°N,

79. 1 8°E). The radar has been developed in two phases. In the initial phase it was commissioned in ST mode using partial power aperture (average power

aperture = 4.8x 106 Wm2) . The ST mode operation, from Feb. 1 992 to July 1 992, has helped in validation of various sub-systems of the radar and also for collection of some interesting observations on atmospheric turbulence, wind field in ST region and also on ionospheric irregularities (see special issue of Indian J Radio & Space Phys. Vol 23, No. 4, 1 994 and Radio Sci. Vol 30, No. 4, July-August 1 995). This radar system became operational in full MST mode in March 1994 with average power aperture

product of 7x108 Wm2. This system has now completed first five years of operations in MST mode. In this period, radar has been used to study

wide range of problems related to atmospheric structure, dynamics and coupling processes in the lower and middle atmosphere and ionospheric field­aligned irregularities. It has also been possible to use Indian MST radar system in spatial domain interferometer (SDI) mode to study the ionospheric irregularities and meteor-trail-associated ionization irregularities.

The objective of the present paper is to summarize various ·scientific pro�ammes at the National MST Radar Facility (NMRF) and to bring out the important scientific results obtained during the first five years of operations in MST mode. Future scientific programmes of NMRF are also summarized. The study would give an idea of the potential of this modern radar facility for atmospheric research and the type of scientific programmes that are currently in progress. This facility is presently being utilised by about 50 scientists from various national laboratories and universities for their research programmes.

2 Indian MST radar The Indian MST radar is a highly sensitive, pulse

coded, coherent VHF phased array radar, operating at 53 MHz with an average power aperture product of 7xl08 Wm2• The basic system has been discussed in detail by Jain et al. 3, Rao et al.4 and Rao et al.5. The system design specifications are presented in Table 1 . Figure 1 shows a functional block diagram of the Indian MST radar system.

The phased antenna .array consists of 1024 crossed 3-element Yagi antennas occupying an area of

130x 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

150 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

Table I -Main specifications of the Indian MST radar

Parameter Location Frequency A verage power aperture product Peak power Maximum duty ratio Number of Vagi antennas Beam width Number of beams for automatic scan Pulse width

Pulse repetition frequency

Maximum number of range bins: Number of coherent integrations Maximum number of FFT points Radar controller

Computer system

Specifications Gadanki (I3.4°N, 79. 1 8°E) 53 MHz 7xlOxW m2

2.5 MW 2.5% 1 024 3° 7*

1 6 and 32 �s coded and 1 -32 �s uncoded (in binary steps) 62.5 Hz-8 kHz (in binary steps) 256

4 to 512 (in binary steps)

5 1 2

PC! AT featuring programmable experiment specifications file 32-bit super mini with vector accelerator (Masscomp­MC56(0)

Note: *Zenith in X and Y polarizations, ± IO° off-zenith i n E-W and N-S plane, and 1 4.8°N looking transverse to B field. This capability has now been enhanced to 1 8 beams which can be chosen from available 82 beams.

positioned at any look angle, but is currently

programmed to position at six look angles, zenith, ± 10° off-zenith in east-west and north-south direction

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 kW to 120 kW each feeding a sub­array of 32 Yagis. To achieve the desired low side lobe 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 principal direction by differential powers of the transmitters and in the other direction by the series feed net work. Some of the important features of the Indian MST radar are l isted below:

(i) The 16 or 32 /J.s pulse can be coded using 16 or 32 baud bi-phase complementary pairs with a

baud length of 1 /J.s, providing a range resolution of 1 50 m.

(ii) The radar system is rendered phase coherent by using a frequency synthesiser with a master oscil lator of stability better than one part in 10

10

(short term) . The same provides transmitter carrier and modulation as well as receiver injection signals.

( i ii) The radar receiver is a phase coherent receiver with two quadrature channels having an overall gain of 1 10 dB, a dynamic range of 70 dB, and a bandwidth matching the baud length of the coded pulse.

(iv) The quadrature outputs of the receiver are given to a signal preprocessor. The unit consists of two identical channels of AID convertor (ADC) , decoder and coherent integrator. The ADC is of l2-bi t resolution to match the dynamic range (70 dB) of the receiver.

(v) The radar preprocessor transfers data to the host computer for further processing. The host is a 32-bit super-minicomputer (Masscomp-MC 5600) which operates in a real-time unix environment.

(vi) The radar controller is a PC/AT which executes an experiment according to the data given in the form of an experiment specification file (ESF).

2.1 Data processing

The radar out put can either be taken in raw data format, i .e . the time series samples of two quadrature channels after coherent integration, or in the form of

Doppler power spectra, after online FFT for each range bin of the selected range window.

The parametrization of the Doppler power spectrum is carried out off-line. The scheme involves (i) removal of d.c., (ii) estimation of the average noise level, (iii) removal of the interference, if any, (iv) incoherent integration and (v) computation of three low order moments as described by Jain et aL. 3 and Rao et al. 5. The three

. moments represent the signal

strength, the weighted mean Doppler shift and half­power width of the spectrum.

The received signal power could be used to

compute the volume reflectivity (11). The mean Doppler shift provides a direct measure of the radial velocity of the scattering irregularities acting as tracer of the background wind. Three components of the wind vector are derived from measurements taken at a minimum of three non-coplanar beam positions. When observations are made at more than three look angles (Table 1 ), the wind vector can be determined

JAIN et al.: MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS 1 51

DISTRIBUTED

1:32 DIVIDER

MODULATOR CODER

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FEEDER NETWORK (TAYLOR ILLUMINATION)

POLARIZATION SWITCHES

32:1 COMBAINER

BROADBAND IF-AMP.

PAT (0-60 dB)

VIDEO AMP.

ADC(12 BIT) 2 Nos.

Fig. I -A block diagram of Indian MST radar system

using the method of least-square. The spectral width provides the variance of the velocity fluctuations from which various turbulence parameters can be determined7.

2.2 Recent developments

Some limitations were experienced during the operations of the Indian MST radar in support of user

scientists experiments. This has resulted in further technical developments of the system which are summarized below: (i) Minimum coherent integration can now be set at

I instead of 4 (see Table 1 ) . This allows large Doppler window of observations as required for observation of field-aligned plasma irregularities at ionospheric heights.

1 52 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

(ii) The number of beams that can be operated in auto-mode has been increased to 1 8 instead of 6 (see Table 1) . These 1 8 beams could be selected out of the available 82 beams between zenith

angle of ± 200, at an interval of 1 0, in two planes, i.e., east-west and north-south of antenna array. This available flexibility has made it feasible to conduct special experiments on (i) radar echo aspect sensitivity to understand the nature of atmospheric scatterers and (ii) atmospheric structure and dynamics during the tropical convection events.

(iii) Two experimental specification files (ESFs) can be operated sequentially in auto-mode. These two ESF can be totally independent of each other. This has provided considerable flexibility in radar operations and optimal utilization of radar time.

(iv) A new software called 'atmospheric data processor (ADP)

, has been developed. This

software makes use of adaptive method of signal tracing as described by Anandan8• This has resulted in better signal tracing and thus makes it feasible to retrieve three low-order moments of radar signal spectrum over larger height range. Figure 2 shows an example of adaptive method of signal tracing.

(v) It has been possible to use Indian MST radar in spatial domain interferometery (SDI) mode. This technique has been applied to echo arising due to backscatter from (a) ionospheric irregularities and (b) meteor trails. This technique and some typical observations are presented in a later section.

3 Scientific programmes The scientific programmes at the National MST

Radar Facility (NMRF) includes four types of experiments: (i) User scientists experiments based on the approved proposals, (ii) Scientific campaigns involving multi-user/multi-technique experiments, (iii) Short duration exploratory experiments and (iv) Common mode observations for long-term data base development. The experiments covered a wide range of atmospheric studies on structure, dynamics and coupling processes and on ionospheric irregularities.

Recently, a number of co-located facilities have been installed at NMRF. These facilities have been described separately by Jain9. These co-located facilities have helped in generating new scientific programmes requiring simultaneous operations of the facilities at NMRF.

3.1 User scientists' experiments

A. Regular experiments:

These experiments are conducted in support of the user scientists' proposals. Some of the problems that are examined under these programmes, can be listed as follows:

(i) Lower atmospheric experiments: (a) Studies on precipitation and drop size

distribution (DSD) using simultaneous observations of LA WP, disdrometer and Indian MST radar.

(b) Studies related to the detection and characterization of structure of bright band which is a layer of enhanced echo intensity around O°C isotherm.

(c) Studies on structure of the atmospheric stable layers.

(d) Studies on vertical shears of horizontal wind

and atmospheric turbulence parameters. (e) Studies of stratosphere-troposphere inter­

change using simultaneous observations of MST radar and balloon-borne ozonesonde.

(f) Aspect sensitivity measurements to under­stand scattering mechanisms.

(g) Study of winds, waves and tides in the tropical troposphere and lower stratosphere.

(ii) Mesospheric studies: (a) Studies on gravity wave dissipation in relation

to the formation of D-region ledge. (b) Day-to-day variability of mesospheric winds

at low latitude and its association with equatorial electrojet.

(c) Climatology of gravity waves in the tropical mesosphere-lower thermosphere.

(d) Detection of meteor-trail-associated radar echoes and studies on characteristics of turbulence associated with such echoes.

(iii) Ionospheric studies: (a) A morphological study of E-region field­

aligned irregularities over tropical latitudes. (b) Formation of ionization layers in the E-region

during nighttime. (c) Studies on QP. echoes associated with back-

scatter from E-region field-aligned irregularities.

(d) Backscatter echoes arising from E-region heights above 120 km.

"'U 'MST RAD,u,R D,I\TA HE�,DER •••••

Date 1917193 Time 11 :438 No. of Range bins 141 No. of FFT pOints 256 No. of coherent integs 64 r,lo. of Incoherent integs 4 Inter Pulse Period 1000 Pulse width 16 Code flag CODED No. of beams 6 Beam position EAST Scancycle 4 Reciever attn o dB Data type SPECTRUM No. of observ. windows 2 Window1 start 24 Window1 length 141 Window2 start 0 Window2 length 0 STC Window length : 0

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154 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

(e) Studies on 3 m irregularities associated with the equatorial spread-F.

(f) Spatial domain interferometric . studies of ionospheric irregularities for their characterization.

B. Exploratory experiments

Under this category of experiments, some new ideas are tried out by the user scientists. Following experiments are carried out under this category.

(a) A new technique based on identification of Brunt-Vaisala frequency from the spectra of the vertical wind measured by MST radar has been developed to derive temperature-height profile.

(b) Demonstration of the operation of MST radar in meteor wind mode. The Doppler spectral observations show multiple peaks demonstrating that meteor trail undergoes instability.

(c) Observations of meteor showers and sporadic­E. The observations show stronger presence of Es on meteor shower days.

3.2 Campaign expefiments

These experiments are multi-user and multi­technique experiments carried out in campaign mode for specific scientific studies. Following campaigns have been carried out.

(i) Convection and precipitation studies for understanding the atmospheric structure and dynamics associated with tropical convection and also to make various precipitation related studies using MST radar and lower atmospheric wind profiler (LA WP).

(ii) Gravity wave campaign for studies on generation, propagation and characterization of gravity waves.

(iii) Spread-F campaign for understanding the generation and dynamics associated with equatorial spread-F (ESF) . These studies would also help in the plasma instability processes associated with ESF irregularities.

3.3 Common mode observations

This programme is aimed at generating long-telm data base for various scientific studies and includes the following:

(i) Daily observations for about 45 minutes between

1 630 and 1 7 1 5 hrs 1ST that is near to the

radiosonde launch time from Chennai ( 13 . 1 ON,

80.2°E) which is the nearest radiosonde station to

Gadanki ( 13 .45°N, 79. 1 8°E) . (ii) Two diurnal cycles observation every month in a

six-beam wind measurement standard mode.

These observations are. carried out from August 1 995 onwards.

4 Highlights of the scientific results

4.1 Radar observations during the passage of convection events

Mesoscale meteorological phenomena such as thunderstorm are common at tropical latitudes. The MST radar has been operated in campaign mode to make high resolution observations on such events. The convective events are found to be associated with higher radar reflectivity, possibly due to enhanced moisture in the convective cell

10.

Large vertical drafts are observed during the passage of such events. Stable layer structures associated with the tropical tropopause seem to weaken during the convective activity. Observations also show significant mass flux, both in upward and downward directions, through the tropopause indicating mass exchange between troposphe�e and stratosphere during th� period of convective activity as shown by Jain et al.

10 (Fig. 3).

4.2 Observations of tropopause 'weakening' and exchange of o�ne between troposphere and stratosphere

Indian MST radar observations over Gadanki are used to study the atmospheric stable layer structure associated with tropical tropopause. Simultaneous ozonesonde observations from Trivandrum are used for determination of height profiles of ozone. These simultaneoLls observations show evidence of stratosphere-troposphere exchange of ozone during tropopause 'weakening' events II. During the event of tropopause 'weakening', there is a clear evidence of decrease m the stratospheric ozone and a corresponding enhancement in the tropospheric ozone, the total being conserved.

4.3 Observation of atmospheric stable layers in ST region

High resolution measurements, both in height and time, zre carried out to monitor stable layers in the troposphere and stratosphere over several diurnal cycles. To have a comprehensive look of the spatial and temporal variations of the stable layer structures,

JAIN et al.: MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS 1 55

DATE: 28 Mar. 1996 8 June 1996 12 Sep.l996

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TIME, hrs 1ST Fig. 3-Time series of mass flux at different height levels for three convection events observed at radar site (after Jain et al., 1998)

height-time-intensity (HTI) maps are drawn using the radar observed signal-to-noise ratio (SNR) in the vertical direction. The SNR (dB) is then plotted sequentially to give continuity in timel2. Figure 4 represent HTI maps for diurnal cycle of observations on six days. Height of meteorological tropopause corresponding to 0530 hrs 1ST (0000 hrs UT) and 1 730 hrs 1ST ( 1200 hrs UT) are marked by arrow on right and left side of the HTI maps, respectively. The HTI maps clearly show multiple stable layer structure of complex nature. The observed multiple layer structures are noticed to be more prominent near the tropopause and in the lower stratosphere. These horizontal structures last for quite some time ( 6- 1 8 h) indicating the large horizontal extent of these structures. The height structures of these multiple layers are observed to vary only slowly and the vertical separation of prominent structures is found to be in the range of 300-700 m. These structures, as seen from Fig. 4, show considerable day-to-day variability. Rottger1 3 and Tsuda et a1. 14, using a range

resolution of 150 m, have reported multiple layer structures at mid-latitudes which are similar to the present observations at Gadanki.

4.4 Detection and characterization of radar bright band

Studies on the radar bright band, a strong enhancement in reflectivity at around ooe isotherm level, in the precipitating atmosphere are of major interest to the radar community. Though observations of the radar bright band began a few decades back, still there is a need to examine the bright band carefully, because it is usually more intense than the one predicted by Fabry and Zawadzkil 5• Estimation of bright band thickness is very important because the cooling rate of the atmosphere is inversely proportional to the thickness of the bright band. Profiles of reflectivity factor and Doppler velocity are used to identify the upper and lower boundaries of the bright band. The boundaries of the bright band are identified as the levels where the maximum curvatures, with opposite sign, of the averaged fall speed profile occurl6. In the reflectivity factor profile,

156 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

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JAIN et at.: MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS 157

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the top of the bright band is the level at which the curvature of the profile changes sign and the base lies where the curvature changes abruptly.

A contour plot of signal intensItIes from precipitation taken on 16 Oct. 1996 is shown in Fig. 5. A clear layered structure, a signature of bright band can be seen at a height centered around 4.2 km and confirms the presence of bright band at this height. The upper and lower edges of the bright band are located at the heights of 4.8 and 3.9 km,

respectively17. The bright band thickness is thus determined to be nearly 900 m, which is lower than the values reported by Leary and Houze18 and Chu et

al. 16 in tropical location§ but higher than the thickness observed by Fabry and Zawadzki15 at higher latitudes. The height of the upper boundary of the band is close to the height of the O°C isotherm as determined from temperature measurements at the nearest radiosonde station.

4.5 New technique for determination of atmospheric temperature from MST radar observation

A new technique based on the identification of Brunt-Vaisala frequency from the spectra of the vertical wind measured by the MST radar has been developed by Revathy et al.19 to derive the temperature-height profile. The derived temperature profile is found to be in good agreement with the radiosonde measurement at Chennai (Fig.6). Error

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Fig. 6---Altitude profile of temperature deduced from MST radar data obtained on 15 Mar. 1995. [The circles represent the temperature from radiosonde data (After Revathy et aI., 1996).]

estimates are found to be reasonable"!!'). This technique has been applied by Revathy et al. 20 to study the diurnal variation of atmospheric temperature at various heights in ST region.

4.6 Retrieval of atmospheric humidity profile Tsuda21 proposed a technique to retrieve humidity

profile from the combined observations of radar and radio acoustic sounding system (RASS). Similar methodology is adopted by Gossard et al. 22 to retrieve

1 58 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

the humidity profile and compared with the humidity profile extracted with the GPS system. Following the method of Tsuda21 , an" experiment was conducted to retrieve the humidity profile from MST radar observations alone. In this method, first temperature height profile is detennined using the method of Revathy et a/19• Humidity profile is then extracted, using the derived temperature profile, following the method given by Tsuda2 1 and Narayana Rao et aP3.

The radar derived humidity profiles are compared with the radiosonde measurements made at Chennai and are shown in Fig. 7. In Fig.7 the line with star corresponds to Qz max taken at 3 .9 km, the line with triangle corresponds to Qz max taken at 1 1 km and the line with circle corresponds to radiosonde measured humidity profile. The Qz max is the reference point of specific humidity taken for initiation. A fairly good agreement is seen between these two measurements. Though the humidity profiles are observed to be fairly matching with the radiosonde measured values, but still there are some limitations of this method. At first, identification of B-V frequency during the disturbed atmospheric conditions, i .e. events like convection, precipitation, etc. is very difficult. The

second limitation is that the specific humidity term is not considered while deriving the humidity profile. The third one is that it is not possible to know the sign of potential refractive index (M) from the MST radar observations. Gossard22 stressed the importance of the sign of the potential refractive index gradients in retrieving the humidity profile.

4.7 Determination of parameters of atmospheric turbulence

Atmospheric turbulence plays a major role in a number of atmospheric phenomena. Sensitive Doppler radars are used for observation of clear-air

.refractivity structures. The observed radar reflectivity is directly proportional to the turbulence refractivity structure constant (Cn), provided half the radar wavelength lies within the inertial sub-range. Thus, the vertical profile of the received power can be converted into a vertical structure constant, e2m and also the eddy dissipation rate, e, which, in turn, is a measure of energy associated with turbulence. The theoretical relationship between refractivity turbulence and eddy dissipation rate have been developed by Gage et ae4, Weinstock25 and Hocking26 . The MST radars provide a unique opportunity to detennine the turbulence parameters such as e2 n , e and also momentum flux, with

excellent time and height resolutions. These' parameters were detennined using Indian MST radar.

Monthly variation of C2n at different heights is shown in Fig.S. Monthly mean Cn value are found to vary by about three orders of magnitude through the course of the annual cycle. From the monthly variation of e2n, it is also observed that there are two minima. One occurring in March, a pre-monsoon month, may be due to the high temperature and low humidity. The other minimum is observed in the winter month of November, which may be due to low humidity gradients. At all heights, it is seen that there is an abrupt change from February to March, demonstrating a seasonal change27 of e2n.

There are two methods by which VHF radars can be used to' measure e. The first method is to measure the absolute strength of the backscattered power and then convert this to e. The second method utilizes the spectral width of the signal received by the radar. The vert ical profile of e estimated from the power method is shown in Fig.9 when a moderate jet stream of about 50-60 ms- 1 is observed at an attitude of IS km. From Fig. 9 it can be seen that e2n and E are minimum near the height of jet stream.

4.8 Effect of aspect sensitivity on measurements of horizontal velocity in troposphere and lower stratosphere

The backscatters arising from refractive index gradients due to presence of isotropic/anisotropic turbulence and Fresnel reflection/scattering are the main mechanisms that give rise to the radar echoes at VHF. The anistropic backscatter and Fresnel reflection/scattering make received echo aspect sensitive. The aspect sensitivity of the radar signal influences the beam pointing direction. This affects determination of the horizontal wind components.

A series of experiments were conducted to make measurements on aspect sensitivity. It can be seen from Fig. to that for small beam zenith angle - 4°, the effective beam tilt angle shows deviation by as much as 1 ° from true zenith angle. For larger beam zenith, say, - 8°, this error is - 0.5°. This deviation in tilt angle reflects in error in horizontal wind measurement by - 24% and - 5% for beam zenith angle of 4° and 8°, respectivell8.

4.9 Observations of atmospheric tides and waves

The MST radar high resolution wind measurements are also used to study the tides and atmospheric waves such as Rossby-gravity (RG) waves and Kelvin waves.

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1 60 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

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-20.0�---.---._--.---r---r---.--__,r--_,--_.--_r--. 2 5 6 7 8 9 10 11 12

MONTHS Fig. 8-Monthly variation of log C2n observed during January-December 1993

Diurnal and semi-diurnal tidal oscillations with amplitudes as large as - 5.8 ms-1 are observed in the zonal wind (see Fig. 1 1 ) above 8 km. The observed amplitudes are much larger than that predicted by the classical theory29. Sasi et al. 30 have shown that, during autumnal equinox season, diurnal amplitudes of 1 -2 ms -I of horizontal winds prevail in the troposphere with a vertical wavelengths of - 3 km in the lower troposphere and - 6 km in the upper troposphere. Sasi et al. 3o have suggested that these

oscillations in the troposphere are manifestation of non-migrating diurnal tides excited by eddy heat flux in the boundary layer and those in the upper troposphere excited by deep convective clouds.

Sasi et at. 31 have computed the vertical flux of zonal momentum associated with equatorial waves from zonal and vertical components of the winds measured by the radar. Momentum flux values of l 6xIO-3, 8xIO-3 and 5 .5xIO-3 m2 are obtained for slow Kelvin ( 1 2 days period), fast Kelvin (5.33 day

JAIN et at .. MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS

1000 1ST 23.0

18.0

E .....- 13.0 ..c Q) <l>

I

B.O

3.0 -19 -17 -16 -14

2 Log en - 8 -7 -6 -5 -4 -3

Pqwer Method Log E

- 8 -7 -6 -5 -4 -3 Width Method

Log E

Fig. 9-Yertical profiles (can) and eddy dissipation rate (E) observed on 17 June 1994. [The parameter E is estimated using the power and spectral width methods.]

21

20

19

] � 18 c.:> ...... � 17

16

15

22 June 1994 East-West

2 4 6

Seff, deg

8 10 12 o 10 20 30 40· 50

R,%

Fig. IO--Plots showing the effect of aspect sensitivity on horizontal wind measurements and present height profiles of effective beam pointing angle (gefT) and corresponding factor R (%) by which zonal (U) and meridional (V) wind components are underestimated for 22 June 1994 [ After Jain et al., 1997.]

161

162 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

DIURNAL <8> SEMIDIURNAL 'v

..... �-.... � -.. �14 ...-' ---� !G- .....

� � -&

12

1&

"� "

10 DI ....,

06

U

� � � r � f- I "

" Clo ot n Ie oa TIME Hrs <1ST)

DIURNAL (b) SEMIDIURNAL

0� � �.

� � � � ..... .....

i a 00 06 1 1 • GO TIME Hrs (1ST)

�

...,2

I Oi

� � �

.� �

� 0Ii

Fig. 1 1-Height-tlme plots of diurnal and semi-diurnal components of tidal oscillation in (a) zonal wind and (b) meridional wind on 26-27 Sep. 1 995 [The shaded area indicates a positive phase (After Jivarajani el aI., 1 997).]

period) and Rossby-gravity (RG) waves (3.43 day period), respectively, in the upper troposphere32 (Fig. 12) . These flux values are somewhat higher than the

values given by tneory and are significant for understanding the coupling between troposphere and stratosphere.

4.10 Observations of turbulence generated by meteor trail

20

----. 15 E .::f. '--' ..... 10 .c 0> <1>

:r: 5

20 b 20

15 15

10 10

5 5

(c /' ( D �

Experiments were conducted to explore the feasibility and promise of Indian MST radar for detecting the meteor trails and use them for studies on dynamics of the lower thermosphere. The MST radar offers the advantage of observing the meteor trail echo signatures simultaneously in different range bins. The meteor trail echo was usually found to

u 0 0 L.,,--...L....-:-' -0.03 0 0.03 -0.03 0 0.03 -0.03 0 0.03 Momentum flux(m2 52 ) Fig. 12-Height profile of vertical flux of zonal momentum. [Filled circle represent the momentum flux values reported by Wallace and Kausky32 (After Sasi et al., 1 999).]

JAIN et a/.. MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS 1 63

4.11 Studies on mesospheric echoes extend over many range bins, separated by 1 .2 Ian

(corresponding to 8 Jls pulse), as seen in Fig. 1 3. The Doppler spectra, for each range bin, show multiple peaks, suggesting that the meteor trail undergoes instability leading to striations similar to that observed in the barium ion c1ouds33.

Observations at meso spheric heights have been carried out using the MST radar. Time history of occurrence of mesospheric echoes34 is shown in Fig. l 4. Presence of two distinct layered structures can be noted around the height range of 67-69 Ian and 73-

IS OJ'S OA 05 OJ [0.1-

E .. I oe.oL---<:�lAl I. • .. c • ..

.e

..II

o • •

� . &.4 • • "I�'�_ ....... _-------.... ---.-.oi It �"

� �

cr' oliI

..

9S 01 " 08 1S A9 North-w (b)-�1J J A

I •

""'I]; "I-II 'l�nJ\'I'A� .A _ " I. I"f rr-. A�' ...,. .v V"" � IT

I'

11

•

o :sz 14 116 lU lao 1 liZ - H l� � 304 :17J 443 Time m unit. of • rnillbee.

$12 Time in unib 0( .e mJllbec.

9� OJ ,I> 04 54 J6 [cd-r 95 OJ 16 O A 43 " ['.lsi..., _16Ur---- ( -:-'01.01-__ ---------­c 1oz.ol--_----_____ -i III 1�.�.._...��---------_; (d) • �012:�----��-----------------1 � "·aJ-----1�iIJf'CX.�.A..f'r_.f'"�-�.., u c � �IOO..ll't�:::::::��::������;���;?����� .z: \t7'�-------...:..�����r_<J-_ _<:::! 11 � '&"4 �

. �

-------�� : � .�-----_.-4�M� ..... .lX.�� 10 : "·OI------""""-�N

� � t3.81--------...:.;,--���'V'� • t:1' '11=============� t:1'n •. 4------------------------� olI oliI

Zl It IT IS '3 U

• T

I liZ z:.4 � � .41 �J% - :szo � 3M 411 448 4.0 $JZ nme In "nlls of • mJlIl.ec. nme In unlls of • rnllllaee.

Fig. 13-Time variations of the inphase (I) and quadrature (Q) amplitudes of meteor echo in Indian MST radar data shown in thick and thin lines, respectively, for each range bin. [The time (1ST) of the data block, year, month, date, hour, min, sec., and beam position are shown at the top. The lower most range bin in each panel, represents floor noise (After Raghava Reddi and Muralidharan Nair, \998).]

91

'17

1S

71

li7

13 11

• .II .. •• _ a A 41 II 41 " .. . . ..... .. .. . . '. . . · ... . ,." .. ..­

••• • • •

• •

__ a_.

•• � •• 1& " a& " .. &. A.. .a " " .. M •••• ,. . • • • • •

. .a .. M" • •

• .. II' ... Ill . ..

& .. ••

• • _ al

• 6.& .. • "

,. I Ill .. . ... A '" """ "

• • • • •

a I

• •• • .a M I

•• I a ..

a • a. a ., . ...

1 • a_a

• a ' ���;:::::::���=::::::t:.:! .. ::!aw�� ..

. .. • .. :::N::_:-:¢':===,::a .. ::.

M - .. �M''''M 2m"t"' ..... --.. ... aM .... •

• . . • • I'

• , , •

12

.. • ..... I .... •

• • • .. . .- .... ---- .. iloilo � .... :===:

.... ,. • • &oM • • I • I.

1l 1.& 'Ill'v1E (lSl) 1�

_" . ".. .. "

-

Fig. 14-Time history of mesospheric echo occurrence [After Jayati Dutta et aI., \998.]

164 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

75 km with a diffused cloud like structure around 85 km. The echoes observed at these heights are intermittent. The observed echoes do not show any aspect sensitivity and are consistent with isotropic turbulence scattering, implying the presence of 3 m scale turbulence in the mesosphere at low latitudes.

4.12 Ionospheric field-aligned irregularities

The MST radar has been operated in ionospheric coherent backscattered mode for mapping the structure and dynamics of the E- and F-region field­aligned irregularities (FA!).

E-region FAl: For observations on the E-region FA!, the radar

beam is oriented at 1 3 .20 due magnetic north to look transverse to the magnetic field at a height of about 100 km. The first results on the E-region FAI have

. 135 R 136 been reported by V Iswanathan et a . , ao et a . and Choudhary et al. 37.

Figure 1 5 shows the height-time-intensity (HTI) plot for the observations taken during nighttime on 1 1 - 1 2 Oct. 1 994. There is a regular E-region band of FAI at about 1 00 km. In addition, there are structures which start in the upper E-region above about 150 km and descend down rapidly to levels below about 1 20 km. The maximum signal intensity is found to be as high as 30-40 dB above the noise level. In contrast to the nighttime, there is a definite pattern in the variation of signal intensity during daytime. The variation is so strong that the signal drops out often for 2-3 h around noontime. The radar beam being transverse to the magnetic field in the meridian plane, the backscatter results from the irregularities propagating almost vertically. These irregularities are believed to be the secondary irregularities generated

HEIGHf - T.UvIE - INTENSITY PLOT

H E I G H T

Km)

11-12 Octoher 1m

19 01 09 TIME (1ST)

-10 -5

G.dcmJci 162.0

154.8

147.6

140.4

133.2

126.0

118.8

111.6

lOU

97.20

90.00

01 Hr 05 Min 26 Sec:

-o 5 10 15

Fig. 15-Height-time-intensity (HTI) map of the backscatter from the field-aligned E-region irregularities observed on 11-12 Oct. 1994

JAIN et af.: MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS 1 65

by the conditions set up by the primary type II irregularities propagating in the zonal direction. It is clear, therefore, that the generation mechanism of type II irregularities is greatly inhibited by the background ionospheric conditions prevailing in the E-region for 2-3 h around noontime. The above HTI characteristics observed over Gadanki, a few degree

's

off the equator are, in many ways, found to be significantly different - from that observed over the equator38.

Doppler spectra observed on the night of 1 1 - 12 Oct. 1994. The height variation of the Doppler velocity shows that it reverses direction from upward to downward at about 1 00 km and again to upward at about 1 1 5 km. The velocities are found to be of the

order of 50 ms -I , which are somewhat greater than normally observed in the nighttime E-region. In general, it is observed that the velocities are

I consistent with the zonal electric field in the height range around 100 km, but not necessarily so below 95 km and above t 1 0 km. It was shown by Krishna Murthy e! at. 39 that the observed drift velocities

Figure 1 6 shows a plot of Doppler velocities derived from the weighted mean Doppler shifts of the

1{ E I G H l'

HEIGHT TII.\.1E VELOCITY PLOT

11 - 12 October 1994 Gadanki . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 1 . • • • . • 1 . . , . . . . . , · r · · · · · · · · · · · · · ·· · · · · · · · · · · · · · · · · · · · · · · · I : : . . . : : : . . . . . : . : . . : . : . . . . : . : : : . " . . . ': : . . ' � : : ' :� : . . " . ; . ' . . . ' . ' . . . : " ' . : . . . . . . r 138.0

I . , . , • • • • • . • • • • , . • • • · 1 ·

·· · · · · ·

· · u

· · · · .�., ... ... � ... IIL.� . . . . <1 iL • • iL . . . 1.1. . . . . 1 . . . . . . . . . . . . . '" . I I I . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . W • • • ..,& .1 . .... n. II .. . . .... � t U lil� . . . . . . 1 �I.I. . . . 128.4 i . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . l'J . . . · ,.,..1 . · "''' '1J. I''�d .1!!I�i..�"1I. ,JOlu� . . . . . . . I . . . . . . . � ! · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · I·tp'· · �·· ...... � · ·�...,.." l . . . . . . 1 1 . . . 1 . . . . " . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . 1111 . "' Ii .ft."''''' .. ���. J. . . J I . . . . l

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . iB . . . . r . a;t.IjiW" •• " ........ , ... ��.�AI,.,JA�.jJIIlI. . . 1 � .� • n+* .... 'A I I . 1 • • • • • • I I • • • � • • • ' I I • • • ' • • • , • • " . I • • • l r@iile44 ' ' •• ii F " . • if' F "111pVpa i " , .. , of ! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . · 81 · · · · · · · · · 4 SO"., tN'S;' 1M· Ii 5AqIi5I'¥ i"�"'f"

I . . · . . . . . . . . · . . . . . . . . . . · . . . . . . . . g . . . . . . . . . . ·r 'PEEl! 44 :::4 ..... " W'P "''''''''!:f� ! · · · · · · ·

i . · · · · · · · · · . . · · · · · · · · · · ·F · r · · · lr �· .,,"" iIIUSZ II qf.I .. ,. .. �( ! 4 � ' I · · · · · · · · · ' · · · ' · · · · " · · · ' · · · · · · · �· l · · · , .... , · · .-"'�" .. L . I I .... -I · · · · '!' · · · · · · · · · · · I · · · · · · · · · · · · r · p¥ ;22 ._ · · · · · Io . !Il · · · · \I ;.1 •• i .. , .. _� .... lli,I •• ,I , . . . . T · · · . . · · ' r ' · 1 ' · · · · . ... , ·I· · r '1 ' 11'· · r ......... . lr . . . . . . , . . ... � J: . . . . · "r · , ,1. .... \� .. I • • .• I ! M!,

I . . . . ) . ',T', I0 " ' " 11,- . • . . . • '- - \ ••. . , L, .... ¥ J . .5, . . . •.•. J 1 11.1. . lib II . .,t 11 . 1. d h I�'II�'p", , ·V 1 1 ill' " l '

80 � . . • . . � .... : rl · · ' - ... . . L • • • • "� ... .L ,, I . . . � .11 . . L IIIa.�II J ItIII.I Jlll J H bh& . LL • . . ' ".1. . 1 .1., . . .. . m! § I _ . . ... .. _ . . . ..... ,1.1 .l • . . 1 .• • • • • '--'ij...�. j ... �_ . • .iI .... .. d', • .I11 ......... JI�.IJ .• �III . IlIiI . �I.u,lu ... ..,..

..... . . .. or _ " 1 0 • • " I ' " I ' , • . • • • • • • • ' " . _ 'I' II I_ iJ--..4'd • " " 1 . . ..... . I ...... �.II .. J�.

I" i

US,8

109.2

99,6

19 01 Hr 01 09

TII.\.1E (1ST) 05 Min 26 Sec

Fig. l6--Height-time variations of the Doppler velocity of the field-aligned E-region irregularities transverse to the magnetic field in the meridional plane observed on 1 1 - 1 2 Oct. 1 994

1.66 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

below 95 km are driven by the neutral wind and the meridional wind component derived from the drift velocity is found to be consistent with the theoretical neutral wind models. The drift velocity in the upper E-region is basically of electrodynamic nature and clearly indicates a height reversal of the electric field.

F-region FAl: For observations on the F-region FAI, the radar

beam is oriented at 14.8° due magnetic north which is the nominal direction looking transverse to the magnetic field at a height of about 330 km. The Leight and time resolutions of the observations are 4.8 km and 32s, respectively. The Doppler power spectral data have been analysed to obtain height-time maps

of signal intensity, Doppler velocity and spectral width40• The radar has been extensively used for mapping the structure and dynamics41 .42 of the 2.8 m scale F-region FAI.

Figure 1 7[(a) and (b)] shows the height-time­intensity (HTI) maps observed on 30 Sep . 1 994 and 1 Mar. 1 996. The signal intensity is given in five shades of signal-to-noise ratio (SNR) in decibels with the noise power reckoned over the Doppler spectral

bandwidth of ± 1 25 Hz. The peak signal intensities are found to be associated with the rising plumes and are in the range of 30-40 dB above the noise level. The intensities are seen to be modulated in time with quasi-periods ranging from 10 to 40 min, which seems to be a manifestation of the commonly occurring medium scale gravity waves. A similar type of multiple periodic plume structures have also been observed at Kwajalein and interpreted as gravity­wave-seeded plasma upwellings43• The HTI maps show both updrafting and down drafting of the plasma structures. The downdrafting observed on 30 Sep. 1 994 [Fig. 1 7(a)] is somewhat unusual in the sense that it extends all the way down to the E-region. The downdrafting observed on 1 -2 Mar. 1 996 [Fig. 1 7(b)], following 2300 hrs LT, is seen to cover the entire height range extending to about 550 km. It is observed, in general, that the downward slopes of the scattering structures match with the associated Doppler velocities. The slopes of the rising plumes, however, do not always match with the corresponding Doppler velocities as has also been noted by Woodman and LaHoz44 .

Figure 1 8 [(a) and (b)] shows the height-time variations of the Doppler velocity transverse to the magnetic field in the meridian plane for the same two

events as the HTI maps presented in Fig. 1 7[(a) and (b)] . The matching of the Doppler velocities to the slopes of the scattering structures in the HTI maps, particularly, during the downdrafting phase of the ESF, makes it possible to study the plasma depleted flux tube motions from the phase velocities of the 3m irregularities. The upward velocities are of the order

of 1 00 ms-I , but could be as high as 300 ms-I in the rising plumes42. In the bottoms ide F-region, the velocities are predominantly downward, particularly, during the later phase of the ESF development. On I Mar. 1 996, the down drafting is observed well into the topside up to a height of 550 km which conforms to the model of Anderson and Haerendel45. The

downdrafting velocities, reaching as high as 100 ms -I ,

are well above the downward drift ot the background

plasma which does not generally exceed 20 ms-I . In the bottoms ide, where (g/Vin) is negligible, any downdrafting faster than that of the background plasma would mean enhanced electrodynamic drift and the downdrafting irregularities are plasma

depletions. The parameter (g/Vin) represents the upward drift of plasma depletion. It is suggested by Laakso et al. 46 that, as the background electric field becomes westward at about 2 1 00 hrs LT, simultaneous updrafting and downdrafting of plasma flow may occur in a depletion channel, resulting possibly in the pinching off of the upper part of the bubble channel. The simultaneous updrafting in the top part and downdrafting in the bottom part of a channel are seen on 30 Sep. 1 994, but no pinching off of the channel is noticed, since the down drafting flow is fairly weak.

5 Radar interferometer technique and sample observations

The radar interferometer technique provides information on the location of the discrete scatterers and their scale sizes along the interferometer baseline. It is also possible to derive the drift of the irregularities along the baseline by tracking their positions as function of time. The technique has been used successfully to study the field-aligned ionospheric irregularities47•

The space domain interferometer (SDI) technique involves transmitting radar pulses with a single high gain antenna and receiving on two (or more)

independent antennas. The cross-spectrum Sl2 (00) of the signals received at the two antennas provides information on the angular position and size of the

H E I o H

T

(Km)

H

E J o H

T

JAIN et al.: MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS

RANGE TIME INTENSITV PLOT 30 September to 0 I October 1994 ( a )

1 8

32

08

2 1

, .

26 C b '

Oadanki

OIdMki

0 1

36

'8

�50.5

457.6

364.7

27 t .'1

1 7'1.0

16, 1

Ht

Min

S,e:

'4 1 ,8

',0'..)

11.4 ()31 ff; � Mm' tj' Sc�'

Fig. 17-Height-time-intensity (HTI) maps of the field-aligned F-region irregularities observed on (<:I) 30 Sep. 1994 and (b) 0 1 Mar. 1996

167

INDIAN J RADIO & SPACE PHYS, A UGUST 2000

scatterer glvmg signai at the Doppler frequency m. The cross-spectrum is given as :

Sl2 (co) = <VI (co) V2 *(co» / <1V1 12 >1n <1V2 12 > 112

Ga d a n k i 3 0 S e p . 1 99!, (0 ) ] ••••••••••••••• • •• •·•• •••••• · ••••• 1:: ' . -••• • • • • • • • 1 . . . . . . . . . . ................. ... .. . . .. wc . . . . . . . . . . . . . . . . .. . .. . . . • • . . . . . .. . . . . . . . . . . . . . . . . . . . ......................... ,u . . . . . . . . . ... . ...... .. . . . . . . . .. .. . . . . .... . . . . . . . . . . . .......................... .>1 . . . . . . . . . . . . . . . .. . , • • • • •• . • . . • . . • . . • • • • • .

. ' O < . . .. . . . .. ..

. . .. .... . . .... . . . . . . . . � .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4(X) . . . . . . . . . . . . .. . . .. . . .. . . ... ...... . . . .. . . . . . . . . . . . . . . . . . . . . .. .. . .. . . . . ... . . . , • • •

. . . . . . .. . . .......................... J. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . ( 1 )

-- . . . . . ... . .. . . ..... .. . .. .... . .. _ - . . . ...... . . . . . . . . .. . . .. . . . . . , . .. . . .. . , .. . . . . . . ... .

� �j ............ �:l;;�! ••••••••••• ; ; � ••••••••••• ; t 1 Z 0 m .·1

I l

t:

I .. .

. .. >:::.�::::� .�:�::.��::.::::::::::: :::::�.=:�:=�:.::�::::::: I

• ,', o; .. . .. . ,... _ .... !-o-. _.".." . "- _ a . .. . . . .. . . . . ..... _ • . • • __ • .•• • • • • .

• • 1 , . . . . . � ... '.k,..> _ . . _ _ . . . ... .. . . . .

£'-.t � . • • . . .• • • • 7't......,... " ' ':\''7., :._� .. . . . . .. .. . . . :.- , ._ - . . . ... . , . . . ' i ; . . .. ..

. � .�.:: :::� :::."I�:�:::���:' ��:�� :: ::: �=::� :::�:�� .. : : : : : � � �L± , :�-f��.... , . } " , • • , 1 JiJ",! l'.1 !!} 11 n .n OJ 01 allf)

.� : ,vi E ( I 5 'r ) l M I1 ' . '; 9 % ( b )

; � I . : :: : .:: : .. ::::·.::.�L§:E���::::���;�-�.:�·: I � l :: : : : : : : : : : : : : : : : :�:::::::::::::.:; : : :: ::: : ::�::-��:: :: l I "j , � ���: :'� �S-: �� 1 Z 0 m .- 1 j . . . . . . . . . . . . . . . . . ... . . _.. i(j .,....... .. . . .. . . . . . . . . .. . . . . . . .. . .. . ... . JOO '

: .

. . . . . . . . . . . . . . . - .. --. . ... ... � ..... . . . . . . . . .. . .. . . . . . - • . . . . . . . . :.: .. . �� � .. f-��:.:.: . . : : :: :::�:;.:: .. : : . . :

�I : ( i �:8<\���+:::: : .. ·:}:::�:::.H . . . . . . • . . ---- .. .... . . ... . . . . . . . . . . . .. . , . "" ' - . . . _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... .1 . . . .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . _ . . . . ...... . , . . . . . . . . . .

. . . . . . . . . " . . . . .... . . . . . . . . . . . . . . . .. , . . . . . . . . . . .... . . . . . . . . , . . . . . . . . . . . . . .

19 1Il 21 II II DO 01 en 0J07

T I M E ( I S T ) Fig. 18-Height-time variations of the. Doppler velocity of the field-aligned F-region irregularities observed on (a) 30 Sep. 1994 and (b) 0 1 Mar. 1 996

where, VI (co) and V2 (co) are the complex spectra of the signals received at the two-phase centres. Tne angular brackets indicate ensemble averaging (time averaging, in practice). Assuming Gaussian statistics to the fluctuations in angle of arrival, SI 2 (co) is expressed as47:

. . . (2)

where, k is the wave number, d the length of the ba"e l ine and e the angle of arrival. For each Doppler frequency ro, the SI2 can be described by its magnitEde or coherence I SJ2 I and. its phase �.ngk ,� For scaHerers, small compared to the scattering

47 volume. the coherence and phase angle are given as

whe: �, fJ[:.) IS the ,mgular SIze and Ow is tr� mean posltion ')f �he target, giving a Doppler shif� w. Th:::s, Lhe c8fi,;',:cnce value of the -,:m5s-spec�n,m corresponding to each Doppler shift provides ?:.�i estimate of how localized the particular scatterer is within the ,adar scattering volume and ¢ is the angular position of the scatterer with respect :':0 the perpendicular bisector of the baseline. From Eq.(4) the position of the scatterer bx along the baseline direction corresponding to a phase difference &!> (;r� radian) 3.t an altitude h can be written as :

ox =h or:p / k d . . . (:5) Hem:e, the rate of change of phasiJ of a panicular

scatterer with time will provide �he drift along the direction of the basel ine.

The Indian MST radar was operated sllccessfully in SDr and the first results on the E-regicn fl.::id-aiigned irregularities (FAI) have been reported )�' Rao �t al. 48. Figure 1 9 shows a sample result or t�e cross­spectrum amplitude (coherence) and phase aion� with the corresponding power spectrum lor the E-regi0;} F AI from the observations made on 30 S ��: L 9°6, 1t can be seen that the coherence is found to be as hIgh as 0.95 indicating that the scatterers are discrete and highly localized. Corresponding to the high coherence values the cross-correlation phase is found to exhibit a remarkable linear relationship with Doppler frequency. This aspect is in conformity with the

JAIN et al .. MST RADAR & ITS SCIENTIFIC PROGRAMMES AND RESULTS 169

1 .0

� � 0.5 0 P-4

0.0 1 .0

� u � � M 0.5 � � 0 U 0.0

+ 1 80 � CIl ro � 0 lA/\

� 1 80 -62.5

It. '" " Ah ",.. ,.. ",..,,-/\ IV Irv !I Y

o +62.5

Doppler Shift (Hz)

Fig. 19-Typical interferometry results showing the power spectrum (top panel), coherence (middle panel) and phase (bottom panel)

theory presented by Van Baelen and Richmond49. As the phase of the returned signal at a particular Doppler shift indicates the location of the scatterer with respect to the mean antenna beam direction, the maximum extent of the linear portion of the phase spectrum can be used to detennine the maximum

angular extent of the radar returns. Using this procedure, the maximum arrival angle as a function of range is plotted in Fig.20. From Fig.20 it can be seen that the arrival angle of the backscattered signal is within OS with respect to the orientation of the beam. The drift velocity of the irregularities along the baseline has been computed by Farley et al.47• The sample velocities are found to be in the range of 30-60 ms-t , but there is considerable uncertainty in the estimates in view of the large scatter in the time vs distance plots. The velocity estimates based on Van Baelen and Richmond49 are found to be quite high

� " co �

1 25.00

1 20.00

1 1 5.00

1 1 0.00

1 05.00

1 00.00

95.00

90.00 +rn-rTmrmT".".,.rrm,.,.",rrrrrrTTTTTTTn'TTTrrTTTT,.,...,..,TTTT1'TTTT'1'rTTT1 0.00 0.05 0. 1 0 0. 1 5 0 .20 0.25 0.30 0.35 9 .. (degret)

Fig. 20-Variation with range of maximum arrival angle of backscatter from E-region field-aligned irregularities

which is believed to be due to the simplified assumptions in their formulation.

6 Future plans Future plans of the NMRF includes: (a)design and

development of new signal processing system, (b) augmentations of present facilities and (c) participation in the international collaboration programmes as discussed below:

6.1 New DSP system

The NMRF has taken up a major development activity on the replacement of old online data acquisition and signal processing system with a new signal processing and timing generation system for MST radar as follows:

(i) Objectives of the new system are : replacement of MST radar data processing hardware with a more general-purpose signal processing system, additional computing power and more functionality in modular manner.

( i i) To change timing and control signal generator to a more robust, programmable and maintainable system.

(iii) Up-gradation of radar controller to a more versatile one with added features.

(iv) Addition of more experimental technique in addition to Doppler beam swinging technique such as (a) Spaced antenna drift (SAD)/Space domain interferometry (SOl), (b) Frequency

170 INDIAN J RADIO & SPACE PHYS, AUGUST 2000

domain interferometry (FOI), (c) Incoherent backscatter and (d) Meteor detection (MD).

6.2 New PC-based local processor systems

The NMRF is developing a new digital signal and data processing (DSP) system for Indian MST radar. To match this new DSP system, it is planned to develop a new local processor system (LPC). These LPC systems act as an interface between radar controller (RC) and radar subsystems and are connected to RC through RC 232C link.

The new local processor (LPC) systems would be PC based. These systems would have digital 110

board interfaces and separate software for radar operation and maintenance. The new design will remove the limitations of the present LPC systems and reduce the down time and would be easy in maintenance. The proposed new LPC systems would incorporate the following features.

(i) Record the status of transmitter analog parameters during operation.

(ii) Control and status monitoring of individual polarization switches.

(iii)Automation of antenna array VSWR measurement.

The project is planned to be initiated this year.

6.3 Addition of RASS capability to LA WP system

It is proposed to add radio acoustic sounding system (RASS) to lower atmosphere wind profiler (LA WP) system9. With the addition of RASS capability to LA WP system, it would be feasible to make high resolution temperature measurements in the lower atmospheric region along with the wind, turbulence and precipitation measurements. Simultaneous temperature measurement using LA WP and RASS system would considerably enlarge the scientific applications of LA WP system.

6.4 Radiosonde and ozonesonde launches from NMRF,

Gadanki

At present radiosonde temperature and humidity data from Chennai are used for interpreting MST radar reflectively (11) data. However, Chennai is about 120 km south-east of Gadanki. It is felt by user

scientists that measurements of temperature and humidity at Chennai may not always be applicable to Gadanki. Therefore, it is planned to have some campaigns of simultaneous radiosonde flights and MST radar observations from NMRF, Gadanki. First

such campaign has been conducted successfully during the period 19 July-14 Aug. 1999. More such campaigns would be planned depending upon the outcome of the first campaign.

It is also proposed to have campaigns of ozonesonde flights from NMRF, Gadanki, to study the exchange of ozone between troposphere and stratosphere during the events of tropopause 'weakening' or 'break'. This is for the understanding of the exchange of ozone between the stratosphere and troposphere and also for the importance of the dynamical and chemical processes in determining the tropospheric ozone.

6.5 Participation in international collaborative programmes

The NMRF would have international collaboration with other MST radar facilities in the world for investigation of global phenomenona. There are three areas in which international collaborative campaigns are planned, viz. E-region QP echoes. gravity waves and turbulence studies. Two campaigns have already

been carried out on QP'echoes and results are planned to be presented at the forthcomi ng international workshop MST9. The NMRF facilities would be optimally utilized for participation in the global observational campaigns such as INDOEX, GAME, TRMM, etc. The forthcoming international campaign in this connection is EPIC (Equatorial Processes Including Coupling).

Acknowledgements National MST Radar Facility (NMRF) is set up

jointly by the Council of Scientific and Industrial Research (CSIR), Defence Research and Development Organisation (DRDO), Departments of Electronics, Environment. Science and Technology and Space, Government of India, with Department of Space (DOS) as a nodal agency. The NMRF is being operated by DOS with partiai funding from CSIR. The authors also acknowledge the efforts of engineering team of NMRF responsible for operating various experimental systems and generating high quality data used by scientific community to arrive at the results which are highlighted in this paper. One of the authors (DNR) wishes to acknowledge the UGC­SVU Centre for MST r::tdar application for providing necessary facilities for carrying out the work.

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