Introduction

High-power radars operating in VHF range have been demonstrated to be a potent instrument for studying atmospheric dynamics, particularly in the lower and middle atmosphere, as well as investigating ionospheric plasma irregularities1. Following the introduction of Mesosphere- Stratosphere-Troposphere (MST) radar technology at Jicamarca Radio observatory by Woodman and Guillen (1974), the atmospheric research community recognized that these radars could be employed to investigate the relatively unexplored middle atmosphere (10–100 km). This was a significant development, as this part of the atmosphere had previously been under-studied due to a shortage of observational instruments. Following this breakthrough, various high-power radars were developed, including the Middle and Upper (MU) atmospheric radar2,3, the Indian MST radar4, Equatorial Atmospheric Radar (EAR)5, and Middle Atmosphere Alomar Radar System (MAARSY)6.

The extent of applications of the radars primarily depend on the frequency band in which they are operated. VHF radars operating around 50 MHz have been extensively used in regional weather prediction models, studying the evolution of convective cloud events and gravity waves, turbulence, thunderstorms and cyclones among many other applications7,8,9,10. These radars are best in terms of continuous wind measurements with high temporal and spatial resolutions. The atmospheric radars detect the backscattered signals resulting from refractive index variations and the corresponding Doppler shift is used to calculate the wind velocities10. The first wind radar profilers were developed around 40–55 MHz11 and provided best results above 1 km and were extensively used in Troposphere—stratospheric–mesospheric studies10,12,13.

The University of Calcutta has established a fully active phased array Stratosphere Troposphere (ST) radar at Ionosphere Field Station, Haringhata with the financial support from the Science and Engineering Research Board (SERB), Govt. of India. This is a major research facility in eastern and north-eastern India as well as South-East Asia. Presently, there are no VHF radars operational in the transition region from the tropical to sub-tropical locations in the South-East Asian longitude sector covering India. Notably, this is the first 50 MHz active phased array radar is installed in an Indian University. The radar is designed to study the neutral atmosphere and ionosphere with a minimum to maximum height coverage of 0.6–1000 km.

The University of Calcutta has a long heritage of research in atmospheric sciences. The CUSTR is a powerful and versatile tool for lower atmospheric and Ionospheric studies. The location of the radar is right on the tropic of cancer and near the northern crest of the Equatorial Ionization Anomaly (EIA). Moreover, there are no such radar located in this transition region from the tropics to the sub-tropics in the south-east Asian longitude sector.

The study of lower atmosphere in this region will help in understanding many interesting physical phenomena with implications to global climate change. Also, this region is characterized by pre-monsoon convective rainfall and Nor’westers/Kalbaisakhi which have a profound influence on the day-to-day human lives.

The CUSTR serves scientific research purposes in various atmospheric phenomena, including tropopause convection process, lower atmospheric turbulence, gravity waves, equatorial/planetary scale waves, and ionospheric E and F region irregularities. It generates data on three-component wind, Doppler spectral width, and signal strength in the lower atmosphere, as well as irregularity drift velocity and backscatter signal strength from ionospheric irregularities. The high-resolution data from the ST Radar is expected to yield valuable insights into the dynamic Stratosphere-Troposphere Exchange (STE) processes, the generation of atmospheric gravity waves, and their influence on regional weather systems. This information can aid in understanding large-scale spatio-temporal weather variability, the impact of global climate changes, and the development of cyclones and depressions over the Bay of Bengal. Additionally, the data can be assimilated into general circulation models to enhance predictions of tropical weather systems, particularly in the Indian region, ultimately improving regional-scale forecasting.

The radar is mainly operated in the Doppler Beam Swinging (DBS) mode with the capability of operating in the Spaced Antenna (SA) mode. The DBS method is employed to derive the velocity wind vectors by utilizing a minimum of three non-coplanar beam orientations (Vertical, East-West, and North-South) to derive the three components of the wind vector. The radial velocity and vertical height represent the corresponding Doppler frequency and range bin. Spatial inhomogeneity in wind fields can introduce errors in the estimation of horizontal winds in the DBS method14,15.

In this paper, we discussed the complete system overview of the 53 MHz radar and its sample scientific results/observations of various regions of the atmosphere obtained with this radar system. The atmospheric boundary layer observations are carried out from 0.6–10 km altitude with a high range resolution of 75 m. The lower atmospheric winds are measured and validated for an altitude range from 1.65–24 km. Mesosphere echoes and ionosphere irregularities were also observed using this system. A prototype version of the ST Radar system with 19 antennas has been tested and the technical details are discussed in16, wind validation results are available in17 and initial ionosphere E-layer irregularities are available in18.

System description

The capabilities of the CUSTR system is (i) ability to measure wind and turbulence in the troposphere and lower stratosphere up to 24 km with high temporal and spatial resolutions (ii) ability to steer the antenna beam to an off-zenith angle of 40° depending on the azimuth angle with 0.1° resolution in both azimuth and elevation angles (iii) ability to study the atmospheric boundary layer (ABL) with high range resolution of 37.5–75 m from the altitude range as low as 600 m above the ground, (iv) ability to study the mesosphere dynamics including meteor‐induced processes, (v) ability to study the dynamics of field‐aligned plasma irregularities, (vi) ability to detect back scattered signals from the ionosphere E and F-regions, and (vii) ability to perform the multi-receiver experiments for spaced antenna and space domain interferometry/imaging of atmospheric and ionospheric echoes. To achieve the above-mentioned capabilities the CUSTR system has been designed to ensure the suitable peak transmitter power, less transmitter and receiver path losses, high averaged transmitted power with maximum duty ratio, pulse width is varied from 0.5 to 128 µs, range resolution maximum of 37.5 m and minimum of 9.6 km, minimal transmit/receive switching time and capability of the multi-channel receiver. Table 1 describes the operational specifications of the ST Radar full system.

Table 1 Specifications of the Radar System.
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The receiver of the CUSTR system gives the radar backscattered signals in the form of in-phase (I) and quadrature phase (Q) signals. The radar received echoes are further cleaned to remove contamination from ground clutter, system bias, and interference, etc. The noise level of the power spectrum is estimated based on the variance decided by a threshold criterion19. The method for estimating noise level utilizes the received Doppler spectrum and properties of white noise, without requiring knowledge of the radar instrument system's noise level. During the computation of spectral moments, the signal in the Doppler spectrum is identified by its peak value20. Extracting the zeroth, first, and second order moments is crucial for determining various atmospheric and turbulence parameters. The zeroth, first and second order moments describes the received signal power, mean Doppler and Doppler width respectively21,22. The moments are used to compute zonal (U), meridional (V), and vertical (W) winds14.

The operational frequency of CUSTR is 53 MHz which was determined by several factors. For height coverage up to the Mesosphere (~100 km), a 50 MHz band is preferred because atmospheric irregularities at that height are larger, and the wavelength of around 6 meters enhances backscattering. Furthermore, VHF frequencies are chosen because in the mesosphere, scattering from turbulent irregularities is significantly enhanced by D-region electrons during the daytime.

The CUST Radar system consists of six major sub-systems, viz, Antenna Array, Exciter, Distribution and Multi-Function Combining Network (DMFCN), Transmit-Receive Modules (TRMs), Radar controller software, and Multi-Channel Receive system. Figure 1 shows the functional block diagram of CUST Radar System.

Figure 1
figure 1

Functional Block diagram of the University of Calcutta ST Radar system.

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Antenna array

The CUST Radar system has a planar antenna array consists of 475 three-element Yagi-Uda antennas grouped into 25 sub-arrays, each sub-array is having 19 three-element Yagi antennas and a metal shelter which comprises of Transmit-Receive Modules, power supplies along with other electronics. The antennas are arranged in a near circular shape spreading over 95 m diameter and an antenna aperture of ~7000 m2. Among the 25 sub-arrays, the central 19 sub-arrays are in hexagonal shape and the remaining 6 sub-arrays are placed at the periphery of the circle. An inter-element spacing of 0.7 λ (where λ is the operating wavelength) is adopted with an equilateral triangular grid to steer the beam up to an off-zenith angle maximum of 40° depending on the azimuth angle. Each antenna in the array consists of 3-elements (1-folded dipole, 1-reflector and 1-director). A half wavelength 50Ω co-axial balanced to unbalanced transformer (BALUN) cable is used and it is electrically connected between the two ends of the folded dipole. Each individual Yagi-Uda antenna has a directive gain of 7.12 dBi with half power beam width of E-plane and H-plane are 65° and ~100° respectively.

The CUST Radar system is fully active phased array system which means each antenna is having an individual TRM with independent phase control and the 475 three sub-element Yagi antennas are aligned in magnetic North-South direction to provide linear polarization, Ionosphere backscatter observation and East-West scanning as well. This configuration provides high flexibility during beam formation and beam steering with a half power beam width of ~ 3.6° and a maximum directive gain of 34.1 dB. Figure 2a shows the illustration of CUST Radar system antenna array with shelters and Figure 2b shows the antenna array geometry with 25 sub-array groups.

Figure 2
figure 2

CUST Radar system (a) antenna array with shelters and (b) Geometry of antenna array with 25 sub-arrays.

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The 475 antennas in the antenna array can be controlled independently during transmission and reception. The 25 sub-arrays can be operated in a combined mode for DBS and can be operated individually for the multichannel receiver experiments for the SA mode as well as interferometry observations.

Exciter

The exciter unit consists of a high stable 20 MHz oven‐controlled crystal oscillator (OCXO), used as reference to generate the pulse-coded transmit (Tx), simulated (SIM) signals, and different clock (CLK) signals for the other sub-systems. The different clock signals are 10 MHz, 20 MHz and 600 MHz. The short-term stability of the OCXO is about 5x10-12 and the phase noise values of the OCXO is 120 dBc/Hz at 20 Hz offset.

The 20 MHz signal is passed through the Band Pass Filter (BPF) to eliminate the unwanted signals before the power division. The 20 MHz clock signal is generated and collected directly from the OCXO and used as reference signal. The 10 MHz and 600 MHz signals are generated from the reference 20 MHz signal by using power division and multiplier networks in respective paths. The 10 MHz clock signal is used for digital receiver purpose and the 20 MHz clock signal is used for the other sub-systems to maintain the synchronization. The Field Programmable Gate Array (FPGA) based Direct Digital Synthesizer (DDS) card takes 600 MHz clock signal as input and generates the range-shifted pulse coded simulated (SIM) signals for testing purpose.

The 20 MHz high stable OCXO output is given to the Phase Locked Loop (PLL) and it has been used to generate the required pulse coded 53 MHz transmit (Tx) signal as well as the 53 MHz Continues Wave (CW) signal. The output level of the pulse coded 53 MHz Tx signal is 0 dBm. The pulsed Tx signal is then fed to the 10W driver amplifier placed near the DMFCN unit. The control and timing pulses are given by the Master Timing and Control Signal Generator (MTCSG).

Distribution and multi-function combining network

The Distribution and Multi-Function Combining Network (DMFCN) plays multi-function role as facilitating transmit/receive switching, transmit signal distribution, receive signals combining into either DBS or multi-channel modes and TR Module calibration/monitoring. The blocks inside the DMFCN is shown in Figure 1 which highlighted in the red color dotted line box. It consists of in-phase power divider/combiner and switches to facilitate different operations. Some of the basic blocks of the DMFCN unit is T/R switch, 25-way power divider/power combiner (PD/PC), 25 numbers of single pole double through (SPDT) T/R switch’s or mode selection switch’s, and Rx / CAL switch SPDT 25 No’s.

In the receive mode, the radar received signal can be switched either in DBS mode or multi-channel mode depending up on the type of experiment conducted. This is facilitated by the 25 SPDT switches. Each SPDT T/R switch has an insertion loss of 1.0 dB (max) and isolation between the ports are 50 dB (min). This T/R SPDT switch is called as mode selection switch and it will act as a mode selection between DBS and SA mode of operation. For DBS mode, the received signals are routed to the 25-way combiner via the SPDT switches and combined and passed through the T/R switch to the DBS receive channel. In the multi-channel mode, the SPDT T/R switches routes the received signals to the Rx / CAL SPDT switches. One set of outputs will be directly sent to the multi-channel / back-end receiver. Another set of outputs are terminated with load and maybe used for calibration purpose if required. The SP25T has an insertion loss of 2.0 dB (max) and port-port isolation is 40.0 dB (min). The Control and timing signals for the DMFCN are provided by MTCSG.

Transmit-receive module

The Transmit-Receive Modules (TRMs) are designed based on the solid-state power amplifier technology to provide long pulse widths and high duty ratios. All the 475 TRMs are placed inside the 25 closed shelters present in the antenna array area. One such shelter along with the chillers is shown in Fig. 3a. Each shelter consists of 19 TRMs, 19-way Power Divider/Combiner, 1:19 switch and Group Level Controller (GLC) along with necessary power supplies such as + 5 V (analog and digital), + 28 V, and + 60 V. Mechanical shelter prevents from entering the different environmental conditions like sun radiation, winds, rain and dust. From the Fig. 3b, it may be noted that the TRMs are attached to the heat sink/cold plate on both the sides. Each TRM dissipates around 125W of power in the form of heat to the heat sink (air duct), likewise, 19 TRMs dissipates a total of 2375W in the form of heat to the heat sink. A chiller is used to remove the heat from TRMs and power supply units. At equilibrium state, the maximum junction temperature of the TRM is observed to be 55 °C.

Figure 3
figure 3

(a) Over view of Hut/Shelter, Chillers, three sub-element Yagi antenna in the antenna array (b) Hut/Shelter housing the TRMs which are mounted on heat sink, Power supply units, GLC and PD/PC.

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The functional block diagram of TR module is shown in Figure 4. It has three major sections, namely, transmit (Tx) section, Receive (Rx) section, and common arm section. The various RF components along with the printed Circuited Boards (PCBs) and Timing and Control Signal Generator (TCSG) card are placed inside the TR module.

Figure 4
figure 4

Functional block diagram of TRM.

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The transmit section consists of three amplifier stages; a pre-driver, driver and a pair of cascaded high-power amplifiers, which all combines to generate a peak output power of 2 kW. The input to the TR Module is − 5 dBm(typical). The pre-driver amplifier provides a gain of 35 dB. The driver amplifier delivers an output of 100 watts with a gain of 18 dB. The high-power amplifier section consists of a power divider, two parallel amplifiers each can deliver an output of 1.2 kW, and a power combiner. The gain of each power amplifier is 15 dB (typical), the efficiency of power amplifier is about 45% (typical) and operated in class AB mode. The combined output is over 2 kW at the output of TR Module vowing to the loss factors of 2-way combiner, T/R switch and dual directional coupler. The transmitter can be operated in different pulse width’s from 0.5–128 µs with 5 % duty ratio (max). TRM also supports the complementary code, Barker code and a few user defied codes.

A dual directional coupler is used at the end of the transmit section, in which the transmitted and backscattered signals are passed through to measure the forward power from the Tx section and reflected power from the antenna. The TRM is having five interfacing connectors, out of which three are RF connectors and two circular connectors.

The receiver section consists of blanking switch, Limiter, a pair of series Low Noise Amplifiers (LNA) and Band Pass Filter (BPF). Blanking switch is used to protect LNA from Tx leakage through the high-power T/R switch during Transmission. The output P1 dB of LNA is 22.6 dBm (typical), the noise figure of the LNA is 0.8 dB and the TRM Rx path noise figure is lass than 3.0 dB. A PIN (Positive-Intrinsic-Negative) diode type of high-speed Blanking switch is used and it provides 50 dB of isolation, which can handle a peak power of +20 dBm minimum. Both blanking switch and high-power T/R switch together offer an isolation of 100 dB or more. This will ensure the safety of LNA with a leakage power of about − 40 dBm at the input of LNA, which does not affect the LNA. In addition to blanking switch, a low loss passive limiter is used before LNA for additional safety.

The common arm section is also called as common input section and it consists of 6-bit digital phase Shifter, 5-bit digital attenuator and a T/R switch. A 6-bit digital phase shifter is used to perform the beam steering with a phase resolution of 5.625° with an insertion loss of 4.0 dB or better. The 5-bit attenuator provides an attenuation of 8 dB with a resolution of 0.25 dB. The low power T/R switch is used to switch the input section between the transmit or receive sections.

Each TRM has been tested thoroughly with different environmental conditions, 72-hrs burn-in test, and endurance test to evaluate the RF performance. The TRM test results for 8 µs complementary coded pulse with 1 µs baud length is shown in Figure 5a. For the same 8 µs pulse width the rise time and fall time are measured as 154 ns and 148 ns respectively. The rise and fall time of all the RF signals are better than 200 ns. The TRM pulse spectrum is measured for the 1 µs pulse width with 2 MHz bandwidth as shown in Figure 5b. The harmonic response and the frequency response bandwidth of the TRM is measured which is shown in Figure 5c and d. The measured Tx bandwidth is about 8 MHz and the sub-harmonic suppression is better than 55 dB.

Figure 5
figure 5

Transmit-Receive Module (a) 8 µs Pulse shape with complementary code (b) 1 µs pulse spectrum with 2 MHz bandwidth (c) Tx path Harmonic response and (d) Tx path frequency response bandwidth.

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Radar controller software

The Radar Controller (RC) software is a computer system with Graphical User Interface (GUI) based application to control and operate the entire radar system by sending commands/instructions, timing and control signals for all the sub-systems. The RC software provides various features such as (i) different operational parameters required for setting up an experiment will be sent to the GLC using RC software via MTCSG in the form of experimental Specification Sheet File (ESF) (ii) creating the phase file for the corresponding azimuth and elevation angles for a given ESF. The phase values in the phase file are loaded to each TRM via ethernet and optical cables from RC PC to GLC. The GLC distributes the phase value to the corresponding TRM through the Communication Port (COM PORT) cable. (iii) RC software communicates with all the sub-arrays individually based on their pre-assigned IP address for sharing the commands and status, (iv) RC software monitors the health parameters of the individual sub-array and TRM for every 30 sec of time and the same will be displayed in the GUI, and (v) RC software will also provide automatic and semi-automatic phase calibration23.

The RC will calculate the phase information for transmit and receive paths by correcting with respect to the calibrated values and transfers them to the individual TRMs. Radar controller and TCSG will work together in co-ordination through the Group Level Controller (GLC). TCSG is based on FPGA card which generates timing and control pulses to control the subsystems, required for the operation/calibration of the radar. The TCSG card in each TRM will have phase shifter controls, attenuation controls, calibration controls and T/R switch controls based on the instructions received from the RC.

The RC GUI uses TCP/IP communication protocol and the GLC uses optical based ethernet communication protocol in the antenna array. Apart from the TRMs health status monitoring, the RC will also monitor and display the shelter health parameters such as temperature, humidity, and smoke detection23.

Multi-channel receive system for signal processing

The CUST Radar receive system has two major sub-systems, (i) multi-channel RF back-end receiver system and (ii) multi-channel digital receiver system. The multi-channel RF back-end receiver system consists of 27 channels, which receives the 53 MHz Rx output from the respective (out-door) Group-level combiners and delivers the same to the Digital Receiver system. Out of 27 channels, one channels is used for combined DBS mode operation, 25 channels are used for SA mode operation and remaining one channels is kept as spare. The major function of this unit is to provide suitable amplification, and band limiting. Typical noise floor at the antenna terminals is about − 100 dBm and 1-dB compression point of the final amplifier will be about +10 dBm. The overall receiver path gain is about 100 dB. Out of which, 20 dB gain is from the front-end section in the TR module and the remaining 80 dB gain is provided by the back-end RF receive channel. The gain of the outdoor 19-way power combiner (PC) is +12 dB (typical) and the RF feeder losses are about − 13.2 dB which is calculated from the antenna cable to back-end receiver input port. The indoor 25-way power combiner gain is +12 dB (typical). Further, it should cater for different dynamic range windows by providing the programmable attenuation. Each receive channel will provide different band widths which are 4 MHz, 6 MHz, 8 MHz, and 10 MHz and the user can select any bandwidth depending on the experiment type. These filters are useful during the astronomical or passive observations, where the antenna provides wide band width to receive the different wavelengths coming from the astronomical objects/sources.

In the RF back-end, each channel consists of a directional coupler, digital attenuator, limiter, blanking switch, band pass filters and four amplifiers each having 20 dB gain and 0.8 dB noise figure. The four low noise amplifiers are combined to provide 80 dB gain and noise figure of 2.2 dB.

The received signals from the back-end receive channels are fed to the multi-channel digital receiver (MCDRX) system for on-line signal processing. All channels are identical and perform the following operations (i) Analog-to-digital conversion (ii) Digital down conversion (iii) Decimation and Filtering to baseband (iv) Pulse compression or decoding (v) Coherent integration (vi) Fast Fourier Transform (optional) and (vii) Incoherent integration (optional). The on-line processed data is transferred to host PC for further on-line or off-line data processing to extract the atmospheric physical parameters.

The MCDRX system configuration is shown in Figure 6, which consists of five Multi-channel Receiver and Signal Processing (MCRSP) units named as Master unit, Slave 1 to Slave 4. These five units are individually connected to five high end PCs as PC 1–PC 5, where one PC will act as a Master and the remaining four PCs acts as slaves. Each MCRSP unit has 6-channels and total of 30 channels, each channel supports direct RF digitization, ADC with 16-bit resolution, coherent integrations from 1–1024 selectable in steps of binary, time series points or Fast Fourier Transforms (FFTs) are 32–4096 selectable in steps of binary, incoherent integrations are 1–100 selectable. It provides data output types as raw / spectrum, moments, wind components and wind products.

Figure 6
figure 6

Configuration diagram of MCDRX system.

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Out of 30 channels, one channel is used for DBS operation, 25 channels are used for SA / Interferometry mode of operation and the remaining are kept spare. In DBS method, one MCRSP unit is used for data acquisition and signal processing. In SA/ Interferometry mode all the five MCRSP units are used for capturing and processing the 25-channel data.

Atmospheric observations

By using the University of Calcutta ST Radar system, some initial observations are made on troposphere-stratosphere region, atmosphere Boundary layer, ionosphere irregularities in E and F-region. The observations are made with standard DBS mode of operation. Apart from the DBS mode there are two other modes of operations are available, they are, spaced antenna drift (SAD) mode and Interferometry/radar imaging mode.

The basic mechanism by which the radar receives the transmitted signal is through scattering by the refractive index perturbations called Bragg scattering24. The atmospheric refractive index is mainly determined by pressure, temperature, and humidity. Refractive index perturbations occur when atmospheric turbulence and waves modify these parameters, and Bragg scattering occurs when turbulent eddies, mainly of the order of half the radar wavelength, brings about refractive index perturbations25.

High resolution atmospheric boundary layer observation

Figure 7 shows the atmospheric boundary layer (ABL) experiment conducted on 06 February 2023, where height-time variation of SNR plot indicates the development of ABL. The CUST Radar system provides the high-resolution data from the lower heights, for which a 2 µs pulse width (Complementary code) with 0.5 µs baud length is used which provides 75 m range resolution. The high values of SNR up to 2 km were observed from 06:00–09:00 IST, which shows the evolution of boundary layer in the winter mornings. The ABL has disappeared between 09:00–11:00 IST. Soon after that, a gradual evolution of the ABL reaching up to 3 km and associated with high value of SNR (up to ~ 40 dB) is noticed from 11:00–16:00 IST. Along with the ABL, there are several thin layered structures observed at different heights especially above 3 km. Among those structured layers, one such thin layer is observed from 06:00–18:00 IST around 3 km height with an enhanced SNR value of more than 25 dB.

Figure 7
figure 7

The Height—Time Variation SNR plot of atmosphere boundary layer observation with 75 m range resolution.

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Stratosphere-troposphere mode of observation

The CU ST Radar is operated in lower atmosphere mode which covers the troposphere-lower stratosphere with an altitude range of 1.65–24 km. Figure 8 shows the height profiles of Doppler Power spectra observed on January 05, 2023 in the East, West, Zenith, North, and South directions. For this observation, the off-zenith beam is tilted at 10° from the Zenith direction. The operational parameters used for this observation, 8 µs pulse width (Complementary code), 250 µs Inter Pulse Period (IPP), 1 µs baud length provides 150 m range resolution, 128 coherent Integrations, spectral computation with 512 FFT points, and 4 spectral averaging was done. The height profiles are observed during clear-air condition and from the Figure 8, it is clear that the height Doppler profiles of the opposite beam directions are mirror image to each other which indicates the expected system functionality.

Figure 8
figure 8

Height—Doppler power spectra profiles of ST Radar with DBS mode of operation for the 5 beam directions on January 05, 2023 (a) East—10° (b) West—10o (c) Zenith—0° (d) North—10° (e) South—10o.

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Preliminary wind comparison

The CUSTR system measures the horizontal wind components (zonal and Meridional winds) and the wind products (wind speed and wind direction) are compared with the GPS radiosonde balloon measurements. Figure 9 shows the primary wind comparison of CUST Radar and India Meteorological Department, Kolkata (IMD-K) measurements on December 21, 2022 at 00 Z/05:30 IST. The CU ST Radar measures the Zonal and Meridional winds in the altitude range of 1.65–24 km with 150 m range resolution but the radiosonde balloon measurements are available with coarse of different range resolutions in the altitude range of 1.65–24 km. A reasonably good correspondence is observed in the zonal, meridional, wind speed and wind direction in low and mid altitude ranges. Whereas, at higher altitudes (above 20 km) there is a significant difference especially in zonal and wind speed. These differences may be due to the different measurement techniques adopted by the two instruments, different range resolutions used for the wind measurements. Overall, the figure shows good agreement between the two measurements.

Figure 9
figure 9

Comparison of Radar and Radiosonde derived (a) zonal wind (b) meridional wind (c) wind speed and (d) wind direction.

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Mesospheric echo observation

The ST Radar system is also capable of observing the Mesosphere echoes because of its peak transmitter power and high receiver sensitivity. Figure 10 shows the Height–Time variation of SNR plot for the mesosphere echoes observed on September 09, 2023 in the zenith beam direction. These echoes are observed by using a pulse width of 64 µs (Complementary code) with 8 µs baud length, inter pulse period of 1500 µs, coherent integrations of 16, incoherent integrations of 4, and FFT of 256 points provides the Doppler spectrum measurements of Mesosphere echoes with a height resolution of 1.2 km. From the figure the echoes are observed in the form of thick discrete patches with maximum SNR in the height range of 70–80 km especially around 75 km. These preliminary results of mesosphere echoes are unique in the region of Kolkata with this ST radar system. The layered properties of mesospheric echoes are important for studying the dynamics of mesosphere region in terms of winds and turbulence1,26,27. Further detailed investigation is required to understand the mesospheric echo phenomenon using this ST Radar system.

Figure 10
figure 10

Height—Time SNR plot of Mesosphere echoes observed on September 09, 2023 in the Zenith beam direction, showing the thick discrete patch structures.

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Ionosphere E—region & F—region observation

Using the CUST Radar system night time Ionosphere E-region and F-region backscattered echoes are also observed. To study the Ionosphere irregularities in the E-region and F-region, the radar beam has tilted towards the Magnetic North direction to satisfy the magnetic field perpendicularity condition. For the observation, a 32 µs pulse width along with 32 µs baud length which gives 4.8 km vertical range resolution and an IPP of 5000 µs is used. The height coverage is kept from 86–750 km to cover the Ionosphere E—region as well as F—region. Figure 11a and b shows the height-time variation of SNR plot and height-time radial velocity plot observed on September 26, 2022. During this observation the antenna beam is tilted 40° towards the North direction. The SNR plot shows the strong E-region and F-region backscattered echoes from 18:00–23:30 IST with a SNR range of − 5 dB to +10 dB. The radial velocity plot shows the velocity components from − 20 to 40 m/s during the observation time28.

Figure 11
figure 11

Ionosphere E & F—region irregularities observed on September 26, 2022 18:00 IST—24:00 IST (a) Height—Time SNR plot (b) Height—Time radial velocity plot.

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Conclusion

The lower VHF 53 MHz ST Radar is a new system installed at University of Calcutta and in the entire eastern and northeastern part of India. It has been designed and operational to carry out the new scientific experiments. The ST Radar system is capable to study the dynamics of the entire atmosphere which is demonstrated from boundary layer to the Ionosphere region. The CUST Radar system provides full electronic beam steering agility and supports 25-channel digital receiver system in an India University at a VHF range for configuring various scientific experiments especially in the space domain interferometry. The atmospheric winds are measured in the Troposphere and lower Stratosphere region and initial wind validation has been carried out. The wind comparison between the radar and IMD-K measurements shows reasonably good correspondence. The sensitivity of the CUST radar system allows to detect the weak signals in the middle and upper atmospheric especially mesosphere echoes and ionosphere irregularities in E and F regions. The multi-channel receiver capabilities of the ST radar system will be tested in spaced antenna and interferometry modes in the future scope of work.