Corresponding author: Z. Chen, Ionosphere Laboratory, School of Electronic Information, Wuhan University, Luojiashan, Wuhan 430072, China. (firstname.lastname@example.org)
 The basic configuration of the Wuhan MST (mesosphere-stratosphere-troposphere) radar, which was designed and constructed by the School of Electronic Information, Wuhan University, is preliminarily described in this paper. The Wuhan MST radar operates at very high frequency (VHF) band (53.8 MHz) by observing the real-time characteristics of turbulence and the wind field vector in the height range of 3.5–90 km (not including 25–60 km) with high temporal and height resolutions. This all–solid-state, all-coherent pulse Doppler radar is China's first independent development of an MST radar focusing on atmospheric observation. The subsystems of the Wuhan MST radar include an antenna system, a feeder line system, all–solid-state radar transmitters, digital receivers, a beam control system, a signal processing system, a data processing system, a product generation system, and a user terminal. Advanced radar technologies are used, including highly reliable all–solid-state transmitters, low-noise large dynamic range digital receivers, an active phased array, high-speed digital signal processing, and real-time graphic terminals. This paper describes the design and implementation of the radar. Preliminary online wind measurements and results of the comparison to simultaneous observations by a GPS rawinsonde are presented as well.
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 Since the first very high frequency (VHF) radar in Jicamarca successfully observed atmospheric winds and turbulence in the 1970s [Woodman and Guillen, 1974], VHF radars have been established successively all over the world to facilitate and boost worldwide atmospheric research. Such VHF radars have developed into the most significant facilities for atmospheric remote sensing that are capable of continuously operating under all weather conditions with fine spatial and temporal resolutions. Currently, a large number of VHF radars for atmospheric research are under operation, situated from the polar region to the equatorial region [Röttger et al., 1990; Fukao, 2007; Hocking, 1997, 2011].
 Among the atmospheric VHF radars, the mesosphere-stratosphere-troposphere (MST) radars, which require an average power aperture product larger than 108 W m2, remain unconventional due to the vast coverage area and high investment cost they entail. In Asia, the middle and upper atmosphere (MU) radar in Japan [Fukao et al., 1980, 1990] and the Gadanki radar in India [Rao et al., 1995; Jain et al., 1995] are the most famous. MU radars are one of the most influential MST radars and have provided numerous important results and findings to further our understanding of the atmosphere and ionosphere. With fine temporal and height resolutions, MST radars around the world provide an outstanding opportunity to extensively and intensively investigate various atmospheric phenomena such as wind measurements [Gage and Vanzandt, 1981; Balsley, 1983], tropopause detection [Reid and Gage, 1996; Yamamoto et al., 2003; Das et al., 2008; Mehta et al., 2008, 2011], gravity waves [Fritts and Alexander, 2003], atmospheric aspect sensitivity [Jain et al., 1997; Tsuda et al., 1986], and ionospheric irregularities [Yamamoto et al., 1991].
 Inspired by the great importance and the achieved contribution of MST radars to atmospheric and ionospheric research, the Ionosphere Laboratory of Wuhan University started the construction of the Wuhan Atmosphere Radio Exploration (WARE) MST radar in January 2008. After more than 3 years of efforts, in March 2011, we implemented the successful trial operation of the WARE radar, which is the first MST radar in the mainland of China.
 The WARE radar is located in Chongyang, Hubei Province, China (114°8′8″E, 29°31′58″N), with a geomagnetic latitude of ~ 23°. The altitude of the radar site is 62 m above sea level. The inclination and declination angles are approximately 44.7° and −3.7°, respectively, at the altitude of 110 km above the radar site. The WARE radar operates as one significant ingredient of the Meridian Space Weather Monitoring Project of China [Wang, 2010] that consists of diverse ground-based remote sensing facilities aligned near the 120°E longitude line for space environment monitoring and forecasting. In this report, the outline of the WARE radar system is first summarized. Then we focus on the system design of the WARE radar, especially on the design and implementation of the antenna, Transmit/Receive (T/R) module module, transmitter, receiver, signal processing, and online data processing. Preliminary observational results are also presented. To validate the uncertainties from system biases and scattering characteristics, wind profiles obtained by the WARE radar are compared with those obtained simultaneously by a GPS rawinsonde.
2 WARE Radar Description
 The Wuhan MST radar is a 53.8 MHz pulse-modulated monostatic Doppler radar with an active phased array system, which has capabilities of radio distance measurement and Doppler velocity measurement. The whole signal processing works in the frequency domain. The frequency spectrum of the echo pulse sequences is analyzed so that the problem of clutter suppression is solved completely. Considering the second return ambiguity, interference, and distant sidelobe of atmospheric turbulence due to pulse compression, a complementary binary pseudorandom sequence phase encoding technique is used in the Wuhan MST radar.
 The operation process of the Wuhan MST radar is as follows. High-frequency (HF) pulse signals are generated by the main controlling computer. These signals are amplified by the phase shifters in the T/R modules, radiated by the antenna feeder, and then synthesized in space in order to concentrate the energy in a certain direction. The radio signals are scattered by the atmospheric turbulence, parts of which are detected by the radar receiving antennas. The received signals are filtered, amplified, phase shifted, and synthesized in the T/R modules before they are transmitted to the digital intermediate-frequency (IF) receivers. At the digital IF receivers, the signals are filtered, amplified, cross phase detected, and finally computed by the signal processor for fast Fourier transform (FFT). The turbulent echo power spectrum is obtained through the above approaches, from which the atmospheric wind fields are calculated and obtained. The technical parameters and performance parameters are tabulated in Tables 1 and 2, respectively.
Table 1. WARE Radar Technical Parameters
53.8 MHz (λ = 5.576 m)
All–solid-state, fully distributed
Low mode 10%
Medium mode 20%
High mode 20%
24 × 24, active phased array
Yagi-Uda aerial, three units, horizontal polarization
Normal beam width
≤4.5° half-power width, pencil beam
Voltage standing wave ratio
Five beams: vertical, off-zenith 0°–20° by 1°
Antenna operation mode
Doppler beam swinging (DBS)
Table 2. WARE Radar Performance Parameters
3.5–10 km (low mode)
11–25 km (medium mode)
60–90 km (high mode)
150 m (low mode)
600 m (medium mode)
1200 m (high mode)
Maximum radial velocity
Radial velocity resolution
Spatial scope of the wind direction
Power aperture product
≥2.3 × 108 W m2
 The overall block diagram of the Wuhan MST radar system is presented in Figure 1.
3 System Configuration
3.1 Antenna and Feeding
 The radar antenna system is composed of 576 Yagi-Uda units with horizontal linear polarization. The overall power of radiation is 0.3 kW × 576 ≈ 170 kW with uniform weighting feeding. To ensure the low sidelobe feature in the scanning direction, one-dimensional weighting is used for receiving. The radar beam can be steered into five directions independently, changing continuously from vertical to a 20° off-zenith angle with a step of 1°. Five directions in total are controlled by the beam control subsystem. The width of the main lobe is approximately 4.5° with a gain of 34.4 dB.
 Each antenna unit consists of three subelements: director, driver, and reflector, which are galvanized iron pipes with spray antirust processing. The antenna main rod length is 2.4 m, and the outside diameter of all subelements is fixed at 50 mm. To ensure accuracy, the height accumulated installation error of each antenna is less than 10 mm. The structure of the antenna is illustrated in Figure 2.
 The array configuration is designed to be 100 m × 100 m in square. The antennas are distributed uniformly in 24 rows and 24 columns. The array spacing is 0.7λ (≈3.9 m) in the row and column directions. The antenna impedance is 50 Ω, matching the impedance of the feed line. The voltage standing wave ratio (VSWR) is less than 1.3 in all designed beam directions. Figure 3 shows a photograph of the actual antenna array, demonstrating the overall construction.
3.2 T/R Module and Preamplifier
 The 600 T/R modules are installed in a 152-component kit located in the antenna array and connected to the fully distributed array. These T/R modules can be categorized into two major types in terms of the quantity of subelements: (1) The large T/R module is composed of a circulator, a phase shifter, a power amplifier, a low-noise amplifier, a directional coupler, an electronic switch, and a beam control unit. There are 24 large T/R modules. They are the core component of the active phased array with the purpose of power amplification and echo signal amplification. (2) The small T/R module only includes a module switch, a power amplifier, and a circulator with a total number of 576.
 A 5 bit digital phase shifter is utilized to improve the accuracy of phase shifting and adjustability of the beam direction. A beam control unit is employed to monitor the status of the phase shifter and transmitted power, which enhance the reliability and maintainability of the T/R module. Forced air cooling is applied in order to ensure long-term, stable, and reliable operation since the maximum duty cycle is up to 20%. The outer shell of the T/R module is made of 5A05 or 6063 aluminum alloy, which is treated by conductive oxidation.
 The principal function of the preamplifier system is to amplify the radio frequency (RF) excited signals (about 10 mW) to 300 W, which is then radiated by 576 antennas after power division. The preamplifier system of the WARE radar consists of a prior-stage RF amplifier, a protection plate, a PIN switch modulator, a radio frequency fault detection circuit, and a power emission device.
 The principal diagram of the T/R module and amplifier is illustrated in Figure 4. Figure 5 shows a real photograph of the T/R module box.
 The backscatter echoes received by the MST radar are very weak, so most signals are lower than the noise level. Consequently, the requirements for the receivers are very high. A digital IF (intermediate frequency) technique is utilized in the Wuhan MST radar. By taking advantage of direct digital sampling and phase detection in the digital domain, phase orthogonality and amplitude consistency are guaranteed, thus greatly improving the observation accuracy and making the equipment more stable and reliable. The receiver consists of a frequency source, a receiving channel, low-noise amplification, a test signal source, error location, and a receiver port. Radio frequency which is delivered from an antenna synthesized network is mixed in the receiver channel and then processed in a digital intermediate-frequency system through digital controlling attenuation. Finally, I/Q output data are transmitted to the signal processor.
 The spectrum data of the wind field obtained from the radar signal processing are transmitted to radar data processing by a peripheral component interconnect (PCI) bus. The main function of data processing includes the following: (1) system control, providing a friendly interface and reliable controlling; (2) preference of working parameters and observation mode; (3) spectrum data processing, mode recognition, moments calculation, and quality control; (4) real-time display of the processing results; (5) radar working state monitoring; and (6) transmission and communication.
 The Wuhan MST radar adopts direct digital sampling. Phase detection is processed in the digital domain, which ensures phase orthogonality and amplitude uniformity. Nonuniformity of the amplitude and phase of I/Q signals could damage the detection capability of the radar and induce echo energy loss. Besides, a mirror spectrum as well as the main spectrum emerges because of nonuniformity, which reflects on a false target. For the IF sampling process, the input signal is one channel. An orthogonal two-channel input signal is processed in the digital domain, which enhances the signal detection capability and improves output zero drift.
 Test signals, which are produced by frequency synthesizers, are sent to receivers by directional couplers and an HF selective switch. The choice of test source is controlled by the control instruction from a signal processing terminal. A majority of parameters of receivers and signal processing terminals could be measured by test signals. When a system fault happens, the fault points could be separated by these signals.
 The principal diagram of the receiver system is illustrated in Figure 6. The main specifications of the receivers are presented in Table 3.
Table 3. Main Specifications of Receivers
≥ 65 dB
Frequency synthesizer single-sideband phase noise
≤ −80 dBc/Hz/10 Hz
Excitation source output pulse peak power
≥ 11.35 dBm
Analog to digital digits
I/Q output amplitude imbalance
≤ 0.05 dB
I/Q output phase imbalance
3.4 Signal Processing
 The atmospheric echoes are extremely weak. In order to get a better result, the processor is capable of capturing the weak echoes and is of high estimated accuracy. The performance of the radar is based on the capability of the signal processor, which is one of the key parts of the MST radar. A special digital signal processing (DSP) chip is adopted to the Wuhan MST radar where coded pulse compression, coherent averaging, FFT, clutter suppression, and spectrum averaging are fulfilled. The echo power spectrum which is the output of the signal processor is the source of subsequent data processing; therefore, the accuracy of signal processing directly influences the validity and reliability of the secondary product. The schematic diagram for the signal processing of the WARE radar is shown in Figure 7.
4 Online Data Processing and Wind Measurements
 Power spectrum data, which are obtained by the signal processing system, are transferred to the data processing system of the WARE radar by a peripheral component interconnect (PCI) bus. The principal functions of an MST radar consist of (1) providing a friendly user interface, (2) providing default operation parameters and observation choices, (3) real-time displaying the results of the wind field, (4) monitoring and displaying the radar status, and (5) data transmission and network communication. The hardware of the data processing system of the WARE radar includes an industrial personal computer and network communication equipment. The radar experimental specifications used for the wind estimation are presented in Table 4. Figure 8 shows the wind vector diagram produced by the data processing system. The detection time is about 9:00 (LT) on 30 August 2011. The detection range is from 2.84 km to 35 km.
Table 4. Radar Experimental Specifications Used for the Wind Estimation
1 µs (low mode)
32 µs (medium mode)
128 µs (high mode)
160 µs (low mode)
320 µs (medium mode)
1280 µs (high mode)
Number of coherent integration
128 (low mode)
64 (medium mode)
8 (high mode)
Number of incoherent integration
10 (low mode)
10 (medium mode)
10 (high mode)
Number of FFT points
256 (low mode)
256 (medium mode)
512 (high mode)
5 min (all modes)
 In general, quality control procedures for wind data rely on principles of internal consistency, physical rules, and statistical methods. For WARE radar data processing, data rationality checks and internal consistency checks are incorporated to eliminate erroneous data points. Data rationality mainly checks whether the observed wind conforms to the laws of physics, and internal consistency checks whether the wind data consist of erroneous points and outliers.
 Comparisons of wind field observations between the WARE radar and a GPS rawinsonde were carried out immediately after the WARE radar was established. Figure 9 shows the comparison of the two kinds of observations obtained at four different local times on 10–12 September 2011. The GPS rawinsonde is released just at the radar array. Five beams (vertical, east, north, west, and south in sequence) of the WARE radar are used for present estimation of winds by the Doppler beam swinging (DBS) method. The off-zenith angles for the east, north, west, and south beams are 15°. The launching time and time at which the rawinsonde reached 25 km are (a) 17:24:26 and 18:43:06, 10 September 2011; (b) 16:56:14 and 18:14:16, 11 September 2011; (c) 6:52:56 and 8:10:27, 12 September 2011; and (d) 9:31:49 and 10:48:21, 12 September 2011. For each case, six low-mode (3.5–10 km) wind profiles and eight medium-mode (10–25 km) wind profiles are averaged to compare with the GPS rawinsonde. The root-mean-square error (RMSE) of the meridional and zonal winds for Figure 9 is presented in Table 5 in the revised manuscript. Clearly, the observations of the WARE radar are reasonably consistent with the rawinsonde observations for the altitude profiles of the wind field, indicating that our newly constructed WARE radar works properly and efficiently to provide reliable observations for further studies. The vertical velocity of wind, which is small in magnitude, is one of the basic atmospheric parameters. The WARE radar can now continuously monitor the vertical velocity profiles. Three profiles of vertical wind observed on 22 June 2012 are presented in Figure 10.
Table 5. RMSE of Meridional and Zonal Winds for Figure 9 (Unit: m/s)
 In this paper we have described the principal features of the Wuhan Atmosphere Radio Exploration (WARE) radar, which is the first MST radar installed in China. The WARE radar is one significant part of the Meridian Space Weather Monitoring Project of China, which is a Chinese multistation chain along 120°E longitude to monitor the space environment. The WARE radar is a fully distributed, all–solid-state, and coherent pulse Doppler radar operating at 53.8 MHz with an average power aperture product of 2.3 × 108 W m2. The phased array of the WARE radar consists of 24 × 24 three-element Yagi-Uda antennas evenly distributed over a total area of 10,000 m2. The radar beam can be steered into five directions independently, changing continuously from vertical to a 20° off-zenith angle with a step of 1°. The characteristics of atmospheric turbulence echoes, including the Doppler velocity and spectrum width, are estimated from background noise and clutter.
 The WARE radar is now continuously on operation and provides valuable observation results day by day. Through the outstanding capabilities of the WARE radar for comprehensive atmospheric research, the WARE radar has great potential to yield new findings, especially for regional atmospheric characteristics. The extensive feature of the Wuhan MST radar also points to the feasibility of low thermosphere if desired.
 The authors are indebted to the Meridian Space Weather Monitoring Project of China for their collaboration. We thank Shaodong Zhang for the continuing support. Thanks are also due to Jingfang Wang and Xiaoming Zhou for invaluable discussions and warm-hearted help on many aspects. This work was supported by the National Natural Science Foundation of China (41204111).