## 1 Introduction  
Ionospheric research burgeoned in the 1920s when [PERSON] and [PERSON] of England confirmed the existence of the ionosphere. Since the end of World War II, a number of high-frequency (HF) radars and ionospheric research programs have been being pursued for measuring important characteristics and applications, such as long-range target monitoring, airborne early warning, and anti-stealth detection [1]. A number of HF radars and their achievements have been reported around the world, such as the new Digisonde-4D of the most famous digisonde family developed by the University of Massachusetts Lowell Center for Atmosphere Research (ULCAR) [2], the Advanced Ionospheric Sounder developed at the National Institute of Geophysics and Volcanology (AIS-INGV) in Italy [3], the DAMSON-a low power Doppler and multipath sounding network in the UK [4], the Canadian Advanced Digital Ionosonde (CADI) digital ionosondes [5], the Jindalee over-the-horizon radar (OTHR) of Australia [6], the TIGER radar of the Super Dual Auroral Radar Network (SuperDARN) network [7], the French OTH radar NOSTRADAMUS [8], _etc._  
Cooperation between the Electronic Information School Wuhan University (Wuhan, China) and University of Paris-Sud (Paris, France) in the fields of ionosphere physics and transmission characteristics of high-frequency channels across Eurasia has been carried out since 1985. In reference to the experience from the STUIO5 system [9], Wuhan University began to develop ground-based ionospheric sounding systems for ionospheric research and HF radio propagation in 2001. The prototype of WIOBSS (Wuhan Ionospheric Oblique Backscattering Sounding System) was developed successfully in 2003. The WIOBSS received the first vertical incident echoes in May 2004 and recorded the first sweep frequency ionogram for backscatter in September 2004 [10]. Initially, the WIOBSS was a monostatic digital ionosonde based on the VXI-Bus (VME eXtensions for Instrumentation). A remote digital receiver with Global Positioning System (GPS) for frequency and clock synchronization was developed to work together with the WIOBSS in 2008 [11]. The receivers can be used for ionospheric bistatic backscatter sounding, oblique incident sounding, as well as sky-wave sea-state sounding. Many kinds of waveforms have been tested on the WIOBSS and the ionograms obtained by different waveforms have been compared and reported [12]. In 2010, the Universal Serial Bus (USB) and high-performance field programmable gate arrays (FPGA) were applied in the new system. The previous radio systems in the WIOBSS family were all single-channel systems, which were applied to observe the ionospheric E-layer, F-layer, and E-region field-aligned irregularities [13,14]. They have the ability to measure the echo amplitude, range, and Doppler shift [15,16], and are used to investigate electron density variations and travelling ionospheric disturbances [17,18].  
To monitor the large-area ionosphere, a new generation of the WIOBSS that incorporate a log periodic antenna array (WIOBSS-AA) is presented in this paper. Firstly, we introduce the hardware frames of the WIOBSS-AA, including the data/control routing solutions, the radio frequency (RF) analog front end, the digital transceiver and the log-periodic antenna array, and so on. Then we outline the new features of the data processing and the digital beam-forming and, finally, make a conclusion based on the initial results recorded by WIOBSS-AA.