## 2 GFZ-Wireless Sensing Unit (GFZ-WSU): Hardware And Communications
### Hardware
\begin{table}
\begin{tabular}{l l} \hline
**ADC Board (GFZ)** & \\ \hline Number of channels & Four (eight Ch. Opt.) \\ AD convertor resolution/effective & 24 Bit, effective 19 Bit (20 Bit in HR Mode) \\ resolution & \\ Input voltage range & +/– 2.5 V \\ Input impedance & 10 k\(\Omega\) \\ Bit weight & 0.3 \(\mu\)V (30 nV with preamp of gain 10) \\ Sample rate & Sample Rate:100 sps (200 \& 400 sps not yet by software supported) \\ Signal bandwidth (-3 dB) & 50 Hz/fs/2 \\ Stop bandwidth attenuation & \(>\)100 dB \\ \hline \end{tabular}
\end{table} 
Table 3: Technical specifications of the various components that make up the ADC board as currently used in the Wireless Sensing Units. The ALIX board ALIX.3D2 [18] has the three roles of analysis, communication, and storage of data (Figure 2b, Table 2). It is made up of an embedded PC (AMD LX800 500 MHz CPU, 256 MB RAM) that uses a Compact Flash card (currently 2 GBytes, but easily increased) as a hard disk. Importantly, the ALIX board is equipped with two positions for WLAN Mini PCI cards (_i.e._, Routerboard R52 wireless 802.11a/b/g, 2.4 and 5 GHz combo cards that use the Atheros AR5414 chipset, [19]), a power supply plug, a serial port USB and 100 MBit/s Ethernet. Power consumption is from 3 to 5 W at 12 V DC. Figure 2d illustrates schematically how the different components (e.g., WRAP, ADC, _etc._) of a GFZ-WSU are organized. The GFZ-WSU is designed to accommodate different kinds of sensors, since the ADC board has four or eight channels and is designed to host contemporarily different modules, thus, leading to the use of instruments considered to be suitable for monitoring different parameters. In particular, for seismic noise measurements, a preamplifier board is added (Table 1), and standard 3D SM-6/B 4.5 Hz external geophones, as well as any other passive sensor, can be connected to the instrument. Furthermore, accelerometric sensors based on MEMS (Micro Electro Mechanical Systems), originally designed to serve as controllers for air bag safety units, but which have also been successfully incorporated into various seismic networks [22], as well as for field acquisition by the exploration sector [23] can be incorporated into the GFZ-WSU and arranged to provide three component data. Actually, the MEMS units tested at the moment have a measurement range of +/- 1.7 g, with a bandwidth of 25 Hz and a noise-level of 0.2 mg [14]. [PERSON] _et al_. [24] successfully exploited a version of the GFZ-WSU instrumented with MEMS units for the monitoring of some strategic infrastructures during the Earthquake Task Force mission following the recent magnitude (Mw) 6.3 Central Italy Earthquake of the 6 th April 2009. However, it is worth noting that the resolution of the MEMS sensors so-far incorporated into the GFZ-WSU is not sufficient for seismic noise measurements and analysis. \begin{table}
\begin{tabular}{l l} \hline Analogue anti-alias filter & \\ Timing & Onboard GPS Receiver for Timing \& Position Data \\ Timing accuracy & Time Base better 2.5 ppm \\ Digital output & USB (1 \(\times\) virtual com-port, 115 kBaud data/GPS) \\ Temperature range & \(-\)20 \({}^{\circ}\)to +70 \({}^{\circ}\)C \\ Power supply & 9 to 18 V (on board DC/DC converter) or +5 V (USB) \\ Power consumption & 540 mW (low power mode) / 720 mW (high resolution mode) \\ \end{tabular}
\end{table} 
Table 3: _Cont._At the present, preliminary tests (not shown here) have been performed with temperature sensors, pressure sensors, and microphones (for low frequency audio signals) with analog output signals up to \(\pm\)2.5 V directly to one of the input channels in the ADC board not connected to a ground-motion recording instrument. For special monitoring applications (e.g., in the case of landslides) where access to main power is limited, the necessary energy can be provided by solar panels.