## 2 GFZ-Wireless Sensing Unit (GFZ-WSU): Hardware And Communications
### Hardware  
The GFZ-WSU consists of three main hardware parts: the digitizer board, the Wireless Router Applications Platform (WRAP, _i.e._, the PC Engines ALIX system board), and the sensors. All components are bought off-the-shelf, with the exception of the digitizer printed circuit board, which has been developed within GFZ Potsdam. This reduces the cost of the GFZ-WSU, leading to them being much less expensive than standard seismometers (about 700 Euro per unit). Figure 2 provides a view and schematic overview of the architecture of a GFZ-WSU, with some technical details listed in Tables 1, 2, and 3. All boards are installed in waterproof outdoor metal cases of reduced dimension and weight (Figure 2a, Table 1). Omni-directional dual-band antennas with a gain of 5 dB are mounted with opposite vertical polarization. The amount of power required by a WSU when all operational activities are fulfilled (recording and real-time communication of data) has been experimentally measured to be about 4.5 W.  
The Analog-to-Digital Converter (ADC) board (Figure 2c, Table 3) was designed as a low-cost solution, but still considers special seismic requirements as there are high resolution, anti-alias filtering, exact time marks and good time stability included in its design. The board is equipped with a 4-Channel ADC (ADS1274) or 8-Channel ADC (ADS1278). These ADCs have a resolution of 24 bits (effectively 19 bits in low-power mode, and 20 bits in high-resolution mode), with the sampling rate selectable from 100 to 400 samples per second (sps), although at present 100 sps is being used. The cut-off frequency of the digital anti-alias filter is close half of the used sampling rate. A GPS unit (Trimble Lassen iQ) provides time and geographical coordinates (Figure 2c). Every second it sends a PPS (pulse per second) to the ATMEGA-2561 micro-controller. This PPS is used to mark the first sample of a second. The GPS time and the position are also transferred to the digitizer board _via_ TAIP strings. The complete power consumption of the board is 540 mW and 720 mW for the low-power and high-resolution mode, respectively, including GPS module and antenna. Figure 2: The prototype GFZ Wireless Sensing Unit (GFZ-WSU). (a) The complete unit. (b) The WRAP board. (c) The ADC board. (d) A schematic overview of the architecture of the GFZ-WSU. Technical details of the various components are listed in Tables 1, 2, and 3. \begin{table}
\begin{tabular}{l l} \hline
**Technical Data:** & \\ \hline Size & 200 \(\times\) 150 \(\times\) 80 mm \({}^{3}\) \\ Weight & 1.1 kg \\ Power Consumption & \(\sim\)5 W@12 V \\ 2 \(\times\) WLAN & 2.4 GHz / 5.7 GHz /LAN (opt.) \\ Power Supply & 10...15V DC \\ ADC & 4 \(\times\) ADC (24 Bit) \\ Sensors & Ext. Geophones / internal 3 Axis MEMS-Accelerometer \\ GPS Receiver \& external GPS Antenna Input \\ \hline \end{tabular}
\end{table} 
Table 1: Technical specifications of the low-cost Wireless Sensing Units. \begin{table}
\begin{tabular}{l l} \hline
**ALIX board** & \\ \hline CPU & AMD L\(\times\)800 (500 MHz) \\ DRAM (dynamic random access memory) & 256 MB SDRAM ( synchronous dynamic random access memory) \\ Operating system & OpenWRT \\ Storage & Compact Flash card, currently 2 GB \\ Power consumption & 3 to 5 W at 12 V DC (excluding miniPCI cards) \\ Safety features & Watchdog timer built into the CPU, LM77 thermal monitors \\ User interface & Three front leds, console I/O redirected to serial port \\ Possible expansions & LPC bus for adding more serial ports, ISA style I/O, GPIO \\  & and I\({}^{2}\)C bus \\ Connectivity & One Ethernet channel (National DP83816), two miniPCI \\  & slots, one serial port \\ BIOS & tinyBIOS version 1.11 \\  & LAN (10/100) \\  & 2 \(\times\) USB \\ \hline \end{tabular}
\end{table} 
Table 2: Technical specifications of the various components that make up the ALIX board as currently used in the Wireless Sensing Units.