## 2 Description of the Multi-Sensor Monitoring System
### SBT80 Sensor Board and TelosB Note  
In our multi-modal sensor system, radar data are supplemented with data obtained using SBT80 sensor motes [8] developed by EasySen. The SBT80 sensor board contains several sensor modalities \begin{table}
\begin{tabular}{c c c c c} \hline
**Parameter** & **Units** & **Minimum** & **Typical** & **Maximum** \\ \hline Operating Frequency & GHz & 24.10 & 24.125 & 24.15 \\ Output Power & mW & & 8.0 & \\ Operating Current & mA & 140 & 175 & 200 \\ Operating Voltage & VDC & & 5 & \\ Incident RF Input Power & dBm & & & +20 \\ Range & m & 1 & 6 & 65 \\ \hline \end{tabular}
\end{table} 
Table 1: Overview of the radar parameters. Figure 2: Range-rate (velocity, m/s) _vs._ range (m) intensity plot of a dismount approximately at 3 m range, as it walks towards the radar transceiver. (**a**) Result with actual dismount movement measured by the radar; (**b**) Simulated version of this scenario, with point targets in the calculations. that can be used for detection and classification purposes. It includes a visual light sensor that we use to trigger other sensors. For our experiments, the visual light sensor is preferable since it detects objects tripping its sensor fairly precisely. In real world situations, these sensors may be replaced by passive infrared sensors. An acoustic sensor provides a raw audio waveform of the surrounding area. A dual axis accelerometer provides vibration information that can be used to infer mechanical characteristics of the target. The SBT80 also includes a dual-axis magnetometer and a temperature sensor, which were not used in the present experiments. An overview of the sensor modalities is given in Table 2. Each SBT80 sensor board sits atop a TelosB mote [5] capable of storing, processing and transmitting sensor data. The TelosB mote is developed by UC Berkeley for low-power research development. Its main components are an 8 MHz 16-bit microcontroller, an IEEE 802.15.4 radio with integrated antenna, a 1 MB serial flash for data logging, and a 12-bit ADC with 8 channels. It supports the TinyOS operating system [9] and it is programmable through an onboard USB interface using the nesC language [10]. The mote has very low power consumption: the microcontroller draws 1.8 mA of current when active and 5.1 \(\upmu\)A when asleep, and the RF transceiver draws 23 mA, 21 \(\upmu\)A, and 1 \(\upmu\)A of current in receive, idle and sleep mode, respectively. Powered by 2 AA batteries, the mote is expected to last weeks with moderate usage. Such energy efficiency, however, comes with severe resource constraints. The microcontroller, MSP430 from Texas Instruments, has only 48 K bytes of program flash memory and 10 K bytes of RAM. The limited memory size presents a significant challenge to our design as the radar and other sensors produce a large amount of data periodically. In order to provide high-quality sensor data without overflowing the RAM, we move the sensor data from the RAM to the larger serial flash after a specified number of runs (20 in our experiments). When the serial flash in a mote is reaching its capacity limit, we transfer the data from the mote to our base station through the mote's wireless radio, which uses the 2.4 to 2.4835 GHz ISM band and has a transmission rate of 250 kbps. The onboard antenna has an outdoor transmission range of 75 to 100 meters and an indoor range of 20 to 30 meters. If the base station is outside the transmission range of the motes, we can run a multi-hop routing protocol on the motes to deliver the data. The mote has a 6-pin and a 10-pin