# Low-Cost Sensor Based on SDR Platforms for TETRA Signals Monitoring
## 1 Introduction
This depends on the characteristics of the monitored signals (bandwidth, frequency range, duration of symbols used for data packaging, transmission technique to access the channel resources, spatial distribution of data using Multiple Input-Multiple Output (MIMO) and beamforming techniques, etc.). Recently, specialists have reported several electromagnetic spectrum monitoring stations based on SDR technology [11; 12; 13; 14; 15]. The main characteristics of an SDR platform that will influence the performance of a monitoring station are as follows: frequency range, sampling frequency and resolution of the digital-analog converter [16; 17; 18]. The applications of SDR platforms have expanded as a result of the benefits offered by software processing, a series of results that have been recently reported in many areas [19; 20; 21; 22]. The key factors that define the figure of merit of a sensor for electromagnetic spectrum monitoring are [7]: low cost; small form-factor; wide-band; distributed; real time; commercial off-the-shelf (COTS) hardware; flexibility. The SDR receiver proposed in [23] has a recording time of 4 s, and the power spectrum is computed via Fast Fourier Transform (FFT), at a resolution of 8192 points. For a sampling frequency of 100 MHz, a frequency resolution of 100 MHz/8192 = 12.207 kHz resulted. Other features are the following: frequency range of (9 kHz-3500 MHz); the noise factor of the receiver is 6 dB; the resolution of the digital analog converter is 16 bits; search speed is 1 GHz/s and real-time acquisition band is at least 16 MHz [23]. A sensor whose structure is based on low-cost commercial off-the-shelf (COTS) hardware components is proposed in [7]. The radio interface is the RTL-SDR USB platform and allows a typical sampling frequency of 2.4 MS/s. The cost of such a sensor is less than USD 100. Comparative evaluations of a Universal Software Radio Peripheral (USRP) SDR platform have shown that the sensitivity is approximately 10 dB lower than the USRP's. The low-cost monitoring system is able to sense, analyze and compress a lossless 2.4 MHz band at a frequency resolution of less than 10 kHz [7]. Another electromagnetic field sensor that also has a radio interface of the RTL-SDR USB platform is presented in [8]. The sensor is configured in two acquisition modes. The first mode allows determination of the power spectral density (PSD) by implementing processing operations at the sensor level (sampling windowing, FFT, averaging, data compression). The second acquisition mode does not use processing at the sensor level, but directly records the In-phase and Quadrature (IQ) components of the raw signal samples. The RTL-SDR radio interface is controlled by a Raspberry Pi device to reduce the cost. Depending on the compression factors and data averaging, transfer rates of the order of 50-100 Kb/s are obtained in the PSD mode, while much higher data transfer rates are obtained, up to the 50 Mb/s, in the IQ acquisition mode [8]. The present paper aims to design and implement an electromagnetic spectrum monitoring station with real-time analysis capabilities to track signals emitted in the TETRA communications standard. The TETRA standard was designed for emergency situations and is used primarily by government agencies [24; 25]. A sensor for monitoring TETRA signals was initially tested by the authors of this paper and the preliminary results are reported in [26]. This sensor has been improved and thus new TETRA signal monitoring capabilities have been developed. The main contributions of the current work are:  
1. _Integration of a Geo-Location module and new support applications have been added_ A low-cost Global Navigation Satellite System (GNSS) module has been integrated into the sensor. A software application has been added to calculate the power of each TETRA channel. The readings from the GNSS module were synchronized with the measurement of the power of the TETRA channels. 2.