1. Data Used and Methodology Conclusions Radio Direction Finding System Using Software-Defined Radio Chatuporn Duangthong, Jirapoom Budtho, and Assoc. Prof. Dr. Pornchai Supnithi Department of Telecommunications Engineering, Faculty of Engineering King Mongkut’s Institute of Technology Ladkrabang References 1.The radio signal from transmitter is received by multiple antenna. 2.The clock signal drives antenna switch that spins the antennas by sequentially selecting one at a time in the order A, B, C, D, etc., at 250 revolutions per second. 3.The radio frequency signal is modulated at this state due to the Pseudo Doppler effect provides PSK modulation output signal and it is demodulated by FM Demodulator in SDR. 4.At the same time, the clock signal is converted to sine wave. There are two sine waves of the same frequency but with different phases for comparing and displaying the direction in compass on MATLAB Graphic User Interface (GUI). [1]S. E. Lipsky, Microwave Passive Direction Finding, New York: John Wiley & Sons, Inc., 1987. [2]P. Gething, Radio Direction-Finding and the Resolution of Multicomponent Wavefields, London: Peter Peregrinus Ltd., 1978. [3]P. Gething, Radio Direction Finding and Superresolution, London: Peter Peregrinus Ltd., 1991. [4]R. Boylestad, Electronic Devices and Circuit Theory, New Jersey: Prentice Hall., 2009. [5]D. N. Aloi and M. S. Sharawi, “Modeling and Validation of a 915MHz Single Channel Pseudo Doppler Direction Finding System for Vehicle Applications,” Electrical and Computor, Engineering Department, Okland University Rochester, Michigan 48309, USA., 2009. [6]Joseph D. Moell and Thomas N. Curlee “Transmitter Hunting Radio Direction Finding Samplified,” New York: McGraw-Hill, Inc., 1987.  In this project, we develop the radio direction finding (RDF) based on software-defined radio based on pseudo Doppler technique.  The experimental results show that the accuracy of the direction estimation is about 16%. Results and Discussions Abstract (may be neglected) xxP_IRI2015 1. Create antenna switcher to receive the radio signal. 2. Interface the antenna switch with SDR. 3. Create SDR programming with MATLAB and SIMULINK. 4. Calculate the angle of arrival from the transmitter. Objectives This research develops the radio direction finding system using software-defined radio (SDR), allowing users to select the frequency in order to search for the direction of the transmitter. This system has 4 antennas placed at 90 degrees apart [5]. The radio signal from the antenna switcher is similar to the received signal from a single antenna rotated in a circle, generating the periodic Pseudo Doppler Effect. The direction in terms of azimuth angle can be calculated by measuring the phase difference between the reference signal and the demodulated signal from antenna switcher using MATLAB. Pseudo Doppler is caused by antenna switch. When antenna is selected sequentially, each antenna receives signal with the difference phases [5]. This state provides function of switching. The discrete phases are modulated on the carrier from transmitter. The output of antenna switch is basically a PSK signal. Figure 1. (A) Multiple Antennas (B) Output signal (A)(B) Figure 2. Spectrum of Simulated and Experimental signal from antenna switch Figure 3. Output from FM demodulator Figure 4. Phase comparison Figure 5. (A)Experimental setup (B)Software implementation (C)Averaged output GUI  Frequency deviation Equation  Pseudo Doppler Equation Phase change Transmitter Receiver  Left picture shows spectrum from simulated signal. Right picture shows experimental result. Both have same Doppler tone on the first harmonic.  FM demodulator output in Figure 3 has similar characteristics to theoretical signal in Figure 1(B).  The phase difference between reference signal and demodulated signal is compared in Figure 4.  The results of phase comparison from experiment in Figure 5 are stored in the array then the average is computed. The averaged computed phase is about 30 degrees. (A) (B) (C) To search for the direction of radio signal or transmitter, there are several systems which can be used to process the arrival of signal. In the past, analog system was used [2], but the improvement and flexibility in digital systems have been commonly recognized. We can use software-defined radio (SDR) and personal computer instead of the analog system. Introduction