Localising Noise Transmitters or Jammers in the UHF band with focus on GPS and Communications Systems
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Detecting and localising noise transmitters in the UHF band implicate difficulties because a great amount of reflectionsand diffractions occur. As one can see inthe theory part, predicting a precise propagation path to an arbitrary signal is a next to impossible undertaking.Therefore it is chosen to focus on being able to predict a probable path of the signal, and then be able to present amost likely placement of the noise transmitter.This paper will suggest a possible solution to the problem described above. The solution uses the fact that reflectedsignals is the same signal with slightly different delays and amplitudes due to the difference in propagation path. Thisis measured by using two antennas, the main antenna is pointing in the direction where the signal is strongest while thesecondary antenna beam is used to scan in the horizontal plane to identify signals as reflections.When the system has identified the different directions as either reflections or the shortest path to the receiver, theresult is presented on a screen using an aerial photo or a detailed map. The system then identifies strong reflectorsthatthe signal bounce off from, from this the signal is traced back to a possible location, in combination with the shortestmeasured path. Areas most probable to hold the noise transmitter is then presented on the map.The way the system measures the delays is to correlate the two signals from the antennas. This implies that the noisesignal need to have relatively good autocorrelation properties. It will be discussed whether this is the case, but theone commercial noise transmitter available on the Internet that could be found that actually indicate how the noisesignal is created, claims to be using pseudo random noise. This type of signals have very good autocorrelationproperties, hence the suggested system will be ideal to track this.Due to the fact that prediction of the path of a signal in the UHF band presents many difficulties regardingreflections,the first part of the paper focuses on collecting information regarding propagation in the UHF frequency band. Most ofthe information is collected from literature concerning mobile communications, as this is a big topic when planningmobile networks as the GSM. A lot of the literature discuss the layout and placement of transmitters for the bestpossible coverage. When trying to detect noise transmitters this has to be turned upside down, because one is measuringa signal and trying to answer where its origin is.To prove the concept a simulation is presented that generates a number of reflections, then uses an antenna to measurethem, before it correlates the different directions to find the shortest path. The simulation is then run a number oftimes to generate probabilities of detecting the correct direction $pm 5^circ$ of the real shortest signal direction.Results from this test show that when receiving a signal where the power is divided between all the reflections in theway $E_0/N$ where $N$ is the number of reflections and $E_0$ is 20 dB above the noise floor, and using an antenna withHPBW of 11.18$^circ$ gives correct measurements in 95% of the cases. In the 5% of cases where the system fails todetect the shortest path the combination of the shortest delayed signal with the strongest signal direction do notproduce a sufficiently high enough correlation peak for the detector to regard as a correlation point.In addition to these results there are done many measurements regarding the effect of antenna main lobe width, antennaside lobes, number of reflections, Gaussian and uniform background noise, and signal strength.