GPS World, February 2016

ANTENNAS CHARACTERIZATION GUIDES UAV NAV yagi antenna connected to the RN XV WiFly module A schematic of this configuration and the flow of information can be seen in FIGURE 1 Given the small size of this payload the flight time achieved during testing was 20 minutes on 4 pounds of batteries two 6 cell 8000 mAh batteries Signal Source Due to restrictions on active interference with GPS signals a 24 GHz Wi Fi router was used as a proxy jammer for all our flight testing The Wi Fi router was placed on the ground at a surveyed location In these tests GPS was used for navigation as we are still developing alternate and GPS jamming resistant navigation Antenna A single the L com HG2409Y yagi antenna was used for this experiment This 24 GHz Wi Fi antenna has a 60 degree beam width both horizontally and directionally as shown in FIGURE 2 As depicted in the OPENING GRAPHIC the antenna was mounted below the vehicle in order to have the clearest view to a ground based signal Furthermore the antenna is placed angled down at 30 degrees in order to have the main lobe of the antenna extend out to the horizon This also leaves a cone underneath the vehicle with a weak signal that was aimed to be leveraged as a null measurement when over the jammer While we are currently using a Wi Fi based system to stand in for GPS we eventually plan to test this system 50 GPS WORLD WWW GPSWORLD COM FEBRUARY 2016 on a true GPS jammer Despite the different frequencies the same methodology and approach will be able to be used when localizing a GPS jammer The biggest change the system will require is the antenna required to make bearing measurements For Wi Fi we have been able to successfully use an off the shelf directional antenna but for GPS either a custom directional antenna or a dualantenna solution will be needed Measurements Throughout the UAVs flight the directional antenna makes RSSI measurements at 2 Hz To calculate bearing from a given location the vehicle simply rotates at a rate of 15 degrees second at that position and combines all of the RSSI measurements using magnetometer data to form the antennas gain pattern This gain pattern can then be used to estimate the bearing of the signal source from that given position In this paper that bearing calculation is done with three different methods max cross correlation and max3 The max method simply finds the maximum RSSI value in the measured pattern and uses that heading as the bearing to the jammer The cross correlation method normalizes the measured pattern and compares it with the known truth pattern for the antenna The truth pattern is shifted by some angle γ The cross correlation is computed for every possible shift γ The shift yielding the highest cross correlation coefficient is taken to be the bearing to the jammer To get our truth pattern we sampled RSSI every 10 degrees at distances ranging from 10 to 40 meters normalized the resulting patterns and took the mean of these normalized patterns The max3 method is an improvement on the max method where the bearing is the mean of the bearing of the two crossings of 3 dB below the maximum RSSI value for the pattern as depicted in FIGURE 3 Flight Area Test flights were performed at the Joint FIGURE 1 Schematic of components on UAV FIGURE 3 Depiction of Max3 method FIGURE 2 Directional antenna gain pattern from datasheet vertical left horizontal right