Emitter Geolocation with Multiple UAVs Nickens Okello Melbourne Systems Laboratory Department of Electrical and Electronic Engineering University of Melbourne, Parkville, VIC 3010, Australia n.okello@ee.unimelb.edu.au Abstract - Geolocation of radar and communica- tion emitters based on time difference of arrivals (TDOAs) can be carried out using a network of three or four unmanned aerial vehicles (UAVs) each of which is equipped with an electronic warfare sup- port (ES) sensor, a global positioning system (GPS) receiver, a precision clock and a limited bandwidth communication system. When the leading edge of an electromagnetic pulse is detected by an ES sensor on board a given UAV, the time of arrival and the UAV’s location are transmitted to a fusion centre. The received measurements from all UAVs within the network are then cross-correlated to determine the time difference of arrivals from which emitter location can estimated. This paper presents two sets of emitter location estimation equations. The first set is for 2-D emitter location estimates for a ground based emitter given time of arrival measure- ments from three netted UAVs. The second set is for an emitter that is locate at an altitude and gives the complete 3-D location estimation equations given measurements from four netted UAVs. Numerical results based on three and four UAVs are presented to validate these equations. Keywords: TDOA, geolocation, deinterleaving. 1 Introduction Goeolocation of radio transmitters is an important need in a wide range of areas including search and rescue of planes and ships, location of illegal or inter- fering emissions, and the location, identification, and targeting of military targets based their radar or com- munication emissions. Geolocation using satellites has received much at- tention [1, 2, 3] and its successful use in search and res- cue is well known through news reports. This approach is the cheapest method available when the search has to be carried out globally on a continuous basis and there is no prior information on the possible location or type of target. Furthermore is suitable when the emitter is a cooperating (or at least non-evasive) target and the emission has a known format that is designed to aid target identification and status evaluation. Further more it is suitable when ground access of the target lo- cation is possible. In military applications, emissions can be evasive or non-cooperating and may even be specifically designed to be elusive. Secondly, the area of interest is gener- ally limited but inaccessible and the search has to be carried out over a very limited period of time. Un- der these conditions, geolocation is better carried out using UAVs that are equipped with ES sensors rather than satellites. Furthermore, UAVs can be equipped with appropriate complementary sensors for emitter identification, and fire power to destroy the target if necessary. In Section 2, we present emission signal character- istics and the signal parameters measured by ES sen- sors. Later in the section we present an overview of deinterleaving, angle of arrival (AOA), time difference of arrival (TDOA), and frequency difference of arrival (FDOA), and how they relate to emitter geolocation and identification. Section 3 is the main body of this paper and contains detailed derivations of geolocation equations for a networks of three and four appropri- ately equipped UAVs that measure and transmit lead- ing edge time of arrivals (TOAs) to a fusion centre. These derivations are adapted from the problem of ge- olocation using geostationary satellites first presented in [1]. In [1] however, the derivations for the case in- volving four satellites is not complete. In this paper, we present complete and accurate derivations involv- ing three and four networked UAVs assuming a three- dimensional spherical earth geometry. In Section 4, we discuss the problem of time of arrival measurement as- sociation. In Section 5, we present performance results based on a network of three or more UAVs and show that accurate geolocation of ground based emitters is possible. 2 Signal Parameters in Geoloca- tion The parameters generally measured by the ES sys- tem for a pulsed signal include carrier radio fre- quency (RF), pulse amplitude (PA), pulse width (PW), time of arrival (TOA), and angle of arrival (AOA). In some systems, polarization of the input signal is measured. Furthermore, frequency-modulation-on- the-pulse (FMOP) is another parameter that can be used to identify a particular emitter and also can be used to determine the chirp rate or phase coding of a pulse compression (PC) signal [4].