a correlative interferometer (CI) AOA system. Correlative interferometry is a widely implemented AOA technique in modern radio monitoring. The correlative interferometer is introduced and discussed in Chapter 4.7.2.2.5 of the ITU Handbook on Spectrum Monitoring, Edition 2011. (NOTE 1 – “Chapter” references in Tables 3-1 and 3-2 refer to the ITU Handbook on Spectrum Monitoring, Edition 2011. Numbers in parentheses in Tables refer to References listed in § 6.) TABLE 3-1 TDOA strengths Simpler antenna requirements The antenna is low cost, low complexity, and may be in size. TDOA receivers may employ a single simple antenna (such as a monopole or dipole). Unlike AOA systems, the antenna does not require high mechanical tolerances and electrical precision, and does not require operational test and measurement for calibration. An benefit is that the antenna may be made small in size and made inconspicuous. This is important when deploying monitoring systems in historical or architecturally restricted sites or when negotiating siting agreements with 3rd parties. Simpler siting and calibration requirements Siting requirements are less restrictive than AOA and require little to no calibration. This allows more flexibility in choosing TDOA sites. As a result, TDOA installations are faster to deploy. In urban installations, additional TDOA receivers may be placed to overcome the shadowing effects of tall structures. In contrast, AOA sites must be chosen to minimize wave front distortion due to re-emanation from local obstacles, ground reflections, and ground conductivity changes. Some AOA antenna arrays must be calibrated after site installation to minimize the resulting frequency and direction dependent errors. Antenna array calibration is one of the most important performance limiting issues in AOA [2]. AOA siting issues are discussed in further detail in Chapters 4.7.2.3.1.2 and 2.6.1.3.
TDOA measurement capability, and one with TDOA measurement capability). For simplicity these three methods are referred to as AOA, TDOA, and hybrid AOA/TDOA. Note that a monitoring site capable of AOA measurement is called an AOA site; a monitoring site capable of TDOA measurement is called a TDOA site, and a monitoring site capable of both AOA and TDOA measurements is called a hybrid AOA/TDOA site. The main characteristics of the three geolocation methods are listed in the Table 1 below. (See Report for a more detailed discussion of the advantages and limitations of TDOA systems.) 1 Geolocation System Characteristics AOA ONLY TDOA ONLY Hybrid AOA/TDOA Minimum number of sites required for geolocation 2 Stations 3 Stations 2 Stations, hybrid AOA/TDOA. One station can be TDOA only Geolocation Accuracy Linear decreasing as the distance to monitoring station increases Approximately constant in between the TDOA sites Deteriorate rapidly in the area outside the TDOA sites Same as TDOA in the area between the hybrid sites Similar to AOA in the area outside the sites Accuracy decreases with increasing distance to emitter Yes Only for transmitters in the area outside the TDOA sites Only for transmitters located far away from the hybrid Independent of signal modulation Yes TDOA does not work against unmodulated signals Difficult for narrowband signals Yes, if there are at least two hybrid sites Data communication requirement Low, 10-30 kbit/s Medium to High, 120 kbit/s – 2 Mbit/s Can be as low as AOA, if only AOA is used or slightly higher than TDOA, if AOA and TDOA are simultaneously used 1 (end) Siting constraints (See § 3) Larger antenna may be harder to erect, possibly limiting site availability Simple Omni, easier to erect Same as AOA for hybrid sites Antenna complexity Multi-element antenna Single antenna Multi-element and/or Single-element antenna Calibration requirements Sometimes (depends on AOA system) No Sometimes (depends on AOA system) As the Table shows, there are a number of advantages and disadvantages to each method. In a specific application, (urban/suburban, permanent/temporary, flat terrain/mountainous, etc.), the requirements for the deployment will determine the optimum configuration.