TECHNOLOGY AREAS: Sensors, Battlespace, Weapons

OBJECTIVE: Develop, analyze and deploy enhanced techniques to improve emitter detection and geolocation performance for improved time-sensitive targeting and Naval Battlespace Awareness

DESCRIPTION: Traditional techniques for emitter geolocation include Angle of Arrival (AOA) for single sensor platform situations and Time Difference of Arrival / Frequency Difference of Arrival (TDOA/FDOA) as a technique involving dual/multiple sensor platforms. These methods provide limited performance due to constraints on platform size, cost, antenna size, instrument errors, processor throughput, signal propagation modeling errors associated with changing geometries, attitude and Doppler changes over a data collection or signal processing time interval, and the types of challenges as itemized below.

Performance parameters associated with emitter geolocation include, but are not limited to �
(1) Time to deploy to region
(2) Signal detection sensitivity
(3) Geolocation accuracy
(4) Time to detect
(5) Time to geolocate to a given accuracy.

Current emitter geolocation technologies fall short on the needed "time and throughput" requirements.

The performance capabilities associated a specific geolocation approach are often interrelated, and scenario and technology dependent.

The time to deploy to region often depends on the system size. For example, technologies which require large antennas and high-power, are often deployed on large Unmanned Air Vehicles (UAVs) or manned aircraft, whereas small antenna and technologies which can be implemented with small weight, size and power requirements may be implemented on small, low-cost UAVs, which can often be deployed in theater more quickly.

Signal detection sensitivity is an important factor for detecting weak signals or even some relatively strong signals at large ranges. While the processing gain for detection can be increased using large antennas, the use of small antennas is often preferred to allow use of smaller, lower cost sensor platforms as mentioned above. The use of large directional antenna may introduce a larger time for detection as compared to the use of omni-directional antennas due to the time to scan or slew a directional antenna to cover a large region of interest. Improvements in signal processing can enable enhancements in signal-to-noise ratio (SNR) compared to traditional "snapshot" algorithms, by enabling longer coherent integration time intervals. For example, at the Global Positioning System (GPS) L1 frequency, the coherent integration time interval for traditional techniques may be limited not to exceed 1 millisecond for some scenario. However, if precise sensor platform navigation and timing information is utilized, then the coherent integration time interval may be extended by a factor of 100 or more for some scenarios, corresponding to a factor of 100 (20 dB) processing gain enhancement for detection of weak signals.

The geolocation accuracy depends on such parameters as the number of sensor platforms, the geometry, the coherent integration time and the observation time. As a numerical example, at the GPS L1 frequency, a �-meter diameter antenna can achieve an ideal geolocation AOA accuracy of about three degrees using the traditional MUltiple SIgnal Classification (MUSIC) algorithm [1]. However, in some cases, the synthetic aperture technique can enable an (effective) antenna size of 50 meters or more, corresponding to a factor of 100 or more improvement in geolocation AOA accuracy. The Cramer Rao bound [2] for the traditional TDOA/FDOA technique typically assumes a single "snapshot" observation, whereas multiple observations over an extended interval of time, can enable improved accuracy which exceeds the Cramer Rao bound for some scenarios.

The time to detect depends on such parameters as the emitted power, the range, the processing gain, and the required time to search a certain region. Dual-platform methods require that both platforms be deployed in suitable geometries within the emitter signal antenna beam, whereas single platform methods may require that after an initial detection occurs, that the platform fly to a second location for triangulation. Depending on the technology, the time to search will depend on whether the sensor is directional and so needs to scan a region through multiple looks, or able to scan a wide region in one look.

The time to geolocate to a given accuracy depends on such parameters as geometry, SNR and observation time. The Cramer Rao bound applicable to a single "snapshot" look, e.g. 1-second, may be significantly exceeded by tracking an emitter signal over an extended interval of time, e.g. 10 to 100 seconds associated with multiple time-varying geometries.

Enhanced techniques for improvement of emitter geolocation performance are sought to enable sensor platforms to operate at much larger stand-off ranges with larger coverage areas for time-sensitive targeting and/or improved naval battlespace awareness. Recent studies have shown that performance may be enhanced through the use of enhanced signal processing techniques (e.g. Kalman tracking filters), improved signal propagation and emitter models, and exploitation of the precise Position-Velocity-Timing (PVT) information provided by GPS carrier phase signal processing. Synthetic aperture signal processing has also been shown to offer great opportunities for improvements in weak signal detection and geolocation accuracy.

It is understood that potential enhancement techniques may not function well for all scenarios, but improved performance as compared to traditional techniques are desired even under restrictive scenario conditions for problems of interest. For example, synthetic aperture techniques may work well for a radar signal emitter, but will not function well for an emitter which only remains turned on for a short interval of time. Long coherent integration time intervals to enhance SNR for detection and geolocation accuracy may not be possible for certain types of signals, waveforms and geolocation methods.

Some of the additional challenges associated with this research are the development of robust techniques which also address --
� Detection and accurate geolocation of low-power emitters ≤ 1 Watt
� Mitigation of error sources, e.g. multipath
� Geolocation of multiple emitters with similar signal characteristics
� Isolating unauthorized emitters from legitimate signal sources
� Geolocation of closely spaced emitters
� Poor geometry conditions
� Geometry, attitude and Doppler changes over signal processing time intervals
� Elimination or mitigation of processing latencies to support real-time operations
� Elimination of latencies, timing and frequency errors
� Moving emitters and emitters with time-varying signal characteristics
� Emitters which turn on and off
� Directional radiators
� Relativistic effects
� Extended (i.e. non-point) emitter sources, e.g. IR images
� Developing efficient search and signal processing techniques, including signal detection, false alarm rejection and selection of signal processing parameters for fast and robust operation

PHASE I: Develop concepts for improved emitter detection and geolocation techniques, algorithms and procedures. Perform trade and sensitivity analyses to demonstrate a strong understanding of stressing scenarios, driving requirements and technology limitations and shortcomings for various scenarios and conditions. Demonstrate proof-of-concept through simulation and error analyses which illustrates significant improvement relative to traditional techniques.

PHASE II: Develop prototype emitter detection and geolocation techniques, technologies, algorithms, and procedures for incorporation into new and/or existing operational systems. Develop requirements for integration of algorithms into new and/or operational systems. Demonstrate significant performance improvements relative to the traditional and state of the art techniques.

PHASE III: Integrate the algorithms and procedures into new and/or operational systems to provide improved emitter geolocation capabilities.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The developed technology could potentially be applied in commercial geolocation services, e.g. location of cell phone transmitters or satellite interference sources.

REFERENCES:
1. Schmidt, R. (1986). Multiple emitter location and signal parameter estimation. "Antennas and Propagation, IEEE Transactions on," 34(3), 276-280.

2. Sonnenschein, A. Hutchinson, W.K. Cummings, W.C. (1993). Geolocation of Frequency-Hopping Transmitters via Satellite. "Aerospace and Electronic Systems, IEEE Transactions on," 29(4), 1228-1236. DOI: 10.1109/7.259526.

3. Broumandan, A., Lin, T., Nielsen, J., & Lachapelle, G. (2008). Practical Results of Hybrid AOA/TDOA Geo-Location Estimation in CDMA Wireless Networks. "IEEE 68th Vehicular Technology Conference. VTC 2008-Fall," Calgary, BC. DOI: 10.1109/VETECF.2008.138.

4. Broumandan, A., Nielsen, J., & Lachapelle, G. (2008). Practical Results of High Resolution AOA Estimation by the Synthetic Array. "IEEE 68th Vehicular Technology Conference, VTC 2008-Fall," Calgary, BC. DOI: 10.1109/VETECF.2008.85.

5. Fowler, M.L., Xi Hu. (2008). Signal Models for TDOA/FDOA Estimation. "Aerospace and Electronic Systems, IEEE Transactions on," 44(4), 1543-1550.

6. Pattison, T. & Chou, S.I. (2000). Sensitivity analysis of dual-satellite geolocation. "Aerospace and Electronic Systems, IEEE Transactions on," 36(1), 56-71.

KEYWORDS: Electronic Warfare; Electronic Support; Emitter Geolocation; TDOA/FDOA; Naval Battlespace Awareness; Detection

Questions may also be submitted through DoD SBIR/STTR SITIS website.