Time-Difference-of-Arrival Geolocation Using Frequency Translators, 16-R9600

Principal Investigator
Don R. Van Rheeden

Inclusive Dates:  01/01/06 – 01/02/07

Background - A frequency translator is a device that receives a signal in one frequency band and retransmits the signal on another frequency band. The goal of this project was to investigate a time-difference-of-arrival (TDOA) geolocation approach that uses frequency translators as TDOA "nodes." Instead of each TDOA site receiving, digitizing, time-stamping, and relaying signal samples over a communications network to a geolocation processor as is commonly done, each frequency translator node would retransmit a signal onto several non-overlapping frequency bands to a central receiver/digitizer. Such an approach would eliminate the need for a high-speed digitizer, a precision time reference, acquisition computer, and network connection at each node. Because a single central receiver would receive frequency translated signal copies concurrently, no precision timing reference is required. This approach has the potential to significantly reduce TDOA node cost, weight, and power requirements.

Approach - The primary objective of this project was to research and develop a frequency translator TDOA geolocation approach. To accomplish this, four primary tasks were completed.

• The cost-benefit tradeoffs of frequency translator hardware components were studied.
• Different multiplexing schemes on the RF downlink were investigated.
• On-air signal testing using commercially available frequency translators was performed. A geolocation algorithm to process frequency translated copies of a signal and produce transmitter location estimates was developed.
• A calibration approach to compensate for biases and other error sources inherent in the frequency translator hardware components was formulated. Group delay characteristics of frequency translators were measured.

Accomplishments - Mathematical models were developed to understand the error sources inherent in the frequency translator approach. A result of this development was the discovery of an approach that allows all frequency translated signals to occupy the same downlink frequency band. This reduces the amount of downlink bandwidth required to relay the frequency translated copies of a transmitted signal to a common receiver/digitizer. A candidate frequency translator hardware architecture that was developed and bread-boarded by SwRI in 2005 was identified. Group delay measurements have been taken and showed that this frequency translator design is suitable for this application. On-air testing with a set of these frequency translators was performed in January 2007. On-air test results validated two TDOA geolocation approaches. The first approach translated signals into separate frequency bands and performed TDOA geolocation on the down-converted signals. The second approach translated signals from two translators into one frequency band, while the third translator signal occupied a separate frequency band. Although TDOA biases were observed because of several microseconds of group delay difference between the translators, results show that with calibration the frequency translator approach can be used to perform TDOA geolocation.

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