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Tightly Coupled Oscillator and GPS Receiver


TECHNOLOGY AREA(S): Electronics 

OBJECTIVE: Develop a highly reliable, externally aided, GPS receiver that leverages precise time information from an atomic clock with increased performance and integrity over typical feed forward time server designs. 

DESCRIPTION: GPS is the ubiquitous source of precise time and stable frequency for many Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance (C4ISR) systems. Time is required for radar, high bandwidth communication, RF sensors, guidance systems, electronic warfare, and in fact, GPS receivers themselves. Many systems utilize a GPS time server with a stable holdover clock, such as a rubidium or other atomic clock, as a backup to maintain accurate time during short GPS outages, disciplining the holdover clock via the GPS receiver 1 Pulse Per Second (1 PPS) when GPS is available. This feed forward approach fails to capitalize on the stability of the holdover clock and to properly safeguard the holdover clock from faulty signals originating from the GPS receiver (possibly due to poor signal environment or an anomalous GPS satellite signal). Current implementations are vulnerable to errors induced into the GPS signal, such as multipath, environmental effects, signal anomalies, or GPS control segment errors. These errors will propagate into the timing output of the GPS disciplined clock unchecked by current disciplining implementations, potentially impacting their client systems. Better overall performance can be achieved if an integrated or coupled approach is applied. This integrated approach would treat the inputs of the holdover clock and the GPS receiver as sensor inputs to be evaluated and weighted to determine the best timing output, as well as deciding when to discipline the holdover clock. More deeply integrated methodologies would further leverage the holdover clock to aid the GPS receiver, increasing the availability and accuracy of the receiver by improving resistance to interference, multipath rejection, three satellite navigation, time to subsequent fix, etc. These methods would lead to a more robust positioning and timing system with better performance and resilience. The above example is one range of different techniques that could be used to better utilize and integrate a stable time and frequency source with a GPS receiver. However, other novel solutions to the stated problem will be allowed and considered. Current GPS receivers do not typically allow the type of modification necessary to integrate external sensors as navigation aids, as this fully integrated approach requires. Instead a Software Defined Radios (SDRs) provides a flexible platform with which to provide capability. It is suggested that algorithms developed for this topic be demonstrated using The Global Navigation Satellite System Test Architecture (GNSSTA) SDR architecture. Though the methodologies and algorithms developed for this topic may be applied to a large number of different clocks, the low Size, Weight, Power, and Cost (SWAP-C) of the Chip Scale Atomic Clock make it a desirable demonstration clock. 

PHASE I: Conduct a feasibility study that identifies the challenges and provides potential solutions for a fully integrated GPS receiver and precision clock design. Provide a fully integrated design and identify hardware and software necessary to build a prototype. 

PHASE II: Develop prototypes and demonstrate capability to TRL 4. Provide test report and analysis detailing all conducted tests and possible solutions to any identified challenges. Deliver 3 prototypes for government evaluation, including all hardware and software necessary to operate and collect data from the prototypes. Prototypes should implement algorithms using both the GPS C/A and P codes at a minimum. 

PHASE III: The purpose of this research effort is to develop a positioning and timing unit that could be useful for both personal navigation (via a low SWAP-C clock) and vehicular applications. The algorithms that are developed would enable an architecture capable of serving as both a high performance GPS receiver and a stable time source for client systems on a variety of platforms. Other applications include telecommunications systems in static installations, autonomous systems, radar platforms, and aviation applications. During this phase, the technical solution should look at applying this technique to other GNSS signals, such as M-code or the new GPS L5 civil signal, as well as the encrypted P(Y) code. 


1: Stevanovic, Stefan, Joerger, Mathieu, Khanafseh, Samer, Pervan, Boris, "Atomic Clock Aided Receiver for Improved GPS Signal Tracking in the Presence of Wideband Interference," Proceedings of the 28th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS

2: Preston, Sarah E., Bevly, David M., "CSAC-Aided GPS Multipath Mitigation," Proceedings of the 46th Annual Precise Time and Time Interval Systems and Applications Meeting, Boston, Massachusetts, December 2014, pp. 228-234

3: Chan, F-C., Joerger, M., Khanafseh, S., Pervan, B., Jakubov, O., "Performance Analysis and Experimental Validation of Broadband Interference Mitigation Using an Atomic Clock-Aided GPS Receiver," Proceedings of the 26th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1371-1379


KEYWORDS: GPS, Precision Timing, PNT, Kalman Filter, GPS Integration, Clock Integration, Sensor Fusion 

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