A Formation Flight Experiment Using Differential Carrier Phase for Precise Relative Navigation

W. R. Williamson, M. F. Abdel-Hafez, I. Rhee, E. J. Song, J. Wolfe, D. Cooper, D. F. Chichka and J. L. Speyer

Abstract:	During the months of September to December of 2001, flight tests of two F-18 aircraft at NASA Dryden Flight Research Center demonstrated formation flight for drag reduction in order to extend the range of the formation. As part of this experiment, a unique instrumentation system with unique algorithms was constructed, tested, and implemented in the flight tests in order to estimate the relative position between the two aircraft to high accuracy in real time. This paper documents the hardware, algorithms, test process, and results of the flight test, which show that the instrumentation system was capable of measuring relative positions accurately without the use of any ground based instruments. The goal of the formation flight project is the development of technologies to allow multiple aircraft to fly in formation to reduce the overall drag of the formation, which will result in fuel savings. To accomplish this task, an autonomous control system is required to maintain the most efficient formation over long periods without overly-taxing the pilot. This control system requires a high-fidelity instrumentation system to measure relative position in order to locate the optimum fuel-saving location. This instrumentation system may not use ground-based instruments since the formation will likely fly over large bodies of water when fully implemented. The level of fidelity required for formation flight for drag reduction is relatively high. In order to achieve drag reduction, one aircraft must fly in the vortex wake of the other aircraft as depicted in Figure 1. The level of drag reduction achieved is dependent upon the location of one aircraft relative to another. For a detailed reference on the effect as well as a method for autonomous control to maximize drag reduction, refer to [1]. In the case of two F-18 aircraft, the size of the vortex wake is approximately 1 meter in diameter. An instrumentation system to measure the relative position must have centimeter level resolution in position as well as fractions of a degree in relative attitude in order to allow the control system to guide the aircraft to the optimal location. A new instrumentation system was developed at UCLA for the purposes of measuring the relative distances between multiple aircraft. The instrument is referred to as the Formation Flight Instrumentation System (FFIS). The new instrumentation systern designed combines a GPS and INS with a wireless communication system in order to estimate both absolute and relative position, velocity, and attitude between the two aircraft. The system is based upon a multi-processor computer linked through Ethernet. The hardware used is described in section 2. In order to achieve the required high fidelity position estimates, the relative integer ambiguity problem between the two moving vehicles must be solved. For flight tests, a new algorithm, the Wald Test [12], was used to estimate the integer ambiguity between the two vehicles. This algorithm is a non-linear, sequential hypothesis-testing scheme, which has been shown to be effective in solving the integer ambiguity problem. A maintenance algorithm was devised to ensure that once the integer ambiguity was resolved, the integers remained fixed even during satellite transitions. This algorithm is discussed in section 3. The GPS and INS are combined in an Extended Kalman Filter (EFK) to estimate vehicle state. Unique blending is used to estimate the state of a follower vehicle relative to the lead vehicle. This blending algorithm uses code and doppler as well as differential carrier phase measurements in order to provide high accuracy relative state estimates. The algorithm is described briefly in section 4. The FFIS was designed, implemented, and tested at UCLA. Tests included software simulation of algorithms, car tests, and Hardware-in-the-Loop Simulation tests. These tests and results of the tests are described in section 5 and 6. Once these tests were completed, the FFIS was ready for flight tests Section 6 shows the results of flight test experiments. During F-18 flight tests, an independent GPS system on board each aircraft combined with a ground based GPS receiver was used to verify the performance of the Wald Test. The verification data was collected from the aircraft and post-processed using batch filters to estimate the relative position accurately. Comparing the results from the batch process to the real-time system shows that the Wald Test did correctly estimate the integer ambiguity. Further, least squares calculations of relative position using the carrier phase measurements in real time agreed with post-processed estimates to within 5 cm of bias and 15 cm of standard deviation during typical flight conditions. In addition, the Wald Test operated well during several regions where the ground based instrumentation did not operate correctly. Finally, the Wald test converged within 30 seconds while operating at 2 Hz.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	988 - 1001
Cite this article:	Williamson, W. R., Abdel-Hafez, M. F., Rhee, I., Song, E. J., Wolfe, J., Cooper, D., Chichka, D. F., Speyer, J. L., "A Formation Flight Experiment Using Differential Carrier Phase for Precise Relative Navigation," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 988-1001.
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