Instantaneous Ambiguity Resolution of MOSAIC/DME: A Single Station Based 3D Positioning System for Alternative PNT

O-J. Kim, C. Kim, J. Song, Y. Kim, C. Kee

Abstract:	The study of MOSAIC/DME, a single station based positioning system, is being performed for the program of Alternate Positioning Navigation and Timing (APNT) which is a part of Next Generation Air Transport System (NextGen). NextGen is a program ready for the future aircraft traffic developed by US Federal Aviation Administration (FAA). And APNT is a developing solution for the continuity and safety of air transport when Global Navigation Satellites System (GNSS) is unavailable, caused by jamming, spoofing or other outages. In NextGen, not only GNSS based infrastructure providing the main role for positioning, navigation and surveillance, but backup or alternative PNT system for aviation is essential to avoid hazardous accidents and economic effects due to the loss of GNSS signals. Therefore infrastructure for APNT should be GNSS independent and terrestrial. But current terrestrial navigation systems, VOR, DME, NDB, VOR/DME, or DME/DME, cannot provide enough performance for the requirement of future air transport. Therefore candidates for APNT have been studied recently by FAA: Optimized DME Network, Wide Area Multi-lateration and Pseudolite Network. MOSAIC/DME also has been being developed with the consideration of GNSS outage to provide positioning, navigation and surveillance in a stand-alone condition. MOSAIC/DME is a DME station with additional antennas to provide carrier phase measurements of signals. And the use of carrier phase measurements, whose noise is mm-level, makes it possible for 3D-positioning even with a single station. Using phase measurements, however, causes cycle ambiguity of wavelength, an obstacle for positioning. But the cycle ambiguity can be handled adopting MOSAIC concept, so that ambiguity can be geometrically avoided or bounded with regard to the separation between antennas. (More details were presented in International Technical Meeting 2012 of ION). This system already presented its accuracy properties to prove that it can provide enough performance for the requirement of APNT. Even though this system can provide enough accuracy, however, the cycle ambiguity itself is not appropriate for aviation. Therefore, ambiguity resolution of this system is required to be studied profoundly. Fundamentally, the ambiguity of this system is geometrically bounded, so its potential solutions have a shape of sphere, whose center is the position of station and its radius is the measurement of DME. This feature means that potential solutions have very large separation between them, which makes the resolution of ambiguity relatively easy. In general, ambiguity resolution of MOSAIC/DME can be aided by other infrastructures providing rough positioning, for example, multi DME stations, VOR/DME or MLAT. The rough position makes only a few potentials around it and ambiguity resolution significantly fast and easy. But, in some cases, ambiguity is needed to be resolved only between station and aircraft. In this case, measurements from station and equipment built-in aircraft can be utilized for resolution. For the improvement of ambiguity resolution, the relation between the antenna formation and ambiguity pattern is analytically studied proving that symmetry formation can cause singularity between potential solutions. And applications of aircraft equipment are considered to resolve the ambiguity instantaneously. One is a barometric altimeter with which all aircraft are equipped, because potential solutions have wide distributed altitudes. And Attitude and Heading Reference System (AHRS) also can be utilized, because each potential solution has a different path trajectory. Compass and inertial navigation system also can be considered for the resolution of cycle ambiguity. After that conventional tests are performed to eliminate false solutions. Monte-Carlo simulations are performed for the verification of methods mentioned above. The considered error sources of measurements are clock error, receiver noise, multipath error, and tropospheric delay. And the simulation result displays that most ambiguities are solved only in an epoch with particular antenna formation and aiding by altimeter. When the measurements from other equipment are considered, it describes much better results. The paper describes the study of ambiguity resolution using a single station with multiple antennas. The simulation results prove the prompt and accurate resolution of cycle ambiguity. And it is clear that if two or more stations are considered, its performance can be much better. As the study of MOSAIC/DME is still being developed, there are still issues, such as air traffic capacity, timing and integrity which should be researched further for APNT.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	521 - 528
Cite this article:	Kim, O-J., Kim, C., Song, J., Kim, Y., Kee, C., "Instantaneous Ambiguity Resolution of MOSAIC/DME: A Single Station Based 3D Positioning System for Alternative PNT," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 521-528.
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