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Using Geostationary Satellites To Help Land Aircraft Safely

Richard Fuller
Aeronautics & Astronautics Department
Stanford University
May 2001

Modern aircraft are technological marvels. Almost every aspect of their design represents our best understanding about science and engineering. However, nearly all aircraft have primary navigation systems based on designs that were first developed 75 years ago. My research presents a technology that will help bring aircraft navigation into the 21st century.

My research studies one of the approaches being developed to aid in the landing and navigation integrity of aircraft. I have studied the use of geostationary satellites to send information to assist in the accuracy and integrity of the Global Positioning System, or GPS. By using geostationary satellites, it is hoped that aircraft operating throughout the country will have very precise and reliable knowledge of position to help navigate efficiently and avoid obstacles near the ground influenced by inclement weather.

GPS is a constellation of satellites used for navigation. Users of GPS employ a receiver that measures the distance or range to the satellites. These ranges are used to calculate the user's position. Knowledge of position, especially altitude, is critical in many phases of aircraft flight. Aircraft have begun to use GPS for a variety of tasks; from aiding other position information sources, to simple navigational awareness. However, GPS lacks the full integrity required for use as a stand-alone aviation due to errors in the signals from natural (the atmosphere) and un-natural (intentional radio jamming) causes.

In my research, I investigate the use of additional satellites, namely geostationary, to aid GPS. Geostationary satellites have a fixed position in the sky relative to an observer on the ground that differs from other types of satellites. This offers a great advantage in providing continuous visibility from a user to the satellite. The basis of my research has two parts: 1) geostationary satellites as a ranging source, in essence a measuring rod from user to satellite; and 2) geostationary satellites as a datalink, a path over which correction data can flow.

The ranging portion of my research utilizes the geostationary satellite signal as another satellite in the GPS constellation. The GPS constellation uses precise clocks on the satellites to provide accurate ranges to the receiver tracking the signals. By providing an additional ranging source, gaps in the GPS coverage where position solutions would be poor or impossible, can be achieved. This research showed that even with a precise clock located on the ground, the position solution could be improved. As a datalink, the geostationary satellite can broadcast corrections to errors in the GPS signals, improving the accuracy of the overall system. This correction data not only showed that the datalink met functional criterion for the signal, but was also able to maintain its usefulness even when the aircraft was going through significant roll angles of up to 60 degrees.

The Wide Area Augmentation System (WAAS) is a GPS-based navigation aid currently under development by the Federal Aviation Administration (FAA). WAAS will provide corrections to aviation users for the GPS clock, its ephemeris (the satellite position), and for the delay in its signal as it passes through the ionosphere. These corrections will be broadcast to users throughout the United States via geostationary satellites. A master station that combines data from a continental network of reference GPS receivers will create these messages. The geostationary satellites serve both as wide-area differential GPS data links as well as additional ranging sources. The data message stream of WAAS enhances the accuracy and integrity of the GPS signal for aviation. Simultaneously, the satellite ranging source increases the percentage of time that the precise signal is available. In this way, WAAS provides needed improvements in four metrics over the standard GPS signal: accuracy, integrity, availability, and continuity.

The ranging function described above, requires an estimate of the position of the geostationary satellite. This dissertation presents a novel technique for generating a satellite position estimate. This technique is designed to provide high-integrity performance in the user position domain and operates in real-time. As such, it contrasts classical orbit determination techniques that have no integrity requirement, are not designed to optimize performance in the user position domain, and usually have no real-time requirement. Our estimator is evaluated using real data from the FAA's National Satellite Test Bed (NSTB).

The WAAS Signal-In-Space (SIS) has a limited data message bandwidth of 250 bits-per-second. This data bandwidth was chosen to balance two concerns. First, the power of the signal must not be so strong that it jams GPS. Second, the signal must provide the minimum amount of information necessary to ensure adequate accuracy and integrity for aviation users over the entire geostationary satellite footprint. The required message loss rate is specified not to exceed a rate of 0.001 (one loss per one-thousand messages) to ensure adequate system continuity and availability. The WAAS message structure is not particularly sensitive to independent message losses below the specified rate. Groups of missed messages (burst-mode) can prove to be a challenge in maintaining a continuous WAAS solution. The effects of burst-mode losses on the quality of the WAAS solution is presented and a Markov model for the burst message loss is developed. This research shows that these burst message losses can be tolerated for WAAS availability, provided that the message loss rate does not exceed a rate of 0.005.

Flight tests were conducted in California and Alaska to establish actual message loss profiles for aircraft. These flight test results were modeled and used in conjunction with NSTB reference station data to establish availability of WAAS solutions for various locations in the United States.