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THE MICROWAVE LANDING SYSTEM

As soon as a reasonably full constellation of Navstar satellites began to arrive in space, the Federal Aviation Administration approved the use of well-design Navstar receivers as a supplemental means of airborne navigation. With that approval, properly equipped airplanes could use the system for point-two-point vectoring and non-precision approach.

While the GPS constellation was being installed, the Microwave Landing System (MLS) was being touted as the favored means for landing airplanes under bad-whether conditions at properly instrumented airports all around the world. A total of 1250 American airports were schedule for Microwave Landing System installations, but, even so, eighty percent of our countries airfields would still have lacked such landing aids. The Microwave Landing System, unfortunately, fell behind schedule and went over budget while clever new approaches were greatly enhancing the capabilities of the Navstar system. With these new concepts in mind, the FAA's experts have essentially abandoned the Microwave Landing System in favor of a Navstar-based approach toward flight vectoring and air traffic control. Roughly one-third of the world's airplanes are based in the United States. Consequently, officials in other countries are expected to rely on the GPS in a similar manner. They are of course, in addition, building and installing space-based navigation systems of their own to replace and accentuate the capabilities of the GPS system.

FUTURE APPROACHES TO AIR TRAFFIC CONTROL

The backbone of the Federal Aviation Administration's rapidly evolving concept for future air traffic control is based on its Wide-Area Augmentation System (WAAS). The WAAS architecture calls for a total commitment two dependent surveillance techniques based on wide-area differential navigation.

If it's proposed architecture successfully materializes, every airplane that flies in the American airspace (excluding hang gliders and ultralights) will probably be equipped with a differential GPS receiver rigged to handle wide-area differential navigation. In a conventional differential navigation system, each differential base station broadcasts pseudo-range and pseudo-range-rate corrections directly to the users within a circular coverage region a few hundred nautical miles in diameter. This approach is conceptually simple and easy to implement, but as many as 500 differential base stations would be required to provide seamless coverage for the lower 48 states. Wide-area differential navigation, by contrast, can provide coverage over a comparable area with only 25 to 30 monitor stations linked to a centrally located master station. As Figure 1 indicates, the widely scattered monitor stations transmit real-time pseudorange measurements and other information to the master station where computer processing algorithms process all the measurements simultaneously as a unit. By processing large matrix arrays of overdetermined measurements, the master station produces and broadcasts information associated with each GPS satellite that is within sight of the United States:

1. 3-D satellite ephemeris corrections

2. Clock-bias errors

3. Real-time ionospheric corrections.

Each local receiver then plucks off the appropriate constants associated with its current navigation solution. Careful computer processing of those values coupled with an appropriate set of conventional real-time pseudo-range measurements allows each user to obtain a dramatically improved navigation solution with essentially differential accuracy over the entire coverage area in real time.

FIGURE 1. The FAA's Wide Area Augmentation System employs 25 to 30 widely dispersed monitor stations that transmit real-time pseudo-range and pseudo-range-rate corrections to a centrally located master station. The master station then computes generalize "differential corrections" that span the entire lower forty-eight states. These values are then transmitted up to a small collection of geostationary satellites serving the system for rebroadcast back down to the users on or near the ground below.

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