The Wide Area Augmentation System (WAAS) is just now finally entering into civil aviation navigation in the United States.  WAAS provides a GPS based means for aircraft to maintain a flight path by issuing level correction vectors.  The end result is that the plane flies on a prescribed level path -- either from a previous flight or a computer generated path -- and follows the path to an accuracy of 30 feet.

Not one to rest on their laurels, NASA is keeping the ball rolling developing an even better system, dubbed the Platform Precision Autopilot (PPA).  One significant advantage of PPA over WAAS is that due to its usage of GPS satellites and traditional techniques WAAS can only operate with 75 degrees of latitude in the northern and southern hemispheres.  For PPA, which NASA plans to use in research planes which travel over Greenland and the Arctic, NASA also uses GPS but it boosts the range by relaying real-time GPS correction-vectors along Iridium’s satellite phone network to allow for navigation anywhere on the global.

NASA makes significant gains in accuracy between PPA and WAAS.  WAAS's accuracy of 30 feet has been beefed up to 15 feet with PPA, a two-fold improvement.  NASA hopes to become even more accurate, and is shooting for an accuracy of a few millimeters.

The final step after grabbing the more accurate GPS data is to combine it with 40 Hz input data from the aircraft's laser gyro-driven Inertial Navigation Unit (INU).  Combining these signals the aircraft's onboard computer outputs positional and guidance information.  This is used to autopilot the plane, but the output is displayed in traditional instrument landing system (ILS) form. Pilots will be able to read and understand it, and take corrective actions if necessary in case of system malfunction or failure.  By implementing ILS, the system can become FAA-certified, paving the way for its eventual adoption on commercial aircraft.

The system was developed at NASA's Dryden Flight Research Center in Edwards, CA, which worked in conjunction with NASA's Jet Propulsion Laboratory (JPL) in Pasadena, CA.  The system is designed to be utilized for NASA's Unmanned Aerial Vehicle Synthetic Aperture Radar (UAVSAR), a radar system designed at NASA's JPL under the guidance of NASA engineer Scott Hensley.  The UAVSAR is a radar system which broadcasts microwaves in the 1.2 GHz range from an L-Band aperture. 

NASA intends to use the UAVSAR for precision mapping of terrain, particularly with unmanned vehicles to map and monitor sites of extreme geologic activity.  The UAVSAR is very flexible and can electronically adjusts its signal, allowing it to be mounted on a wide variety of vehicles, but it requires a system like PPA to maintain a steady enough altitude for it to get good images.

The UAVSAR will be mounted aboard NASA's C-20A Gulfstream III, which will be used as a test of PPA's accuracy and whether it operates sufficiently for the UAVSAR system's readings.  NASA plans to log 140 hours of test flights before August 2008.   Since the Gulfstream III operates outside civilian air space it will not need a permit to use the UAV which takes 90-days due to a somewhat archaic processing system.  The test platform will allow NASA to instantly map hot zones of geologic activity.  Satellite SAR systems currently exist, but they only flyby a location with 24 to 45 days, so being in the right place at the right time for short-term events is unlikely.

NASA continues to lead the way in international aviation and its PPA and UAVSAR systems are no exception.  The PPA is especially promising to not only allow cutting edge research flights, but also promises to evolve and America's next generation of civilian aircraft safer.