NASA Develops Super-Accurate Nav System for Scientific Plane
The UAVSAR underbelly pod is in clear view as NASA's Gulfstream III research aircraft banks away over Edwards Air Force Base during aerodynamic clearance flights. As a Multi-Role Cooperative Research Platform, the heavily instrumented twin-turbofan aircraft provides long-term capability for efficient testing of subsonic flight experiments for NASA, the U.S. Air Force, other government agencies, academia, and private industry. Originally designated a C-20A by the Air Force, the aircraft was declared excess by that service and transferred to NASA Dryden at Edwards AFB, Calif., in September 2002.
Credit: NASA Dryden Flight Research Center Photo Collection

NASA has developed a way for an aircraft to repeat the same level flight path anywhere in the world, after any time interval, to a constant positional accuracy within 15 feet (5 meters) in any direction.

Development of the highly accurate navigation technique was needed to allow NASA's Dryden Flight Research Center at Edwards, Calif. to use its C-20A Gulfstream III as a flying testbed for a new, compact imaging radar pod that uses a technique called repeat pass interferometry to produce highly detailed microwave images of the earth's surface.

The radar pod, developed by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., contains an L-Band synthetic aperture radar that uses microwaves in the 1.2 gigahertz range to detect and measure small deformations in the Earth's surface that are of interest to scientific researchers. Such measurements include the movements of glaciers, earthquake fault lines, landslides, and expansion of volcanoes.

JPL has designed the 10-foot-long pod so it can be mounted on a variety of aircraft. These will include unmanned aircraft which can loiter for long periods over areas of interest, or can fly particularly dangerous missions -- say, over the domes of volcanoes expected to erupt imminently. That's why JPL has called the project the Unmanned Aerial Vehicle Synthetic Aperture Radar (UAVSAR).

As might be expected, the new navigational technique -- developed by NASA Dryden in cooperation with JPL -- makes use of Global Positioning System (GPS) signals.

Positional accuracy within 1 foot

But where the Wide Area Augmentation System now entering service in the United States for civilian aviation navigation provides positional correction vectors accurate to within 30 feet, UAVSAR needs correctional vectors 30 to 50 times more accurate than this so its radar can be "steered" electronically to maintain the requisite millimeter-scale resolution.

Another problem with commercial GPS systems is that the NavCon satellites they use only provide signal coverage to 75 degrees latitude in both the northern and southern hemispheres, said Scott Hensley, chief scientist for the UAVSAR program.

But UAVSAR will be used to measure the movements of glaciers in Greenland and Antarctica, located at latitudes greater than 75 degrees. So JPL has developed a way to send real-time GPS correction vectors via Iridium satellite phone circuits, allowing coverage over the entire globe.

To achieve the required navigational accuracy, the real-time GPS signal that the UAVSAR pod receives is fed to the Gulfstream III's flight management computers.

The Platform Precision Autopilot

There, in a new system developed by NASA Dryden and called the Platform Precision Autopilot (PPA), it is combined with 40-times-a-second inputs from the aircraft's laser gyro-driven inertial navigation unit (INU) to produce very accurate positional and guidance information, explained James Lee, NASA's principal investigator for the Platform Precision Autopilot.

When fed through the aircraft's navigation receiver to its flight director, the combined GPS and INU signals show positional and guidance information as an instrument landing system (ILS) approach-mode display, familiar to instrument-rated pilots. The ILS display shows a cross-track localizer vector to provide lateral guidance and a glideslope vector to provide vertical guidance.

By having the PPA provide ILS signals through the flight director, NASA Dryden has ensured that the Gulfstream III's modified navigational system remains FAA-certified as being safe for flight.

Additionally, said Lee, Dryden's pilots know the pitch-angle, roll-angle, and yaw-angle limits programmed into the Gulfstream III's flight director. "So even if our (PPA) system is shut off, we know what the aircraft will do" in terms of its handling characteristics, he said.

UAVSAR's electronic steering

To make things simpler, JPL chose to have UAVSAR's 1.5 meter-by-0.5 meter radar antenna steered electronically -- that is, a microprocessor modifies its radar signal to compensate for changes in the carrying aircraft's attitude -- rather than mechanically, said Hensley.

Had JPL chosen to steer the antenna mechanically, the steering system would have to be modified every time the UAVSAR pod was fitted to a different aircraft. But with electronic steering the pod can be transferred from aircraft to aircraft without modification.

To ensure the accuracy of the electronic steering system and the radar's imaging resolution, JPL also has given the UAVSAR pod an inertial navigation system, which works in conjunction with its real-time GPS input to produce positional fixes accurate to within 10 to 20 centimeters.

The 19 test flights that NASA Dryden has performed so far with the PPA-equipped Gulfstream III/UAVSAR combination have proved that more than 90 percent of the time the aircraft can repeat exactly the same level, great-circle flight path within a 15-foot radius. Effectively it follows a 10-meter-diameter tube in the sky.

"That was our requirement," said Lee. However, the "contour plots" that Dryden has developed of the aircraft's exact position during each repeat pass show that 20 to 30 percent of the time the Gulfstream III stays within 3 feet (1 meter) of the center of its flight path in the previous pass.

Quick-reaction flying needed

NASA Dryden and JPL will conduct another 140 hours of flying before UAVSAR flight-testing finishes at the end of August 2008. But Hensley said the pod will remain on the NASA Gulfstream III for the foreseeable future.

The reason for this is that obtaining FAA permission for a UAV to operate in U.S. civilian airspace normally requires filing a flight plan 90 days in advance. Flying the manned Gulfstream III requires no such notice.

"If there's an earthquake, we want to be up in the air quickly," said Hensley. The 90-day UAV advance-filing period "can change, but right now the logistics in the NAS (U.S. National Airspace System) are simpler with a conventional aircraft."

Satellites usually provide more suitable mounts for SAR sensors than do aircraft, because they are unaffected by air movement and their orbits are much more predictable navigationally than aircraft flight paths. However, SAR satellites only overfly the same location once every 24 to 45 days.

"The processes we're watching happen at timescales less than 30 days," said Hensley. By mounting UAVSAR on an aircraft, "we can control repeat time from hours to years."