2008 Annual Report - NASA Airborne Science Program

UAVSAR
Trajectory
Control
A
Platform Precision Autopilot
(PPA) has been developed to
enable an aircraft to repeatedly
fly nearly the same trajectory
hours, days, or weeks later. This
capability allows accurate earth deformation
measurements through precise repeatpass
interferometry, a key element for the
success of the NASA Unmanned Aerial
Vehicle Synthetic Aperture Radar (UAVSAR)
program. The PPA uses a novel approach to
interface with the NASA Gulfstream III by
imitating the output of an Instrument Landing
System (ILS) approach. This technique
minimizes, as much as possible, modifications
to the baseline GIII. In addition, the safety
features of the aircraft’s autopilot are
retained. The PPA finished all phases of flight
testing in early 2008.
Objective
The objective of the PPA is to enable repeat
pass flights within a five meter radius tube
over a 200 kilometer course in conditions of
calm to light turbulence for over 90 percent
of the time. In order for JPL’s synthetic
aperture radar to generate the best images,
it is important to operate on a steady
platform. Hence, as a secondary goal, the
PPA has to minimize motion of the GIII
during data collection runs. The end product
is a “care-free, user-friendly” autopilot suitable
for deployment and operation by the flight
operations engineer.
Approach
The PPA uses a Kalman filter to generate a
real-time navigation solution with information
from the GIII systems and a differential GPS
unit located in the UAVSAR pod. The realtime
position solution is used to compute
commands (Guidance and Control modules)
which in turn drive two modified ILS testers.
The ILS tester units produce modulated RF
signals fed to the onboard navigation receiver.
These correction signals then allow the GIII
autopilot to fly a simulated, constant-altitude
ILS approach to meet the PPA requirements
for UAVSAR operations.
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