AN INNOVATIVE DEEP SPACE APPLICATION OF GPS TECHNOLOGY FOR FORMATION FLYING SPACECRAFT



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INTRODUCTION

Applications of Global Positioning System
(GPS) receivers have proliferated well
beyond the original vision of its system
architects. Receivers are now being built
which can provide absolute measurements
such as translational position, velocity, time,
as well as attitude and attitude rate.
Moreover, technology is emerging which
will make GPS receivers capable of routinely
delivering highly accurate measurements for
most of these states. Thus GPS receivers
hold the promise of satisfying nearly all the
guidance, navigation and control (GN&C)
sensing requirements for Earth orbiting
spacecraft in a single integrated, reliable,
low mass, low volume, and low power
package.


Other Separated Spacecraft Missions

Many other NASA separated spacecraft
missions could potentially benefit from the
AFF technology being studied for missions
such as DS-3. They include Earth orbiting
imaging missions such as TOPSAT (Figure
2), and the New Millennium EO-1 (Earth
orbiting mission 1) Landsat Co-flyer (Figure
3), gravity mapping missions such as
GRACE (Figure 4), deep space sparseaperture
RF telescope such as ALPHA
(Figure 5), and MUSIC (Figure 6), a
moderately filled aperture interferometer
constellation. All of which requires
moderate to precision formation sensing
capabilities.


THE AUTONOMOUS FORMATION FLYING GNandC SENSOR

While the Jet Propulsion Laboratory (JPL)
has had an active role in developing and
demonstrating GPS technology for Earth
orbiter and ground-based tracking
applications, the AFF represents a
fundamental new area of application of GPS
technology. It is based in part on several
novel and innovative uses for the
TurboRogue GPS receiver which do not
actually involve the tracking of GPS
satellites. The TurboRogue GPS receiver
was originally developed at JPL2,3 for precise
positioning and timing applications, and is
currently commercially available from Allen
Osborne and Associates.


Clock Calibration
When two AFF sensors are used to selftrack,
each sends out a 1-way signal and
each receives a 1 way signal from the other
simultaneously. The combination of these
two 1-way signals enables the clock offset
and distance between the two AFF sensors
to be uniquely determined