> May 9, 2000

Dr. Yoaz Bar-Sever

MS 238-600

Jet Propulsion Laboratory

4800 Oak Grove Drive

Pasadena, CA 91109-8099

Dear Dr. Bar-Sever,

The revolutionary new technology you demonstrated with the IGDG software
holds great promise for the Solid Earth and Natural Hazard Program, and
probably for many other NASA programs. I was delighted that the AIST
program also recognized the importance of this software by funding the
development a NASA global differential GPS service. The IGDG technology
promises continuous, global, realtime, three-dimensional positioning of
an IGDG-compatible platform, to an instantaneous accuracy that varies
from cm-level to about 30 cm. Such accuracy is currently achievable only
with post-processing of GPS data that occurs days to weeks after the
fact. This capability will have significant benefits to NASA and to
society in general. The benefits fall into three broad categories:

1\. Enabling new science products

> \- through autonomy and intelligent platform control
>
> \- through rapid response to natural events

2\. Enabling improved environmental forecasting

> \- through reduction in data latency
>
> \- through on-board science processing and intelligent data reduction

3\. Cost savings and flexibility due to the simplification of ground
operations and reduced data bandwidth from real time on-board
processing.

The benefits extend to any mission that currently requires any kind of
post-processing for positional accuracy in space-borne, airborne, marine
or terrestrial environments. A variety of free-flyers -- ocean
altimeters, lidars and synthetic aperture radar (SAR) mappers,
multi-spectral imagers -- seek orbit accuracy from centimeters to
decimeters. While for many it is not needed in real time, the ability to
achieve such accuracy autonomously on-board would save time and expense
on the ground. In addition, real time kinematic geodesy and gravimetry
could be much simplified with sub-decimeter positioning in real time,
and readily extended to remote locations and large regions. A variety of
mobile science instruments worldwide could generate finished products in
real time, ready for interpretation, with enormous savings in analysis
and infrastructure. The scientific appeal of seamless worldwide
positioning offering post-processing performance in real time can hardly
be overstated. As a working marine geophysicist I was challenged by
numerous applications which required a real-time wide area sub-decimeter
positioning system in remote regions.

Possibly the most stringent positioning requirements for the Solid Earth
and Natural Hazard's Program come from Interferometric SAR (InSAR)
applications. Repeat pass InSAR requires consecutive flights along a
tightly constrained flight path. When an earthquake occurs, there is an
immediate need to assess the displacement in the affected area. This
targeting operation requires real-time control and accurate real-time
knowledge of the spacecraft or aircraft state. The solid Earth and polar
science communities place high scientific value on repeat pass SAR
mapping of land and ice masses to detect surface change. Emerging
applications in the domain of natural hazard reduction will also place a
high priority on InSAR technology and therefore upon advanced
navigational technology. Other mission concepts call for constellations
of spacecraft that will challenge ground operations. To be effective
these formations need to coordinate their science instruments, or
control their orbits to achieve their combined science goal. The AM
Constellation comprised of Landsat, EO-1, SAC-C and Terra in a repeat
ground track formation is the first example of this scenario, albeit
with low accuracy requirements.

Real-time data from the ground network of GPS receivers will provide the
ability to monitor volcanic inflation precisely and in real time.
Similarly, the spatial distribution of motions before, during, and after
major earthquakes will be precisely known almost immediately. Existing
and proposed dense networks of GPS receivers \-- California and Japan,
or the newly proposed Plate Boundary Observatory of nearly two thousand
GPS receivers will directly benefit from the ability to process
displacement estimates in situ.

Finally, IGDG is well positioned to be a trailblazer in the effort to
transition NASA's remote sensing infrastructure into a self-sustaining
enterprise. This will allow the ESE to remain responsive to new and
increasing demands on its resources without incurring further burden on
the US taxpayer.

Sincerely,

John LaBrecque, Manager

> Solid Earth and Natural Hazards Program