RGPS Bulletin - 10/16/97				
R. Kwok(ron@rgps1.jpl.nasa.gov)

The purpose of these bulletins is to report on the progress of 
data acquistion and processing of the ScanSAR imagery into various 
geophysical products (Lagrangian ice motion, ice age and thickness, 
and backscatter histograms). Since this the first of such bulletins,
I give a brief description of the recent history of the RGPS.

1. Brief History

The RADARSAT synthetic aperture radar provides large scale coverage of 
the Arctic sea ice cover on a 3-day basis with its wide-swath ScanSAR mode. 
We are processing these datasets into geophysical fields in the RADARSAT
Geophysical Processor System (RGPS). The direct observations we extract
from the sequential SAR imagery are Lagrangian ice motion of points on
the ice cover and records of the backscatter histograms within areas
enclosed by polygons defined by these points (cells). The vertices of
these cells are tracked from fall freeze-up to the onset of melt in the
spring. In the winter, we interpret a local increase in cell area as
the addition of open water or thin ice; ridging or rafting decreases
the cell area. The age of the new areas in each cell are kept as
entries in an ice age histogram. Ice thickness is computed using ice
age and the cumulative freezing-degree days experienced by the thin ice
in each cell. The temporal evolution of backscatter in each cell to
estimate the dates of melt onset in the spring and freeze up in the
fall. Open water fraction is estimated in the summer.

RADARSAT was launched in Nov of 1995 into a 24-day exact repeat orbit.
The routine collection of the Arctic snapshot for the RGPS, a  3-day repeat 
coverage of the Arctic Ocean, using the 460 km swath ScanSAR mode, began 
in late October 1996. The data from the region of the Arctic outside the 
Alaska SAR Facility (ASF) reception mask are being downlinked to the 
reception station in Tromso. Currently, these data from Tromso are not 
available to US investigators due to on-going negotiations regarding
data costs.

2. RGPS Processing

Cycle 15 (a cycle is one 24-day period corresponding to an exact
repeat interval), between Nov 4, 1996 through Nov 28, 1996, was the
first RADARSAT cycle to provide routine mapping of the Arctic. Since
then, the Arctic snapshots have been collected successfully through the
end of August 1997. This data collection was interrupted by the
Antarctic Mapping Mission (AMM). This was a two month period (Sept and
Oct, 1997) when the radar was in the south-looking mode and
dedicated to the mapping of Antarctica. The collection of the Arctic
snapshot will resume in early November. 

Small volumes of uncalibrated ScanSAR image frames were made available 
to the RGPS by ASF for validation and verification since April 1997. In
July, the image frames from Cycle 15 were  delivered to the RGPS. Cycle
16 data were delivered in September. However, these datasets were
geometrically and radiometrically uncalibrated. The final processor for
producing calibrated ScanSAR data will not be operational at ASF until
mid-December.        

In the interim, we have produced several preliminary data products,
using data from cycles 15 and 16, for format verification and experimentation.
The products include: 1. Lagrangian ice motion; 2. Ice cover
deformation; 3. Ice age histograms; 4. Ice thickness histograms
and 5. Backscatter histograms.
These data products and a description of their format are available at 
the RGPS website at JPL (http://www-radar.jpl.nasa.gov/rgps).  

The currrent plan is to start operational processing of the RADARSAT
data immediately after calibrated data are available and to have an
RGPS working group (RGPSWG) meeting in early 1998. The RGPSWG will 
address issues of science validation, data usage and  processing.


3. Products on the RGPS Website (http://www-radar.jpl.nasa.gov/rgps)

These are preliminary products produced from uncalibrated imagery, so
the user should be careful in interpreting the results. The format
of the products are stored in the 'Documents' directory in the RGPS website.
We are adding to the products at the end of every the processing of a snapshot.
The products, at the time of this writing, include the following:
- Lagrangian ice motion 
- Backscatter histogram 
- Ice Deformation
- Ice Age histogram
- Ice Thickness histogram