11 The RADARSAT Geophysical Processor System
255
11.4
The RGPS System
The mapping approach uses the RADARSAT ScanSAR Wide B mode to provide a
close to 6-day repeat coverage of the Arctic Ocean and adjacent Seas. The swath width
of this mode is approximately 460 km. The drift of the orbit over 7 days, within the
24 day repeat cycle, is modest and always eastward. In our approach, the passes from
days 1,2 and 3 are used for the first map (Fig. 1). This mapping is completed in 72 h.
Later maps are made on days 7/8/9, 13/14/15, and 19/20/21. Each map takes less than
107 min radar on-time. In the following 24-day cycle, we repeat the mapping starting with passes from days 1,2, and 3. The interval between the sequential maps is 6
days. This sequence is repeated for all cycles. The maps are constructed predominantly with descending passes and gaps are filled with ascending passes. Over a 24day repeat period, 423 min ScanSAR data are required to map the Arctic Ocean for
the purposes of the RGPS. This represents a daily data rate of close to 14500 X 460
km ScanSAR images. Some features of the system are described below.
Throughput/Storage. The ice motion tracker presents the largest computation load
on the system. At a grid spacing of close to 5 km, each Arctic map contains approximately half a million observations of ice motion. The RGPS throughput is designed to
slightly exceed the daily rate of data collection, in order to accommodate a certain
amount of reprocessing of the data. The computation capability required to sustain
this throughput is provided by application accelerators which provide an aggregate
floating point performance of approximately 1 Gflops. We store approximately 30 days
of image data in the system for efficient pairing of the image frames for motion tracking. In addition, storage space is required to accommodate the database tables
described in the previous section. A total storage capacity of 100 GB is available for
such purposes.
Ice Motion Tracking. The Lagrangian observations are sensitive to errors introduced by the tracker because errors in location propagate through the time sequence.
In the RGPS, all results from the automated tracking procedure are visually inspected by an operator before the observations are recorded. To make this process more
efficient, the operator is provided with the capability to view areas of large deformation as designated by the tracker and to correct the tracker results where appropriate.
Other Data Sources. The system assimilates wind and pressure data produced by
the National Meteorological Center and regrids them into a polar stereo graphic projection. Also, the 2-m temperature field (described previously) is provided to us by the
POLES project. These data sets are used in the analysis of the SAR data.
Archive and Distribution. The RGPS does not archive and distribute data products.
The RGPS products are delivered to the Alaska SAR Facility where they are archived
and distributed to users.
Flexibility. A goal in system development is to provide the flexibility for ease of
incorporation of new algorithms. We envisage that new algorithms and methods to
enhance current algorithms will be proposed as the science community develops
experience working with SAR data and the current set of geophysical products. The
system is designed such that the addition of new algorithms is a relatively simple procedure.
255
11.4
The RGPS System
The mapping approach uses the RADARSAT ScanSAR Wide B mode to provide a
close to 6-day repeat coverage of the Arctic Ocean and adjacent Seas. The swath width
of this mode is approximately 460 km. The drift of the orbit over 7 days, within the
24 day repeat cycle, is modest and always eastward. In our approach, the passes from
days 1,2 and 3 are used for the first map (Fig. 1). This mapping is completed in 72 h.
Later maps are made on days 7/8/9, 13/14/15, and 19/20/21. Each map takes less than
107 min radar on-time. In the following 24-day cycle, we repeat the mapping starting with passes from days 1,2, and 3. The interval between the sequential maps is 6
days. This sequence is repeated for all cycles. The maps are constructed predominantly with descending passes and gaps are filled with ascending passes. Over a 24day repeat period, 423 min ScanSAR data are required to map the Arctic Ocean for
the purposes of the RGPS. This represents a daily data rate of close to 14500 X 460
km ScanSAR images. Some features of the system are described below.
Throughput/Storage. The ice motion tracker presents the largest computation load
on the system. At a grid spacing of close to 5 km, each Arctic map contains approximately half a million observations of ice motion. The RGPS throughput is designed to
slightly exceed the daily rate of data collection, in order to accommodate a certain
amount of reprocessing of the data. The computation capability required to sustain
this throughput is provided by application accelerators which provide an aggregate
floating point performance of approximately 1 Gflops. We store approximately 30 days
of image data in the system for efficient pairing of the image frames for motion tracking. In addition, storage space is required to accommodate the database tables
described in the previous section. A total storage capacity of 100 GB is available for
such purposes.
Ice Motion Tracking. The Lagrangian observations are sensitive to errors introduced by the tracker because errors in location propagate through the time sequence.
In the RGPS, all results from the automated tracking procedure are visually inspected by an operator before the observations are recorded. To make this process more
efficient, the operator is provided with the capability to view areas of large deformation as designated by the tracker and to correct the tracker results where appropriate.
Other Data Sources. The system assimilates wind and pressure data produced by
the National Meteorological Center and regrids them into a polar stereo graphic projection. Also, the 2-m temperature field (described previously) is provided to us by the
POLES project. These data sets are used in the analysis of the SAR data.
Archive and Distribution. The RGPS does not archive and distribute data products.
The RGPS products are delivered to the Alaska SAR Facility where they are archived
and distributed to users.
Flexibility. A goal in system development is to provide the flexibility for ease of
incorporation of new algorithms. We envisage that new algorithms and methods to
enhance current algorithms will be proposed as the science community develops
experience working with SAR data and the current set of geophysical products. The
system is designed such that the addition of new algorithms is a relatively simple procedure.