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ALOS Systematic Observation Strategy
ALOS Systematic Observation Strategy
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The PALSAR Observation Strategy
The PALSAR acquisition strategy features routine observations at four* pre-selected sensor modes (Table 1). The mode selection represents a compromise solution where scientific requirements, user requests, programmatic aspects and satellite operational constraints have been taken into consideration.

PALSAR sensor default modes
Sensor mode Polarization Off-nadir
angle
Pass
designation
Caverage Time
window
Observation
frequency
Fine Beam
Single pol.
HH 34.3° Ascending Global Dec-Feb 1-2 obs/year
Fine Beam
Dual pol.
HH+HV 34.3° Ascending Global May-Sept 1-4 obs/year
Fine Beam
Polarimatric
HH+HV+
VH+VV
21.5° Ascending Regional March-May 2 obs/2 year
Fine Beam
Polarimatric
HH+HV+
VH+VV
23.1° Ascending Regional April-May 1 obs/2 year
ScanSAR
5-beam
short burst
HH 20.1°-
36.5°
Descending (a) Global
(b) Regional
Jan-Dec (a) 1 obs/year
(b) 8 obs/1 year
Fine Beam
Dual pol.
HH+HV 49.0° Ascending the Arctic Circle June-Oct 1-3 obs/year

To assure spatially and temporally homogeneous data collection over regional scales, acquisitions are planned in units of whole (46-day) repeat cycles, during which only one of the available default modes is selected. The PALSAR strategy is furthermore separated into one plan for ascending (evening) passes, and one for desceding (morning) ditto.

Ascending acquisitions (evening, ~22.30)
The PALSAR ascending mode plan comprises repetitive, global-scale observations with a constant off-nadir angle of 34.3° in both single polarisation (HH) and dual polarisation (HH+HV). To maintain mode-consistency in the multi-annual time series to be acquired, single-pol observations are scheduled during the northern hemisphere winter, and dual-pol observations around the summer months.

The minimum requirement for any land area on Earth is to perform at least one single-pol and one dual-pol acquisition annually, and in addition, two dual-pol acquisitions during consecutive 46-days cycles on a bi-annual basis to enable interferometric applications. Most areas are however to be acquired significantly more often than this, typically 3-5 times per year. In general, regions in the eastern hemisphere (Asia, Australia, eastern Europe and Africa) within the coverage of the Data Relay Satellite (DRTS) are acquired most frequently, while the western hemisphere (the Americas, western Europe and Africa) is restrained by the recording and down-link capacity of the on-board data recorder (HSSR).

To promote research relating to SAR polarimetry and polarimetric interferometry, polarimatric observation campaigns are planned once every two years, during which selected regions around the globe are acquired in full polarimetric mode during two consecutive cycles. As polarimetric operations at large off-nadir angles is not possible however, acquisitions will be performed at 21.5°.

For a detailed view of which regions that are scheduled for ascending acquisitions by PALSAR during a particular cycle, click here.

Descending acquisitions (morning, ~10.30)
To minimize resource conflicts with PRISM and AVNIR-2, which only can be operated during day-time passes, the descending acquisition plan for PALSAR is principally limited to low data-rate (120 Mbps) ScanSAR observations at HH polarization.

The ScanSAR scenario comprises one global coverage on an annual basis, and in addition - given the LHH-band sensitivity to detect inundation phenomena - intensive monitoring over a number of selected regional-scale wetland environments of global significance. To adequately capture the hydrological changes that occur throughout the year, ScanSAR observations will typically be performed every 46-days during 8-9 consecutive satellite cycles (12-13 months).

For a detailed view of which regions that are scheduled for descending acquisitions by PALSAR during a particular cycle, click here.


* In addition to the 4 default modes, one more mode (Fine Beam HH at 21.5°) will be used for some limited, local-scale observations in descending mode.

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