ASTER
Helpful hints on xAR input parameters Before going further, you are encouraged to read the information provided in the Obtaining ASTER Data and A description of the ASTER scheduling process pages.Purpose of this web page
This page was developed for two main reasons.
Although this page generally refers to DARs, most of the statements are relevant for STARs as well. Before they are transferred to the Science Sheduling Support Group (SSSG), who examine them and submit them to the ASTER GDS, STARs will be created by ASTER Science Team members. They should use the same caution in creating STARs that they use in creating their own personal DARs.
General caveat
In the absence of compelling reasons to the contrary, the ASTER User should use the default values where they are provided by the DAR Tool, for all but two sets of xAR parameters. The two exceptions are Maximum Acceptable Cloud Cover (for cloudy targets) and the VNIR and SWIR Gain Settings (for unusually dark or bright targets).
Information and helpful hints about specific parameters
xAR status parameters
Observation urgency
By setting this parameter to "Urgent", you indicate that a short-lived important
phenomenon is occuring in the targets AOI during the xAR Lifetime of the DAR. This
increases the DARs priority and therefore the likelihood that an ASTER observation
will be scheduled over the AOI during the xAR Lifetime. However, by defining a DAR as
Urgent you do three things:
Area of Interest (AOI) parameters
Coverage method
Most ASTER Users will choose to select the default value "Normal" for this
parameter. The value "Sampled" was developed to allow ASTER to acquire a smaller
number of images from anywhere within a large AOI. The locations of those scenes would not
be random, they would partly be determined by the locations of AOIs for other xARs in the
region.
Allow cross-track fragmentation
ASTER Users are advised to select the default value "Yes" for this
parameter, unless scientific reasons dictate that the entire AOI be observed
simultaneously. By selecting "No", you decrease the likelihood that your target
will be observed. Because of the details of the EOS-AM1 orbit and the ASTER scheduling
algorithm, opportunities for observing targets wider than about 25 km (in the cross-track
direction) without fragmentation may be somewhat rare. [Of course AOIs greater than 60 km
wide cannot be observed without fragmentation.]
Full AOI duration required
ASTER Users are advised to select the default value "No" for this parameter,
unless scientific reasons dictate that the entire AOI be observed simultaneously. By
selecting "Yes" (to prevent down-track fragmentation), you decrease the
likelihood that your target will be observed. Because of the details of the ASTER
scheduling algorithm, opportunities for observing long targets without fragmentation may
be somewhat rare.
Observation timing parameters
xAR lifetime
ASTER Users are strongly advised to choose a xAR lifetime of at least 16 days (the
EOS-AM1 orbit repeat cycle) . By choosing a lifetime less than this, you may guarantee
that your AOI is never observed, especially at low latitudes.
Acquisition window (AW) duration
ASTER Users are strongly advised to choose an AW duration of at least 16 days (the
EOS-AM1 orbit repeat cycle) . By choosing a lifetime less than this, you may guarantee
that your AOI is never observed, especially at low latitudes.
AW repeat cycle
When choosing an AW repeat cycle, please bear in mind the number of scenes that you
are asking ASTER to acquire. You may not want to exhaust your ASTER resource allocation
only partway through the DARs lifetime.
Cloud cover parameter
Maximum cloud cover
For AOIs in regions with generally heavy cloud cover,
ASTER Users may want to choose a value for maximum cloud cover that is
larger than the default. However, note that you can increase the value
of this parameter after submitting the DAR, using the Modify function of
the DAR Tool, but you cannot decrease it.
The ASTER Scheduler assigns a lower priority to scenes that are predicted to have a higher cloud cover (in NOAA global weather forecasts) than the xAR has requested. The ASTER Ground Data System (GDS) in Japan will not generate Level 1B products for scenes that it determines (during Level 1A processing) to have more cloud cover than the maximum specified.
Viewing Geometry parameters
Minimum sun angle and
Maximum sun angle
For daytime observations, ASTER Users are strongly advised to choose the default
values ("0°" and "90°" respectively) for these two parameters. If
you are not careful, your combination of observation timing parameters and non-default
values for sun angles can guarantee that your AOI is never observed.
Use specific look angle
ASTER Users are very strongly advised to select the default value "No" for this
parameter. By selecting "Yes", you greatly decrease the likelihood that your
target will be observed, due to the details of the ASTER scheduling algorithm.
Minimum look angle and
Maximum look angle
ASTER Users are strongly advised to select the default values for these two parameter.
Non-default values can greatly decrease the likelihood that your target will be observed,
due to the details of the ASTER scheduling algorithm.
Suggestions for ASTER gain parameters
This table gives suggested values for the gain setting parameters for several land target types.
Band | Possible Values | Desert | Ice | Others |
VNIR #1 | "high", "normal", "low - 1" | "normal " | "normal" | "high" |
VNIR #2 | "high", "normal", "low - 1" | "normal " | "normal" | "high" |
VNIR #3 | "high", "normal", "low - 1" | "normal" | "normal" | "normal" |
SWIR #4 | "high", "normal", "low - 1", "low - 2" | "normal" | "high" | "normal" |
SWIR #5 | "high", "normal", "low - 1", "low - 2" | "normal" | "high" | "normal" |
SWIR #6 | "high", "normal", "low - 1", "low - 2" | "normal" | "high" | "normal" |
SWIR #7 | "high", "normal", "low - 1", "low - 2" | "normal" | "high" | "normal" |
SWIR #8 | "high", "normal", "low - 1", "low - 2" | "normal" | "high" | "normal" |
SWIR #9 | "high", "normal", "low - 1", "low - 2" | "normal" | "high" | "normal" |
More information available
If you need more detailed information on all xAR parameters, please refer to the online DAR Tool Tutorial.