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xAR Hints

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.

  1. When creating a DAR, it is possible for the ASTER User to enter xAR parameters that guarantee that the DAR will never be satisfied. There are several pitfalls of which the ASTER User should be aware. These pitfalls are explained here and strategies for avoiding them are described.
  2. Because the SWIR and VNIR bands of ASTER have limited dynamic range, it is necessary to specify the gain settings to be used for observations in each DAR. Suggested gain setttings for different types of target are listed here.

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 target’s AOI during the xAR Lifetime of the DAR. This increases the DAR’s 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:

  • automatically set the beginning of the xAR Lifetime to the time of submission,
  • limit the length of the xAR Lifetime to a maximum of 18 days, and
  • increase the amount of ASTER resources that are charged against your ASTER User account for each scene acquired by a factor of three (thus decreasing the total number of scenes you can ever acquire).

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 DAR’s 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

Important :
It is easy to ruin a DAR by choosing a non-default value for any of these parameters. This is based on experience with STARs.

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 " "low-1" "high"
VNIR #2 "high", "normal", "low - 1" "normal " "low-1" "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 ASTER DAR Tool User Guide.

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Updated: 09/07/2004 12:00 PM
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