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Understanding Emission Angle

Shyam Mohan

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Hello all,

As per the definition of Emission angle given in http://ode.rsl.wustl.edu/moon/pagehelp/quickstartguide/index.html?seach_page.htm#filterbyanglerange, higher values of emission_angle indicate more oblique viewing. Consider the two LROC products M1099037220LC and  M1108467737LC with a very close emission angles of 1.78 deg and 1.76 deg respectively. However M1099037220LC appears to be very oblique with elliptical shaped craters and M1108467737LC appears to be nadir looking with almost circular craters. Can you please explain why is it so ? The screenshot of the LROC images are attached for you reference.

Thank You,




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The definition for emission angle in ODE is correct. The image where the craters look circular has the same value for scaled pixel width and height. The other image has a scaled pixel width that is twice that of the pixel height making the craters look elliptical. You can verify this by following the link in ODE to the team site and viewing the metadata there (see attached images). This metadata is also provided in the index table of the PDS archive.

Please let us know if you have any more questions.

PDS Geosciences Node




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Hello Jennifer,

Thank you for your time and clear explanation. I could confirm the scaled pixel width(p_width) from the spacecraft altitude(h) and camera parameters. p_width=2*h*tand(FOV/2)/x =2*tand(2.8502/2)*144.07*10^3/5064 =1.42 where FOV is the camera field of view and x is the total line samples. The scaled pixel width of the images, M1099037220LC and  M1108467737LC are 1.44 and 1.42 meters/pixel respectively. The are close to each other since their altitudes are very close (144 and 141 Km).  I have two follow up questions,

1. The scaled pixel height of M1099037220LC(0.75) is lower when compared to that of M1108467737LC(1.42). Is it because the spacecraft was at lower velocity in case of M1099037220LC that the downscan resolution or the scaled pixel height was lower? I am asking this out of curiosity.

2. Now that an image can have different scaled pixel width and height. What do you mean by Map Resolution? The definition in the label file says "The calculated pixel resolution at the center of a NAC or WAC observation."  How is Map resolution and scaled pixel width or height related ? Is there any document that explains these parameters in detail.

3. As per the definition of slew angle in the label file, it is the Angle of LRO off nadir when observation acquired. Is it not same as emission angle? How is slew angle related to emission angle ?

I am referring to the label file  in the below link, http://lroc.sese.asu.edu/data/LRO-L-LROC-3-CDR-V1.0/LROLRC_1012/INDEX/INDEX.LBL. Please let me know if you need any clarity on the questions that I am asking.

Thank You,



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