IPB

Welcome Guest ( Log In | Register )

10 Pages V  « < 8 9 10  
Reply to this topicStart new topic
DSCOVR, Formerly Triana, a.k.a. Goresat
JRehling
post Jan 24 2017, 06:11 PM
Post #136


Senior Member
****

Group: Members
Posts: 1911
Joined: 20-April 05
Member No.: 321



If I were to try to build a robust solution to this, I think I'd try the following. In large part, I think this follows more or less the algorithm that we people use in inspecting an image of the Earth.

Preparatory indexing:
1) Make an index of the shapes of coastlines at the resolution of ~ 5km/pixel. In particular, index segments where coastlines change orientation such as the Strait of Hormuz, the east coast of Somalia, the Baja peninsula, Gibraltar, southern Italy, Tierra del Fuego, Newfoundland, Michigan, etc.

Processing a single image:
2) At the time the image was taken, make a list of the coastline segments that are located within ~60 of the sub solar point. Perform a transformation to adjust them to how they should appear from the direction of the Sun, which will approximate the geometry of DSCOVR.

3) Run edge detection on the image, excluding any edges that are bounded by white regions, which are probably clouds.

4) Match the detected segments against the projected indexed segments from (2).

5) If three or more segments are matched (possibly two that are far apart), you now have a good registration between the image and the Earth.

Probably the tricky step is (4), but there's research on this.
Go to the top of the page
 
+Quote Post
scalbers
post Jan 24 2017, 07:53 PM
Post #137


Senior Member
****

Group: Members
Posts: 1224
Joined: 5-March 05
From: Boulder, CO
Member No.: 184



Interesting to consider this procedure. I wonder how this solution would work at the limb. I've been able to match the coastlines and other features fairly well. Clouds in my matching were also useful to check. However the extreme limb is where things appeared to drift off, possibly due to the setting of the reference limb in the atmosphere as mentioned earlier. It seems this might work OK with the raw data (however that would be available) and less well with the displayed web images or L1B image data. It's also helpful to consider the actual position of DSCOVR that can be around 10 degrees from where the sun is located.

http://stevealbers.net/albers/allsky/outerspace.html


--------------------
Go to the top of the page
 
+Quote Post
JRehling
post Jan 25 2017, 05:30 PM
Post #138


Senior Member
****

Group: Members
Posts: 1911
Joined: 20-April 05
Member No.: 321



QUOTE (scalbers @ Jan 24 2017, 12:53 PM) *
I wonder how this solution would work at the limb.


I wonder how possible it is to capture the limb. We know that we can see stars and the Sun when they would, on an airless globe, be below the horizon. That means, conversely, that vantage points in space have a view of points on the surface beyond the literal horizon, which means that other points must be projected to other locations. In principle, this means something very messy is happening at the limb. And a small displacement near the limb corresponds to a large difference in position on the map. The devil is in the details as to the magnitude of that effect, or if it affects such a tiny boundary around the disk as to be negligible.
Go to the top of the page
 
+Quote Post
scalbers
post Jan 25 2017, 06:20 PM
Post #139


Senior Member
****

Group: Members
Posts: 1224
Joined: 5-March 05
From: Boulder, CO
Member No.: 184



Using my simulated imagery (link 2 posts above) as an example, it seems possible to determine how far the (often obscured solid surface) limb is located below the top of the visible atmosphere. With a non-zero phase angle, the shading effects can also be considered. This is the more significant aspect I think with the visible atmosphere extending perhaps 30-60km above the limb.

Refraction is also of interest as you note. The actual lateral displacement of the limb (and locations nearby) from refraction is only about 1km, smaller than the camera resolution. This small amount can still allow another 100km or so of land to be squeezed into theoretical visibility near the limb.


--------------------
Go to the top of the page
 
+Quote Post

10 Pages V  « < 8 9 10
Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 24th February 2017 - 07:30 PM
RULES AND GUIDELINES
Please read the Forum Rules and Guidelines before posting.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted. Do not reproduce without permission. Read here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators.
SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society and is funded by donations from visitors and members. Help keep this forum up and running by contributing here.