[Spice_discussion] Geometry Engine

[Lee Elson]

As part of an ongoing effort to increase the functionality of 
the SPICE Toolkit, we at NAIF are planning the development of 
SPICELIB routines that will allow the user to determine time 
windows or "schedules" when certain geometric constraints are 
met. To make this development process as efficient as 
possible, we'd like to determine which geometric constraints 
would offer the most "bang for the buck" in terms of useful 
functionality. Listed below are brief descriptions of what 
we've come up with (in a loose arrangement of decreasing 
priority):

1)"Distance" -- determine time intervals during which the 
apparent distance between 2 specified bodies, as viewed from a 
specified observing body, satisfies a specified constraint 
(e.g. less than, equal to, greater than, between values)

2)"Phase"-- determine time intervals during which the apparent 
observer-target-Sun angle satisfies a specified constraint.

3)"Eclipse"-- determine time intervals during which one body 
is eclipsed by another as viewed by an observer.

4)"Occultation/transit"-- determine time intervals during 
which one body is occulted/transited by another as viewed by 
an observer.

5)"Time from periapsis/apoapsis"-- determine time intervals 
during which a body in orbit is within a specified time 
interval from periapsis/apoapsis.

6)"Elevation"-- determine time intervals during which the 
elevation of an object, in a specified reference frame, 
satisfies a specified constraint.

7)"Viewed Latitude/longitude"-- determine time intervals 
during which a viewed location (latitude/longitude) satisfies 
a specified constraint.

8)"Nadir separation"-- determine time intervals during which 
the angle between an observed object and the nadir direction 
satisfies a specified constraint.

9)"Separation"-- determine time intervals during which the 
angular separation of 2 bodies (as seen by an observer) 
satisfies a specified constraint.

10)"Incidence/emission/solar longitude"-- determine time 
intervals during which the incidence/emission/solar longitude 
angle satisfies a specified constraint.

11)"Orbital longitude"-- determine time intervals during which 
the orbital longitude of an object, relative to an observer, 
satisfies a specified constraint.

12)"Angular velocity"-- determine time intervals during which 
the angular velocity of one object relative to another 
satisfies a specified constraint.

13)"Apparent diameter"-- determine time intervals during which 
the apparent diameter of a body satisfies a specified constraint.

14)"Elongation"-- determine time intervals during which the 
elongation (angular separation of a body from the sun) of a 
body satisfies a specified constraint.

15)"Relative velocity"-- determine time intervals during which 
the velocity of one body relative to another satisfies a 
specified constraint.

16) "User supplied"-- determine time intervals during which a 
function provided by the user satisfies a specified constraint.

In addition to the time oriented "windows/schedules" discussed 
above, it is possible (but more work) to have other solutions. 
An example might be to find a range of viewed latitudes and 
longitudes for a specified time window.

The main purpose of this note is to solicit your input. Are 
the 16 constraints listed above of any use to you? Are there 
others that would be more useful?

Regards,
Lee Elson
Lee.Elson at jpl.nasa.gov
818-354-4223