[Spice_discussion] Geometry Engine

Lee Elson Lee.Elson at jpl.nasa.gov
Wed Feb 19 09:00:33 PST 2003

```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
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

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