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Re: ultiment squint profile



At 01:53 PM 11/24/2001, joseph Murray wrote:
>Do we know what the actual Sub Satellite Point will be under inertial
>wheel stabilization?

I've never seen anything official on this, so I presume its not decided.

>Does the Stabilization point on the surface of the earth have to be on a
>line which between the center of the earthy and the satellite position"

I interpret your question as "does the antenna boresight have to point 
toward the center of the earth".  Answer to that is no.

>How much dynamics are involved in the stability of that pointing ( not at
>perigee )

Well, that's an interesting subject.  It depends much on the type of 
attitude sensor(s) available.  If the sensors were available that could 
provide continuous measurement of the attitude, then attitude could be 
maintained in a fixed position very precisely.

I believe that AO40 does not have the kind of attitude sensor(s) that would 
allow them to get good measurements while sitting still in arbitrary 
attitudes.  In particular, I believe they need to sense the EDGE of the 
earth, and the sensor doesn't swivel.  There has been some discussion that 
indicates they intend to rock the spacecraft back and forth (perhaps in a 
triangular pattern) so that they get periodic measurements of the edge of 
the earth.  Such a scheme would mean that the point at which the antenna 
boresight intersects the earth would move around a roughly triangular sort 
of pattern on the earth's surface.  I inquired awhile back about the 
anticipated magnitude and period of this rocking, but never got an 
answer.  I presume that's because it just had not  been determined at that 
time.

Don't take any of this as official position of anyone official.  I'm just a 
bystander who is sharing some discussions I've had with various folk.

>Folks with higher fixed latitudes North or South of the equator would I
>am sure like to have a means to calculate their predicted squint angle,
>horizon to horizon for a given satellite pass.

This rocking means that the link budget will vary some over the rocking 
cycle, and the effect of that on the link will of course be largest when 
you are near the edge of the earth (from the viewpoint of the satellite), 
which is when the satellite is near the horizon from your point of view.

I sure look forward to the transition to 3D stabilized operation.



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