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Re: RE: About ALON/ALAT of AO-40




>Wayne replies:
>
>It depends on the time frame you consider.  In early February I started
>planning a portable AO40 station for a hamfest on 24 March.  Using the
>widely quoted "1 degree per day" prediction I could expect good squint
>angles, about 10-16 degrees.  Using the actual "0.8 degree per day" change
>over that 6 week time period, I now expect squint angles of 17-22 degrees.
>The squint will be above 20 degrees near apogee for half of the hamfest.
>Yes, I can make contacts at 22 degrees squint, but I want the public
>response to be "AO40 is good", not "AO40 is barely usable".  I am proceeding
>with the demonstration, but I will shut down when conditions no longer give
>people the "AO40 is good" impression.

Wayne,
You're making this harder than it needs to be.  In addition, you seem to be 
asking for a degree of daily ALON/ALAT prediction precision that simply 
doesn't exist.  There's a reason why that table from December says 
"APPROXIMATE AO-40 ALON/ALAT Schedule."

There are two issues here.  First, when will the sun ALLOW us to get back 
to ALON/ALAT = 0/0.  The answer to this has always been mid-April.  No 
matter how good the magnetorquing is, we can't go back to 0/0 until 
then.  The sun and AO-40's orbit are extremely well calibrated. The 
geometry of the satellite's orbit with respect to the sun changes as the 
sun moves along the ecliptic and the orbit precesses.  Thus, the amount of 
allowable advancement in ALON per day varies somewhat.  As a rule of thumb, 
though, if it takes 365.25 days to go 360 degs around the sun, then that's 
pretty close to an average allowable advancement of 1 deg/day (neglecting 
precession effect).

The second issue relates to the speed at which we can actually advance the 
ALON.   This depends on the RPM of the satellite, eclipse times, the 
efficiency of our TORQUE routines, whether we have to correct for an ALAT 
drift, time out to take pictures and calculate attitude, etc....  Remember, 
we're still calibrating this satellite.  It should hardly be surprising, 
given these variables, that the value we have reported for the advancement 
of ALON per day has varied a little.

The fact still remains that we can't go to 0/0 until mid-April.  At the 
moment, we are repeatedly banging the limits of the solar sensors on every 
orbit, so every orbit is as close to the optimum ALON as it can be, given 
the sun's position.  We have about 30 degs left to go and about 30 days to 
do it in.  Assuming we can maintain our current optimum setup, that sure 
sounds close to 1 deg/day to me, right on schedule.  The man-hours on three 
continents required to do all this is considerable to say the least.

Whether the current ALON/ALAT of better than 330/0 is "good" for a 
DX-pedition, is a side issue.  From my perspective, I think the signals now 
are quite good for a substantial period of time.  Assuming a constant 
squint, the change in footprint, for example, between 40,000 km of altitude 
and 58,000 km of altitude is not great, but the path loss for the 
additional 18,000 km is a potentially big factor.  If you have a good setup 
for your DX station, perhaps this is not a problem.  However, for 
demonstration purposes, the booming signals currently heard with low 
squints at relatively low altitude are plenty impressive.  The time of day 
for the pass will also change over time (getting progressively earlier) as 
the earth continues around the sun, so this may be a factor in planning as 
well.  Unfortunately, the number of stations on the satellite, though 
considerably increased recently, is still such that any good DX site is 
going to work them all in a couple of hours.




-- 
  ________________________________________________________________________
  Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
    Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
  ________________________________________________________________________

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