[amsat-bb] Re: Satellite in eclipse

Gould Smith gouldsmi at bellsouth.net
Tue Jan 15 13:47:00 PST 2008

Hello Darin,

The eclipse period depends upon the orbit of the satellite.  In the last 
AMSAT Journal I had a graph of the changing eclipse period of AO-51.
The ability to charge the batteries depends upon the surface area of the 
satellite that has solar cells, how efficient the solar cells are, the 
length of time the solar cells are exposed to sunlight and the angle of the 
sun on the solar cells.

With AO-51 we can set the TX output power level for both transmitters.  In 
my role of managing the satellite I am constantly monitoring the total power 
consumption of the satellite and making sure that the satellite batteries 
will sustain that power level during each eclipse period. The batteries are 
recharged each time the satellite is in the sun.  Determining the TX power 
levels and what modules are on at any particular time and factoring in the 
eclipse time and sun time plus sun angle are all part of managing the 
satellite and part of being a command station.
Some smaller satellites may not have space to add the additional control 
circuitry, it is a trade off.

AO-51 has gone from periods of nearly half an hour of eclipse each orbit to 
no eclipse period over the last 3.5 years.

Essentially it goes back to the basic design of the satellite.  Knowing the 
basic orbit the satellite will be in, how much surface area is needed to 
support X amount of TX power during Y long eclipse periods.  Then designing 
a battery charger/regulator to support this and provide the ability to alter 
whatever is needed to maintain an operational satellite.

If you interested in the actual systems AMSAT offers a book on AO-51 with 
descriptions of all the hardware systems and specifications.

Gould, WA4SXM
AO-51 Command Station

----- Original Message ----- 
From: "PDC" <yet.another.squid at gmail.com>
To: <amsat-bb at amsat.org>
Sent: Tuesday, January 15, 2008 3:19 PM
Subject: [amsat-bb] Satellite in eclipse

> At great risk of being thwapped with a clue bat, I'm going to ask this
> newbie sort of question that may seem stupid but it has me stumped.
> I read/hear about satellites being in eclipse for too long and running out
> of battery/shutting down/whatever.  Now, since most of the LEO satellites
> are in orbits of 100 minutes +/-, I don't understand how they could be in
> eclipse for more than ~50 minutes.  So the question is...
> For the satellites that have this eclipse issue, is it that their 
> batteries
> are essentially expired and can't power the satellite for ~50 minutes 
> (that
> makes sense to me) based on the charge they get from having the solar 
> panels
> exposed for ~50 minutes, or is there some mechanism that is totally
> non-obvious to me that allows a LEO satellite to be in the dark for 
> periods
> of time much longer than half of each orbit thereby creating a situation
> where the batteries have to power it in the dark for a very long period of
> time relative to the orbital period?
> If it's the former, well, that's obvious enough I guess.  If it's the
> latter, could someone draw me a picture or point me to a URL that explains
> how that is possible?
> Thanks.
> Darin
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