[amsat-bb] Re: AO-51 S band Question

Andrew Glasbrenner glasbrenner at mindspring.com
Wed Jul 21 13:37:16 PDT 2010

Reid Crowe wrote:
> Drew,
> Do you think the bird will be able to support 2.4gig in the future?
> -Reid
Hi Reid,

I hope you don't mind if I copy this answer to the -bb as well. It's a 
good question. The batteries on AO-51 have lost significant capacity 
since launch six years ago, and in order to protect and lengthen their 
life as long as possible, we are limiting the load during eclipse by 
shutting off the UHF transmitter. We are also using the PL to only turn 
on the UHF transmitter when in use. Both of these actions are controlled 
by the flight software, specifically the PL and power management routines.

Since the S band transmitter on AO-51 is an added "experiment", the ways 
in which it reacts with the IHU and flight software are different than 
the UHF transmitters. The PL cannot be used to turn S on or off, only to 
gate the audio. The power management software is unable to control the 
state of the S band transmitter as well. The S transmitter is a fixed 
output, that draws about 730 mA. This is an equivalent load to running 
the 435.300 transmitter at over 1 watt. In other words, it is not an 
insignificant load.

Since the return of eclipses this summer, after a long period of total 
illumination, we have discovered that the S transmitter will pull the 
battery voltage too low during eclipses. This activates a software 
watchdog that shuts the S transmitter off until commanded back on from 
the ground. As a work around, we have been using the automatic mode 
scheduling software to re-enable the S transmitter a few minutes after 
exiting eclipse each orbit. This has worked very well. The downside is 
the scheduler only holds 18 events, as it was designed for the weekly 
mode changes. At 1440 minutes per day, and 100 minutes per orbit, you 
can see we need to upload a new schedule every day to keep S running 
every orbit. As long as the satellite is in the sun, there is no problem 
running the S downlink.

So, you can see why we have been frugal with S activity this summer, as 
it is a real load on the command stations to plan the daily schedule, 
encode it, and upload it to the satellite daily (at 5AM). The good news 
is we return to a no eclipse period at the end of August, and I plan to 
run S at a ~minimum~ of 1 week a month until eclipses return in 2011 for 
many years. This works out OK over the summer, as there are lots of 
folks on right now from their vacation spots, and out on the roads 
giving out rare grids.

I hope this long-winded, probably-too-detailed explanation answers your 

73, Drew KO4MA

BTW, this AM Mark N8MH raised the 435.300 output to 1 watt.

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