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AO-16 in repeater mode

In ANS this week the note on AO-16 is a bit incomplete regarding the 
current state of the satellite.  Here is some further detail.

The on-board computer has not failed. What is no longer working is the 
computer's ability to access the EDAC SRAM needed to run uploaded 
software.  The boot loader (MBL) software was burned into ROM prior to 
launch and continues to run.  The processor itself works OK and is 
running that MBL software just fine.  One of the unique features of MBL 
is that it does not use any SRAM but runs entirely in processor 
registers.  It also does not use DMA or interrupts. One of the design 
goals was for it to continue to run even when several hardware things 
had failed - and that has proven to work well now that the SRAM access 
no longer works.

With the computer still processing instructions and running MBL it 
remains possible to send commands directly to various hardware modules 
using the AART bus (another innovation in these satellites).  It is that 
capability which allowed the mode to be switched.  Further, we know the 
computer continues to successfully run MBL because any glitch in 
processing would likely reset the computer. That would cause MBL to 
restart and reset the hardware to the default, turning off the U TX, and 
resetting the downlink to digital. Should that happen it might be 
possible to get it back into the repeat mode, and perhaps do that more 
than once. But at this point it's anyone guess how long enough hardware 
will work to allow more use from this rugged old bird.


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