On 6/23/13 10:47 AM, Chris Albertson wrote:
Magnetic cores were not invented until the 1950's and realy cam into use as
tubes were beibg replaced by SS. But there isnot reason yu can't build a
tube computer with core memory. I have actually seen and used a computer
that had one megabyte of core memory. The stuff was still in use in the
late 1970s 1MB was a lot of RAM in 1975.
Lots of rotating drum memory and acoustic delay lines were used back then.
I don't know that the whole GPS thing is very memory intensive anyway.
You've only got a few dozen variables to work with, it's all in the
math, but presumably you'd that as a parallel or pipeline
implementation, rather than a ALU and memory. It's not like you're
going to be changing the algorithm often (e.g. the programming language
is solder)
The almanac is a bit more of a challenge, but if you allow "externally
supplied" that might make it easier. But even that isn't all that big,
11 or 12 numbers times 32 or so spacecraft.
You can have very good reliability with tube circuits. It was just that
few people wanted to pay for it. Down time was cheaper. It is not hard to
add redundancy to a circuit but it does have a huge cost multiplier effect.
4x or 5x the price.
Not necessarily. That is, your redundancy doesn't have to be something
like triple modular redundancy. You can do more efficient error
correcting codes, use clever arithmetic, or make use of the fact that
your computer can run faster than you need the computation to be done,
so you can get temporal redundancy. Serial computation can also be
done. Remember that the nav message is only coming in at 50 bits/second.
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