Tolerating Delays

By Mike Deliman

This may sound a bit funny, but in the space industry, we're
constantly playing catch-up.  We're either looking at or for things
that happened millions or hundreds of millions of years ago, sending
rockets off to get to where something will have been just in time to
take a picture of or bore a hole into it, or designing new rockets for
flight 5 years from now with computer bits that would have been
considered top-of-the-line 5 or 10 years ago.  When we're recovering
data and sending commands from and to deep space probes, we point our
antennae to where the probe is supposed to be 30 minutes from now and
start sending our data now; the idea is by the time the data actually
gets there the craft will be where it was expected and receive the
commands, and send it's data back to us.

This process I just
described – of anticipating where a craft is, transmitting before it's
there – and it transmitting back – is pretty much how the Deep Space
network is currently used.  It takes huge amounts of planning, all done
in advance, to set up the multiple sessions that allow one successful
exchange like that to work out.  People consult tables of times when
craft will be "visible", consult tables of one-way light distances to
find transmission times.  When they think they know when they need
time, and how much data they expect to exchange (how much  time they
need),  they contact the folks who run the big antennas.  if the time
slot is available, arrangements are made, and that one set of
transmissions can take place.  The folks who run the Deep Space Network
and their customers do this sort of thing all the time – it's how they
try to make the most efficient use of their giant antennae.

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