[sdiy] VERY cool/bizarre: Buchla noise

[David Cornutt]

On Sep 30, 2004, at 2:28 AM, Magnus Danielson wrote:
>

> On the same time is the LACK of true randomness in pseudo-random 
> generators
> also used EXTENSIVELY. In GPS is the C/A code 1023 bits/chips long and
> repeating 1000 times a second (1.023 Mchips/s). Each satelite has it's 
> own
> unique pattern and the "randomness" allows for them to become 
> spread-spectrum.
>

The NASA TDRS satellites use a similar technique; in this case it's 
done mainly
for noise immunity.  They have an additional "coherent" mode which adds 
an
interesting twist.  When they set up a link with a Shuttle in coherent 
mode, the
TDRS encodes the "forward" link (data going to the Shuttle) with a 
short sequence.
The Shuttle strips off the sequence and then uses the sequence it 
received to
encode the "return" (telemetry to the ground) link.  By measuring the 
time/phase
lag between the sent sequence and the received one, the TDRS calculates 
how
far away the Shuttle is from it.  Combining that with antenna pointing 
data, the
TDRS can calculate the Shuttle's position -- a no-radar-needed tracking 
system.