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HRPT (was: RE: uo-36 tnc's)



Several people asked about this, so a few brief notes
may be in order.

The application is the digital HRPT downlink from the
NOAA weather satellites. The operational frequencies
are typically 1698 and 1707 MHz.

The downlink is Manchester-encoded BPSK at 665.4 kbps.
NOAA (and lots of other agencies) solved the Doppler
problem long ago. They transmit a residual carrier
(typically about 8 dB down) - enough for a synchronous
detector to lock on to, but not enough to mess up the
spacecraft power budget too badly. Cheap, simple, no
fuss, no hassle. Obviously not an AMSAT design. :-)

With only a couple of frequencies to worry about, and
Doppler already taken care of, there is no great need
for frequency agility. I'm using a PLL synthesizer for
the LO because I have one available.

My RF hardware will owe a lot to the receiver published
by S53MV (VHF Communications 3/97, UKW-Berichte 4/95).
Matjaz uses a fairly low 1st IF of 36 MHz with a TV IF SAW
filter, followed by a PLL detector. Such a low IF requires
some care in the RF hardware, but it's not impossible.

For the computer interface I'm trying to convince myself
that USB is the answer, but I'll probably initially just
latch 8 bits at a time and haul them in through the
parallel port. Since all it's doing is recording, this
shouldn't tax a modern parallel port (or my doorstop
computer) too badly.

As with my current APT system I'll record the pass, and
then do all further processing offline in software. The
underlying data formats, for example, favour 10 bit quan-
tities...

Documentation? Have a look at:

   http://www2.ncdc.noaa.gov/docs/klm/html/c4/sec4-1.htm

Laura Halliday VE7LDH     "Que les nuages soient notre
Grid: CN89mg               pied a terre..."
                                   - Hospital/Shafte
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