[Lf] "long" baseline LF reception

[Stewart Nelson]

Hi Jake and all,

It would be great if the receiving sites could somehow just
use a conventional SSB receiver, capture the audio with their PC's
sound card, and forward the data to a central site.  IMO,
this would encourage at least ten times as many hams to
participate.

It appears that my suggestion of using the LORAN "QRM" for
synchronization was not taken seriously (I admit that it is
quite complex, but still believe it is feasible).

Here is another scheme which should be pretty easy to process.
It is based on an invention by Peter Martinez, G3PLX.  All you
need is a GPS receiver with a one pulse per second output.
These are readily available for about $100.  

The 1 PPS is simply coupled, through a small capacitor, into
the antenna input of the receiver!  Listening to the
output, you'll hear a click once per second.  The clicks all
sound about the same.  But if you look with a scope, the
pulses don't look the same, unless the receiver LO is a precise
multiple of 1 Hz.  That's because the phases of the various
components of the audio pulse are a function of the instantaneous
LO phase when the 1 PPS pulse arrives.  These phase changes can
be tracked with software to determine, and compensate for, the
variations in the receiver's LO.  Effectively, you are "locking"
the receiver by post processing its output.  This also provides
time stamps and compensation for error in the soundcard's
sampling rate.  The time of the start of recording must be known
within 0.5 seconds, and the receiver's frequency must be known
within 0.5 Hz.  Receiver drift must be less than about 10 mHz
per minute.  After capturing just the GPS pulses for about one
minute (for calibrating the impulse response of the receiver),
the antenna is also connected and the combined signals are
recorded.  Subsequent software processing, which need not be
real-time, converts the output to what would have been received
if the LO and soundcard timebases were precise.

This method could also be used with other frequency standards.

73,

Stewart KK7KA