Dear Klaus, Laurence, LF Group,
I have been doing some experiments with WSPR on 2 PCs linked by audio
cables. I would certainly agree with KL1X about the importance of clock
accuracy - I found that an error of 5 seconds was enough to completely
prevent decoding. Fortunately, the clock on my shack PC is accurate
enough
that setting it against an MSF-controlled clock once every several hours
is
OK. I also found that the "QSO version" of WSPR in WSJT 7 and the WSPR
1.01
beacon software will correctly decode each other's signals, although in
either case the QSO mode requires reception of all the overs of the QSO
in
order to display the callsigns, due to the format used. When using the
beacon software for reception, be aware that it may take some minutes for
the decode to appear after the signal has been received - this seems to
be
due to the "off line" nature of the signal processing used.
Recent experience here is that the WSPR mode offers advantages compared
to
CW, either manual or QRSS, on the 500kHz band due to the low SNR and the
fading experienced. Many signals are either always below the audible
noise,
or not audible for long enough for aural CW copy without large numbers of
repeats. With QRSS, and a sufficiently long dot period, one can indeed
detect very weak signals, but the fading often prevents receiving, for
example, a complete callsign without losing some symbols and so also
requiring repeats. QRSS works much better on 136k, where the fading
period
is much longer.
WSPR certainly decodes signals that are too weak for aural reception. In
sensitivity terms, I think it is comparable to QRSS 3. WSPR has the
advantage over QRSS 3 that the message duration is shorter. The 2 minute
transmission period of WSPR is a reasonable match to the fading period
experienced on 500kHz, so there is a good chance of sending the message
successfully before the signal fades out. I have yet to see any readable
trans-atlantic CW or QRSS signals at this QTH, while copy of WE2XGR//2
using
WSPR was reasonably consistent, producing a decode for about 25% of the
transmissions. The redundancy in the data should also make WSPR
relatively
resistant to errors caused by QRN impulses, although the QRN level has
been
low here recently. For beacon purposes, WSPR also has the huge advantage
that reception, logging and reporting is automated, so the operator can
go
to bed sometimes!
While WSPR does work well, I don't think it is the final word in LF/MF
digital modes - In particular, the information in a QSO is largely
restricted to "rubber stamp" exchanges. So I will certainly be interested
in
trying other modes, and look forward to see the results of Klaus' work.
Cheers, Jim Moritz
73 de M0BMU
