wa1...@att.net wrote:
Hi Jim,
Is BPSK the modulation used on such an 8bps link for DSN?
BPSK, typically with a lot of residual carrier. And, at slow rates, the
data is modulated on a (e.g. 16 kHz) subcarrier, which is then phase
modulated on the main carrier with a mod index to create the desired
ratio between power in the data signal and power in the carrier. Typical
mod indices would be 0.2 to 1.3 radian (peak).
If so, any idea what limit of C/N is used? And in what BW?
For the SDST, which we've been using for the last 10-15 years, threshold
(the signal level at which the carrier can be acquired and tracked) is
<-156 dBm. Tracking loop BW is on the order of 20Hz for small signals.
NF is <2.5dB
You can read more at
nmp-techval-reports.jpl.nasa.gov/DS1/SDST_Integrated_Report.pdf
(actually, googling SDST will turn up a fair amount of data)
The challenge in operating is that you don't necessarily have a-priori
knowledge of where the SDST is listening (temperature and doppler
uncertainty, mostly).. so we sweep the uplink signal over a range that
hopefully covers where it is. So there's a tradeoff between loop
bandwidth being wide enough to allow a decent sweep rate and narrow
enough to allow working at low signal levels. Because of the long light
time delay, the sweep and acquisition process is open loop (that is, we
assume we swept across the appropriate frequency, that the SDST acquired
and is following, and we can send the data)
Newer radio designs do things like change the bandwidth, or use
parallel detection.
My interest in short-term stability has always been for more DX on the
mm-wave ham bands. My best results were running QRSS with <1Hz BW on
241GHz but have often wondered if BPSK would be better than OOK.
For a given transmitter power, BPSK has a higher average power than OOK.
The problem is that it takes a more sophisticated receiver than a
power detector. OOK has the virtue of simplicity.
-Brian, WA1ZMS
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