On 03/18/2013 07:09 PM, Lux, Jim (337C) wrote: > Another case where SP might be ok is in converting from fairly low precision > gridded observations and doing some sort of model function retrieval, and > perhaps regridding. > > If your measurements of ocean backscatter are on 10km grids, and the > uncertainty of the measurement after model function retrieval is 0.5 m/s out > of a maximum value of 50 m/sec, SP is probably more than accurate enough, > since the random noise in the input data set is enormous compared to any > roundoff precision things. > > Likewise, if you're taking data that's unevenly spaced from an orbiting > sensor (e.g. Seawinds on QuikScat) and regridding to a 10km mesh, I doubt > that going to DP will make any difference in the interpolation. > > SAR (synthetic aperture radar) data processing is another case where SP is > probably good enough. If the radar is collecting data with a 12bit ADC, it's > unlikely that you'd need more than 24 bits of mantissa even after all the > range processing. SAR is a notorious bandwidth hog, as well. 1 Gbit/s kinds > of rates aren't unusual. For on board processing, often they do some form of > block floating point, so the math is really all integer. > > But even in FP, the dynamic range of the signal isn't all that great. > Compare the 9.4 GHz radar cross section of an aphid head on is about 5E-6 > cm^2 (from Riley, IEEE Proc, 1985), or 5E-10 m^2. Let's consider something > really big as a target, like the moon with a RCS of around 1E13 square meters > (I just took the cross sectional area..the moon's round and rough, so its RCS > is lower). That's a range of 24 orders of magnitude, which would justify > DP, but, on the other hand, it's unlikely we'll be looking for individual > aphids on the moon. > > If you were processing all the RF signals available in the HF band you might > need DP. The instantaneous dynamic range can be around 130dB within a small > subband, and that's on the order of 21-22 bits, and if you look at the entire > 2-30 MHz spectrum, you probably have more range than can be accommodated in > the 24 bit mantissa of a SP floating point. > > Jim Lux
I would be happy to be corrected, but isn't an SNR of 1000:1 considered to be excellent for a spectrograph (I'm thinking of particular datasets from Victor Blanco and CFHT). I don't believe any reduction pipeline or analysis for such data benefits from DP. _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
