radargrams with the data values

A JPEG format browse image based on a data product is plotted. The following parameters are parsed from each RDR frame. RDR Field Variable(s) ----------------- ------------------ RANGE_SHIFT sft ECHO_SAMPLES_REAL re ECHO_SAMPLES_IMAGINARY im SUB_SC_EAST_LONGITUDE slon (orbit start) elon (orbit end) SUB_SC_PLANETOCENTRIC_LATITUDE slat(orbit start) elat (orbit end) The real and imaginary parts of the signal are then joined via a traditional log-scale transform:

 sig = 10 * log10 (re^2 + im^2)
I’m confused about the axes : took time on x axes and sig values on y axes

 The sig is sorted numerically and the mean value for the lowest
 100 sig values is added to a running average noise value.
| _*how to find running average noise value*_|

 The mean noise is subtracted from each pixel value, transforming 
 the 'sig' variable into 'san', or 'signal above noise.'

 san = 10*log10(10.^(0.1*sig)-10^(0.1*noise));

 The san values are scaled so the pixel brightness values
 correspond linearly to an absolute range of 0 to 40 decibels.
How to do this one : 
 Finally, the radargram is shifted to adjust for the RANGE_SHIFT

Can Anyone tell me how to plot jpeg image with san values