[gradsusr] define a field from existing field

[Jian-Jian Wang]

Hi,

I have an existing field, "sat_rain" , and would like to define a new field "sat_detect", making it to be constant of 1 if sat_rain > 0.
I tried:

if (prec > 0) define sat_detect = 1

sat_detect = if(prec,>,0,2,-a)

none of them works.  Would you please provide some advice?
Thank you in advance. 

Jay