Random noise at a given position during a standard 180 second scan is
3npB
.
This is set mainly by detector read noise but also shows
some signature of noise due to sky
brightness fluctuations.
Average images using 10 or more scans have a random noise of approximately
1npB
.
Systematic error is estimated by the degree to which the measured polarization brightness
approaches zero in the far field.
This value is 3npB
.
The systematic error in the inner field is set by the accuracy of the known value for the
calibration tilt plates.
Uncertainty comes from lack of precise knowledge of the tilt plate orientation.
The estimated error is 
10%.