This may answer some of your questions, but not all. Additional information may exist in the book.
Also take into account that both Voyager spacecrafts could perform the same measurements before they diverged. The results could be compared.
Following excerpts are from the book Deep Space Craft: An Overview of Interplanetary Flight, Dave Doody, Springer Science & Business Media, 2010, parts of which are available in the free preview (use chapter links to the source of the two excerpts):
5.6.2 Extensible Booms
[...] A potential source of error in the magnetometer instruments’
measurements is the amount of twist in the long fiberglass
magnetometer boom established after deployment. To compensate, a
magnetic coil, which can be energized on command to create a magnetic
field of known strength and orientation to calibrate the instruments,
is mounted on the spacecraft.
6.4.8 Calibrations and Ground Truth
Scientific measurements are made by using instruments that have
quantifiable error. Calibrations are carried out by instruments on a
spacecraft to acquire baseline data for comparison with an actual
observation, thus allowing instrument errors to be quantified.
Prior to carrying out an infrared spectral measurement of a target,
the IR spectrometer will be aimed toward a spot of deep space free of
any bright objects in its field of view. An absolute reference value
is obtained, and any defects in the instrument’s sensors can be
recorded and later included in data analysis.
For the same reason, radio science experiments always begin and end
with a measurement of the spacecraft’s unobstructed, unmodulated radio
tones lasting tens of minutes before and after encountering the
Many imaging instruments may be aimed toward a special calibration
target mounted on the spacecraft bus. Voyager’s calibration target was
a rectangular plate mounted below the bus (see Appendix A, page 294)
coated with a material of known grayscale and albedo values. The
spacecraft’s scan platform could aim the cameras so that the target
plate would fill the field of view for calibration.
The same target plate on Voyager serves as the thermal radiator for
the spacecraft’s electrical system regulator, so Voyager’s infrared
instrument, the infrared radiometer-spectrometer (IRIS) could be
The operation of every science instrument and experiment includes some
sort of procedure or other means for calibrating its measurements.