  Gas modulation refractometry (GAMOR) is a technique based on a
dual-Fabry-Perot (FP) cavity (DFPC) for assessment of gas refractivity,
density, and pressure that can alleviate significant limitations of
conventional refractometry systems, predominantly those related to drifts.
Repeated assessments of the beat frequency when the measurement cavity is
evacuated provide conditions under which the methodology is immune to the
linear parts of the drifts in the system, both those from length changes of the
cavities and those from gas leaks and outgassing. This implies that the
technique is solely influenced by the non-linear parts of the drifts. This work
provides a description of the principle behind the GAMOR methodology and
explicates the background to its unique property. Based on simple models of the
drifts of the temperature in the cavity spacer and the residual gas in the
reference cavity, this work predicts that a GAMOR system, when used for
assessment of refractivity, can sustain significant temperature drifts and
leakage rates without being affected by noticeable errors or uncertainties. The
cavity spacer can be exposed to temperature fluctuations of 100 mK over 103 s,
and the reference cavity can have a leakage that fills it up with gas on a
timescale of days, without providing errors or uncertainties in the assessment
of refractivity that are 3 x 10^(-12), which, for N2, corresponds to 0.01 ppm
(parts per million) of the value under atmospheric pressure conditions, and
thereby 1 mPa. Since well-designed systems often have temperature fluctuations
and leakage rates that are smaller than these, it is concluded that there will,
in practice, not be any appreciable influence from cavity length drifts, gas
leaks, and outgassing in the GAMOR methodology.
