Gas-Filter Correlation Radiometry (GFCR)
Gas-Filter Correlation Radiometry (GFCR) is a remote sensing technique used to measure the amount of a gas of interest in the section of an atmosphere located within the field of view of an instrument that employs this technique. It has been used since the 1960's in airborne and space-borne devices; most notably for the MOPITT instrument on-board NASA's Terra polar orbiting satellite.
As the name suggests, a sample of the target gas fills a container, usually named correlation cell, located within the path of the incoming electromagnetic radiation and acts as a spectral filter. The radiation incident upon the cell is only the part of total incoming radiation that passes through a narrow band-pass filter matching the instrument spectral sensitivity range. This range is chosen so that it contains the absorption lines of the gas, which is the reason why filtering takes place.
The absorption line is a particular case of a spectral line. It is a very narrow range in the electromagnetic spectrum where the light wavelengths are absorbed and re-emitted by the molecules of a gas. The emission takes place in different directions not related to the incident one. Depending on the position of an observer of this effect with respect to the light source and gas, emission or absorption of light can be noticed. The former takes place if the source is not visible and the observer "sees" only some of the re-emitted light. The latter case, which is the one we are interested in, takes place when the observer "sees" the source but only part of the re-emitted light. Below is a simplified graphical representation of an instrument spectral band with two (there are typically tens of thousands) absorption lines of a gas as a function of transmittance coefficient, which takes values from 0 to 1, with wavelength.
When the radiation passes through the cell containing the gas sample, the part of it corresponding to the wavelengths of absorption lines is partially absorbed. What remains is measured by a detector and associated electronics. A schematic representation of a GFCR system is represented above.
Separating the background radiation from that due to the spectral properties of the gas is a very difficult task due to the complex nature of the background signal and very small magnitude of the spectral contribution of the gas. In order to obtain a signal that can be directly linked to the gas content of the sensed atmosphere, we need to vary the conditions in the correlation cell and combine the measurements in a suitable way. This is normally done in a GFCR by modulating the density of the gas sample in the correlation cell, thus changing the width and strength of the absorption line and, implicitly, the transmittance of the cell. Further, the output of the system at the lowest and highest density values are demodulated from the detector signal and used to produce the gas-interpretable measurement.
The gas-interpretable measurement is a ratio between the difference signal (Sdiff) between the output at the lowest and highest cell densities and the average signal (Sav) obtained from the same measurements. This result can be explained by the fact that Sdiff gives information only at the absorption lines, thus linking spectrally to the gas of interest, and Sav provides a measure of the background radiation. Hence, the more sample gas there is in the sensed atmosphere, the higher the difference-to-average ratio signal becomes.
As regards to the modulation methods, there are several that have been implemented and validated in aircraft and spacecraft-borne instruments, such as selective chopper, pressure or length modulations. Synodon has devised and is using a new technique in its realSens&trade system called Simultaneous View Correlation Radiometer (SVCR).