Outgoing Longwave Radiation OLR from the Top of the Atmosphere ToA is measured by a CERES satellite looking down on ToA equipped with a sensor as one end of a thermocouple with its other end kept at a steady temperature generating a voltage scaling with the temperature difference at its ends.
The CERES instrument is calibrated by determining a gain factor from sensor temperature to radiance letting the instrument look at a black body of known temperature $T_B$ with assumed Planck radiation $\sigma T_B^4$ while recording the sensor temperature. With the gain factor so determined the instrument reports radiance from ToA from a reading of sensor temperature. This is the simplest form of calibration assuming linearity. Very primitive technique, where the details of the instrument do not matter. It is like measuring intensity of rainfall using your hands to collect water calibrated to a faucet. The accuracy is at best 1 W/m2 or 0.4% of the same size as estimated Earth Energy Imbalance from CO2.
A pyrgeometer measuring Downwelling Longwave Radiation from the atmosphere to the Earth surface also uses a sensor as one end of a thermocouple with the other end kept a base temperature, and also measures a voltage scaling with temperature difference. The calibration is here different because the outgoing radiation from the sensor can no longer be included in the calibration process, but has to be supplied through a Planck formula $\epsilon\sigma T^4$ with $T$ sensor temperature and $\epsilon$ sensor emissivity. The accuracy is at best 5 W/m2 again too big to detect global warming if present.
OLR and DLR are thus measured in a similar way, but with different forms of calibration the difference being that OLR faces empty space ay 0 K, while DLR faces the Earth surface. The accuracy is not enough to decide any size of global warming, although it is claimed that trends can be detected.
In both cases Planck's Law in the form $\sigma T^4$ is used, which in the case of DLR is incorrect because the correct form is $\sigma (T^4-T_E^4)$ with $T_E$ Earth temperature expressing negative DLR.
Summary: Measurements of OLR and DLR are made to detect global warming. The accuracy of the instruments is not good enough to detect any warming if present. DLR measurements can be questioned since an incorrect Planck Law is used. OLR and DLR as radiance as process variable fluctuate and as such are difficult to measure.
EEI is a cornerstone of global warming alarmism, and so measuremennt of EEI has become a prime task for instrument technology, which does not seem to have delivered. The effect of EEI on surface temperature is unknown and impossible to measure and DLR is a misconception based on an incorrect form of Planck's Law.
ChatGPT on objective of CERES:
CERES connects to global warming because it:
-
Measures the planetary energy balance directly at TOA.
-
Detects changes in OLR and OSR (reflected shortwave) caused by greenhouse gases, aerosols, clouds, and ice.
-
Provides the evidence that Earth is currently taking in more energy than it loses — the physical basis of global warming.
ChatGPT on objective of measuring DLR :
- Provide a direct measure of the atmosphere’s infrared emission to the surface, essential for closing the surface energy budget, quantifying the greenhouse effect, tracking climate change, and validating models.
We read that the objective of CERES is to support global warming alarmism by measuring and reporting EEI attributed to CO2. But the objective is not reached, because (i) the accuracy of the measurement is not better than 1 W/m2, which is the expected size of EEI, and (ii) attribution to CO2 to is not credible because it is swamped by changes of cloud cover. We read that the objective of measuring DLR by a pyrgeometer is to quantify greenhouse effect. Both cases amounts to "chasing after wind" using "ghost detectors".