left over noise,
love to see their error analysis how they got there,
very hard measurement to do
their value is 2.725 K @ 160Ghz says wiki
classic equation for noise is P=kTB
where
k is boltzmans constant 1.3807 E-23 [J/K]
T is temperature [K]
B is Bandwidth [Hz]
k error bounds;
The Boltzmann constant (kB or k), which is named after Ludwig Boltzmann,
is a physical constant relating the average kinetic energy of particles
in a gas with the temperature of the gas.[2] It is the gas constant R
divided by the Avogadro constant NA:
k = R/NA
The Boltzmann constant has the dimension energy divided by temperature,
the same as entropy. As of 2017, its value in SI units is a measured
quantity. The recommended value (as of 2015, with standard uncertainty
in brackets) is 1.38064852(79)×10−23 J/K.
https://en.wikipedia.org/wiki/Boltzmann_constant
T error bounds;
one has to calculate the noise temperature of the receiver, using MFGR
data on the first amplifier, or measure it, one can bracket the value,
1.7 K cryogenic amp here;
http://www.lownoisefactory.com/products/cryogenic/06-2a/
T error bounds;. Then there is an additional factor due to the antenna
pattern, picking up backlobes or sidelobes of the warm earth. the
pattern can be calculated and measured withing a few dB, except for
backlobes which rely on phase cancelations.
B error bounds; this can be measured.
so two main sources of error are the antenna pattern w backlobes, and
noise temperature of the reciever.
note I have not specified a frequency band yet,
nor addressed the noise power from stars in the main beam
https://en.wikipedia.org/wiki/Cosmic_microwave_background
from wiki
The cosmic microwave background was first predicted in 1948 by Ralph
Alpher and Robert Herman.[19][20][21] Alpher and Herman were able to
estimate the temperature of the cosmic microwave background to be 5 K,
though two years later they re-estimated it at 28 K. This high estimate
was due to a mis-estimate of the Hubble constant by Alfred Behr, which
could not be replicated and was later abandoned for the earlier
estimate. Although there were several previous estimates of the
temperature of space, these suffered from two flaws. First, they were
measurements of the effective temperature of space and did not suggest
that space was filled with a thermal Planck spectrum. Next, they depend
on our being at a special spot at the edge of the Milky Way galaxy and
they did not suggest the radiation is isotropic. The estimates would
yield very different predictions if Earth happened to be located
elsewhere in the universe.[22]
even more from wiki;
The interpretation of the cosmic microwave background was a
controversial issue in the 1960s with some proponents of the steady
state theory arguing that the microwave background was the result of
scattered starlight from distant galaxies.[29] Using this model, and
based on the study of narrow absorption line features in the spectra of
stars, the astronomer Andrew McKellar wrote in 1941: "It can be
calculated that the 'rotational temperature' of interstellar space is 2
K."[30] However, during the 1970s the consensus was established that the
cosmic microwave background is a remnant of the big bang. This was
largely because new measurements at a range of frequencies showed that
the spectrum was a thermal, black body spectrum, a result that the
steady state model was unable to reproduce.[31]