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In 1992 the COBE DMR team, including George F. Smoot and Charles L. Bennett, used radiometer antennas to compare antenna temperatures between sky directions $60^{\circ }$ apart at $31.5$, $53$ and $90\mathrm{GHz}$, with two nearly independent channels per frequency. Repeated sky coverage supported least-squares solution map reconstruction.^[ref]

The analysis fitted magnetic response and removed the assumed kinematic angular dipole and its second-order Doppler effect contribution. Channel sums retained sky structure; channel differences estimated noise. Cross-channel correlations, different frequency maps and Milky Way latitude cuts tested the interpretation. Residual large-angle structure supported Cosmic microwave background anisotropy; Galactic contamination particularly limited the cosmic microwave background quadrupole. Brightness fluctuations alone were not sufficient evidence of primordial structure.^[ref]

The two differences serve different purposes. Comparing two sky directions measures angular variation. Comparing two independently calibrated receiver maps observing the same sky tests noise and consistency. A common sky signal cancels in the latter comparison if their antenna beam patterns and response scales match; a systematic error shared by both channels need not cancel. Separating observer motion, local emission and intrinsic Cosmic microwave background anisotropy requires these additional tests.

The cosmic microwave background is nearly isotropic thermal electromagnetic radiation. Its spectrum connects radio receiver measurements with the universe's thermal history: redshift preservation of a thermal photon spectrum cools a blackbody spectrum as space expands. The 1965 Princeton analysis described this relation.^[ref]

A radiometer cannot label a received photon as cosmic, atmospheric or instrumental. Its measured power combines the incoming radiation with the receiver response. The useful question is which comparisons and controls can isolate a sky contribution. This makes the Penzias-Wilson microwave background experiment an example of both observational cosmology and measurement calibration.

David Todd Wilkinson helped construct a receiving horn and radiometer for absolute microwave-background measurements. The 1965 hot-universe paper explicitly credited the Princeton instrument to him and P. G. Roll, and explained the 3-centimetre wavelength as a compromise between atmospheric absorption and longer-wavelength foregrounds. Its measurements were still pending when the paper interpreted the separately reported microwave excess. This separates the team's experimental preparation from the evidence already available for relic radiation.^[ref]

Noise-equivalent radiance is the incident radiance change producing signal-to-noise ratio one for a specified spectral and electrical bandwidth. A linear radiometer with voltage response $G=dV/dB$ and RMS voltage noise ${\sigma }_V$ has $\mathrm{NER}={\sigma }_V/|G|$. Its convention must distinguish per-sample noise from a noise amplitude per square-root bandwidth.

Source paper: www.whatdotheyknow.com/request/physics_past_exam_papers_2015_16/response/973997/attach/3/Cowell%20Attachment.zip?cookie_passthrough=1. The dated cover identifies the Part C sitting of 14 June 2016. All eight options are treated below, using the native radiometer plot for the graphical estimates.

A calibrated multispectral filter radiometer is a natural choice for rapid thermal mapping: a few selected bands provide higher photon throughput and faster imaging than spreading the light over many spectrometer channels. Several bands can help separate temperature, dust emissivity and atmospheric effects. A spectrometer would be justified if identifying composition or resolving spectral structure were a central additional objective; one broadband radiance alone cannot generally determine both temperature and unknown emissivity.

Thermal infrared radiation is preferable to reflected visible light for measuring temperature. The Rayleigh criterion setsA band near $10\mu \mathrm{m}$ is therefore a reasonable compromise between useful planetary thermal emission and the required diffraction-limited resolution. At that wavelength, the nominal surface Rayleigh criterion separation is about $415\mathrm{m}$; at $15\mu \mathrm{m}$ it is about $623\mathrm{m}$ and fails the target. The exact bands must avoid or model strong atmospheric absorption and account for the dust spectrum. The $500\mathrm{m}$ sampling pitch is still not a guarantee of faithful reconstruction of arbitrary $500\mathrm{m}$ features: detector sampling and the point spread function must be considered together.

In 1965, Arno Penzias and Robert Woodrow Wilson reported measurements with a 20-foot horn-reflector antenna at $4.080\mathrm{GHz}$. They manually switched a traveling-wave maser receiver between the horn and a helium-cooled reference. From $6.7\mathrm{K}$ zenith antenna temperature, atmospheric and antenna/ground contributions left an excess $3.5\pm 1.0\mathrm{K}$.^[ref]

Elevation scans estimated the atmospheric term. Taping horn seams produced no measurable change; a nearby transmitter tested ground response. These controls addressed distinct possible contaminations.^[ref]

The comparison does not make all nuisance contributions disappear. A reference can establish a response scale, but atmospheric emission and pickup through unwanted antenna directions are also real incident radiation. In his later account, Robert Woodrow Wilson describes these competing contributions as central to a radio astronomy measurement.^[ref]

The Princeton group proposed primordial thermal radiation as an interpretation and called for more wavelength measurements. Their separate $3\mathrm{cm}$ radiometer had not yet produced results.^[ref] A single-frequency excess therefore does not by itself establish a complete Planck spectrum. Reproducibility across independent instruments, observing directions and frequencies tests the interpretation further.

Constructed, jointly with a coauthor, a radiometer and receiving horn for absolute thermal-radiation measurements at $3\mathrm{cm}$. The associated cosmic microwave background paper explained the wavelength choice as a compromise between atmospheric absorption and longer-wavelength foregrounds; results from this instrument were still pending.^[ref]

A radiometer measures electromagnetic radiation through its received power. A radio radiometer can express that power as a noise temperature, allowing an unknown source to be compared with calibrated reference temperatures.

For a stable linear radiometer, known cold and hot radiances give $B_S=B_C+(B_H-B_C)(V_S-V_C)/(V_H-V_C)$. Put $a=(B_H-B_C)/(V_H-V_C)$ and $b=-a(V_S-V_C)/(V_H-V_C)$. Then $\delta B_S=a\delta V_S+b\delta V_H-(a+b)\delta V_C$; all three measured voltages contribute to the retrieved uncertainty.

For a stable radiometer with affine output $S=g(T+T_r)$, let $g>0$ be the fixed response coefficient and $T_r$ the receiver noise temperature. Two reference loads with known equivalent input noise temperatures $T_c<T_h$ give outputs $S_c$ and $S_h$. Subtracting their outputs eliminates $T_r$; dividing source and reference differences eliminates $g$:This two-load radiometer calibration assumes unchanged response and receiver noise temperature between readings, with the loads and source referred to the same input. It also eliminates an additional constant output offset. Physical load temperatures must first be converted to the applicable equivalent input noise temperatures when the classical thermal approximation fails. The Penzias-Wilson microwave background experiment gives an original worked example.