| Summary: | The High-Resolution Infrared Radiation Sounder (HIRS) has been operational since 1975 on different satellites. In spite of this long utilization period, the available information about some of its basic properties is incomplete or contradictory. We have approached this problem by analyzing intrusions of the Moon in the deep space view of HIRS/2 through HIRS/4. With this method we found: (1) The diameters of the field of view of HIRS/2, HIRS/3, and HIRS/4 have the relative proportions of <inline-formula> <math display="inline"> <semantics> <mrow> <mn>1.4</mn> <mo>°</mo> </mrow> </semantics> </math> </inline-formula> to <inline-formula> <math display="inline"> <semantics> <mrow> <mn>1.3</mn> <mo>°</mo> </mrow> </semantics> </math> </inline-formula> to <inline-formula> <math display="inline"> <semantics> <mrow> <mn>0.7</mn> <mo>°</mo> </mrow> </semantics> </math> </inline-formula> with all channels; (2) the co-registration differs by up to <inline-formula> <math display="inline"> <semantics> <mrow> <mn>0.031</mn> <mo>°</mo> </mrow> </semantics> </math> </inline-formula> among the long-wave and by up to <inline-formula> <math display="inline"> <semantics> <mrow> <mn>0.015</mn> <mo>°</mo> </mrow> </semantics> </math> </inline-formula> among the shortwave spectral channels in the along-track direction; (3) the photometric calibration is consistent within 0.7% or less for channels 2–7 (1.2% for HIRS/2), similar values were found for channels 13–16; (4) the non-linearity of the short-wavelength channels is negligible; and (5) the contribution of reflected sunlight to the flux in the short-wavelength channels can be determined in good approximation, if the emissivity of the surface is known.
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