Modelling the sidelobe contributions to ATMS measurements

NPOESS is a tri-agency (DOC, DoD, and NASA) United States program to provide the next-generation operational, environmental, polar-orbiting satellite system administered through the NOAA Integrated Program Office (IPO). The Advanced Technology Microwave Sounder (ATMS) is one of the NPOESS instruments.

Microwave radiometers have wide beamwidths and significant sidelobes, which entail the possibility of contaminating the signal of interest, which comes from a far-away scene, with signals from nearby structures.

 In the case of a satellite-borne instrument, these local sources are often located at large angular distances from the boresight of the antenna, and physically very near the instrument itself.

During the pre-launch testing of the ATMS instrument, the measurements of beam patterns consisted of taking 'cuts' (at orthogonal polarizations) through the antenna beam, as shown in Figure 1.

These one-dimensional profiles have been processed into a two-dimensional array, as shown in perspective view in Figure 2 and in Azimuthal Equidistant projection in Figure 3, which facilitate their integration for determining compliance with requirements such as beam efficiency, and allow the convolution of the antenna pattern with both the Earth hemisphere, and with nearby obstructions.

Using a simple model of the environment on the NPOESS Preparatory Project (NPP) spacecraft, and accounting for the changes in the pointing of the antenna as a result of the scanning motion, as indicated in Figure 4, this paper attempts to predict any angular dependency, or asymmetry in sidelobe radiation which might mask a scene-dependent signal.

Results will be presented for a few of the frequency channels of the ATMS.