The Advanced Technology Microwave Sounder (ATMS) Antenna Beam Analysis Software uses a suite of test data to calculate the ATMS instrument beam pointing, and secondarily, the polarization state, in the coordinates of the onboard optical cube. The software uses measurement inputs of RF radiation pattern performance made at multiple rotational viewpoints and across a range of frequencies, rotation matrices to characterize physical rotation between the instrument measurement states and the optical cube, and correction rotation matrices used to eliminate gravity effects through an independent characterization.

The JPSS (Joint Polar Satellite System) ATMS instrument accepts microwave energy from a specified direction at a moment in time. Knowledge of where the acceptance angle is — the instrument “beam pointing” — is critical to using the instrument data products properly. Historically, characterizing the beam pointing involved making antenna pattern measurements and extrapolating the resulting beam peaks in orthogonal planes. The primary disadvantages to this prior art technique involve test errors due to anechoic chamber test zone wavefront uncertainty, errors due to asymmetric gravity sag on the instrument while being tested at various beam position orientations and plane rotations, and errors due to RF multipath within the test zone and chamber.

The expected input data consists of sets of azimuth/elevation RF radiation pattern cuts, at multiple f rotations, and over a frequency sweep if time gating of RF multipath is desired, or just at discrete frequencies. The input data format is the facility’s native MI Technologies MI-3000 antenna measurement system database format.

Five key steps are included in the data analysis when applied to a set of measured data files: (1) data input of measured UUT (Unit Under Test) and standard gain horns (SGH) of mechanically known polarization orientation; (2) display and time gating of the measured antenna patterns; (3) beam pointing and chamber wavefront analysis; (4) SGH frequency response and polarization calculations; and (5) UUT polarization tilt angle analysis.

All results are displayed graphically as a function of frequency, and are averaged at the desired frequency to determine the tilt angle estimate. The software has been validated using two methods: self-validation, which also highlights any operator error, and (2) independent validation of a separate simpler and known antenna. Results were within the accuracy requirements of the ATMS instrument.

Unique features include direct importing of the measurement data files in their native MI-3000 database format, and Excel templates populated with the results of predefined characterization laser tracker test routines measured of the instrument, the latter involving both the test orientation and a separate characterization of the gravity effects.

This work was done by Kenneth Hersey of MEI Technologies for Goddard Space Flight Center. GSC-16784-1

NASA Tech Briefs Magazine

This article first appeared in the November, 2015 issue of NASA Tech Briefs Magazine.

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