A method of estimating the pointing error of an offset-conic-section reflector antenna equipped to receive in both right circular polarization (RCP) and left circular polarization (LCP) exploits a phenomenon that, heretofore, has been regarded as a minor nuisance. The phenomenon in question is a misalignment between RCP and LCP beams and manifests itself as opposing squints of the beams. The method enables the generation of pointing-error estimates at high rates for use as feedback in high-bandwidth antenna tracking-control systems, without need for the complex radio-frequency circuitry of monopulse or traditional sequential-lobing antenna tracking-control systems. The method was conceived especially to be used at terrestrial stations to track LCP and RCP signals transmitted by satellites.

These Curves Were Calculated for Gaussian RCP and LCP beams 3° wide, with 0.3° squint between beams.

When an antenna is mispointed in one direction, the RCP signal normalized by its peak value is stronger than the corresponding normalized value of the LCP signal. When the antenna is mispointed in the opposite direction, the normalized LCP signal is the stronger one. Zero pointing error in this context is defined as the antenna-aiming direction for which the normalized RCP and LCP signal levels are equal. The difference between normalized signal levels can be characterized as an S-curve function of the pointing error; this curve is given by

S( φ) = 2{[R( φ)/RP] + [L( φ)/LP]}/{[R( φ)/RP] – [L( φ)/LP]},

where φ is the angular pointing error; R( φ) and L( φ) denote the levels of the RCP and LCP signals, respectively, at φ; and RP and LP denote the peak levels of these signals. The figure presents an example of the normalized signal levels and of the S-curve function for small pointing-error angles. The S-curve function can be inverted so that the pointing error can be estimated from the normalized signal levels.

In the present method, one obtains the normalized signal levels from measurements of levels and differences between levels of satellite signals received through the antenna while switching rapidly (electronically) between the reception in RCP and LCP. “Satellite dish” antennas equipped with circuitry to receive both RCP and LCP and capable of rapid switching between RCP and LCP are commercially available. This method of reception amounts, in effect, to a method of electronic sequential lobing. The faster the electronic switching between RCP and LCP, the greater the bandwidth of the pointing- error signal.

This work was done by Arthur Densmore of Caltech for NASA’s Jet Propulsion Laboratory. Under the Electronic Components and Systems category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel,
NASA Management Office–JPL;
818-354-7770.

Refer to NPO-19938.


This Brief includes a Technical Support Package (TSP).

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This article first appeared in the April, 2002 issue of NASA Tech Briefs Magazine.

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