Large aperture, high-frequency antenna measurements take time, so errors due to thermal drift may affect accuracy. The NSI-SW-5105 Motion Tracking Interferometer (MTI) option is a software enhancement that addresses this problem and increases measurement accuracy by correcting for thermally induced amplitude and phase measurement drifts. The MTI technique is much more sophisticated than the traditional tie-scan technique since it can monitor and correct for thermal variations in the antenna, antenna mount, planar scanner, and RF equipment and cables. Unlike the tie scan, MTI provides a multi-degree of freedom measure of the relative rigid body motion between the scanner and test antenna during the test. Additionally, the MTI system provides an estimate of the measurement uncertainty.

MTI measurements are made at 4 spatial points in the higher energy region of near-field radiation at prescribed measurement times defined by the user. At the end of the data acquisition, NSI's MTI system software allows the user to analyze the data and make decisions on how to correct the thermal errors.

The MTI processor measures the relative azimuth, elevation, and Z motion between the scanner and antenna. The MTI data is acquired by periodically interrupting the normal data acquisition process and then measuring four spatially separated points. The MTI scan is performed at a single frequency with an un-steered beam, even in the case of multi-frequency and multi-polarization measurements. The MTI measurements are phase and distance normalized to remove frequency dependence so that the results apply to all polarizations and frequencies.

Additional information and applications of the MTI technique can be found in the following publications.

- AMTA94 Technical Paper, "550 Ghz Near-Field Antenna Measurement System for NASA Submillimeter Wave Astronomy Satellite"

- AMTA01 Technical Paper, "A Large Aperture 650 GHz Near-Field Measurement System for the Earth Observing System Microwave Limb Sounder"

- NSI Patent, "Three-axis Motion Tracking Interferometer for Measurement and Correction of Positional Errors Between an Article Under Test and a Measurement Probe"

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NSI-SW-5105 - Motion Tracking Interferometer Software