The Reflector Array Maintained in Delta Junction, Alaska

by Phil Utley and Wade Albright

ASF has maintained an array of tri-hedral reflectors in the fields at Delta Junction since 1991 to aid in calibration and ground truth evaluation of several international environmental satellites. These reflectors provide a known radiometric return to measure the strength of the satellite signal.

Reflector positions are well known, so they are useful for measuring geographical accuracy and geometric distortions. When used this way, the reflectors help tie the satellite images from space to earth.

Many of the reflectors are mounted on ring bases, which allows easy and accurate orientation toward the origin of any satellite signal.

The trihedral shape of the reflectors causes a triple bounce of the incident rays of the radar beam, which makes them all appear to reflect from the apex no matter where they impinge upon the reflector panel.

In other words, the distance between the satellite and all points on the reflector appear to be the same.

A corner reflector returns forward scatter rather than the typical backscatter; it actually turns the incident beams back on a parallel path to the satellite.

This allows for a brighter return because it simulates a longer illumination time than the single point would normally have.

This brighter return allows calibration engineers to easily identify the reflector in an image and perform impulse response measurements, such as resolution, peak-to-side-lobe ratio, and radar cross section, which would usually be lost in the clutter of a normal target.

These measurements help determine the optimal processing options for a given application.

ASF calibration engineers used the Delta reflector array to assist with calibrating two European Remote Sensing satellites (ERS-1 and ERS-2), the Japanese Earth Resource Satellite (JERS-1), and multiple beam modes of the Canadian Radarsat-1 (R-1) satellite.

The reflectors are useful to monitor long-term stability of the satellites and provide ground control points for producing Alaska digital elevation maps.

ASF is exploring the idea of allowing its users to utilize this array of corner reflectors for optimizing their own geolocation or image focusing algorithms.

Interested parties should contact ASF User Services to discuss possible collaboration with ASF’s calibration team on using the corner reflectors.

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