waves. The polarization of the reflection can be used to infer the general shape of the objects detected.

Calibration of the data acquired by using sampling radars can be achieved by a number of techniques. These include radar range measurements of fragments of known size, shape, and mass, and the use of orbital calibration spheres. The Haystack radar used both of these techniques. In this case, the range measurements indicated that irregular fragments reflected similarly to spheres but a broad distribution of possible signal returns must be considered in interpreting the data. Existing calibration spheres, as well as the Orbital Debris Radar Calibration Spheres (ODERACS), were also used in calibration.

Future efforts to sample the debris population with ground-based radars may be the most effective means to collect data on medium-sized debris in LEO. Improvements in this capability can be achieved by (1) performing more debris sampling with existing radars; (2) siting new radars so they can detect low-inclination debris populations; and (3) using high-powered, short-wavelength radars to detect smaller debris.

Increasing the amount of time that radars spend sampling debris is basically a problem of allocating the resources needed to carry out additional searches. Continued sampling efforts with existing radars can increase statistical confidence in existing data and, over time, could provide information on changes in the debris population. However, the Haystack, Goldstone, and Arecibo radars, which were not designed to detect debris, have other users preventing them from being used full-time for debris detection and are expensive to operate. For these reasons, the Haystack Auxiliary Radar (HAX) was recently built specifically to detect debris. HAX, which is located near the Haystack radar, will not be as sensitive as Haystack, but its slightly larger field of view and lower

BOX 2-4 The ODERACS Experiment

The ODERACS experiment was launched from the U.S. Space Shuttle in March of 1994 and provided calibration for a number of Earth-based radar and optical sensors. In this experiment, six aluminum spheres (two 5 cm in diameter, two 10 cm in diameter, and two 15 cm in diameter) were released into LEO. One sphere of each type had a polished surface whereas the other had a rough surface. This experiment demonstrated the validity of sampling debris with a radar and helped calibrate both radar and optical sensors. A similar future experiment will release three spheres and three dipoles to further calibrate the sensors. The dipoles are intended to calibrate polarization measurements, which are important for determining debris shape.

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