The Orbital DEbris RAdar Calibration Spheres (ODERACS)

The Orbital Debris Radar Calibration Spheres (ODERACS) spaceflight experiments were designed to provide small, low earth orbiting (LEO) calibration targets for the ground-based radar and optical systems used for orbital debris measurements. The primary objective was to calibrate the Haystack Long Range Imaging Radar (LRIR) and validate the JSC Orbital Debris Analysis System (ODAS). These measurements and resulting data processing were a complete success.

The Haystack radar is used for orbital debris measurements in an unorthodox way. Instead of moving the radar dish to track satellites, the dish "stares" in a fixed direction. Debris objects that fly through the radar beam produce echoes that are recorded on magnetic tape. Analysis of the echo data must take into account that debris objects can fly through the radar beam from any direction and can cross the beam at any position. Entirely new analysis tools were developed to deal with this novel kind of data. Because of the complexity of this process, it was determined that an end-to-end calibration of the radar and the associated data processing system was essential in order to be confident that the debris data derived from the radar were valid. The question relating to radar operation was: Is the radar calibrated correctly, such that the measured radar cross section is the correct one? The question relating to the data processing system was: Are the physical sizes and orbital parameters calculated from the Haystack radar data correct?

The best way to do these calibrations was to place known objects into orbit and measure them with the Haystack radar. From a practical point of view, metal spheres were the best choice for test objects, since their radar cross section is independent of aspect angle, and their shape makes them easy to deploy from the Space Shuttle Orbiter bay. A problem with spheres is that they return only one polarization of radar waves, the Principal Polarization (PP). For this reason, the Haystack radar could be calibrated only for PP returns using spheres. Since irregular debris objects return both PP and Orthogonal Polarization (OP) signals, it was necessary to make an independent calibration of the radar for its response to OP returns. Wire dipole targets were used for this purpose, since they reflect exactly equal OP and PP polarizations.

ODERACS Sphere Delivery System

The first mission, ODERACS 1, calibrated the PP response of the Haystack radar and the data processing system by launching calibrated metal spheres. The second mission, ODERACS 2 calibrated the OP response relative to the PP response by launching small wire dipoles. Spheres were also launched on the second mission to provide additional tests of the radar calibration and data processing system, and to provide orbital markers to help ground-based radars locate and track the tiny dipoles.

The ODERACS 1 flight experiment, flown on Discovery STS-60 in February 1994, deployed six spheres: two 6-in. diameter, two 4-in. diameter, and two 2-in. diameter. The spheres remained in orbit from 8 to 13 months and completely burned up upon reentry. ODERACS 2 which was flown on Discovery STS-63 in February 1995, deployed three spheres (2-, 4-, and 6-in. diameters) and three dipoles (two 5.255 x 0.040-in. dipoles, and one 1.740 x 0.040-in. dipole). The ODERACS 2 targets remained in orbit from 17 days to 13 months.

 

ODERACS Deployment, 9 February 1994

 

Other Images:
Haystack and HAX radar domes in Massachusetts; 453 KB.
Close up view of HAX radar dome under construction; 684 KB.

 

For further information on ODERACS contact Eugene G. Stansbery (eugene.g.stansbery@nasa.gov)