Publications

Over the past several years, the Air Force has been developing new devices and technology for the detection and tracking of earth orbiting satellites. This technology has been targeted to provide an upgraded capability for an operational space surveillance system called GEODSS. Currently, a number of GEODSS systems are deployed around the world as part of the world-wide space surveillance system operated by the US Air Force. Each GEODSS site is currently equipped with 1-meter class telescopes and EBSICON detector systems which represent 1970's technology. The Air Force is now in the process of upgrading the GEODSS system to achieve the performance offered by state of the art detector systems. Under Air Force sponsorship, Lincoln Laboratory has developed a new generation of sensitive, large format, frame transfer CCD focal planes for GEODSS. These focal planes have been installed in a new generation of cameras and are currently undergoing testing at the Lincoln Laboratory Experimental Test Site (ETS). When equipped with the new focal plane and camera technology, the modest sized GEODSS telescopes have considerable capability to conduct large coverage, sensitive searches for earth crossing asteroids. Theoretical analysis has indicated that the CCD equipped GEODSS telescope will be capable of achieving a limiting magnitude of 22, over a 2 sq/deg field of view, with about 100 seconds of integration. This is comparable to the sensitivity of considerably larger telescopes equipped with current cameras. In addition to the high sensitivity, the CCD is configured for frame transfer operations which are well suited to asteroid search operations. This paper will present the results of the initial system tests conducted at the ETS and will discuss how this technology fits into a concept of operations for a planetary defense system based on the Air Force developed technology.

A monolithic two-dimensional array of surface-emitting AlGaAs diode lasers with dry-etched vertical facets and parabolic deflecting mirrors was mounted junction-side up on a W / Cu microchannel heatsink and evaluated under continuous-wave (CW) operating conditions. Both the facets and parabolic deflecting mirrors were etched using chlorine ion-beam-assistd etching. Threshold current densities of different sections of the array were consistently around 240 A/cm (to the second power), and measured CW differential quantum efficiencies were in the 46-48% range. CW power densities as high as 148 W/cm (to the second power) were achieved with an average temperature rise of less than 25 degrees C in this junction-side-up configuration.

Optimal searches for a fixed object are discussed and the rigorous analytical results of discrete search theory are presented. They show that the totally optimal, the uniformly optimal, the locally optimal, and the fastest searches are identical under not too restrictive assumptions. The mathematical formalism is illustrated by an Earth-approaching asteroid search and optimal searches for such objects are explicitly constructed. The approximation that Earth-approaching asteroids are fixed is equivalent to having a very high (>or=100 square degrees/hr) search rate. Generalizations to other types of astronomical search are briefly mentioned.

A prototype observatory was constructed near Socorro, New Mexico to search for and observe earth-approaching asteroids. Hardware modifications were made so that the discrimination of artificial satellites at an angular speed of 15 arcsec/sec can be used to observe minor planets at an angular speed of 0.01 arcsec/sec. Assuming no correlations between time of perigee passage and time of year, a recovery of 25 asteroids per year is expected.

Tropospheric angle-of-arrival and amplitude scintillation measurements were made at X-band (7.3 GHz) and at UHF (0.4 GHz). The measurements were made using sources on satellites with 12-day orbits. The angle of arrival of the ray path to a satellite changed slowly allowing observations of fluctuations caused by atmospheric irregularities as they slowly drifted across the ray path. The fluctuations were characterized by the rms variations of elevation angle and the logarithm of received power (log power). Over a one-year period, 458 hours of observation were amassed spanning every season, time of day, and weather conditions. The results show strong scintillation occurrences below 1 to 2 degrees elevation angles characterized by a number of random occurrences of multipath events that produce deep fades, angle-of-arrival fluctuations, and depolarization of the received signal. The log power fluctuations ranged from 1 to 10 dB rms at elevation angles below 2 degrees to less than 0.1 dB at elevation angles above 10 degrees. The elevation angle fluctuations ranged from 1 to 100 mdeg at elevation angles below 2 degrees to less than 5 mdeg at a 10 degrees elevation angle. Comparable fluctuations in elevation angle are expected for bias refraction correction models based upon the use of surface values of the refractive index.