Charles C. Kilgus

Prelaunch performance of the NASA altimeter for the TOPEX/Poseidon project

The TOPEX/Poseidon radar altimeter satellite applies advances in remote sensing instrumentation to reduce long wavelength measurement errors to dramatically lower levels. The TOPEX altimeter measures the range to the ocean surface with 2-cm precision and accuracy through the use of both Ku- and C-band radars, a high pulse repetition frequency, an agile tracker, and absolute internal height calibration. Dual pulse bandwidths for both frequencies make it possible to quickly acquire the surface and begin tracking after crossing the land/ocean boundary. The altimeter requirements and the elements of the altimeter design that have resulted in meeting these requirements are presented. Prelaunch test data, based on the use of a radar altimeter system evaluator to simulate the backscatter from the ocean surface, are presented to demonstrate that the TOPEX altimeter will meet these requirements and provide the data necessary to the understanding of basin scale mean circulation. >

Four-meter lunar engineering telescope

The 16-meter diffraction limited lunar telescope incorporates a primary mirror with 312 one-meter segments; 3 nanometer active optics surface control with laser metrology and hexapod positioners; a space frame structure with one-millimeter stability; and a hexapod mount for pointing. The design data needed to limit risk in this development can be obtained by building a smaller engineering telescope on the moon with all of the features of the 16-meter design. This paper presents a 4.33-meter engineering telescope concept developed by the Summer 1990 Student Program of the NASA/JHU Space Grant Consortium Lunar Telescope Project. The primary mirror, made up of 18 one-meter hexagonal segments, is sized to provide interesting science as well as engineering data. The optics are configured as a Ritchey-Chretien with a coude relay to the focal plane beneath the surface. The optical path is continuously monitored with 3-nanometer precision interferometrically. An active optics processor and piezoelectric actuators operate to maintain the end-to-end optical configuration established by wave front sensing using a guide star. The mirror segments, consisting of a one-centimeter thick faceplate on 30-cm deep ribs, maintain the surface figure to a few nanometers under lunar gravity and thermal environment.

Evaluation of a CCD star camera at Table Mountain Observatory

An instrument has been designed to demonstrate 5 arc second autonomous, all-stellar attitude determination on a NASA Spartan spacecraft. The instrument includes a CCD star camera that provides centroid measurements of up to 5 simultaneous star images with 10 frames of data per s. The performance of the camera and processing techniques have been studied in an extensive test program including star measurements taken under realistic conditions at the Jet Propulsion Laboratory Table Mountain Observatory. Results of these tests are presented and compared to laboratory and simulation data. At slue rates less than 0.1 deg/s the camera accuracy was found to vary from 5 to 15 arc seconds per image, depending on star magnitude. Q

Gulf Stream dynamic topography measured from space

On March 9, 1989, the Navy-sponsored GEOSAT radar altimeter satellite completed 50 cycles over its 17-day exact repeat ground track. Results obtained from analyzing the mission data set were presented at the Eleventh GEOSAT Users Meeting held March 23 at the Johns Hopkins University Applied Physics Laboratory (APL). Publication of the results is planned for a special 1989 issue of the Journal of Geophysical Research, which will focus on GEOSAT. Development of a synthetic geoid processing technique that allows the total dynamic topography of the Gulf Stream front and eddy system to be measured was reported by scientists from the Naval Ocean Research and Development Activity (NORDA), Woods Hole Oceanographic Institution (WHOI), Harvard University and APL. The “synthetic geoid” is realized by subtracting the climatology of the surface topography along a ground track (the mean circulation topography) from the mean of the altimeter data along the track. The technique dramatically increases the scientific value of radar altimeter data by removing the distortion and signal reduction caused by subtracting a mean sea surface that includes the mean circulation.