The similarities and differences between four water vapor products currently available to correct for the wet tropospheric effect in altimeter data are quantified. Two of the water vapor data sets are derived from satellite measurements (Nimbus 7 scanning multichannel microwave radiometer (SMMR) and Tiros-N operational vertical sounder (TOVS)), and two are from operational analyses (European Center for Medium Range Weather Forecasts (ECMWF) and Fleet Numerical Oceanography Center (FNOC)). Subsequently, these analyses serve as the basis from which to assess the simulated effect of these products and their uncertainties on our ability to resolve significant sea level signals in the tropical Pacific Ocean via satellite altimetry. The time frame 1979-1983 was selected for study because it was the only period in which four water vapor data sets overlapped. This period also provided an interesting contrast between a relatively regular seasonal cycle for 1979-198! and the strong E1 Nifio-Southern Oscillation (ENSO) event in 1982-1983. Water vapor product intercomparisons showed the greatest consistency between the two satellite data sets, SMMR and TOVS. The standard deviation of the mean seasonal cycle for these two range corrections was 2 to 4 cm compared against an rms difference of 1 to 2 cm between the seasonal cycle of the two data sets. An additional 1-to 2-cm difference was also present between the two mean fields of the data. The largest rms differences, 4 to 8 cm, were found to occur between either the satellite and operational data or between the two operational analyses. Detailed spatial structure apparent in both the SMMR and TOVS data sets corresponded well with underlying sea surface temperature fields and features such as the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Although the spatial resolution of the ECMWF data was coarser than either satellite product, many of these same features were also present. The FNOC water vapor correction was the clear outlier of the four products. Tropical Pacific model sea level solutions for 1979-!983 were degraded with the SMMR and ECMWF water vapor products to simulate altimeter measurements unadjusted for a water vapor correction. The low-frequency, large-scale influence of a wet troposphere was analyzed on seasonal and interannual time scales. Seasonally, the SMMR and ECMWF wet tropospheric effects induced a slight increase in the amplitude of the simulated sea level retrievals without seriously distorting the spatial structure. Interannual anomalies of the water vapor correction during the height of the 1982-1983 E1 Nifio reached 12 cm in the eastern tropical Pacific, superimposed on simulated sea level anomalies of 20-30 cm. As the equatorial atmospheric convection shifted from the western to the eastern Pacific during this ENSO episode, the associated increase of columnar water vapor had the effect of significantly decreasing the simulated altimeter measurement of the elevated sea level characteristic of El Nifio in the ea...
A system utilizing Global Positioning System signals for three-axis attitude determination of a low-Earth-orbiting spacecraft is described. Measurements of wavefront aspect made by observing the phase differences of the same signal received at two separated antennas are processed by a Kalman filter to produce an estimate of vehicle attitude, body rate, disturbance torque, and antenna phase center shift. Factors affecting system accuracy are examined, including global positioning system satellite visibility, number of receiving antennas, and attitude error observability, which is strongly a function of vehicle dynamic motion. The Global Positioning System hardware suite is described as is a typical application to a momentum biased, nadir pointing spacecraft. A high-fidelity computer simulation is used to demonstrate that the global positioning system sensor can lend adequate attitude determination accuracy to this class of missions.
This paper presents a description of the Hubble Space Telescope (HST) Near Infrared Camera and Multi Object Spectrometer (NICMOS) Cooling System (NCS), the evolution of the cutting edge technology involved, a comparison of predicted versus on-orbit thermal performance, as well as possible future space applications. The NCS hardware consists of the NICMOS Cryogenic Cooler (NCC), an Electronics Support Module (ESM), a Capillary Pumped Loop (CPL)/Radiator assembly, and associated interface harnessing. The NCC is a state-of-the-art reverse Turbo-Brayton cycle mechanical cooler employing micro turbo machinery, driven by advanced power conversion electronics, operating at speeds up to 450,000 revolutions per minute to remove heat from the NICMOS instrument. The ESM provides command, control, and power distribution to the NCS, as well as providing the primary interface to the existing HST electronics. A two-phase CPL system removes heat from the NCC and transfers it to the radiator mounted externally on the HST aft shroud. The system was installed during Servicing Mission 3B via extravehicular activities in March 2002. The NCS revived the NICMOS instrument, which experienced a reduced operational lifetime due to an internal thermal short in its dewar structure, and restored HST scientific infrared capability to operational status.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.