In-orbit optical performance assessment of Geostationary Ocean Color Imager
After Geostationary Ocean Color Imager (GOCI) was launched at June 27th, 2010, it is operated to observe the Korean peninsula on the geostationary orbit as one of the three main payloads in COMS satellite. Generally, the performance indicators are Ground Sample Distance (GSD), Modulation Transfer Function (MTF), and Signal to Noise Ratio (SNR) that verify the optical performance of payload. Most of all, the MTF performance is widely used since it is closely related to reliability and accuracy of observational target information in aspects of satellite image application. Especially, in case of ocean color sensor, for confirming the ability of GOCI to separate the slight difference of ocean color information, on-orbit MTF performance test and SNR assessment should be performed.
MTF measurement using spotlight
For Earth observation satellites such as SPOT, it is important to measure the modulation transfer function (MTF) in order to quantify the spatial resolution of the imaging system. This measurement is useful to decide to focus the telescope or to make a deconvolution filter whose purpose is to enhance image contrast by ground processing. The paper presents an MTF measurement method used during the in-orbit checkout of SPOT3 and the corresponding results. This method relies on a spotlight pointed to the satellite. The image of the spotlight nearly corresponds to the point spread function (PSF) of the system, but this PSF is undersampled because the sampling rate is limited by the pixel spacing. The samples are too sparse to describe the PSF completely and to obtain the MTF directly by computing the PSF Fourier Transform. Indeed, aliasing falsifies the computations. At this step, unless several spotlights are used, the use of an MTF model is mandatory: the model, including a parametric defocus, enables computation of the PSF at a high sampling rate and then the problem is to match the measured samples to the samples derived from the model. The MTF of the system corresponds to the MTF of the model which leads to the best matching. The paper reviews the context of work, the measurement method and, for SPOT3, the MTF model used, the experimentation and the results obtained.<<ETX>>
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.