Hiromi Oaku

Basin scale estimates of sea surface nitrate and new production from remotely sensed sea surface temperature and chlorophyll

The highly variable nature of T-N relationships in oceanic waters has restricted nitrate (N) measurements from remotely sensed sea surface temperature (SST) to small time and space domains. Here we show that if changes in T-N relationships resulting from phytoplankton (chlorophyll a) are taken into account, remote sensing can be exploited to provide high resolution maps of sea surface nitrate (SSN) that are valid over much larger scales than has been previously possible. We illustrate the potential of the method for monitoring basin scale, interannual variations in SSN in the north Pacific Ocean using co-registered imagery of SST and chl a and demonstrate the usefulness of such data for estimating basin scale annual new production.

A method for estimating sea surface nitrate concentrations from remotely sensed SST and chlorophyll a-a case study for the north Pacific Ocean using OCTS/ADEOS data

Proposes a method to estimate sea surface nitrate (N) from space using satellite measurements of sea surface temperature (SST) and chlorophyll a (chl a). The procedure relies on empirical relationships between shipboard measurements of N and its predictor variables, temperature (T) and chl a in surface and near surface waters. Although N appears to be controlled primarily by T, the addition of the biological variable chl a helps improve N prediction by reducing local and regional differences in the character of the temperature-nitrate (T-N) relationship. The authors have applied these empirical algorithms to SST and chl a data from the Ocean Color and Temperature Scanner (OCTS) on board the Advanced Earth Observation Satellite (ADEOS). The results clearly suggest that measurements of SST and chl a now possible by modern-day ocean satellites could be exploited usefully to extend the resolution of shipboard N measurements over large spatial and temporal scales. Systematic errors in estimates of N that could result from errors in satellite estimates of SST and chl a are examined through sensitivity analyses.

Calibration and Validation of the Ocean Color Version-3 Product from ADEOS OCTS

We present calibration and validation results of the OCTS’s ocean color version-3 product, which mainly consists of the chlorophyll-a concentration (Chl-a) and the normalized water-leaving radiance (nLw). First, OCTS was calibrated for the inter-detector sensitivity difference, offset, and absolute sensitivity using external calibration source. It was also vicariously calibrated using in-situ measurements for water (Chl-a andnLw) and atmosphere (optical thickness), which were acquired synchronously with OCTS under cloud-free conditions. Second, the product was validated using selected 17 in-situ Chl-a and 11 in-situnLw measurements. We confirmed that Chl-a was estimated with an accuracy of 68% for Chl-a less than 2 mg/m3, andnLw from 94% (band 2) to 128% (band 4). Geometric accuracy was improved to 1.3 km. Stripes were significantly reduced by modifying the detector normalization factor as a function of input radiance.

The sea surface temperature product algorithm of the Ocean Color and Temperature Scanner (OCTS) and its accuracy

The Ocean Color and Temperature Scanner (OCTS) aboard the Advanced Earth Observing Satellite (ADEOS) can observe ocean color and sea surface temperature (SST) simultaneously. This paper explains the algorithm for the OCTS SST product in the NASDA OCTS mission. In the development of the latest, third version (V3) algorithm, the OCTS match-up dataset plays an important role, especially when the coefficients required in the MCSST equation are derived and the equation form is adjusted. As a result of the validation using the OCTS match-up dataset, the algorithm has improved the root mean square (rms) error of the OCTS SST up to 0.698°C although some problems remain in the match-up dataset used in the present study.

Calibration of the ocean color and temperature scanner

Calibration results of the ocean color and temperature scanner (OCTS) on board the Advanced Earth Observing Satellite (ADEOS) are presented. The authors have evaluated the OCTS responses to internal calibration sources (i.e., lamps, electric voltages, and sunlight), natural targets (i.e., night data and uniform targets in the daytime), external calibration source (i.e., underflight of a calibrated airborne sensor), and a theoretical calibration source modeled for the atmospheric radiation. The authors then compared the acceptability of these calibration sources.

Calibration of advanced visible and near infrared radiometer

Calibration results of the advanced visible and near infrared radiometer (AVNIR) on board ADEOS are presented. First, the AVNIR responses to the internal and external calibration sources are evaluated, and their short- and long-term stabilities are summarized. Second, absolute radiometric calibration of AVNIR is conducted by using the NASA JPLs calibrated airborne optical sensor, and the results from several different calibration methods (preflight calibration, and in-flight calibrations using the on-board data, the AVIRIS, and the vicarious method) are compared. Third, the geometric accuracy and the image quality are summarized.

POLDER-OCTS preflight cross-calibration experiment using round-robin radiometers

This joint article presents the POLDER-OCTS preflight cross-calibration procedure and data set. POLDER is a radiometer developed by CNES devoted to the measurement of the polarization and directionality of the Earths reflectances and OCTS is an ocean color and temperature scanner developed by NASDA. Both radiometers are onboard the ADEOS satellite to be launched in 1996. The preflight POLDER-OCTS cross-calibration experiment was carried out by NRLM, NASDA, and CNES from March to April in 1994 using round- robin radiometers. The cross-calibration results show the agreement between NRLM/NASDA and CNES radiometers better than 6% regarding POLDER integrating sphere at CNES in Toulouse and better than 5% regarding OCTS integrating sphere at NEC in Yokohama. Calbration of OCTS integrating sphere by NEC agreed with cross calibration by NRLM/NASDA within 3%. The calibration of CNES round-robin radiometer is guaranteed at 3.5%.

SAR calibration using frequency-tunable active radar calibrators

In this paper, the impulse response function (IRF) of a synthetic aperture radar (SAR) image of frequency-tunable reference point targets [e.g., active radar calibrator (ARC)] is analyzed. The frequency-tunable ARC is an effective SAR calibration device that can yield a larger radar cross section (RCS or /spl sigma/) than a corner reflector, and it displaces the response to a desirable background area for isolation from brighter man-made targets (e.g., buildings). SAR calibration accuracy is degraded by frequency shift, however, because of less correlation gain and broader IRF. We compared the theoretically derived IRF with the measurement data and drew the following conclusions: first, the location shift and the peak gain loss can be theoretically estimated within 4.2 m and 1.6 dB (one standard deviation); second, the peak calibration method is degraded by the defocused IRF; third, the integral method, which is not sensitive to defocusing, is recommended for SAR calibration; and fourth, the frequency shift should be less than 40 Hz for the satellite-based L-band SAR calibration.

Calibration of advanced visible and near infrared radiometer (AVNIR)

Abstract Initial on‐orbit calibration results of the advanced visible and near infrared radiometer (AVNIR) onboard ADEOS were presented. When ADEOS returned over the Japan ground station mask on Sept. 11996, AVNIR was first activated to evaluate performance. AVNIR calibration toas then conducted using the reference signal sources (internal lamps, natural target data, NASAs airborne sensor data, etc). The evaluation indicated that AVNIR has sufficient sensitivity to function as a high‐resolution imager and is useful for the land monitoring, though some noises are to be removed.

OCTS absolute calibration using AVIRIS

The absolute calibration of the Ocean Color Temperature Scanner (OCTS) was conducted using the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) over the clear ocean test site. The radiance from the ocean heavily depends on the sun and the sensor incident angles, so the scattering characteristics of the ocean surface were first evaluated for AVIRIS data. The calibration coefficient was then calculated by comparing two sensors.