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Dive into the research topics where S. D. Desai is active.

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Featured researches published by S. D. Desai.


Marine Geodesy | 2004

One-Centimeter Orbit Determination for Jason-1: New GPS-Based Strategies

Bruce J. Haines; Yoaz E. Bar-Sever; Willy Bertiger; S. D. Desai; Pascal Willis

The U.S./French Jason-1 satellite is carrying a state-of-the-art GPS receiver to support precise orbit determination (POD) requirements. The performance of the Jason-1 “BlackJack” GPS receiver was strongly reflected in early POD results from the mission, enabling radial accuracies of 1–2 cm soon after the satellites 2001 launch. We have made further advances in the GPS-based POD for Jason-1, most notably in describing the phase center variations of the on-board GPS antenna. We have also adopted new geopotential models from the Gravity Recovery and Climate Experiment (GRACE). The new strategies have enabled us to better exploit the unique contributions of the BlackJack GPS tracking data in the POD process. Results of both internal and external (e.g., laser ranging) comparisons indicate that orbit accuracies of 1 cm (radial RMS) are being achieved for Jason-1 using GPS data alone.


Marine Geodesy | 2010

The Harvest Experiment: Calibration of the Climate Data Record from TOPEX/Poseidon, Jason-1 and the Ocean Surface Topography Mission

Bruce J. Haines; S. D. Desai; George H. Born

We present a 17-year calibration record of precise (Jason-class) spaceborne altimetry from a California offshore oil platform (Harvest). Our analyses indicate that the sea-surface-height (SSH) biases for all three TOPEX/Poseidon (1992–2005) measurement systems are statistically indistinguishable from zero at the 15 mm level. In contrast, the SSH bias estimates for the newer Jason-1 mission (2001–present) and the Ocean Surface Topography Mission (2008–present) are significantly positive. In orbit for over eight years, the Jason-1 measurement system yields SSH biased by +94 ± 15 mm. Its successor, OSTM/Jason-2, produces SSH measurements biased by +178 ± 16 mm.


Marine Geodesy | 2010

Assessment of the Jason-2 Extension to the TOPEX/Poseidon, Jason-l Sea-Surface Height Time Series for Global Mean Sea Level Monitoring

Brian D. Beckley; Nikita P. Zelensky; S. A. Holmes; Frank G. Lemoine; Richard D. Ray; Gary T. Mitchum; S. D. Desai; Shannon T. Brown

The Jason-2 (OSTM) follow-on mission to Jason-1 provides for the continuation of global and regional mean sea level estimates along the ground-track of the initial phase of the TOPEX/Poseidon mission. During the first several months, Jason-1 and Jason-2 flew in formation separated by only 55 seconds, enabling the isolation of inter-mission instrument biases through direct collinear differencing of near simultaneous observations. The Jason-2 Ku-band range bias with respect to Jason-1 is estimated to be −84 ± 9 mm, based on the orbit altitudes provided on the Geophysical Data Records. Modest improved agreement is achieved with the GSFC replacement orbits, which further enables the isolation of subtle (<1 cm) instrument-dependent range correction biases. Inter-mission bias estimates are confirmed with an independent assessment from comparisons to a 64-station tide-gauge network, also providing an estimate of the stability of the 17-year time series to be less than 0.1 mm/yr ± 0.4 mm/yr. The global mean sea level derived from the multi-mission altimeter sea-surface height record from January 1993 through September 2009 is 3.3 ± 0.4 mm/yr. Recent trends over the period from 2004 through 2008 are smaller and estimated to be 2.0 ± 0.4 mm/yr.


Marine Geodesy | 2010

Sub-Centimeter Precision Orbit Determination with GPS for Ocean Altimetry

Willy Bertiger; S. D. Desai; Angie Dorsey; Bruce J. Haines; Nate Harvey; Da Kuang; Ant Sibthorpe; Jan P. Weiss

We assess the accuracy of JPLs estimated OSTM/Jason-2 Global Positioning System (GPS)-determined orbits based on residuals to independent satellite laser ranging (SLR) data, compared with orbits produced by different software from different data (SLR/DORIS), Geophysical Data Record version C (GDR-C) orbits, and altimeter crossover tests. All of these tests are consistent with sub-cm radial accuracy: high elevation SLR residual standard deviation lies at 6.8 mm, RMS differences from GDR-C in the radial component typically fall below a cm, and altimeter crossovers from JPL orbits have a variance 89 mm2 smaller than altimeter crossovers from GDR-C orbits. Although RMS differences between radial components of different orbit solutions typically lie below a cm, we observe systematic dependences on both time and geography. The improved precision and accuracy of JPLs OSTM/Jason-2 orbit solutions rely on a new algorithm for applying constraints to integer carrier phase ambiguities. This algorithm is sufficiently robust to improve solutions despite half-cycle carrier phase identification issues in OSTM/Jason-2s BlackJack receiver. Although Jason-1 receiver performance differs, our algorithm should extend to Jason-1 processing (during the time span of nominal GPS receiver operations).


Marine Geodesy | 2004

Monitoring Measurements from the Jason-1 Microwave Radiometer and Independent Validation with GPS

S. D. Desai; Bruce J. Haines

The Jason-1 Microwave Radiometer (JMR) provides measurements of the wet troposphere content to correct the altimetric range measurement for the associated path delay. Various techniques are used to monitor the JMR wet troposphere path delays, with measurements of zenith troposphere content from terrestrial GPS sites used as an independent verification technique. Results indicate that an unexpected offset of approximately +4.1 ± 1.2 mm (drier) emerged in the JMR measurements of wet path delay between cycles 28–32 of the Jason-1 mission, and that the measurements may be drifting at a rate of approximately −0.5 mm/year. These anomalies are shown to be caused by a −0.7 K offset in 23.8 GHz brightness temperatures between cycles 28–32, and a 0.16 ± 0.04 and −0.45 ± 0.08 K/year drift in the 18.7 and 34.0 GHz brightness temperatures, respectively. Intercomparison of the 3-Hz JMR brightness temperature measurements show that they have been drifting with respect to each other, and that a dependence on yaw-steering regime is present in these measurements. An offset of 0.5 m/s between cycles 28–32 and a drift of approximately 0.5 m/s/year in the JMR wind speed measurements is also associated with these anomalies in the 1-Hz brightness temperatures. These errors in JMR wind speeds presently have a negligible impact on the retrieved JMR path delays.


Progress in Oceanography | 1997

Effect of long-period ocean tides on the Earth's polar motion

Richard S. Gross; Ben F. Chao; S. D. Desai

Abstract The second-degree zonal tide raising potential is symmetric about the polar axis and hence can excite the Earths polar motion only through its action upon non-axisymmetric features of the Earth such as the oceans. In the long-period tidal band, spectral peaks occur at the fortnightly tidal frequencies in power spectra of polar motion excitation observations from which atmospheric effects have been removed. Different observed polar motion excitation series are studied, and different methods of removing atmospheric effects from the polar motion excitation observations are explored in order to assess the robustness of the resulting empirical models for the observed effect of long-period ocean tides on polar motion excitation. At fortnightly frequencies, the various observed empirical models are found to agree with each other to within about 1σ. However, improved observations of the effect of fortnightly ocean tides on polar motion excitation are required since the observed effects at the Mf and Mf′ tidal frequencies are not consistent with the expectation that the oceans should have the same relative response to the tidal potential at these two nearby tidal frequencies. The observations are then compared with predictions of three ocean tide models: two purely hydrodynamic models and one estimated from Topex/Poseidon altimetric sea surface height measurements. At the fortnightly tidal frequencies, the three ocean tide models predict polar motion excitation amplitudes that differ from each other and from the observations by factors as large as 2, and phases that differ by more than 100°. This illustrates the need for improved models for the effect of long-period ocean tides on polar motion excitation.


Marine Geodesy | 2003

Jason-1 Geophysical Performance Evaluation Special Issue: Jason-1 Calibration/Validation

P. Vincent; S. D. Desai; J. Dorandeu; M. Ablain; B. Soussi; Philip S. Callahan; Bruce J. Haines

The Jason-1 satellite was launched on 7 December 2001 with the primary objective of continuing the high accuracy time series of altimeter measurements that began with the TOPEX/Poseidon mission in 1992. To achieve this goal, it is necessary to validate the performance of the Jason-1 measurement system, and to verify that its error budget is at least at the same level as that of the TOPEX/Poseidon mission. The article reviews the main components of the Jason-1 altimetric error budget from instrument characterization to the geophysical use of the data. Using the Interim Geophysical Data Records (16DR) that were distributed to the Jason-1 Science Working Team during the verification phase of the mission, it is shown that the Jason-1 mission is performing well enough to continue studies of the large-scale features of the ocean, and especially to continue time series of mean sea-level variations with an accuracy comparable to TOPEX/Poseidon.


Marine Geodesy | 2004

Assessment of the Jason Microwave Radiometer's Measurement of Wet Tropospheric Path Delay Using Comparisons to SSM/I and TMI

Victor Zlotnicki; S. D. Desai

The Jason microwave radiometer (JMR) provides a crucial correction due to water vapor in the troposphere, and a much smaller correction due to liquid water, to the travel time of the Jason-1 altimeter radar pulse. An error of any size in the radiometers measurement of wet path delay translates as an error of equal size in the measurement of sea surface height, the ultimate quantity that the altimetric system should yield. The estimate of globally-averaged sea surface height change associated with climate change, requires that uncertainties in the trends in such a global average be accurate to much better than the signal of 1–2 mm/yr. We first compare the JMR observations to those from the TOPEX/Poseidon radiometer (TMR) over approximately six months, since the intent of Jason is to continue the 10-year time series of precision ocean surface topography initiated by T/P. We then assess the stability of the JMR measurement by comparing its wet path delay to those of other orbiting radiometers over 22 months, specifically the Special Sensor Microwave Imager aboard the Defense Meteorological Satellite Program (DMSP-SSM/I) series of satellites, and the Tropical Rainfall Mapping Missions Microwave Imager (TMI), as well as the European Center for Medium Range Weather Forecastings (ECMWF) atmospheric numerical model estimate of water vapor. From the combined set, we obtain a robust assessment of the stability of JMR measurements. We find, that JMR is in remarkable agreement with TMR, only 2.5 mm longer, and 6–7 mm standard deviation on their difference in 0.5 degree averages; that JMR has experienced a globally-averaged step-function change, yielding an apparent shortening in wet path delay estimates of 4–5 mm around October 2002 (Jason cycles 28–32); that this step-function is visible only in the 23.8 GHz channel; and that the 34 GHz channel appears to drift at a rate of −0.4K/year. In addition, we find that, while in 2002 there was no evidence of sensitivity to the Jason satellites attitude (a correlation of the wet path delay with yaw state), in 2003 there are strong (2–3 mm, up to 7 mm globally averaged) changes associated with such yaw state. These JMR issues were all found in the first 22 months of Jasons geophysical data records (GDR) data, and thus they apply to any investigations that use such data without further corrections.


Marine Geodesy | 2003

Statistical Evaluation of the Jason-1 Operational Sensor Data Record Special Issue: Jason-1 Calibration/Validation

S. D. Desai; P. Vincent

The Jason-1 satellite altimeter mission represents a first step towards operational oceanography from satellite altimeter missions. An operational data product, the Operational Sensor Data Record (OSDR), provides measurements from the on-board altimeter and radiometer within 3–5 h of real time. This data product is a wind and wave product that is aimed towards near-real–time meteorological applications. A higher accuracy and more detailed data product, the Interim Geophysical Data Record (IGDR), that is better suited to detailed scientific studies of ocean topography, is available no sooner than 2–3 days from real time. The measurements reported on the OSDR primarily differ from those on the IGDR in that the OSDR reports measurements derived from on-board processing of the altimeter waveforms, while ground retracking of the waveforms is performed for the IGDR. The altimeter-derived measurements on the OSDR are validated through a statistical evaluation of the differences between data on the OSDR and IGDR. In doing so, the impact of ground retracking of the altimeter waveforms is also illustrated.


Marine Geodesy | 2003

Near-Real–Time GPS-Based Orbit Determination and Sea Surface Height Observations from the Jason-1 Mission Special Issue: Jason-1 Calibration/Validation

S. D. Desai; Bruce J. Haines

The Jason-1 Operational Sensor Data Record (OSDR) is intended as a wind and wave product that is aimed towards near-real–time (NRT) meteorological applications. However, the OSDR provides most of the information that is required to determine altimetric sea surface heights in NRT. The exceptions include a sufficiently accurate orbit altitude, and pressure fields to determine the dry troposphere path delay correction. An orbit altitude field is provided on the OSDR but has accuracies that range between 8–25 cm (RMS). However, tracking data from the on-board BlackJack GPS receiver are available with sufficiently short latency for use in the computation of NRT GPS-based orbit solutions. The orbit altitudes from these NRT orbit solutions have typical accuracies of < 3.0 cm (RMS) with a latency of 1–3 h, and < 2.5 cm (RMS) with a latency of 3–5 h. Meanwhile, forecast global pressure fields from the National Center for Environmental Prediction (NCEP) are available for the NRT computation of the dry troposphere correction. In combination, the Jason-1 OSDR, the NRT GPS-based orbit solutions, and the NCEP pressure fields can be used to compute sea surface height observations from the Jason-1 mission with typical latencies of 3–5 h, and have differences with those from the 2–3 day latency Interim Geophysical Data Records of < 5 cm (RMS). The NRT altimetric sea surface height observations are potentially of benefit to forecasting, tactical oceanography, and natural hazard monitoring.

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Bruce J. Haines

California Institute of Technology

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Willy Bertiger

California Institute of Technology

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William Bertiger

California Institute of Technology

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Yoaz E. Bar-Sever

California Institute of Technology

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Aurore Sibois

California Institute of Technology

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Jan P. Weiss

California Institute of Technology

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John Wahr

University of Colorado Boulder

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Ant Sibthorpe

California Institute of Technology

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Dzulkefly Kuang

California Institute of Technology

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Nate Harvey

California Institute of Technology

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