Papers by Christopher Ruf
IGARSS 2000. IEEE 2000 International Geoscience and Remote Sensing Symposium. Taking the Pulse of the Planet: The Role of Remote Sensing in Managing the Environment. Proceedings (Cat. No.00CH37120)
A novel technique has been developed to verify the absolute accuracy and relative stability of a ... more A novel technique has been developed to verify the absolute accuracy and relative stability of a spaceborne microwave radiometer's calibration using a statistical analysis of the measured Earth brightness temperatures (TBs). This procedure relies solely on the final, main beam-referenced TBs and so tests the complete end-to-end system calibration, including instrument temperature and non-linearity corrections, the stability of reference TB calibration standards, and far-side lobe antenna pattern corrections. A very stable cold reference TB calibration point results which is known with high absolute accuracy. The data processing steps required to produce the cold reference TB are described here. The technique has been tested previously at a nadir viewing angle of incidence. Its application, at oblique angles, to both conical and cross-track scanning imagers is considered here.
IGARSS '98. Sensing and Managing the Environment. 1998 IEEE International Geoscience and Remote Sensing. Symposium Proceedings. (Cat. No.98CH36174)
The standard on orbit data processing used by all current and upcoming satellite altimetry missio... more The standard on orbit data processing used by all current and upcoming satellite altimetry missions TOPEX/Poseidon, GEOSAT Follow On, and Jason-1) results in a radar altimeter footprint which is generally much smaller than the footprint of the microwave radiometer that is used to correct for path delay of the radar signal by tropospheric water vapor and clouds. As a result, the quality of ocean topology retrievals can be compromised in coastal environments even when the altimeter itself is still operating nominally. For example, the diameter of the TOPEX altimeter footprint is 3-5 km at typical wind speeds, but the radiometer has a 35 km footprint at 21 GHz. Because of this size disparity, wet path delay corrections are generally not considered reliable within approximately 50 km of a major coastline. We present here an antenna deconvolution procedure which is specifically intended to improve near-coastal retrieval of the wet tropospheric path delay correction by satellite altimeters. The procedure has immediate applications with archived and current TOPEX/Poseidon data, and will be applicable to the upcoming GFO and Jason-1 missions, for the imaging of ocean currents and tides in the littoral zone.
2006 IEEE International Symposium on Geoscience and Remote Sensing
The Correlated Noise Calibration Standard (CNCS) is an electronic device that generates two chann... more The Correlated Noise Calibration Standard (CNCS) is an electronic device that generates two channels of broadband microwave noise with a programmable degree of complex correlation. It can supply signals to the input of a microwave radiometer with independently variable vertical, horizontal, 3 rd and 4 th Stokes brightness temperatures. An X-Band version of the CNCS has been evaluated with the NASA Goddard Airborne Earth Science Microwave Imaging Radiometer (AESMIR). AESMIR measurements were made with a highly over constrained set of CNCS polarimetric states. This allows for the simultaneous solution of both non-ideal characteristics of the CNCS system as well as of the complete polarization mixing calibration equation for AESMIR. Absolute calibration accuracy of AESMIR is estimated to be 0.2 K for its 3 rd and 4 th Stokes channels.
2006 IEEE MicroRad
ABSTRACT
[Proceedings] IGARSS'91 Remote Sensing: Global Monitoring for Earth Management
The measurement of soil moisture and ocean salinity requires long wavelengths and therefore large... more The measurement of soil moisture and ocean salinity requires long wavelengths and therefore large antennas in space. It may be possible to achieve a practical system in space to make these measurements by using aperture synthesis to reduce the collecting area needed in orbit. The sensor envisioned is a three frequency radiometric system (1.4, 2.65 and 5.0 GHz) which would
2013 IEEE Aerospace Conference
2015 18th International Conference on Information Fusion (Fusion), 2015
We investigate the capability of GPS signals of opportunity to detect and localize targets on the... more We investigate the capability of GPS signals of opportunity to detect and localize targets on the sea surface. The proposed approach to target detection is new, and stems from the advantages offered by GPS-Reflectometry (GPS-R) in terms of spatial and temporal sampling, and low cost/low power technology, extending the range of applications of GPS-R beyond remote sensing. Here the exploitation of GPS signals backscattered from a target is proposed, to enhance the target return with respect to the sea clutter. A link budget is presented, showing that the target return is stronger than the background sea clutter when certain conditions are verified. The findings agree with the only empirical measurement found in literature, where a strong return from a target was fortuitously registered during an airborne campaign. This study provides a first proof-of-concept of GPS-based target detection, highlighting all the potentials of this innovative approach.
2019 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting, 2019
This paper presents a novel method to calibrate the transmit antenna pattern of the GPS satellite... more This paper presents a novel method to calibrate the transmit antenna pattern of the GPS satellites, using the zenith antenna mounted on each of the 8 small satellites of the NASA CYGNSS mission. The zenith measurements are calibrated using an end-to-end calibration. The pattern of the zenith antenna is calibrated using the multiple published GPS transmit antenna patterns. A weighted spherical harmonic fitting algorithm is used to minimize the impacts by the measurement noise. The weighted RMSD of two independently calibrated patterns using GPS PRN 21 and 28 is only 0.31 dB. The 8 calibrated CYGNSS zenith patterns are used to accurately and precisely calibrate the GPS transmit antenna pattern over the full terrestrial service volume for all 32 GPS satellites. The calibrated patterns have 0.5 × 0.5 degree resolution while the published patterns have a 2 × 10 degree resolution. The calibrated patterns are significant to resolve many open challenges in Earth remote sensing using the ref...
Introduction: Large amounts of water ice exist on Mars, but the presence of pure liquid water is ... more Introduction: Large amounts of water ice exist on Mars, but the presence of pure liquid water is inhibited by the planet's low temperature [1]. However, liquid saline water or brines can be present because many salts can depress the freezing temperature below current values [1-8]. The Phoenix Mars Mission discovered salts [9] in the soil of its landing site in the martian Arctic capable of depressing the freezing temperature and water vapor partial pressure enough to form liquid brines under today's local environmental conditions [11]. Phoenix found physical and thermodynamical evidence for liquid brines at its landing site [10,11]. Since the Phoenix initial discoveries, independent results suggesting the presence of liquid brines on Mars have been mounting [12-19]. The theoretical idea that freeze/thaw cycles lead to the formation of liquid brines where ice and salts coexist near the surface [11], suggests that liquid brines are common on Mars. The instrument proposed here ...
A new microwave radiometer detector has been developed that is capable of detecting low levels of... more A new microwave radiometer detector has been developed that is capable of detecting low levels of Radio Frequency Interference (RFI) and of reducing its effect on the measured brightness temperatures. The Agile Digital Detector (ADD) digitizes the pre-detection radiometer signal, performs digital sub band filtering, and then measures the first four moments of the signal’s probability density function. The second moment reproduces the square law output of a conventional analog detector. The ADD can discriminate between RFI and natural thermal emission signals using higher order moments of the signal. The ADD uses spectral filtering methods to remove the RFI.
The NASA Earth Venture Cyclone Global Navigation Satellite System (CYGNSS) is a constellation of ... more The NASA Earth Venture Cyclone Global Navigation Satellite System (CYGNSS) is a constellation of eight observatories in a 35° inclination, ~530 km altitude Earth orbit. Each observatory carries a 4-channel bistatic wind scatterometer receiver. Measurements of the ocean surface scattering cross section are converted to 10 meter-referenced wind speed. The mission improves the temporal sampling of winds in tropical cyclones (TCs) with a revisit time of 2.8 hours (median) and 7.2 hours (mean) at all locations between 38 deg North and 38 deg South latitude. Operation at the 1575 MHz GPS L1 frequency permits wind measurements in the TC inner core that are often obscured from other spaceborne remote sensing instruments by intense precipitation in the eye wall and inner rain bands. The potential for improved storm surge forecast skill is examined using simulated CYGNSS science data products for Hurricane Irene. We present and compare ADCIRC 2DDI storm surge hindcasting results of Hurricane ...
1. University of Michigan2455 Hayward St., Ann Arbor, MI, 48109-2143 USA734-764-6561(V), 734-936-... more 1. University of Michigan2455 Hayward St., Ann Arbor, MI, 48109-2143 USA734-764-6561(V), 734-936-0503 (F), [email protected](E)2. University of Central Florida, 3. National Oceanic and Atmospheric Administration, 4. NASA Marshall Space Flight CenterAbstractThe Hurricane Imaging Radiometer (HIRAD) is an innovative radiometer which offers new and unique remotely sensed observations of both extreme oceanic wind events and strong precipitation. It is based on the airborne Stepped Frequency Microwave Radiometer (SFMR) [Uhlhorn and Black, 2004]. The HIRAD instrument advances beyond the current nadir viewing SFMR to an equivalent wide-swath SFMR imager using passive microwave synthetic thinned aperture radiometer (STAR) technology [Ruf et al., 1988]. This sensor operates over 4-7 GHz, where the required tropical cyclone remote sensing physics has been validated by both SFMR and WindSat radiometer [Bettenhausen et al., 2006; Brown et al., 2006]. HIRAD incorporates a new and unique array antenn...
Shortly after its launch in December of 2016, the Cyclone Global Navigation Satellite System (CYG... more Shortly after its launch in December of 2016, the Cyclone Global Navigation Satellite System (CYGNSS) constellation began producing estimates of sea surface wind speeds. With its unique remote sensing method and use of eight satellites, the constellation is able to produce winds with shorter revisit times and in more intense weather than other sea surface wind missions. As a part of the CYGNSS calibration and validation (cal/val) process, these estimates are routinely compared to other estimates in order to understand their uncertainties as well as to diagnose and develop corrections for calibration issues. This paper presents trending of CYGNSS data, focusing primarily on comparisons to numerical weather forecast model analysis wind speed estimates and inter-comparison of simultaneous observations within the CYGNSS constellation.
In 2012, the National Aeronautics and Space Administration (NASA) selected the CYclone Global Nav... more In 2012, the National Aeronautics and Space Administration (NASA) selected the CYclone Global Navigation Satellite System (CYGNSS) mission coordinated by the University of Michigan (UM) as a low-cost and high-science Earth Venture Mission. The CYGNSS mission was originally proposed for ocean surface wind speed estimation over Tropical Cyclones (TCs) using Earth-reflected Global Positioning System (GPS) signals, as signals of opportunity. The orbital configuration of each CYGNSS satellite is a circular Low Earth Orbit (LEO) with an altitude ~520 km and an inclination angle of ~35°. Each single Delay Doppler Mapping Instrument (DDMI) aboard the eight CYGNSS microsatellites collects forward scattered signals along four specular directions (incidence angle of the incident wave equals incidence angle of the reflected wave) corresponding to four different transmitting GPS spacecrafts, simultaneously. As such, CYGNSS allows one to sample the Earth’s surface along 32 tracks simultaneously, ...
The Cyclone Global Navigation Satellite System (CYGNSS) project, NASA's first Earth Venture M... more The Cyclone Global Navigation Satellite System (CYGNSS) project, NASA's first Earth Venture Mission, is on schedule to launch in late-2016. CYGNSS will implement a spaceborne earth observation system designed to collect measurements of ocean surface winds through measurements of variations in the direct vs. reflected Global Positioning System (GPS) signals. The mission will provide data to enable the study of the relationship between ocean surface properties, moist atmospheric thermodynamics and convective dynamics; factors thought to be fundamental to the genesis and intensification of tropical storms. Key information about the ocean surface under and around a tropical storm is hidden from existing space borne observatories due to signal attenuation in the frequency bands in which they operate by the intense tropical cyclone precipitation, thus obscuring the ocean’s surface. This plus poor temporal sampling are driving factors behind the fact that while tropical storm track for...
The eight micro-satellite Cyclone Global Navigation Satellite System (CYGNSS) constellation was l... more The eight micro-satellite Cyclone Global Navigation Satellite System (CYGNSS) constellation was launched on December 15, 2016. Each of the observatories carries a 4-channel GNSS-R receiver tuned to receive signals reflected by the Earth's ocean surface from which near-surface wind speed is estimated. The mission is focused on providing high temporal and spatial sensing of the wind conditions under and near developing tropical storms and cyclones. CYGNSS is studying the relationship between ocean surface properties, moist atmospheric thermodynamics, radiation and convective dynamics to determine how a cyclone forms, whether it will strengthen, and how much. A recap of launch and early operations is presented via a somewhat humorous analogy to parenting octuplets, with lessons learned included throughout. Topics include the roller-coaster ride of false labor (launch delays); the excitement of the birth, er, launch; the euphoria of seeing all eight μSats born alive and breathing; t...
IEEE Transactions on Geoscience and Remote Sensing
Ocean microplastic concentrations are known to vary significantly by location, with especially hi... more Ocean microplastic concentrations are known to vary significantly by location, with especially high levels in the North Atlantic and North Pacific gyres. Most direct measurements come from plankton net trawling made in these regions; concentrations in other regions have been estimated by microplastic transport models that depend on large-scale ocean circulation patterns. However, global measurements of microplastic distribution and its temporal variability are lacking. A new method is presented for detecting and imaging the global distribution of ocean microplastics from space. The method uses spaceborne bistatic radar measurements of ocean surface roughness and relies on an assumed reduction in responsiveness to wind-driven roughening caused by surfactants that act as tracers for microplastics near the surface. Annual mean microplastic distributions estimated by the radars are generally consistent with model predictions. The spaceborne observations are also able to detect temporal changes that are not resolved by the models. For example, seasonal dependencies are observed at midlatitudes in both Northern and Southern Hemispheres, with lower concentrations noted in the winter months. Time lapse images at finer spatial and temporal scales reveal episodic bursts of microplastic tracers in the outflow from major river discharges into the sea. This new method will provide better monitoring of ocean microplastics and will support future model development and validation.
2014 IEEE Geoscience and Remote Sensing Symposium
One of the greatest challenges in the evaluation of microwave radiometer RFI detection methods is... more One of the greatest challenges in the evaluation of microwave radiometer RFI detection methods is a lack of suitable ground truth about the RFI environment; the false alarm rate can be readily estimated without it, but probability of detection cannot. A method is introduced to derive the probability distribution of RFI amplitude anywhere on the globe, thus characterizing the RFI environment. The method constructs low-level RFI characteristics which are difficult to detect from the highlevel RFI characteristics that are more easily detected. The conceptual and theoretical basis of the method is presented, followed by possible future augmentations and improvements.
IGARSS 2020 - 2020 IEEE International Geoscience and Remote Sensing Symposium
The Cyclone Global Navigation Satellite System (CYGNSS) measures GPS signals specularly reflected... more The Cyclone Global Navigation Satellite System (CYGNSS) measures GPS signals specularly reflected from Earth's surface to remotely sense ocean surface roughness and wind speed. The mean square slope (mss) is a key physical parameter that relates the ocean surface properties (wave spectra) with the CYGNSS measurement of the normalized bistatic radar cross section (NBRCS). An approach to model the mss for validation with CYGNSS mss data was developed by adding the contribution of a high frequency tail to the IFREMER WAVEWATCH III (WW3) mss. It is demonstrated that the ratio of CYGNSS mss and the modified WW3 mss can be used to diagnose potential calibration errors that exist in the Level 1 calibration algorithm. This approach can help to improve CYGNSS data quality, including the Level 1 NBRCS and Level 2 ocean surface wind speed and roughness.
Remote Sensing
This article presents the methodology for an improved estimation of the sea surface wind speed me... more This article presents the methodology for an improved estimation of the sea surface wind speed measured by the cyclone global navigation satellite system (CYGNSS) constellation of satellites using significant wave height (SWH) information as external reference data. The methodology consists of a correcting 2D look-up table (LUT) with inputs: (1) the CYGNSS wind speed given by the geophysical model function (GMF); and (2) the collocated reference SWH given by the WW3 model, which is forced by winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) organization. In particular, the analyzed CYGNSS wind speeds are the fully developed seas (FDS) obtained with the GMF 3.0, and the forcing winds are the ECMWF forecast winds. Results show an increase in sensitivity to large winds speeds and an overall reduction in the root mean square difference (RMSD) with respect to the ECMWF winds from 2.05 m/s to 1.74 m/s. The possible influence of the ECWMF winds on the corrected wind...
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Papers by Christopher Ruf