SF52A-01 10:20h
Promise of Climate Benchmarking Using Radio Occultation
While a growing body of evidence points towards the anthropogenic increases of greenhouse gases as the cause of twentieth century warming, the world's most sophisticated climate models still show substantial uncertainty in predictions of future warming even when given the same forcing constraints. Successive controlled intercomparisons by efforts such as the IPCC assessment reports and the Coupled Model Intercomparison Project bear this out. The U{.}S{.} Climate Change Science Program has called for a sustained effort to reduce the uncertainties of climate forecasts by climate models, as these climate models are necessary tools in making strategic societal decisions. Efforts to improve models by increasing the sophistication of sub-grid scale parameterizations have not substantially reduced forecast uncertainties. Testing climate models by monitoring the climate using benchmark quality measurements is a promising alternative approach, which additionally places significant new requirements on the U{.}S{.} climate observing effort. GPS radio occultation and radio occultation at 22- and 183-GHz are among the strong candidates for benchmark data types. A climate benchmark measurement is one which is absolutely calibrated, sensitive to ill-constrained feedbacks in the climate system, and is easily accessed and understood for all time. The requirement for absolute accuracy dictates that the measurement be tied to S{.}I{.} standards. In the case of radio occultation, that unit is the second, the most precisely defined of the S{.}I{.} units. Biases induced by undersampling the diurnal cycle are easily overcome with GPS occultation because of the ease with which constellations of GPS LEO receivers can be deployed. Among the least constrained feedbacks in the climate system is the water vapor-longwave radiation feedback. We will show the GPS occultation is sensitive to this feedback in the lower troposphere, albeit weakly so. Radio occultation at 22- and radio occultation at 183-GHz are more sensitive to the water vapor-longwave feedback in the lower and upper troposphere, respectively. Finally, GPS occultation observations can stand alone for all time because successive generations of GPS receiver instrumentation are interpretable in the same way as those of GPS/MET, the first GPS occultation mission flown in 1995-7. We will present a case for the science requirements of GPS, 22-GHz, and 183-GHz radio occultation missions and data processing. We also call for climate observing system simulation experiment (climate OSSE) for radio occultation to demonstrate the viability of radio occultation as a benchmark data type.
SF52A-02 10:35h
Recent results of open-loop tracking of GPS occultations in the lower troposphere and implications on measuring specific humidity
GPS occultation measurements from CHAMP and SAC/C have demonstrated the ability to measure atmospheric temperature with accuracy better than 1K and a precision of 0.1K in the upper atmosphere and lower stratosphere. Most challenging has been the accurate detection of the occulted GPS signal in the lower troposphere due to atmospheric defocusing causing signal attenuation, atmospheric multipath causing strong signal dynamics, and atmospheric ducting causing temporary signal disappearance. Theoretically, accurate determination of the occulted GPS phase delay and its amplitude in the lower troposphere can lead to a determination of the atmospheric refractivity from which specific humidity can be inferred to better than 0.5 g/kg with a vertical resolution approaching 100m. Because of the signal temporary disappearance and its dynamics, traditional closed-loop or semi-closed-loop (also known as flywheeling) techniques for measuring the atmospheric refractivity in the lowest 5 km proved problematic, where systematic biases under-estimating the atmospheric refractivity and specific humidity are created. "Open-loop" tracking in this region, where the atmospheric Doppler shift is modeled and used inside the receiver, has been suggested as a means of resolving these problems. Recently, open-loop tracking has been implemented and tested on the SAC/C GPS occultation receiver. This talk will present an overview of the challenges and difficulties encountered with traditional techniques of detecting lower tropospheric water vapor from GPS occultations and evaluate improvements with the recent "open-loop" data from SAC/C.
SF52A-03 10:50h
Developments in the Applications of GNSS and GNSS-like Signal Measurements to Atmosphere and Ocean Remote Sensing at ESA
Over the last few years several research and development activites have taken place in the frame of the European Space Agency (ESA) programmes to exploit GNSS and GNSS-like signal measurements for remote sensing of the atmosphere and of the oceans. These included: - the definition of a space mission, the Atmosphere and Climate Explorer (ACE+), for active sounding of water vapour in the troposphere and of temperature in the troposphere and stratosphere, using Ku- and Ka-band GNSS-like signals in a radio occultation approach - the development and validation of multiple instrument breadboards, proving the technical feasibility of the ACE+ concept; - the further development of instrumentation for GNSS radio occultation, namely of a second-generation GRAS (GNSS Receiver for Atmospheric Sounding) using Galileo signals in addition to GPS; - research on radio occultation retrieval algorithms and on the limits of performance caused by tropospheric turbulence, as well as on possible approaches to reduce its impact; - airborne experiments with GPS signals reflected at the sea surface, aiming in particular at the retrieval of directional sea surface roughness, in terms of full directional mean square slope from delay-doppler maps of the reflected signals, in addition to altimetric information. The status and results of these activities will be reviewed, emphasizing key achievements, open issues and possible avenues for further research and development.
SF52A-04 11:05h
An Active Orbiting Microwave Spectrometer
We present an overview of a satellite-to-satellite occultation system concept operating at cm and mm wavelengths to profile atmospheric water, temperature, the geopotential of atmospheric pressure surfaces and clouds. The system is essentially an orbiting active microwave limb viewing spectrometer and with suitable choice of frequencies, it can characterize other constituents such as ozone. The unique features of this system are global and diurnal coverage, similar performance in clear and cloudy conditions, high vertical resolution (~200 m), a wide dynamic range such that it can profile water from near the surface to the mesopause, very high precision ~1-3% over most of this altitude range and absolute accuracy (perhaps to 1%) and lack of drift. Ozone profiles will have similar performance from the upper troposphere into the mesosphere. Our analysis indicates that such a system will yield dramatically higher vertical resolution, precision and accuracy than present and planned passive radiometric systems in both clear and cloudy air. It will complement other observations for weather applications and is particularly well suited for climate because of its self-calibrating nature. We will discuss the expected performance of such an orbiting system including in particular the effects of scintillations associated with atmospheric turbulence and how to mitigate them. Our simulations indicate that scintillations will not limit the performance in the upper troposphere and above but they will likely limit performance in lower troposphere particularly in the boundary layer with a strong tradeoff between precision and vertical resolution. Time permitting we will also discuss a proof of concept mission and a constellation of microsatellites carrying these instruments focused on the hydrological cycle and monitoring of climate.
SF52A-05 11:20h
GPS Ocean Topography of Monterey Bay
Eight successful aircraft flights were performed over Monterey Bay in Aug. 2003 with the goal of producing eddie-resolving, 4-D ocean topography measurements. Two flights on each of 4 days covered an approximately 50 km by 70 km area. About 170 Gbytes of open-loop data were recorded on each flight and were later processed by a GPS software receiver. The aircraft position was determined from GPS, while the difference in arrival times between the direct and ocean-reflected GPS signals gave the height of the aircraft above the ocean surface at each of the GPS specular reflection points; these data were combined to form a topographic map of the Monterey Bay area. This paper will present results of the data analysis and preliminary topographic maps. An extrapolation of the results to a global spacecraft mission will also be presented.
SF52A-06 11:35h
3D Tomography of Ionospheric Perturbations Produced by Earthquakes Using Global Positioning System
The recent development of Global Positioning System led to establish dense regional networks of bistatic GPS receivers providing today a powerful ionospheric observing system. Now the ionosphere can be imaged by tomographic methods using GPS data. Therefore the ionospheric perturbations can be characterized by monitoring Total Electronic Content (TEC). These disturbances have multiple sources located adove and below ionospheric layers. The most known are the Travelling Ionospheric Disturbances (TID) produced by internal gravity waves. But some ionospheric disturbances are also due to infrasonic waves. We focus this study on ionospheric perturbations generated by infrasonic waves exited by seismic waves, resulting from the coupling between Earth and the atmosphere. We present a spectral analysis of TEC GPS data, the 3D tomographic method and its application to post-seismic perturbations. By removing background noise we are able to monitor acoustic post-seismic waves, generated by the rupture process and the seismic surface waves, that reach the ionosphere. Especially, we show the observations for the Denali earthquake of $3^{rd}$ November 2002 and the Hokkaido earthquake of $25^{th}$ September 2003 using respectively the Californian networks (SICGN) and the Japan network (GEONET). Both the horizontal and vertical propagation of the waves are vizualized in the 3D tomographic movies. The observed waves arrive with a timing and a propagation velocity coherent with expected waves and we purpose an interpretation in terms of infrasonic waves in the atmosphere, generated both near the epicenter and at further distance, at the level of the Rayleigh waves front. Finally we present the improvement of the 3D tomographic methods with the advent of the Galileo system and possible application in seismology.
SF52A-07 11:50h
The Application of Rapidly-Sampled GPS Signal Amplitudes to Remote Sensing of Ionospheric Irregularities
Cornell University has developed a GPS scintillation receiver in which the amplitude and phase of the L1 GPS signal can be measured at up to 50 samples per second. This capability enables a variety of measurements and experiments to remotely sense properties of the disturbed ionosphere. Fast variations in the GPS signal are primarily produced by a diffractive process whose source is Fresnel-scale-length electron density irregularities. These variations can be used to infer ionospheric drift velocities, the scattering heights, and the spatial and temporal properties of the scintillation patterns. In addition these receivers can be used in conjunction with other remote sensing instruments such as imagers, radar, and ionosondes to understand the origin, life-time, and dynamics of the irregularities. We will present examples estimating ionospheric drift velocities, scattering heights, irregularity decay rates, and the size, shape, orientation, and time-scale of scintillation patterns.
SF52A-08 12:05h
In Situ Electron Density in Low Earth Orbit From Radio Occultation Data
Global Positioning System (GPS) radio occultation data, collected in low earth orbit (LEO), provide valuable information about the vertical structure of electron density in the ionosphere as well as temperature, pressure, and water vapor in the neutral atmosphere. It is well known that profiles of ionospheric electron density can be derived from the observed total electron content (TEC) between the GPS transmitter and the LEO receiver. Recently, we discovered that the data can also provide accurate, model-independent estimates of the electron density at the LEO orbit altitude, provided that the collection of occultation data include the top point where the tangent height is close to the orbit altitude. More precisely, the electron density at the orbit altitude is obtained from the observed TEC data using linear regression of the square of the TEC near the orbit altitude. Thus, the electron density at the LEO at the beginning of an ionospheric occultation can be derived from the occultation data. We are currently applying the approach to GPS occultation data from the Challenging Mini-Satellite Payload (CHAMP), and will present the results of statistical comparisons with in situ electron density measurements from the Planar Langmuir Probe on board CHAMP.