George Antoine Hajj

Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System

The implementation of the Global Positioning System (GPS) network of satellites and the development of small, high-performance instrumentation to receive GPS signals have created an opportunity for active remote sounding of the Earths atmosphere by radio occultation at comparatively low cost. A prototype demonstration of this capability has now been provided by the GPS/MET investigation. Despite using relatively immature technology, GPS/MET has been extremely successful [Ware et al., 1996; Kursinski et al., 1996], although there is still room for improvement. The aim of this paper is to develop a theoretical estimate of the spatial coverage, resolution, and accuracy that can be expected for atmospheric profiles derived from GPS occultations. We consider observational geometry, attenuation, and diffraction in defining the vertical range of the observations and their resolution. We present the first systematic, extensive error analysis of the spacecraft radio occultation technique using a combination of analytical and simulation methods to establish a baseline accuracy for retrieved profiles of refractivity, geopotential, and temperature. Typically, the vertical resolution of the observations ranges from 0.5 km in the lower troposphere to 1.4 km in the middle atmosphere. Results indicate that useful profiles of refractivity can be derived from ∼60 km altitude to the surface with the exception of regions less than 250 m in vertical extent associated with high vertical humidity gradients. Above the 250 K altitude level in the troposphere, where the effects of water are negligible, sub-Kelvin temperature accuracy is predicted up to ∼40 km depending on the phase of the solar cycle. Geopotential heights of constant pressure levels are expected to be accurate to ∼10 m or better between 10 and 20 km altitudes. Below the 250 K level, the ambiguity between water and dry atmosphere refractivity becomes significant, and temperature accuracy is degraded. Deep in the warm troposphere the contribution of water to refractivity becomes sufficiently large for the accurate retrieval of water vapor given independent temperatures from weather analyses [Kursinski et al., 1995]. The radio occultation technique possesses a unique combination of global coverage, high precision, high vertical resolution, insensitivity to atmospheric particulates, and long-term stability. We show here how these properties are well suited for several applications including numerical weather prediction and long-term monitoring of the Earths climate.

Ionospheric electron density profiles obtained with the Global Positioning System: Results from the GPS/MET experiment

The Global Positioning System Meteorology (GPS/MET) experiment, which placed a GPS receiver in a low-Earth orbit tracking GPS satellites setting behind the Earths limb, has collected data from several thousands of occultations since its launch in April 1995. This experiment demonstrated for the first time the use of GPS in obtaining profiles of electron density and other geophysical variables such as temperature, pressure, and water vapor in the lower atmosphere. This paper discusses some of the effects of the ionosphere, such as bending and scintillation, on the GPS signal during occultation. It also presents a set of ionospheric profiles obtained from GPS/MET using the Abel inversion technique, and compares these profiles with ones obtained from the parameterized ionospheric model (PIM) and with ionosonde and incoherent scatter radar measurements. Statistical comparison of NmF2 values obtained from GPS/MET profiles and nearby ionosondes indicates that they agree to about ∼20% (1-sigma) in a fractional sense. The high vertical resolution, characteristic of the occultation geometry, is reflected in the GPS/MET profiles which reveal ionospheric structures of very small vertical scales such as the sporadic E.

A Technical Description of Atmospheric Sounding by GPS Occultation

Abstract In recent years, the global positioning system (GPS) has been exploited via radio occultation techniques to obtain profiles of refractivity, temperature, pressure and water vapor in the neutral atmosphere and electron density in the ionosphere. The GPS/MET experiment, which placed a GPS receiver in a low-Earth orbit, provided a wealth of data which was used to test this concept and the accuracy of the retrievals. Several investigations have already demonstrated that the retrieval accuracies obtained with GPS/MET is already comparable, if not better, than the more traditional atmospheric sensing techniques (e.g., radiosondes). Even though the concept of atmospheric profiling via radio occultation is quite a simple one, care must be taken to separate the numerous factors that can affect the occulted signal. These include the motion of the satellites, clock drifts, relativistic effects, the separation of the ionosphere and the neutral atmosphere, and the contribution of the upper atmosphere where sensitivity of the GPS signal is weak. In addition, care must be taken to use proper boundary conditions, use proper smoothing intervals and interpolation schemes to avoid retrieving artificial atmospheric structures, and most importantly detect and correct phase measurement errors introduced by sharp refractivity gradients in the atmosphere. This work describes in some detail the several steps involved in processing such data. In particular, it describes a system that was developed at the Jet Propulsion Laboratory and used to process the GPS/MET data. Several examples of retrieved refractivity, temperature and water vapor profiles are shown and compared to analyses from the European Center for Medium-range Weather Forecast (ECMWF). Statistical comparisons of GPS/MET and ECMWF temperatures for data collected during June 21–July 4, 1995, indicate that differences are of order 1– 2 K at northern latitudes where the ECMWF analyses are most accurate.

Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning System

Recent radio occultation measurements using Global Positioning System satellite transmitters and an orbiting receiver have provided a globally distributed set of high-resolution atmospheric profiles, suggesting that the technique may make a significant contribution to global change and weather prediction programs. Biases in occultation temperatures relative to radiosonde and model data are about 1 kelvin or less in the tropics and are generally less than 0.5 kelvin at higher latitudes. Data quality is sufficient to quantify significant model errors in remote regions. Temperature profiles also reveal either an equatorial Rossby-gravity or an inertio-gravity wave. Such waves provide a fundamental source of momentum for the stratospheric circulation.

Imaging the ionosphere with the global positioning system

Observing the Global Positioning System with a satellite in low earth orbit in an occulting geometry provides a powerful means of imaging the ionosphere. Tomographic imaging of the ionosphere from space and ground is examined using singular value decomposition analysis. The resolution and covariance matrices are examined, and simulations are performed that indicate that space data are significantly more effective than ground data in resolving both horizontal and vertical structures, such as the E layer, can be probed with occultation data.©1994 John Wiley & Sons Inc

A comparison of water vapor derived from GPS occultations and global weather analyses

Despite its fundamental importance in radiative transfer, atmospheric dynamics, and the hydrological cycle, atmospheric water is inadequately characterized particularly at a global scale. Occultation measurements from the Global Positioning System (GPS) should improve upon this situation. Individual occultations yield profiles of specific humidity accurate to 0.2 to 0.5 g/kg providing sensitive measurements of lower and middle tropospheric water vapor with global coverage in a unique, all-weather, limb-viewing geometry with several hundred meters to a kilometer vertical resolution. We have derived water vapor profiles from June 21 to July 4, 1995, using GPS occultation data combined with global temperature analyses from the European Center for Medium-Range Weather Forecasts (ECMWF) and reanalyses from the National Centers for Environmental Prediction (NCEP). The zonal mean structure of the profiles exhibits basic climatological features of tropospheric moisture. Specific humidity biases between the GPS results and the ECMWF global humidity analyses in the middle to upper troposphere are ∼0.1 g/kg or less. Occultation results below 6 km altitude are generally drier than those of ECMWF with the bias generally increasing toward warmer temperatures. Near the height of the trade wind inversion, the ECMWF analyses are significantly moister than the occultation results due to vertical smoothing and overextension of the boundary layer top in the analyses. Overall, the occultation results are drier than the NCEP reanalyses with a marked exception near the Intertropical Convergence Zone (ITCZ) where occultation results are wetter by more than 10%. The occultation results are significantly wetter near the ITCZ and drier in the subtropics than the classical moisture climatology of Peixoto and Oort. Similarities between the NCEP and the Peixoto and Oort near-ITCZ differences suggest that a common analysis/model problem may be responsible. The generally wetter Peixoto and Oort results in the subtropics are due in part to moist radiosonde biases. Discrepancies between these data sets are significant and limit our ability to resolve uncertainties in moisture control and feedbacks in a changing climate.

Observing tropospheric water vapor by radio occultation using the Global Positioning System

Given the importance of water vapor to weather, climate and hydrology, global humidity observations from satellites are critical. At low latitudes, radio occultation observations of Earths atmosphere using the Global Positioning System (GPS) satellites allow water vapor profiles to be retrieved with accuracies of 10 to 20% below 6 to 7 km altitude and ∼5% or better within the boundary layer. GPS observations provide a unique combination of accuracy, vertical resolution (≤ 1 km) and insensitivity to cloud and aerosol particles that is well suited to observations of the lower troposphere. These characteristics combined with the inherent stability of radio occultation observations make it an excellent candidate for the measurement of long term trends.

A novel approach to atmospheric profiling with a mountain‐based or airborne GPS receiver

The delay induced by the Earths atmosphere on the Global Positioning System (GPS) signal has been exploited in the last decade for atmospheric remote sensing. Ground-based GPS measurements are traditionally used to derive columnar water vapor content, while space-based GPS measurements, obtained by a receiver in a low-Earth orbit tracking GPS satellites occulting behind the Earths atmosphere, yield accurate, high-resolution profiles of refractivity, temperature, and water vapor. A GPS receiver on a mountain top or an airplane with a “downward looking” field of view toward the Earths limb is a novel concept presented here. We describe a generalized ray-tracing inversion scheme where spherical symmetry is assumed for the atmosphere, and the refractivity is modeled as piecewise exponential, with scale height changing from one atmospheric layer to the next. Additional refractivity data, derived from a model, might be introduced above the receiver as an a priori constraint, and are treated as properly weighted additional measurements. The exponential scale heights and a normalizing value of refractivity are retrieved by minimizing, in a least squares sense, the residuals between measured bending angles and refractivity and those calculated on the basis of the exponential model and ray-tracing. As a first validation step, we illustrate results comparing refractivity and temperature profiles obtained by this generalized ray-tracing scheme against those derived via the Abel inversion for the GPS/MET experiment. Additionally, we present results for a hypothetical situation where the receiver is located within the atmosphere at a height of 5 km. For the last case we investigate the accuracy of the retrieval both below and above the receiver at a set of locations in the atmosphere ranging from middle to tropical latitudes. The main objective is that of establishing whether the bending measurements have sufficient strength to allow for retrieval of refractivity below and possibly above the receiver location. Our findings suggest that accurate profiles of refractivity at heights ranging from the Earths surface to slighly above the receiver location can be derived by GPS data collected from within the atmosphere.

Demonstrating soil moisture remote sensing with observations from the UK TechDemoSat-1 satellite mission

The ability of spaceborne Global Navigation Satellite System (GNSS) bistatic radar receivers to sense changes in soil moisture is investigated using observations from the low Earth orbiting UK TechDemoSat-1 satellite (TDS-1). Previous studies using receivers on aircraft or towers have shown that ground-reflected GNSS signals are sensitive to changes in soil moisture, though the ability to sense this variable from space has yet to be quantified. Data from TDS-1 show a 7 dB sensitivity of reflected signals to temporal changes in soil moisture. If the effects of surface roughness and vegetation on the reflected signals can be quantified, spaceborne GNSS bistatic radar receivers could provide soil moisture on relatively small spatial and temporal scales.

GPS radio occupations coming of age: Spacecraft launches add two new instruments for climate monitoring

Global Positioning System (GPS) radio occultations are active limb soundings that measure the time delay of a GPS signal propagating through the atmosphere. This time delay can be related to vertical profiles of atmospheric refractivity from which highly accurate profiles of geopotential height, temperature, pressure, and specific humidity are derived. With their global coverage, self-calibrating nature, penetration through clouds, and high vertical resolution, atmospheric radio occultations are coming of age and hold great promise for weather prediction and climate monitoring.