E. R. Kursinski

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.

The Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS)

The Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS) is designed to observe Earth’s climate. It extends and overcomes several limitations of the GPS radio occultation capabilities by simultaneously measuring atmospheric bending and absorption at frequencies approximately 10 and 100 times higher than GPS. This paper summarizes several important conceptual improvements to ATOMMS made since OPAC-1 including deriving the hydrostatic upper boundary condition directly from the ATOMMS observations, our much improved understanding of the impact of turbulence and its mitigation, and a new approach to deriving atmospheric profiles in the presence of inhomogeneous liquid water clouds. ATOMMS performance significantly exceeds that of radiometric sounders in terms of precision and vertical resolution and degrades only slightly in the presence of clouds and it does so independently of models. Our aircraft-to-aircraft occultation demonstration of ATOMMS performance will begin in 2009 representing a major step towards an orbiting observing system.

Simulating the influence of horizontal gradients on retrieved profiles from ATOMS occultation measurements - a promising approach for data assimilation

The Active Tropospheric Ozone and Moisture Sounder (ATOMS) is envisaged to provide information about the vertical distribution of atmospheric refractivity, volume absorption coefficients, temperature, pressure, water vapor pressure, clouds, and ozone via observations of the phase and amplitudes of LEO—LEO occultation signals at certain frequencies. The retrieval of these products assumes that the atmosphere in the vicinity of the ray path tangent points are spherically symmetrical. Since the tropospheric horizontal distribution of water vapor can be quite variable over relatively short distances, this assumption is far from perfect and may result in very large errors in retrieved profiles of volume absorption coefficients and water vapor pressure (not considering clouds). We assess close to worst-case errors in retrieved profiles of refractivity and volume absorption coefficients — from which temperature and water vapor pressure can be derived — by simulating the occultation measurements in cases where the signals propagate through a model of a weather front, including moisture. If the retrieved profiles are compared to the corresponding model profiles following the loci of the ray path tangent points, fractional errors in volume absorption coefficients can exceed 70%. An alternative comparison involves a linear mapping of the two-dimensional structure in the occultation plane into a one-dimensional profile, mimicking the occultation geometry as well as the subsequent data inversion process. The maximum fractional errors in volume absorption coefficients, comparing the retrieved profiles against such mapped profiles, are about 10%. Corresponding fractional errors in refractivity are about 10 times smaller. The linear mapping approach is simple and fast, and seems to be a good candidate as an observation operator for future data assimilation of occultation measurements.

The Mars Atmospheric Constellation Observatory (MACO) Concept

The Mars Atmospheric Constellation Observatory (MACO) represents an innovative approach to characterizing the present Martian climate from the surface into the thermosphere including the hydrological, CO2, and dust cycles together with the energy and momentum budgets. The mission concept is based on a constellation of satellites forming counter-rotating pairs for observing satellite-to-satellite microwave occultations to determine vertical profiles of water vapor, CO2, temperature, pressure, and wind. Satellite radio occultation, used in previous missions such as Mars Global Surveyor (MGS), provides precision, accuracy and vertical resolution typically 1 and sometimes 2 orders of magnitude beyond that of passive radiometers. Furthermore it can measure absolute pressure versus height (which is unobservable by radiometers) and thus remotely determine seasonal CO2 changes and winds. The microwave observations are supplemented by IR observations by a Dust and Ice Sensor (DIS). With the addition of a UV spectrometer, MACO can characterize the upper atmosphere’s composition and thermodynamic structure as well as escape rates. With a three satellite constellation, MACO will sample the Martian atmosphere with more than 80 occultations each day and, with observations from rapidly precessing orbits over at least one Martian year, will characterize the diurnal and seasonal cycles.

An Active Microwave Limb Sounder for Profiling Water Vapor, Ozone, Temperature, Geopotential, Clouds, Isotopes and Stratospheric Winds

We summarize our findings on the performance of a radio occultation system operating at cm and mm wavelengths selected to profile atmospheric water, ozone and other constituents such as water isotopes as well as temperature, the geopotential of atmospheric pressure surfaces and clouds. Furthermore winds in the upper stratosphere can be determined from the Doppler shift of the line center. Our analysis indicates that such a system will yield dramatically higher vertical resolution, precision and accuracy than present and planned passive radiometric systems.

An Overview of the University of Arizona ATOMS Project

Results from the now well known GPS/MET experiment have demonstrated the capabilities of radio occultation techniques for remotely sensing certain atmospheric properties (Ware et al. 1996; Rocken et al. 1997; Kursinski et al. 1997). The GPS/MET experiment was designed to provide vertical atmospheric profiles of temperature, pressure, density, and geopotential height from determinations of the vertical refractivity profiles. Indeed, for altitudes from a few km above the surface to about 40 km, extremely accurate profiles were obtained with very good vertical resolution. At altitudes below about 5–8 km, an ambiguity may exist however, when using just the GPS frequencies, due to the presence of water vapor. Water vapor, as well as dry air, affects the refractivity of the atmosphere, and the two variables cannot be separated without additional assumptions, or additional information. In polar regions where water vapor amounts are generally quite low, accurate temperatures can usually be retrieved by assuming the water vapor to be zero. In tropical regions, where low tropospheric temperature profiles are quite constant from day to day, water vapor profiles may be recovered from the refractivity profiles by assuming the temperature profile is known. It is in mid-latitudes where this ambiguity is most important. Techniques have been developed to overcome some of this difficulty, but uncertainties still exist in the recovered water vapor profiles, especially in data sparse regions. The ATOMS (Active Tropospheric Ozone and Moisture Sounder) project was conceived in order to provide a totally independent measurement to use for water vapor retrievals. A preliminary study indicated that the use of phase measurements as in the GPS/MET experiment, but near a water vapor absorption line where the refractivity undergoes a rapid variation, would not provide the necessary sensitivity due to the small mixing ratio of water vapor. However amplitude measurements at various frequencies within and near the line would provide a method to retrieve vertical water vapor profiles. In this paper we report on the progress we have made to date in developing this technique. By using various frequencies within both the 22 GHz and 183 GHz water vapor absorption lines, it will be shown that usable profiles may be recovered over a wide range of altitudes. A similar study will be presented using the 195 GHz ozone absorption line to recover ozone profiles from the mid-troposphere to well up into the stratosphere.

Applications of COSMIC Radio Occultation Data from the Troposphere to Ionosphere and Potential Impacts of COSMIC-2 Data

What: More than 130 people representing 15 nations met to highlight accomplishments in global positioning system (GPS) radio occultation (RO) operations and algorithm development, meteorology, climate, and ionospheric applications using COSMIC data. When: 30 October–1 November 2012 Where: Boulder, Colorado APPLICATIONS OF COSMIC RADIO OCCULTATION DATA FROM THE TROPOSPHERE TO IONOSPHERE AND POTENTIAL IMPACTS OF COSMIC-2 DATA

Deriving atmospheric water vapor and ozone profiles from active microwave occultation measurements

The GPS/MET experiment was the first active atmospheric microwave occultation experiment using the existing GPS Li and L2 frequencies to measure the atmospheric refractive index. One major limitation to this tecimique is that the presence of water vapor in amounts typically found in the lower troposphere (below 5-7 km) causes an ambiguity between the contributions of dry air and moisture to the refractive index. Additionally the profiles of other gases, such as ozone, cannot be measured using the Li and L2 frequencies. A new satellite remote sensing technique to independently monitor atmospheric water vapor and ozone is under development. It will include small satellites with both transmitter and receiver capabilities on each. The frequencies will be located around the 22 and 183 GHz water vapor and the i95 GHz ozone absorption lines. The receivers will also have the capability to observe the Li and L2 GPS frequencies. Simulation studies show that this new active occultation technique has the potential to provide accurate profiles of water vapor and ozone, as well as refractivity, temperature and pressure.

Single frequency processing of atmospheric radio occultations

Tracking of the radio signals broadcast by the Global Positioning System (GPS) satellites as they are occulted from a GPS receiver by the Earths atmosphere can provide high resolution vertical profiles of atmospheric refractivity, temperature and water vapour. Most implementations of this radio occultation technique use two GPS frequencies to correct for ionospheric effects. However, during most soundings, one of the frequencies is degraded by the introduction of the so-called Anti-Spoofing (AS) encryption mode. A retrieval method is discussed in this work for periods when only one of the two frequency signals has good quality. This method uses only the frequency with higher signal-to-noise ratio. We illustrate the quality of the atmospheric profiles obtained from such single frequency retrievals using GPS/MET data from the periods where the AS was turned off and the two frequencies were available. The results enable us to ensure the quality of a climate record of thousands of radio occultations collected by GPS/MET during the period with AS encryption, and the data processing of future missions with similar constraints, like IOX, can be performed.

Atmospheric profiling via satellite to satellite occultations near water and ozone absorption lines for weather and climate

Significantly reducing weather and climate prediction uncertainty requires global observations with substantially higher information content than present observations provide. While GPS occultations have provided a major advance, GPS observations of the atmosphere are limited by wavelengths chosen specifically to minimize interaction with the atmosphere. Significantly more information can be obtained via satellite to satellite occultations made at wavelengths chosen specifically to characterize the atmosphere. Here we describe such a system that will probe cm- and mmwavelength water vapor absorption lines called the Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS). Profiling both the speed and absorption of light enables ATOMMS to profile temperature, pressure and humidity simultaneously, which GPS occultations cannot do, as well as profile clouds and turbulence. We summarize the ATOMMS concept and its theoretical performance. We describe field measurements made with a prototype ATOMMS instrument and several important capabilities demonstrated with those ground based measurements including retrieving temporal variations in path-averaged water vapor to 1%, in clear, cloudy and rainy conditions, up to optical depths of 17, remotely sensing turbulence and determining rain rates. We conclude with a vision of a future ATOMMS low Earth orbiting satellite constellation designed to take advantage of synergies between observational needs for weather and climate, ATOMMS unprecedented orbital remote sensing capabilities and recent cubesat technological innovations that enable a constellation of dozens of very small spacecraft to achieve many critical, but as yet unfulfilled, monitoring and forecasting needs.