Larry J. Romans

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

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.

Initial results of combining GPS occultations with ECMWF global analyses within a 1DVar framework

We present results of combining occultation refractivity profiles from GPS/MET with ECMWF global analyses in a 1DVar framework in order to separate the wet and dry contributions to refractivity and assess their impact on the analyzed temperature, surface pressure and specific humidity fields. We find significant zonal mean temperature, surface pressure and humidity differences between the 1DVar solutions and the ECMWF analyses reflecting biases between the GPS refractivities and ECMWF analyses. Large profile-to-profile temperature discrepancies in the tropical lower stratosphere are due to waves not represented in the analyses. The 1DVar solution is generally drier than ECMWF particularly in the southern subtropics. Lack of moisture above 300 hPa in the present model caused the solution to make large adjustments in low latitude surface pressure and tropospheric temperatures to increase upper troposphere densities and compensate for the missing upper level moisture. The discrepancies between the solution and the background and observational data sets represent roughly a 2-sigma level of agreement rather than the 1-sigma level desired in a 1DVar solution. Given the simplicity of our error covariances, our results are promising as a first step. In the future, the error covariances need to be refined and, in particular, to vary with location.

Initial Results of GPS-LEO Occultation Measurements of Earth’s Atmosphere Obtained with the GPS-MET Experiment

The radio occultation technique, which has been repeatedly proven for planetary atmospheres, was first utilized to observe Earth’s atmosphere by the GPS-MET experiment (launched in April 1995), in which a high performance GPS receiver was placed into a low-Earth orbit. During certain phases of the mission, more than 100 occultations per day are acquired. A subset of this occultation data is analyzed and temperature in the neutral atmosphere and electron profiles in the ionosphere are obtained. Comparing about 100 GPS-MET retrievals to accurate meteorological analyses obtained from the European Center for Medium-range Weather Forecasting at heights between 5–30 km, temperature differences display biases of less than 0.5K and standard deviations of 1–2K in the northern hemisphere, where the model is expected to be most accurate. Furthermore, electron density profiles obtained for different geodetic locations and times show the main features that are expected in the ionosphere.

Relative time and frequency alignment between two low earth orbiters, grace

The two GRACE (gravity recovery and climate experiment) spacecraft were launched into a near polar circular orbit around the earth in March of 2002. The two spacecraft serve as test masses to measure the earths gravitational field. Both spacecraft carry ultra-stable oscillators (USO) with an Allan deviation of a few parts in 10/sup -13/ for Tau=1 to 1000 s. The USOs drive both the microwave links and GPS receivers. To cancel out long term errors on the USOs a linear combination of the 1-way microwave links is used (dual-one-way). In order to form the dual-one-way measurement and cancel our long term USO error, time must be synchronized between the two spacecraft to about 150 picoseconds. This synchronization is accomplished using the GPS data. For each spacecraft, the GPS data are used to solve for the orbital positions and the difference between the on-board clocks and a ground reference clock every 5 minutes. The relative clock is determined by the difference of these two solutions. Validation of the relative clock accuracy includes the solutions from overlapping data arcs which are typically less than the 150 picosecond goal and unique combination of the one-way microwave links that allows independent comparison of the GPS determine relative frequency of the USOs to a measurement made by the microwave link.

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.

A novel GPS-based sensor for ocean altimetry

A technique for a novel application of Global Positioning System (GPS) signals to ocean altimetry is described. The entire Earth surface is divided into a triangular grid of points nearly-uniformly spaced. The sea surface heights at the grid points are determined using the GPS signals reflected from the surrounding area. Practical grid size and data sampling rate are determined. An efficient estimation scheme is devised to solve for the thousands of parameters. A simulation analysis shows that global ocean altimetric information can be recovered to better than 10 cm with only 1 day of reflected GPS signals.