Genesis GPS Occultation Observations Level 2

The Genesis GPS Occultation Observations Level 2 data set contains
profiles of atmospheric temperature, pressure, refractivity, and water
vapor pressure with resolution of about a kilometer, derived from
LEO-GPS (low-earth-orbiter satellites and satellites of the Global
Positioning System) radio occultation (limb sounding) data. Similar
profiles from NCEP and ECMWF analyses are also included. LEOs
collecting occultation data include GFZ's CHAMP (CHallenging
Minisatellite Payload satellite) and CONAE's SAC-C. These are the
Level 2 products of this radio occultation data set.

The full set of products available include
* Level 0: Raw GPS data
* GPS data for orbit determination in RINEX format
* Orbit products
* Level 1A: Atmospheric phase delay and signal amplitude
* Level 1B: Atmospheric doppler shift and bending
* Level 2: Atmospheric refractivity, temperature, pressure, water vapor
pressure profiles, and comparisons to weather analysis and
radiosondes.

CHAMP and SAC-C each carry a JPL Blackjack GPS receiver and a
rearward-facing antenna to record the signals of GPS satellites
setting behind the Earth's limb as the signal passes through the
atmosphere. (SAC-C also carries a forward-facing antenna, currently
inactive.) Typically over 200 of these "occultations" occur per day
per LEO with fairly uniform global distribution. By measuring the
precise phase delay experienced by the GPS signals, and using precise
clock-offset and orbit information, the atmospheric component of the
phase delay can be extracted. Assuming local spherical symmetry about
the tangent point, inversion of the atmospheric phase delay phase
measurements during an occultation yields atmospheric refractivity
profiles, which can be converted to temperature and pressure profiles
between 60 km and the middle troposphere, and, with independent
knowledge of temperature, into water vapor density in the middle and
lower troposphere.

Valuable properties of radio occultation measurements of atmospheric
profiles include:
* Self-calibrating, making them ideal for climate detection.
* Sub-kilometer vertical resolution.
* Sub-Kelvin temperature accuracy below 45 km.
* All-weather operation
* Independent height and pressure data allowing computation of
geopotential heights and derived wind fields
* Concurrent global coverage with a small constellation

Comparison of CHAMP and SAC-C to National Center for Environmental
Prediction (NCEP) and the European Center for Medium-range Weather
Forecast (ECMWF) analyses show that GPS occultation temperature
profiles are consistent with the models to better than 0.5 K in the
mean, and better than 1.5 K in standard deviation.