Genesis GPS Occultation Observations Level 1A

The Genesis GPS Occultation Observations Level 1A data set consists of
LEO-GPS (low-earth-orbiter satellites and Global Positioning System
satellites) radio occultation (limb sounding) atmospheric phase delay
and amplitude data for use in computing atmospheric refractivity,
temperature, pressure, and water vapor profiles. LEOs collecting
occultation data include GFZ's CHAMP (CHallenging Minisatellite
Payload satellite) and CONAE's SAC-C. Data are available at the two
GPS frequencies, 1.2 and 1.6 GHz, the latter at a time resolution of
0.01 or 0.02 seconds. These are the Level 1A 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.

Geometric optics inversion techniques primarily use the phase delay,
but physical optics inversion can also make precise use of signal
amplitude variation to account for diffraction effects. Each Level 1A
data file contains atmospheric phase delay and amplitude data from a
single occultation at a rate of 50 or 100 samples/sec at the 1.6 GHz
frequency. 1.2 GHz data is also included, but may be noisier and at a
lower sample rate.