Genesis GPS Occultation Observations Level 1B

The Genesis GPS Occultation Observations Level 1B data set consists of
LEO-GPS (low-earth-orbiter satellites and Global Positioning System
satellites) radio occultation (limb sounding) atmospheric doppler and
bending 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, at a time resolution of 1 second or
better. These are the Level 1B 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.

Smoothed atmospheric doppler and bending angle are useful intermediate
products between the Levels 1A and 2 products. These products are
provided at a sample rate of 1/sec or better for the 1.6 GHz
frequency, and at a rate of 1/sec for the 1.2 GHz frequency. The 1.2
GHz data has lower SNR than the 1.6 GHZ data, and is used to subtract
the effect of the ionosphere. Atmospheric bending with the effect of
the ionosphere removed, and impact parameter are also provided.
Bending angle and impact parameter are computing assuming local
spherical symmetry.