Christian Marquardt

Atmosphere sounding by GPS radio occultation: First results from CHAMP

The first radio occultation measurements of the CHAMP (CHAllenging Minisatellite Payload) satellite using Global Positioning System (GPS) signals have been performed on February 11, 2001. By the end of April 2001 more than 3000 occultations were recorded. Globally distributed vertical profiles of dry temperature and specific humidity are derived, of which a set of 438 vertical dry temperature profiles is compared with corresponding global weather analyses. The observed temperature bias is less than ∼1 K above the tropopause and even less than 0.5 K in the altitude interval from 12 to 20 km at latitudes >30°N. About 55% of the compared profiles reached the last kilometer above the Earths surface. In spite of the activated anti-spoofing mode of the GPS system the state-of-the-art GPS flight receiver aboard CHAMP combined with favorable antenna characteristics allows for atmospheric sounding with high accuracy and vertical resolution.

Prospects of the EPS GRAS Mission For Operational Atmospheric Applications

Abstract Global Navigation Satellite System (GNSS) Receiver for Atmospheric Sounding (GRAS) is a radio occultation instrument especially designed and built for operational meteorological missions. GRAS has been developed by the European Space Agency (ESA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) in the framework of the EUMETSAT Polar System (EPS). The GRAS instrument is already flying on board the first MetOp satellite (MetOp-A) that was launched in October 2006. It will also be on board two other MetOp satellites (MetOp-B and MetOp-C) that will successively cover the total EPS mission lifetime of over 14 yr. GRAS provides daily about 600 globally distributed occultation measurements and the GRAS data products are disseminated to the users in near–real time (NRT) so that they can be assimilated into numerical weather prediction (NWP) systems. All GRAS data and products are permanently archived and made available to the users for climate applications and sc...

Validation and Data Quality of CHAMP Radio Occultation Data

Since the first successful radio occultation measurement taken by the GPS receiver onboard CHAMP, more than 40,000 vertical soundings of stratospheric temperature and tropospheric humidity have been processed at GFZ Potsdam. During the first year of the radio occultation experiment, several multi-week batches of quasi-continuous measurements have been collected, allowing for detailed validation against other meteorological measurements. This paper presents the first validation results and discusses some conclusions regarding the data quality of CHAMP radio occultation data drawn from these.

GPS Radio Occultation with CHAMP

The GPS (Global Positioning System) radio occultation experiment onboard the German CHAMP (CHAllenging Minisatellite Payload) satellite was activated in February 2001. By the end of 2001 about 36,500 occultations were recorded. We review the occultation data processing at the GFZ and discuss selected first results. 1) CHAMP allows for atmospheric sounding with high accuracy despite of the Anti-Spoofing (A/S) mode of the GPS. 2) There are advantageous consequences for the GPS data processing due to the termination of the Selective Availability (SA) mode of the GPS. It is possible to reduce the GPS ground station acquisition rate for double difference processing. The application of space-based single differencing technique for precise occultation data processing became feasible. 3) The state-of-the-art GPS flight receiver onboard CHAMP combined with high-gain occultation antenna allows for atmospheric sounding deep into the lower troposphere. Possible applications and improvements of occultation data analysis in the lower troposphere are discussed.

The EUMETSAT Environment for Precise Orbit Determination

In the frame of the processing of data from the GNSS Receiver for Atmospheric Sounding (GRAS) instrument onboard the MetOp satellites, EUMETSAT has developed an environment for operational Precise Orbit Determination (POD) processing and validation. The design of this POD environment, in which several components for validation, monitoring, reprocessing and calibration are centralized, makes it well-suited for its usage in future EUMETSAT missions involving POD, such as Sentinel-3, part of Europe’s Global Monitoring for Environment and Security (GMES). The purpose of this paper is to describe this environment, consisting of two different platforms (operational and offline) using GRAS processing as an example of its capabilities. The Navigation Package for Earth Observation Satellites (NAPEOS) infrastructure from ESA is used as the core POD software.