Thomas P. Yunck

The GPS flight experiment on TOPEX/POSEIDON

The precision orbit determination (POD) experiment on TOPEX/POSEIDON using the Global Positioning System (GPS) is yielding concrete results. Orbit consistency and accuracy tests indicate that GPS is routinely providing satellite altitude with an accuracy of better than 3 cm. Here we review the GPS experiment, its basic concepts, POD techniques and key results, and discuss the possible cost and performance benefits that may flow to future missions.

Generating climate benchmark atmospheric soundings using GPS occultation data

Atmospheric soundings derived from Global Positioning System radio occultations (GPSRO) acquired in low-Earth orbit have the potential to be global climate benchmark observations of significant value to the Global Climate Observing System (GCOS). Geophysical observables such as atmospheric pressure and temperature are derived by measuring propagation delay induced by the atmosphere, a measurement whose fundamental unit-the second-is absolutely determined by calibration against atomic clocks. In this paper, we analyze the sources of systematic and random error for GPSRO soundings to determine the steps needed to establish GPSRO as a climate benchmark observation. Benchmarks require specific processing strategies and specific forms of documentation so that confidence in the accuracy and precision of the measurements is assured. Establishing calibration traceability to absolute standards (SI-traceability) is an essential strategy. We discuss a wide range of error sources in a geophysical retrieval, such as orbit determination error, signal delay in the Earths ionosphere, and quality control strategies. Uncalibrated ionospheric delay is identified as the error source deserving the most attention in establishing SI-traceability of the retrievals, to meet stringent climate observation requirements of 0.5 K accuracy and 0.04 K stability. Profile comparisons from the recently launched COSMIC constellation establish strong upper limits on systematic error arising from the individual instruments. These encouraging results suggest that GPSRO should become a permanent resource for the GCOS. These highly precise and accurate instruments can be deployed on future Earth Observation satellites at a low per-sensor cost and minimal interference to existing and planned observational programs.

GPS Radio Occultation as Part of the Global Earth Observing System

Radio occultation measurements of the atmosphere using transmissions of the Global Positioning System (GPS) are discussed in the Decadal Survey for Earth Science released in 2007. Several successful examples of RO missions are currently in orbit: CHAMP, SAC-C, COSMIC and GRACE. RO retrievals have the fortunate characteristic of being based on time delay measurements, whose fundamental unitiquestthe secondiquestis absolutely calibrated using atomic clocks. Due to the absence of bias or long-term drift, multi-decadal time series of GPS RO retrievals are natural to develop for climate monitoring of atmospheric properties from the troposphere to the stratosphere. Highly accurate temperature profiles with high vertical resolution (50 m-200 m) are retrieved from the stratosphere to the mid-troposphere. Water vapor profiles are available from approximately 5 km altitude to the surface. The technique has sufficient vertical resolution to resolve the planetary boundary layer over much of the globe. Understanding the spatial sampling properties of GPS RO is important when bringing these data into the broader Earth observing context. In this paper, we discuss the unique relationship between vertical and horizontal resolution and describe the technology development needed to achieve maximum benefit for climate and weather applications.

CICERO: Nanosat arrays for continuous Earth remote observation

CICERO (Community Initiative for Continuous Earth Remote Observation) is a grassroots effort to deploy a nanosat-based cellular observing system to acquire global Earth and environmental data from low Earth orbit at extremely low cost. CICERO will place micro-sensors on nanosats (or cells) in large numbers for diverse remote sensing applications. Scores of cells will combine to form a global supersensor of enormous power. Produced by the dozen, these tiny craft will cost little to build and launch. The cellular model will improve measurement sensitivity by an order of magnitude at a fraction of today’s system costs. GeoOptics has developed an array of novel cellular observing technologies, from sensor and nanosat designs to unique on-orbit cell configurations, to maximize information return. The approach consolidates diverse sensing techniques into integrated sensors to yield a breadth of products. CICERO will offer frequent launches to expedite deployment of new sensors.