ION GNSS 2012
Session A6: GNSS and the Atmosphere 2

Title: Detection of Atmospheric Turbulence in GPS-RO Amplitude Spectra
Author(s): E. Barlow, P. Axelrad, S. Palo, University of Colorado Boulder; L. Cornman, R.K. Goodrich, National Center for Atmospheric Research
Date/Time: Friday, September 21, 2012, 1:50 p.m.
Room: 103/104 (NCC)

The study of atmospheric turbulence has suffered from a lack of observations, as most existing data sources are either unreliable, poorly distributed, or both. The advent of GPS radio occultation (GPS RO) provides an opportunity to study the behavior of the terrestrial atmosphere with a global distribution and scientific reliability. Better knowledge of air turbulence also has significant practical value in the realm of commercial aviation. Every year, millions of dollars in extra aircraft maintenance and personal injury are incurred by air turbulence, and additional millions are cost by inefficiencies due to the necessity of rerouting flights to avoid recently discovered turbulence (Kulesa, 2003). This is particularly relevant when noting that often discovery of regions of turbulence is done unintentionally when an aircraft flies through it; this generates a report which is passed on to other aircraft on the same or a similar flight path. A valid detection metric that is obtainable remotely would allow airlines and air traffic controllers to avoid these inefficiencies, and more importantly, increase aviation safety.
This paper presents results of a new method for measuring turbulence in the Earth´s atmosphere using GPS RO data derived from COSMIC. The detection method depends on the effects caused by turbulence on the propagation of the GPS signal, which can be observed in the spectrum of the observed GPS signal amplitude. The retrieval is based on a model derived from weak scattering theory, developed by Cornman (2011). Validation for COSMIC turbulence detections is attempted in two ways: comparison using in-situ measurements of turbulence from accelerometers mounted on commercial aircraft, and comparison using turbulence detection data from ground-based mesosphere-stratosphere-troposphere/stratosphere-troposphere (MST/ST) RADAR systems. The MST/ST RADAR data are not yet available. Analysis of coincident COSMIC RO data and the aircraft in-situ is complete. We expect to have the RADAR data analyzed by the time of the presentation.

The first product of this work is the continued development of an estimation method to be applied to COSMIC data to return meaningful information on turbulence. The estimated parameters are the location of the turbulence, determined as a path coordinate called ?, and a relative spatial intensity parameter, called Cn2??. Determining these parameters requires knowledge of the ray path through the atmosphere, which is determined using ray tracing methods developed from geometric optics, aided by the precise orbit determination conducted by COSMIC of both the COSMIC constellation and the GPS constellation.

The in-situ aircraft data analysis failed to conclusively validate the turbulence sensing method, but this is due to the inherent limitations of the data set. The in-situ aircraft data is biased in its distribution in a number of ways; specifically, it is more concentrated in areas of heavy traffic (i.e., along a finite set of existing flight paths), and commercial air pilots are charged with avoiding air turbulence. The analysis has unfortunately shown that this particular data source is not the ideal tool for validation of other turbulence detection methods, unless those methods are somehow automatically collocated with the in-situ collection points. The end result is that the in-situ data would be expected to show a lower overall level of turbulence in a region for a particular time period than actually exists. While this expected trend was observed in the data, it is not conclusive, and the analysis of this data will focus on specific case studies identified in the large investigation.

The MST/ST radar data are collected by a number of fixed radar installations around the globe. Because of the constellation design of COSMIC, occultations are primarily distributed in mid-latitudes, which in turn motivates the choice of radar site for comparison. For this study, radar sites in Aberstwyth, Wales, and Harrow, Ontario were targeted for comparison. The MST/ST radar data will provide a more useful data set, as it represents the accumulated observations in a single location over many years. As a result, the likelihood of finding meaningfully collocated data between the radar and COSMIC occultations is much higher. The study will examine overall turbulence in the region of the MST/ST radar system, as well as examining specific case studies of events of air turbulence for further investigation.

For both types of data, case studies that are identified will be instances of turbulent event detection by the weak scattering theory model estimator collocated in space and time with an event, or a number of events, recorded in the external data sources. A high number of such collocated cases will allow for extraction of meaningful calibration of the turbulence intensity, whereas a moderate or small number of cases will still provide useful examples to validate the technique.

References:

Cornman, L. B., R. K. Goodrich, P. Axelrad, and E. H. Barlow. "Progress in Turbulence Detection via GNSS Occultation Data." Atmospheric Measurement Techniques Discussions 4.3 (2011): 3401-297.

Kulesa, G.J., D.J. Pace, W.L. Fellner, J.E. Sheets, V.S. Travers, and P.J. Kirchoffer, 2003: The FAA Aviation Weather Research Programs Contribution to Air Transportation Safety and Efficiency. Proc. 19th Conf. on Interactive Information Processing Systems for Meteorology, Oceanography and Hydrology, AMS, Boston.

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