Regional Tropospheric Tomography Based on Real-Time Double Difference Observables

N. A. Nicholson, S. Skone, M.E. Cannon, G. Lachapelle and N. Luo

Abstract:	Tomography techniques have been developed for recovering 3-D wet refractivity distributions over regional GPS networks. Most techniques employ single path slant wet delay (SWD) observations as model input. In order to recover SWD observables with sufficient accuracy, a number of error sources must be removed including orbital uncertainty, satellite and receiver clock biases, and ionospheric propagation delays. Double difference, ionosphere-free GPS observations with fixed L1 and L2 ambiguities are necessary as well as precise orbit products and station coordinates. After error sources apart from the neutral atmospheric range delay are mitigated, single path delays must be recovered from the double-differenced observations. One technique to recover single-path SWD information uses a ‘zero-mean’ assumption while another employs a batch estimate of the average total atmospheric delay (and average gradients) overlying a reference station. In both cases, limiting assumptions are made about spatial variations of properties in the neutral atmosphere. This paper investigates the feasibility of an alternate approach - using double difference residuals as tomography model input directly without first recovering single-path SWD information. A double difference tomography model has been developed at the University of Calgary. Simulations indicate that integrating through estimated wet refractivity fields to estimate zenith wet delay agree with truth to better than 1 mm when network station heights vary by 400 m. Regular and inversion atmospheric profiles are recovered using the model. Simulations using flat network geometry recover smoothed vertical profiles but estimated zenith wet delay values still agree with truth to within 3.0 mm. GPS data from the Southern Alberta network is processed with MultiRefTM Network RTK Software to form double difference SWD observables. Tests using real data infer that the magnitude of positioning misclosures can be reduced by 18 – 20 % using model refractivity fields to form estimates of DD SWD. The development and validation of incorporating double differenced observables directly into tomography models expands the real-time capabilities of existing network processing software at the University of Calgary, by allowing an option to estimate 3-D wet refractivity distributions of the overlying region.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	269 - 280
Cite this article:	Nicholson, N. A., Skone, S., Cannon, M.E., Lachapelle, G., Luo, N., "Regional Tropospheric Tomography Based on Real-Time Double Difference Observables," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 269-280.
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