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On-the-fly ambiguity resolution using an estimator of the modified ambiguity covariance matrix for the GNSS positioning model based on phase data

100%

Cellmer S.

tom Vol. 47, No. 3

81--90

On-the-fly ambiguity resolution (OTF AR) is based on a small data set, obtained from a very short observation session or even from a single epoch observation. In these cases, a classical approach to ambiguity resolution (e.g. the Lambda method) can meet some numerical problems. The basis of the Lambda method is an integer decorrelation of the positive definite ambiguity covariance matrix (ACM). The necessary condition for the proper performing of this procedure is a positive definiteness of ACM. However, this condition is not satisfied in cases of very short observation sessions or single epoch positioning if phaseonly observations are used. The subject of this contribution is such a case where phase-only observations are used in the final part of the computational process. The modification of ACM is proposed in order to ensure its positive definiteness. An estimator of modified ACM is a good ACM approximation for the purpose of performing the LAMBDA method. Another problem of short sessions (or a single epoch) positioning is the poor quality of the float solution. In this paper, a cascade adjustment with wide-lane combinations of signals L1 and L2 as a method of solving this problem is presented.

Sensitivity of ADOP to changes in the single-baseline GNSS model

100%

Odijk D. , Teunissen P. J. G.

tom Vol. 42, No. 2

71--96

The ADOP (Ambiguity Dilution Of Precision) is a measure for the precision of the carrier phase ambiguities involved in precise relative GNSS positioning. By computing the ADOP one may get knowledge in whether ambiguity resolution can be expected successful or not, already in a stage before the GNSS data are collected. In Odijk and Teunissen (2008) compact closed-form expressions have been derived for the ADOP of single-baseline GNSS models. In this paper these expressions are used to study the impact of certain changes in these models, as there are the observation time span, the weighting of the ionospheric delays, the number of frequencies, the weights of the phase and code data, the number of satellites, elevation-dependent observation weights and taking linear combinations of data.

Resisting Contextual Information: You Can't Put a Salient Meaning Down

88%

Peleg O. , Giora R. , Fein O.

nr 1

13-44

Two experiments support the graded salience hypothesis (Giora 1997, 1999, 2003; Peleg et al. 2001, 2004), which assumes that early processing involves distinct mechanisms-linguistic and contextual-that do not interact but run parallel. While contextual processes make up an integrative, top-down mechanism that benefits from linguistic and extra-linguistic information, the linguistic mechanism is modular (Fodor 1983). Using Vu et al.'s (2000) materials, Experiment 1 shows that the sentential position of a target word (initial vs. final) is crucial for the operation of the global, predictive mechanism, whose effects, accumulated in prior discourse, mask lexical effects in final, but not in initial position. Experiment 2 shows that even in a sentential position that favors contextual effects (i.e. sentence final position, see Peleg et al. 2001, 2004), lexical access is not affected: Salient meanings are activated upon encounter of the lexical stimulus, regardless of contextual information to the contrary. Taken together, these findings argue against direct access models, which suggest that context can selectively activate the appropriate meaning, regardless of salience (see Vu and Paul 1998; Vu et al. 2000).