From Phil Thu Sep 26 09:52:19 1996 From: Phil (Phil) Date: Thu, 26 Sep 1996 9:52:19 PDT Subject: [IGSMAIL-1439] GNAAC NCL now producing P-Sinex Message-ID: ****************************************************************************** IGS Electronic Mail Thu Sep 26 9:52:19 PDT 1996 Message Number 1439 ****************************************************************************** Author: Phil Davies Subject: GNAAC NCL now producing P-Sinex Beginning this week 0872, weekly P-Sinex files are submitted to IGS Data Centres by Associate Analysis Centre NCL (Newcastle, England), starting with the week 0867 Polyhedron solution. This is in addition to the weekly G-Sinex we already deposit. Because the GNAAC definition is open to various interpretations, I briefly summarise below the procedure we have initially adopted to create these files. (i) Stage one - Global network estimation Weekly A-Sinex files from COD, EMR, ESA, GFZ, JPL and SIO are used. Significant station constraints are removed where present to give unbiased A-networks. The remaining orientation constraints of each are augmented to ensure loose (several m) SDs of orientation. A variance scaling factor is applied to each A-network. A-networks are combined by stacking normal equation blocks (assuming no correlation between A-networks) to estimate the NCL G-network. No frame parameters are estimated. Stations estimated by fewer than three ACs are excluded from the G-network. A posteriori statistical testing on the G-network includes (a) MINQE variance component estimation to test the A-network scale factors, which may be updated after a hypothesis test; (b) three-dimensional data-snooping on the A-network station observations, primarily to trap AC antenna height blunders; (c) Overall chi-square test. The G-network estimation is iterated as necessary following these procedures. An ITRF Core constraints block is added to the loose (i.e. approximately free) G-network to give the constrained G-network which is written out in a G-Sinex file NCLwwwwG.SNX. The accompanying SUM file shows Helmert transformations between A and G networks, residual sizes, excluded observations, and station information discrepancies. (ii) Stage two - Polyhedron assembly Currently R-Sinexes from EUR, GSI and PGC are used (SIR and ASI having no unique stations). Significant station constraints are removed where present to give unbiased R-networks. The covariance of each is augmented to give large SDs of all seven Helmert parameters. An ad hoc variance scaling factor is applied to each R-network. Also, an 'extra' R-network is formed from a combination of the non-Global A-network station estimates using the Core station set as Anchor stations; any stations which also appear in a 'real' R-network are deleted from this extra block. R-networks are adjusted to the G-network by backsubstitution of G-network coordinates and covariance for the R-network Anchor station parameters. The Polyhedron is a concatenation of the G-network and the adjusted R-networks. The full Polyhedron covariance matrix is computed. The loose and ITRF-constrained Polyhedra are obtained in this way from the loose and constrained G-networks respectively. Note that using this approach Global station coordinates and (co)variances in the Polyhedron are unchanged from their G-network values, and that Polyhedron coordinates do not depend on R-network variance scaling. Adjusted R-networks take their reference frame definition from the G-network Anchor station positions. The ITRF-constrained Polyhedron estimate and full matrix is written out in a P-Sinex file NCLwwwwP.SNX. This includes the same a priori constraints block as the G-Sinex, which can be removed by users if required to give the loose Polyhedron; this has the same reference frame definition as the loose G-network. The P-Sinex includes almost all the stations in the input A and R Sinexes; those also included in the G-Sinex are considered 'first order' stations. At the moment the accompanying SUM file does not contain any comparison statistics - I intend to include these 'gradually' over the next weeks. [Mailed From: "Philip Davies" ]