03/17/03 12.540 Lec 11 1 12.540 Principles of the Global Positioning System Lecture 11 03/17/03 12.540 Lec 11 2 Statistical approach to estimation view Prof. Thomas Herring ? Summary –Look at estimation from statistical point of –Propagation of covariance matrices –Sequential estimation 1 ? 03/17/03 12.540 Lec 11 3 Statistical approach to estimation ? Examine the multivariate Gaussian distribution: ? probability density function, we maximize the likelihood of the estimates (MLE). ? This is just weighted least squares where the weight matrix is chosen to be the inverse of the covariance matrix of data noise f (x) = 1 (2p) n V e - 1 2 (x-m ) T V -1 (x-m ) (x - m) T V -1 (x - m) By minimizing the argument of the exponential in the Multivariant Minimize gives largest probability density 03/17/03 12.540 Lec 11 4 Data covariance matrix ? If we use the inverse of the covariance matrix of the Gaussian distribution. ? ? Difficult question to answer completely ? Issues to be considered: the noise (discussed later in course). sometimes be treated as noise-like. angles noise in the data, we obtain a MLE if data noise is How do you obtain data covariance matrix? – Thermal noise in receiver gives on component – Multipath could be treated as a noise-like quantity – Signal-to-noise ratio of measurements allows an estimate of – In-complete mathematical model of observables can – Gain of GPS antenna will generate lower SNR at low elevation 2 03/17/03 12.540 Lec 11 5 Data covariance matrix ? In practice in GPS (as well as many other fields), the data covariance matrix is somewhat arbitrarily chosen. ? Largest problem is temporal correlations in the measurements. Typical GPS data set size for 24- hours of data at 30 second sampling is 8x2880=23000 phase measurements. Since the inverse of the correlations requires the inverse of 23000x23000 matrix. ? To store the matrix would require, 4Gbytes of memory ? correlations over a time short compared to 24-hours), covariance matrix is required, fully accounting for Even if original covariance matrix is banded (ie., the inverse of banded matrix is usually a full matrix 03/17/03 12.540 Lec 11 6 Data covariance matrix ? use samples every 5-minutes (JPL method) measurement so that equivalent to say 5-minute sampling (ie., (GAMIT method) assumptions (discussed more near end of course). ? what can we say about noise in parameter estimates? Methods on handling temporal correlations: – If measurements correlated over say 5-minute period, then – Use full rate data, but artificially inflate the noise on each sqrt(10) higher noise on the 30-second sampled values – When looking a GPS results, always check the data noise Assuming a valid data noise model can be developed, 3 ? 03/17/03 12.540 Lec 11 7 Propagation of covariances y = Ax with V xx x V yy =< yy T >=< Axx T A T >= A < xx T > A T V yy = AV xx A T ? Given a data noise covariance matrix, the characteristics of expected values can be used to determine the covariance matrix of any linear combination of the measurements. Given linear operation : as covariance matrix of 03/17/03 12.540 Lec 11 8 Propagation of covariance known. – least squares – Covariance matrix for post-fit residuals from least squares – Covariance matrix of derived quantities such as estimates. ? Propagation of covariance can be used for any linear operator applied to random variables whose covariance matrix is already ? Specific examples: Covariance matrix of parameter estimates from latitude, longitude and height from XYZ coordinate 4 ? 03/17/03 12.540 Lec 11 9 ? Propagation of covariance can be applied to the ? Notice that the covariance matrix of parameter estimates is a natural output of the estimator if A T V -1 A is inverted (does not need to be) ?x = (A T V yy -1 A) -1 A T V yy -1 y < ?x?x T >= (A T V yy -1 A) -1 A T V yy -1 < yy T > V yy -1 T V yy -1 A) -1 V ?x?x = (A T V yy -1 A) -1 Covariance matrix of parameter estimates weighted least squares problem: A(A 03/17/03 12.540 Lec 11 10 Covariance matrix of estimated ? Notice that for the rigorous estimation, the inverse of the data covariance is needed (time consuming if non-diagonal) ? To compute to parameter estimate covariance, only the covariance matrix of the data is needed (not the inverse) ? In some cases, a non-rigorous inverse can be done with say a diagonal covariance matrix, but the parameter covariance matrix is rigorously computed using the full covariance matrix. This is a correct (just not the best estimates that can found). ? This techniques could be used if storage of the full covariance parameters non-MLE but the covariance matrix of the parameters should be matrix is possible, but inversion of the matrix is not because it would take too long or inverse can not be performed in place. 5 ? 03/17/03 12.540 Lec 11 11 Covariance matrix of post-fit residuals ? Post-fit residuals are the differences between the observations and the values computed from the estimated parameters ? Because some of the noise in the data are absorbed into the parameter estimates, in general, the post-fit residuals are not the same as the errors in the data. ? In some cases, they can be considerably smaller. ? The covariance matrix of the post-fit residuals can be computed using propagation of covariances. 6 03/17/03 12.540 Lec 11 12 Covariance matrix of post-fit residuals covariances: e v y = Ax + e ?x = (A T V yy -1 A) -1 A T V yy -1 y v = y - A?x = I - A(A T V yy -1 A) -1 A T V yy -1 Amount error reduced 1 2 3 è ? í í ? ? ˙ ˙ e Eqn 1 V vv =< vv T >= V yy - A(A T V yy -1 A) -1 A T ? This can be computed using propagation on is the vector of true errors, and is vector of residuals 444 444 03/17/03 12.540 Lec 11 13 Post-fit residuals ? Notice that we can compute the compute the covariance matrix of the post-fit residuals (a large matrix in generate) ? v=Be; why can we not compute the actual errors with e=B -1 v? ? (there is in fact one inverse which would generate the true errors) ? Note: In this case, singularity does not mean that there is no inverse, it means there are an infinite number of inverses. Eqn 1 on previous slide gives an equation of the form B is a singular matrix which has no unique inverse 03/17/03 12.540 Lec 11 14 Example as D 0 1 2 3 4 5 6 0.0 10.0 20.0 30.0 40.0 50.0 Data Time Dt Postfit error bar somewhat reduced ? Consider the case shown below: When a rate of change is estimated, the slope estimate will absorb error in the last data point particularly t increases. (Try this case yourself) Postfit error bar very small; slope will always pass close to this data point Example of fitting slope to non-uniform data distribution 7 ? ? 03/17/03 12.540 Lec 11 15 ? Propagation of covariances can be used to determine longitude and radius. q is co-latitude, l is longitude, R is radius. DN, DE and DU are north, east and radial changes (all in distance units). DN DE DU è ? í í í ? ? ˙ ˙ ˙ = -cos(q)cos(l) -cos(q) l) q) - l) cos(l) 0 X / R Y / R Z / R è ? í í í ? ? ˙ ˙ ˙ A 1 2 3 DX DY DZ è ? í í í ? ? ˙ ˙ ˙ Covariance of derived quantities the covariance of derived quantities. Example latitude, Geocentric Case : sin( sin( sin( matrix for use in propagation from Vxx 4444444 4444444 8 03/17/03 12.540 Lec 11 16 V 1 0 0 0 V 2 0 0 0 V 3 è ? í í í ? ? ˙ ˙ ˙ -1 = V 1 -1 0 0 0 V 2 -1 0 0 0 V 3 -1 è ? í í í ? ? ˙ ˙ ˙ Estimation in parts/Sequential estimation ? A very powerful method for handling large data sets, takes advantage of the structure of the data covariance matrix if parts of it are uncorrelated (or assumed to be uncorrelated). ? 03/17/03 12.540 Lec 11 17 Sequential estimation ? Since the blocks of the data covariance matrix can be separately inverted, the blocks of the estimation (A T V -1 A) can be formed separately can combined later. ? Also since the parameters to be estimated can be often divided into those that effect all data (such as station coordinates) and those that effect data a one estimations (shown next page). time or over a limited period of time (clocks and atmospheric delays) it is possible to separate these 9 03/17/03 12.540 Lec 11 18 Sequential estimation ? Sequential estimation with division of global and local parameters. V parameter estimates), V xg is covariance matrix of prior parameter x g and x l are local parameter estimates, x g + are new global parameter estimates. y x g è ? í ? ? ˙ = A g A l I 0 è ? í ? ? ˙ x g x l è ? í ? ? ˙ x g + x l è ? í ? ? ˙ = A g T V -1 A g + V xg -1 ( ) A g T V -1 A l A l T V -1 A g A l T V -1 A l è ? í í ? ? ˙ ˙ -1 A g T V -1 y + V xg -1 x g A l T V -1 y è ? í ? ? ˙ is covariance matrix of new data (uncorrelated with priori estimates with estimates 03/17/03 12.540 Lec 11 19 Sequential estimation parameters, x l x g 03/17/03 12.540 Lec 11 20 ? As each block of data is processed, the local , can be dropped and the covariance matrix of the global parameters passed to the next estimation stage. ? Total size of adjustment is at maximum the number of global parameters plus local parameters needed for the data being processed at the moment, rather than all of the local parameters. Summary ? We examined the way covariance matrices and be manipulated ? Estimation from a statistical point of view ? Sequential estimation. ? Next class continue with sequential estimation in terms of Kalman Filtering. ? Reminder: Paper topic and outline due Wednesday. 10