A method for operating a plurality of GNSS (= Global Navigation Satellite System) receivers, wherein for position determination, the GNSS receivers receive GNSS code signals belonging to signals sent from a multitude of satellites, and wherein each received GNSS code signal is correlated with a reference code signal generated by the receiving GNSS receiver to obtain an autocorrelation function (ACF obs), wherein a multitude of function values of the autocorrelation function (ACF obs) at different discrete chip spacings are analyzed and used in obtaining a test metric, wherein using the test metric, a decision is made whether the received GNSS code signals are suitable for position determination or unsuitable for position determination due to satellite signal deformation, and wherein the GNSS code signals found unsuitable for position determination are excluded from position determination, wherein for each satellite i, with i being a satellite index, using the test metric includes the following steps,: a) for each time epoch t k in which the GNSS receivers receive GNSS code signals belonging to the signal of the same carrier frequency sent from the satellite i, forming a vector X i,k of measurement values X m,i,k, with m being a chip spacing index, and k being a measurement time epoch index, and the respective measurement value X m,i,k being derived from normalized and bias-removed function values of the autocorrelation functions (ACF obs) of the GNSS code signals received at the GNSS receivers, combined for a multitude of the GNSS receivers; b) obtaining the correlation matrix R i,n at time epoch t n from the epoch-wise measurement value vectors X i,k obtained over a plurality of n time epochs t k with k = 1,...,n, with n being a monitoring time epoch index, which results in matrix X i, n = X → i, 1 ⋮ X → i, n, in particular with R i,n ∼ X T i,n X i,n with X T i,n being the transpose matrix of X i,n, c) doing a principal component analysis on correlation matrix R i,n, thus obtaining an eigendecomposition of matrix R i,n into a multitude of eigenpairs, namely pairs of eigenvector and eigenvalue, d) calculating for time epoch t n a signal quality monitor test metric SQM i,n by transforming vector X i,n to vector T i,n via multiplication with principal component matrix PC i,n and forming the square sum of vector T i,n 's entries, wherein PC i,n contains at least two eigenvectors of R i,n, with the eigenvectors chosen having the largest eigenvalues of all eigenpairs, e) comparing SQM i,n to a predefined threshold value THR, and if SQM i,n exceeds THR, the entirety of GNSS code signals received at all GNSS receivers belonging to the signal (13, 14) of the same carrier frequency sent by satellite i in time epoch t n are considered unsuitable for position determination due to satellite signal deformation, wherein for subsequent time epochs having passed, the signal quality monitor test metric SQM i,n is determined again, with n increasing by 1 for each time epoch having passed. a) for each time epoch t k in which the GNSS receivers receive GNSS code signals belonging to the signal of the same carrier frequency sent from the satellite i, forming a vector X i,k of measurement values X m,i,k, with m being a chip spacing index, and k being a measurement time epoch index, and the respective measurement value X m,i,k being derived from normalized and bias-removed function values of the autocorrelation functions (ACF obs) of the GNSS code signals received at the GNSS receivers, combined for a multitude of the GNSS receivers; b) obtaining the correlation matrix R i,n at time epoch t n from the epoch-wise measurement value vectors X i,k obtained over a plurality of n time epochs t k with k = 1,...,n, with n being a monitoring time epoch index, which results in matrix X i, n = X → i, 1 ⋮ X → i, n, in particular with R i,n ∼ X T i,n X i,n with X T i,n being the transpose matrix of X i,n, c) doing a principal component analysis on correlation matrix R i,n, thus obtaining an eigendecomposition of matrix R i,n into a multitude of eigenpairs, namely pairs of eigenvector and eigenvalue, d) calculating for time epoch t n a signal quality monitor test metric SQM i,n by transforming vector X i,n to vector T i,n via multiplication with principal component matrix PC i,n and forming the square sum of vector T i,n 's entries, wherein PC i,n contains at least two eigenvectors of R i,n, with the eigenvectors chosen having the largest eigenvalues of all eigenpairs, e) comparing SQM i,n to a predefined threshold value THR, and if SQM i,n exceeds THR, the entirety of GNSS code signals received at all GNSS receivers belonging to the signal (13, 14) of the same carrier frequency sent by satellite i in time epoch t n are considered unsuitable for position determination due to satellite signal deformation,