This paper discusses about GNSS-based positioning and timing in challenging propagation conditions for receivers featuring an array of antennas with an arbitrary distribution. In particular, the positioning and timing problems are treated as multiple-input multiple-output (MIMO) estimation problems in which all the GNSS signals received by the array of antennas are jointly exploited in the corresponding domain. The MIMO-GNSS processing approach is targeting the derivation of robust solutions in the presence of realistic signal impairments introduced by the propagation channel or any other external interference. The maximum likelihood estimator (MLE) in the MIMO-GNSS framework is reviewed, and simulation results in realistic challenging scenarios are presented, showing the benefits introduced by the exploitation of arrays of antennas in both positioning and timing.