1. A system for monitoring at least one performance parameter of a gas turbine engine installation comprising: first sensor means for supplying a signal representative of the current value of each monitored performance parameter; second sensor means for supplying a first plurality of signals representative of the current values of an associated plurality of operating parameters of the engine installation being monitored, said plurality of signals being selected so that a signal ##EQU9## provides a current estimate of each monitored performance parameter where y denotes the estimated value of each monitored performance parameter, x.sub.i denotes a series of m independent performance parameters each having a current value dependent on the current value of selected ones of said plurality of signals supplied by said second sensor means and c.sub.i denotes a set of m scalar simulation coefficients; first signal processing means responsive to said plurality of signals supplied by said second sensor means, said first signal processing means including means for supplying a second plurality of signals representative of the current value of each of said independent performance parameters x.sub.i, means for supplying a third plurality of signals representative of each of said simulation coefficients c.sub.i and means for combining predetermined signals of said second and third plurality of signals to supply a signal representative of the current value of each estimate y of said monitored performance parameters; and, second signal processing means responsive to each of said signals representative of the current value of each estimate y of said monitored performance parameters and responsive to each of said signals supplied by said first sensor means for supplying a difference signal representative of the difference between the actual current value of each of said monitored performance parameters and the current estimated value of that same performance parameter.