A phonic wheel system for a gas turbine engine , the system comprising: a phonic wheel comprising: a circular disk having first and second opposing faces , the circular disk defining a root surface that extends between and circumscribes the first and second faces ; a first plurality of projections extending from the root surface and oriented substantially parallel to an axis of rotation (A) of the disk, the first plurality of projections circumferentially spaced substantially equally from one another and each having a first physical configuration; and at least one second projection extending from the root surface and positioned between two adjacent first projections , wherein the at least one second projection is disposed on the root surface at an angle relative to the first plurality of projections ; and a sensor adjacent the phonic wheel and configured to sense passage of the first plurality of projections and the at least one second projection as the phonic wheel rotates; characterised in that : the at least one second projection has a second physical configuration different from the first physical configuration; and the sensor is further configured for producing, in response to sensing the first plurality of projections and the at least one second projection , a signal pulse comprising a series of first pulses and at least one second pulse, wherein the at least one second pulse produced in response to sensing the at least one second projection is substantially equalized with the series of first pulses produced in response to sensing the first plurality of projections due to the second physical configuration being different from the first physical configuration. a circular disk having first and second opposing faces , the circular disk defining a root surface that extends between and circumscribes the first and second faces ; a first plurality of projections extending from the root surface and oriented substantially parallel to an axis of rotation (A) of the disk, the first plurality of projections circumferentially spaced substantially equally from one another and each having a first physical configuration; and at least one second projection extending from the root surface and positioned between two adjacent first projections , wherein the at least one second projection is disposed on the root surface at an angle relative to the first plurality of projections ; and a sensor adjacent the phonic wheel and configured to sense passage of the first plurality of projections and the at least one second projection as the phonic wheel rotates; characterised in that : the at least one second projection has a second physical configuration different from the first physical configuration; and the sensor is further configured for producing, in response to sensing the first plurality of projections and the at least one second projection , a signal pulse comprising a series of first pulses and at least one second pulse, wherein the at least one second pulse produced in response to sensing the at least one second projection is substantially equalized with the series of first pulses produced in response to sensing the first plurality of projections due to the second physical configuration being different from the first physical configuration. the at least one second projection has a second physical configuration different from the first physical configuration; and the sensor is further configured for producing, in response to sensing the first plurality of projections and the at least one second projection , a signal pulse comprising a series of first pulses and at least one second pulse, wherein the at least one second pulse produced in response to sensing the at least one second projection is substantially equalized with the series of first pulses produced in response to sensing the first plurality of projections due to the second physical configuration being different from the first physical configuration. <EOS>