Abstract: A ship steering system has a multiplicity of selectable steering stations. A continuously rotating helm at each steering station generates digital rudder control signals which cause a predetermined rudder movement for each 360.degree. of helm rotation. Steering stations are selected through a multiplexer controlled at a command steering station. The steering system is operable from the command steering station in a plurality of modes, which includes auto-pilot, helm (full-follow-up), and tiller (non-follow-up). Helm and/or tiller modes of operation are available at steering stations remote from the command steering station. The system provides for an emergency takeover from the helm or autopilot modes by automatically establishing the tiller mode when the tiller stick is moved. When in the autopilot mode an alarm system provides an alert should the course deviate from a selected course tolerance.

Abstract: A global positioning system (GPS) receiver includes a phase locked loop that locks to the carrier signal from the GPS satellite at an offset frequency in order to recover the GPS data. The loop includes a phase shifter driven by a system clock through a synchronous clock inversion circuit. The phase of the system clock is periodically inverted synchronously with respect to the clock in order to compensate for the offset loop frequency so that the loop does not lose lock because of the offset error.

Abstract: A squaring loop for a GPS receiver locks to a carrier that is bi-phase modulated with a PRN acquisition code and a data message. The receive signal is down converted and hard limited and phase measurements are made between the rising edges of a local clock and every edge of the hard limited carrier. Phase measurements made with respect to extraneous or missing transitions resulting from the data bits that would result in phase locking instability are removed by a phase evaluation comparison. Phase measurements that are outside of limits determined in accordance with the noise characteristics of the channel are excluded thereby removing measurements that would cause the instability. The phase measurements outside the limits are resonably attributable to data bit transitions rather than to noise.