Towed vehicle systems in the past have been powered by direct current requiring increasingly high voltages as- power requirements have increased. As vehicle capabilities were augmented, more telemetry channels were required. Multi-wire cables capable of transmitting the vehicle power and' carrying the telemetry channels are satisfactory for shallow applications, but greatly restrict the operational range of a towed vehicle due to cable size and inherent drag, telemetry channel cross talk, ground isolation, and noise. Powering the vehicle with alternating current via a single coaxial cable, which could also be shared with multiplexed telemetry, is an alternate approach to meet the demand for increased vehicle capability and operational range. A general description of an AC power system which could transmit up to 15 kw of power and multiplexed data over a single coaxial tow cable several miles long to a towed vehicle is presented. Primary power limitations of the system are attributed to the maximum allowable voltages before corona occurs. Telemetry is primarily limited by the frequency attenuation characteristics of the tow cable.
In recent years, there has been an increasing interest in developing deep towed research vehicles with multifunctional capabilities: Naturally, with increased capability, a correspondingly greater amount of power and telemetry is required, thus taxing existing power and telemetry techniques to their limits. At the Westinghouse Oceanic Division, we have been concerned with the design and development of multifunctional deep towed research vehicles, such as the Ocean Bottom Scanning Sonar (OBSS), typically used for such operations as ocean floor exploration in sample gathering and basic research.
In a shallow water configuration, a multi-conductor tow cable (such as shown in Figure 1) allows for the simplest techniques for power and signal transmission to the vehicle. Quite often DC power is used directly through the cable to operate the electronics and various motor functions on the vehicle. If the number of telemetry functions to and from the vehicle is small, separate wires or wire pairs can be used for each telemetry channel. For more complex vehicles, multiplex telemetry is required to keep the size of the multi-conductor cable reasonable. FM multiplex telemetry, for example, has been increasingly used by numerous agencies, including ourselves, for handling many control and display information channels over one or two coaxial cables, thereby minimizing the size of multi-conductor cables.
However, for deep water operations, multi-conductor cable becomes impractical due to its weight and drag area. Analog signals over long wire pairs are impractical due to signal attenuation and channel cross talk, hence all telemetry must be multiplexed over coaxial cables for long distance transmission.