Theoretical studies and field tests led to specifications for and subsequent development of an all weather, 24-hour operation, range-range type marine navigation system capable of reliably operating much farther from shore than previously available systems of this general type. It utilizes a single, relatively narrow band VHF or UHF channel, specially designed matched-filters, low-noise solid-state circuitry, and computer control to derive real-time position information in a semiautomatic manner at ranges up to 250 miles from shore.
The expanded offshore exploration for oil is typical of the major oceanographic programs which have an urgent need for an accurate marine navigation system capable of reliable operation at ranges of at least 250 miles from shore, 24 hours a day.
Experience has shown that older pulsed range-range type systems operating at VHF/UHF frequencies provide accurate and reliable navigation data. If the range of such systems can be extended to the required 250 miles, they overcome such problems as sky wave interference and having to maintain lane counts, typical factors which complicate and sometimes limit the usefulness of other longer range systems.
The National Bureau of Standards (NBS}l and others ' have shown (through theory, field tests, and extrapolation of data) the median propagation path losses to expect for VHF/UHF frequencies at distances far beyond the radio horizon. Although much greater than free space losses, they are not enough to prohibit operation at ranges of 250 miles, or more, with reasonable sized transponder. systems using modern matched filter and computer. data processing techniques to substantially enhance received signal-to-noise ratios. In fact, the data shows that each dB improvement in system sensitivity should give at least six miles increase in detectable signal range.
In certain areas of the world, marine operators have been able to use the older model Shoran systems reliably at ranges beyond the radio horizon, up to 150 miles, by incorporating new low-noise receiver preamplifiers and higher gain antennas. Controlled experimental tests by Texas Instruments and GSI, using a UHF CW transmitter and narrow band receiving system operating at distances well over the radio horizon, have confirmed that the predicted propagation path attenuation levels are consistently present. Parallel tests using a Shoran system with the above described changes showed that VHF/UHF range signals can be transmitted at nominal levels and reliably received "around-the-clock", under in climate as well as ideal weather conditions.
The results of the experimental tests coupled with system analysis and design studies led to the development of the modernized range-range type radio-location (navigation) system for marine use described herein. Field tests of the system have confirmed that its improved characteristics allow operation at much longer ranges from shore (over 250 miles versus approximately 125 to 150 miles for current systems). It accomplishes this while using no more peak power than the older Shoran and while requiring only one "clean, splatter-free" 2.87 MHz bandwidth UHF channel, compared to three 4 to 6 MHz bandwidth channels for the older systems.
