Atomic Frequency Standards in Offshore Navigation and Surveying
Authors: Sender Friedhelm 
Published: October 2008

Atomic frequency standards now attained a perfection that they already have become a valuable tool in oceanographic work. Their most significant application is in navigation and positioning. They are now used to improve synchronisation in Loran and Omega ground systems. (1, 2).

ANA+) was the first navigation system introducing atomic frequency standards aboard the vehicle. The various ANA systems for aircraft and ships operation are described in detail especially in view of the synchronisation problems between atomic frequency standards situated in different geographical locations.

Discussed are possibilities for improving the accuracy of satellite positioning and enable the realisation of a worldwide anticollision system as well as absolute time event measurements in sonar, seismic and other inner space survey by using atomic frequency standards.

Introduction

Atomic frequency standards, in the following called AFS, are devices with a very stable frequency output. Since the definition of frequency is l/second there is a direct relationship to time. To convert an AFS into an atomic clock only a convenient frequency counter must be added which integrates the frequency output into a time display.

An atomic clock turned out to be much more accurate than the inherent earth clock. (3) That is because there are some irregularities in the earth's orbital motion round the sun and the nutation of its axis. So we got several time definitions since the introduction of atomic time in 1964.

The time scale widest in use today is UT 2. This is based on the rotation of the earth round its axis with corrections to polar motion and seasonal variations caused by mass distribution alterations. Most national time standards are presenting UT C This is atomic time corrected by astronomical observations, as the U.S. Naval Observatory, Washington D.C.

Atomic time is presented only by a few standard transmitters, like WWVB and WWVL. Other time scales are mean side-real time and ephemeris time.

Time information is very important for Ships as any seagoing ship must have a chronometer. In the present stage this chronometer is only needed for astronavigation where time information is paramount.

The time accuracy, as obtained by chronometers, is in the order or a tenth of a second. With atomic clocks it will be improved to orders better than one micro second. What progress in navigation or other oceanographic operation can be obtained by this improvement?

Basic Theory

The gain in accuracy with atomic clocks cannot give much improvement in astronavigation in the present state. The limitations are set by the sextants normally used. Only complicated and expensive stabilized electronic systems can lead to any progress, but there are other limitations set by gravity anomalies and the time lag in definition of the time standard out of the earth rotation by the Internal Bureau of Standards.

The most significant advance may be expected in conjunction with radio navigation aids. Radiowave propagation velocity is about 300 meters in one microsecond. As all bearing measuring systems are not too accurate, the important radio navigation system is oriented in time or time difference domain.