Navigation by the use of inertial information is now an accepted technique for military aircraft and for missiles. In civil aircraft the application of inertial techniques are, as yet, limited to defining the vertical and providing a memory of azimuth direction; that is, as aids to flight rather than a primary or even sole navigational aid. Although the principles applied—the inertial properties of matter and the laws of gravitation—are the same, the accuracy of the inertial sensors differs by some three orders of magnitude, and the cost by some two orders, between these two extremes.

Mr. A. Stratton of the Royal Aircraft Establishment first of all presents in perspective the range of application of inertial techniques that are available for civil use and suggests how the information obtained by inertial means can be related to that from other airborne sources. He shows that even if the expense of an accurate inertial platform of ‘navigational’ quality should not be justified, considerable advantage over existing sources obtains in terms of accurate attitude reference, and instantaneous velocity and acceleration, by the use of a lower-grade platform in conjunction with other navigation aids.

The objects of this paper are to present in perspective the range of application of inertial techniques that are available for civil use and to suggest how the information that can be obtained by this means relates to that from other airborne sources.

Inertial navigation systems have become of great importance in the navigation of military aircraft and in recent years there has been a growing interest in their application to civil transports. A special problem arises with this class of equipment in that the axes require to be set before navigating and a particular difficulty arises with the azimuth. This involves penalties in special pre-flight routines or in extra cost and complexity of equipment which are of concern to the airline operator who is attempting to assess the usefulness of inertial techniques. The purpose of.this paper is to discuss the general principles on which azimuth alignment schemes can be based with reference to the likely penalties involved.

The paper is primarily concerned with systems where aircraft accelerations are measured by accelerometers mounted on a gyro stabilized platform and then integrated to obtain velocity. The velocity may be either used directly in a navigation computer or mixed first with velocity from another source, such as doppler radar, as described in Ref. 1. However, all dead reckoning systems require a knowledge of heading and in cases where a better accuracy is required than can be obtained by reference to the Earth's magnetic field, a low-drift gyroscope can be used as an azimuth memory. Some of the alignment schemes discussed would be applicable to these heading references as well as to stable platforms with accelerometers. With most inertial systems, alignment to the Earth's magnetic field would be made before fine alignment commenced.

In the next decade there will be an increasing demand for more accurate self-contained navigation systems due to more stringent air traffic control requirements, particularly with the introduction of the supersonic transport. Although inertial systems have been fairly widely used in military aircraft and missiles, the only self-contained systems in service in civil aircraft are dead reckoning systems based on measuring the aircraft's velocity and heading and drift angle. A big improvement has been effected in the measurement of velocity and drift angle by the introduction of doppler radar. The accuracy achievable with doppler is not matched by the accuracy of existing heading reference systems and the overall accuracy is, at present, inadequate for future requirements.

An inertial navigation system is a possible alternative but its cost will be two to three times that of a doppler heading reference system.

This paper is concerned with the effect on airline operation of the introduction of inertial equipment, from the engineering viewpoint. It is difficult to avoid entirely all operational considerations but the main object here is to stress the economic and engineering aspects arising from the use of this equipment.

During the first years of the last war the navigational problem was how to get to the target at all. In such conditions solutions to the navigation problem are priceless. Towards the end of the war it came to be accepted that, in transport operations at least, guidance to destination was almost a matter of course. Navigational performance ceased to be regarded as a question of success or failure and became a question of efficiency. This development in air transport we owed most of all to the radio compass.

As the volume of civil traffic grew, it rapidly became clear that the major limitation on navigational efficiency of air transportation was (and it still is) air traffic control. Although there has been considerable interest in the optimum trajectory particularly in connection with turbine aircraft, the development of refined operating techniques has been damped by restrictions on their application by A.T.C. The flat assertion by some would-be supersonic aircraft manufacturers that the supersonic (unlike the subsonic) jet will not take NO for an answer from A.T.C. once it is committed to a flight programme has recently been a most valuable stimulus.

The A.T.C. problem is well known. It is generally accepted that the capacity of an air space depends on the quality of navigational data, of the data link between aircraft and control centre and of the data correlation capabilities of the centre.

A radar installation, designed to serve as a navigational aid, is in course of being set up at the mouth of the Gironde. It is one of a long series of port- or river-radars installed in similar conditions. So far, these have been entrusted to port officials. They are used, almost exclusively in reduced visibility, to give ships accurate information about their position, and in particular to keep them informed of the presence of ships in their vicinity. Their chief function, in association with radio-telephony, is to allow traffic to be controlled at times when visual signals cannot be distinguished.

The Gironde enterprise is more ambitious. The intention is to entrust the radar to pilots so that they may guide shipping, in both clear weather and fog, without having to go on board. This method is to be used between the sea and Le Verdon, and in particular at the bar, the trickiest section of this stretch. It should entail a considerable reduction in the cost of pilotage, the chief of several reasons for this being that there would no longer be any need for a pilot-vessel. It is worth examining whether this is a sensible plan and under what conditions it can be made effective, in the Gironde or elsewhere.

That a pilot should use radar to direct a ship from some distance away is a completely revolutionary idea.

The following is an abbreviated version of the 29th Haldane Memorial Lecture given by Sir Frederick Brundrett in the University of London Senate House on 7 March 1963. Sir Frederick is Scientific Adviser to the Civil Service Commission.

The following discussion took place at an Ordinary Meeting of the Institute held in London on 15 February 1963.

Like Mr. R. S. Blicq (Journal 15,456) we have also been concerned in supplying details of the Sun's position to engineers on radar sites who use radiation from the Sun for, at present, two purposes:

(a) Measuring or checking the radiation pattern of the aerial in the vertical plane.

(b) Lining up the aerial in azimuth or elevation.

For these purposes the required accuracy of computation is usually about ±0°·1 in azimuth and elevation. Greater accuracy is never required, but less accuracy is sometimes adequate, up to ±1°.

We have found that graphical methods appeal most to engineers, and that even when a full table of values is supplied to them they prefer to display this information in graphical form. We have accordingly evolved a simple method of constructing graphs of elevation against time, and azimuth against time.

In order to achieve the required accuracy of ±0°·1 in elevation we have constructed a graticule of size 21 in. × 40 in. A skeleton version of this is shown in Fig. 1. The engineer has to know his latitude, and to find, from the Nautical Almanac or other source, the time of local noon (i.e. when the Sun is due south) and the mean declination, δ, of the Sun during the period of observation, which usually lasts up to three hours.

Precise navigation upon the ice-cap depends largely upon the accurate steering of a chosen course. This operation can be rendered difficult by three major problems commonly faced by Arctic and Antarctic expeditions. First, the absence of topographic features as points of reference; secondly, the loss of precision of the magnetic compass inside a moving vehicle and in areas adjacent to the Magnetic Pole; thirdly, the dependence of the astro-compass and similar instruments upon an unobscured Sun.

A simple method by which these difficulties are overcome was tested and found satisfactory during an inland traverse from Wilkes Station in November–December 1960 into an area not previously explored by land. The principle of this method is the use of a periscope-like arrangement of mirrors combined with a suitable sighting system to enable the driver to use his track astern to steer a straight course ahead.

The original device was installed upon a vehicle (a ‘Weasel’), already fitted with an astro-compass and a magnetic compass in order to function as the navigation vehicle of the train. The rear vision mirror taken from a ‘Traxcavator’ was used as the main ‘Track mirror’. It was pivoted at its centre point through the shorter axis. The mirror was mounted on a frame of two half-inch steel pipes bolted on to the vertical wall of the well between the windscreen and the forward hatch of the Weasel.

In his Presidential Address (Journal 16, 1) Dr. G. E. R. Deacon, C.B.E., F.R.S., says: ‘We have learnt from the Journal that one of the most frequent sources of error is a poor horizon, and recent (my italics) experiments by our American colleagues have shown that much can be gained by fitting a more powerful telescope in the sextant so that stars can be used while the horizon is better illuminated. They showed that it is better to look at the star directly and to make the horizon the reflected image.’

Dr. Deacon has been misled. In the ‘old’ Discovery from 1925 onwards (and before that in various surveying ships, whilst on passage), in which I was Navigating Officer, the horizon was always brought up to the star. A ‘sounding’ sextant also was always used. This is lighter than an ordinary observing sextant and mine had a large, aluminium telescope, of much greater power than the star telescope of an ordinary sextant, and with a very large object glass.

It was, however, graduated only to minutes of arc. To overcome this the altitude was pre-set for every contact to fixed intervals of, I think, 5′; this had the additional advantage that if the time of the middle contact agreed with the mean of the others it formed an excellent check on the accuracy of the sights and could be seen at a glance.