THE GEE SYSTEM
By W. F. BLANCHARD
(Edited by Jerry Proc)

INTRODUCTION

Watson-Watt's demonstration in 1935 of the possibilities of radar for detecting aircraft caused considerable work to be put in hand in the UK on the development of high power pulse transmitters and, of equal importance, methods of presenting aircraft returns to operators. It could only be done by visual presentation and it required the design of stable, accurate time bases for cathode ray tubes. In 1935, good cathode ray oscilloscopes (or oscillographs as they were called) were still laboratory instruments and were by no means widely available or cheap. The few television sets then available were expensive, virtually hand-made and unreliable.

One major and common problems in designing any hyperbolic navigation system was the measurement of time. Since no means of directly measuring a millionth of a second was available in that era, it forced designers to use continuous-wave phase comparison with its attendant problem of ambiguity. Once it became possible to transmit very short pulses, the possibility of designing an unambiguous system was realized immediately. But there was another stumbling block. There was no way of measuring short time intervals that could be used in an operational system by relatively unskilled operators.

DEVELOPMENT

It was the development of reliable cathode ray tubes and their associated time bases that provided the solution. In October 1937, R. J. Dippy, who was on Watson-Watt's staff at the time, conceived a system using pulse transmitters and a cathode ray tube display that would measure the difference in time of arrival of two pulses sent out from two transmitters placed about ten miles apart and with a baseline at 90 degrees to a runway. Synchronized pulses would be sent out from both transmitters and the delay between reception of them would be seen on the CRT display. When there was no delay they would be seen as a single pulse and the aircraft would be on the right bisector of the baseline; in other words, lined up with the runway. If it was off course, one way or the other, there would be a delay and, by identifying which of the two pulses was leading, the pilot could tell on which side of the runway he was and turn accordingly.

Watson-Watt records that he thought it was quite feasible and, further, that there was a need for it, but there were even more urgent needs and he reluctantly had to shelve it for the time being. Perhaps it was just as well. The device would have had to be pilot interpreted and the CRT's of the era were very dim and could only be read in daylight by using a black-out hood and letting one's eyes adjust to the darkness. One cannot imagine a pilot letting down from the brilliant light above clouds trusting only what he could see on a dim tube! At night it might not have been so bad, of course, but almost no night flying was being done at that time. Several years later BABS (Blind Approach Beam System) did virtually the same thing, but was navigator- interpreted.

In 1938, Dr. R. V. Jones, apparently unaware of Dippy's earlier proposal, also suggested the use of pulse transmitters to form a hyperbolic system, but without success. Dippy's idea lay dormant until 1940 when mounting evidence of poor navigation brought it up again. It had been refined, and now appeared as a navigation rather than a landing aid. Dippy's new proposals were for a master station with up to three slave stations around it on 80 mile baselines that would provide almost all-round cover. He thought it would work up to 100 miles from the master, but the first trials in late 1940 showed it was much better than that  Later flight trials achieved ranges of 300 miles. Dippy was awarded British Patent 581602 in December 1942 for his invention.

The principle of Dippy's system of navigation by using three transmitters to ascertain position was originally called Trinity. The holy three in this case being the three transmitters that constituted the RAF's first radar navigation aid. To mask the real name of GEE , the system was called the "Goon" box because Gee meant  "grid"  - ie the electronic grid of latitude and longitude derived from the combination of three signals received by the aircraft.

It is interesting that the major uses initially predicted for the system were more or less local - the accurate assembly of large numbers of bombers after take-off and their post mission precise recovery to base or alternate airfields. One reason for this was that the range of the 30 MHz signals was at first greatly underestimated, as it had been for the Knickebein system operating in the same frequency band. Later, it was thought it might provide navigators with enough fixing on their way to the Continent to establish accurate winds for later dead reckoning.  It was also assumed that the signal would be jammed over Germany within three months of it on the air debut and would be of no value for bombing thereafter. Actually it was more than 5 months before that happened and it was one of the main aids used in the ' 1,000-bomber' raid on Cologne in May 1942.

SIGNAL CHARACTERISTICS

The operating principle of Gee was the transmission of short (6 microsecond) pulses at frequencies around 30 MHz (later extended up to 80 MHz., Signals sent from a master station were received at up to three slave stations and was used to synchronize their own transmissions. The slaves operated on the same frequency as the master (see figure 1).  A slave transmission would therefore normally be received after the master, but on the baseline extension behind the slave the difference would be zero and the pulses would overlap.
 

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Figure 1 - A sample of the GEE lattice.

 

Because the slave transmitter could not actually transmit instantaneously on receipt of the master signal, a fixed delay was built into the slave.  Some method of identifying one slave from another was needed and it was done by a combination of differing delays and making the slaves visually different in appearance on the operator's CRT. Thus, the master (A) always appeared at the start of both traces of a twin trace presentation. The B slave after the master on the top trace, the C slave after the master on the lower trace, and the D slave appeared on both traces but was a double pulse. The correct A pulse for starting the time base was selected by arranging the A transmitter to transmit twice as fast as the others but making every other pulse a double one. The final appearance of the time base was as in Fig.2
 
 

gee_p299.jpg
Figure 2 - The appearance of GEE signals on the face of a CRT.

 

The A1 pulse (or 'A-ghost' as it was known) was also used as a chain identifier by making it blink in a pattern unique to each chain. This was required because the Gee receiver was wideband and sometimes signals from two chains on adjacent frequencies could be seen at the same time. The time base itself was not locked directly to the master pulse but was generated by a free running oscillator that could be offset slightly with a manual control. If it was not properly synchronized the pulses would all drift to the left or right., and the drill was to stop them drifting with a tuning control while bringing the A pulses to the left-hand edge of the time base. Small pedestals were then brought under the slaves the operator wished to use, and an expanded time base was then flashed up.

After final alignment of the pulses, a timing display was switched in and the time delays for each slave visually counted. This could be done accurately because the signals were switched off and a noise free display was obtained. For various reasons, time was not counted in microseconds (as was done in Loran) but in 'Gee units' where one unit was equivalent to 66.66 microseconds. A measurement accuracy of 1 microsecond was achievable, representing a position line accuracy of about 150 meters at best; two such providing a fix accuracy of around 210 meters , although other errors in the system might double this. At longer ranges, 350 miles for example, the error ellipse was about 6 miles by 1 mile. While not remarkable by today's standards, it was revolutionary at the time and far in advance of any other method of fixing.

GEE EQUIPMENT
 

gee_indcator.jpg
A GEE Mk II receiver/indicator with cabling

 
gee1.jpg
Gee station components. (Photo courtesty of  "Signals Collection '40-'45" web page).

 
 
 
 
 
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Gee set installation in a Lancaster bomber mockup. (Photo courtesty of  "Signals Collection '40-'45 " web page).

 
 
gee_indicator_s.jpg Closeup of Gee Indicator. Click on image to enlarge. (Photo by Jerry Proc) 

GEE CHAINS

Several Gee chains were established in the UK and after 1944, abroad.  There were, in 1948, four in the UK, two in France and one in Germany. It developed into one of the most widely fitted airborne radio navaids of the day, becoming standard in the US Eighth Air Force, as well as in the RAF. Gee transmitters had a radiated power output around 300 kw and operated in four frequency bands between 20 and 85 MHz. At those frequencies, the useful range of the system was limited to approximately 150 miles at ground level and 450 miles for high flying aircraft.
 
 

gee_cover.jpg

Coverage: Gee coverage in the UK in August 1948 indicating the various chains. The coarse cross-hatching indicates maximum coverage at 10,000 feet altitude.

In 1946 it was proposed to the Provisional International Civil Aviation Organization (PICAO) Conference on civil navaids of that year as a standard civil aid. A new receiver was designed that had a direct readout and was thus more suitable for pilot operation and, considerably later (1954), a fully automatic receiver appeared. Unfortunately, the digital computer that could have made it a really viable system had not yet appeared on the scene and civil aviation turned to VOR/DME, the last Gee chain being taken out of service in 1970. It is the author's opinion that Gee was one of the great missed navaid opportunities of the post-war years. It was at least as accurate as anything else in use, and far more accurate than most; nor did it suffer from sky wave and static problems anywhere nearly as badly as did many of the lower frequency systems. It was perfectly suited to aviation, its line-of-sight range being no drawback, and there is not the slightest doubt that, had investment been made in more modern transmitters and receivers, it could have been made fully automatic and even more accurate.

REFERENCE:

1)The Journal Of Navigation - Chapter 4.
W.F. Blanchard, Royal Institute of Navigation;
Vol 44, No. 3; Sept 1991. Used with permission.

2) Winning the Radar War. Jack Nisen. Macmillan of Canada , 1987.

3) "Signals Collection '40-'45" web page: www.qsl.net/pe1ngz

4) Electronic Navigation Systems. Philco Ford. 1967

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Oct 24/12