GEE MK1, THE MISSING LINK

THE GEE NAVIGATION EQUIPMENT

The Gee system was the brain child of R. J. Dippy, the concept initially put forward as a blind landing aid as early as 1937. When in 1940 it was realised that bombing Germany killed more cows than people, with the bombers seldom coming within five miles of the target, the principles of the blind landing aid were adopted but applied as a navigation aid, resulting in the Gee navigation system with a range of several hundred miles.

Any person having an interest in WWII radio and Navigation aids will, I am sure, be aware of the Gee equipment, however they will probably quote Indicator 62A and Receiver R1355 when asked the question 'what does the Gee equipment consist of'?


This is of course Gee MK2 and if there is a MK2 there must have been a MK1 and also a prototype system. Like many other early MK’s of equipment they often disappear from the face of the earth and this is so of Gee MK1.

A few years ago an unusual receiver turned up, an R1324. It had holes in the front panel, initially suggesting it had been’ got at’ by some enthusiastic amateur, but it did have the familiar aircraft mounting attachments and it warranted further investigation.


After a lot of enquiries amongst other collectors it emerged it was a Gee MK1 receiver and the holes in the front panel were used for locating detonators to destroy the equipment to stop it falling into enemy hands. If there is a Gee MK1 Receiver there must have been an Indicator and other associated equipment. A search on the internet found nothing about Gee MK1 other than a possible number for the Indicator which turned out to be Type 60.

Enquiries at the Hendon Aircraft Museum Library eventually unearthed a copy of the AP1766P which, although a big step forward, did not give any circuit details of any of the equipment other than a setting up procedure and the basic layout diagrams.

Contact had been made with the ERT (Electronic Restoration Trust) who had set up a trust to collect information and equipment on some or the rare radio, radar and navigation aids from WWII. They became involved in the Gee MK1 project and their members unearthed a vast amount of documentation on the early Gee or ‘G’, as it was initially called, from the National Archives, The documentation obtained by ERT contains a large quantity of drawings, circuit diagrams and memos relating to the early years of the ‘G’ development and flight trials.

The information obtained was fascinating to read, giving circuit information of the flight trials equipment but little on the MK1 production equipment or circuits. The prototype circuits were somewhat unusual, bearing little resemblance to Gee MKII so the circuits were reconstructed together with suitable power supplies, CRT and some modern circuits to investigate and find out what would actually appear on the CRT.

As well as giving the basic layout details of Gee MK1 the AP1766 obtained from Hendon did give some screen shots and it was found these agreed almost exactly with those obtained from the reconstructed prototype equipment. This was a big step forward as it indicated that the production equipment, i.e. Indicator 60, differed little from the flight trials prototype. This, together with some other information relating to the layout and construction, enabled a experimental Indicator 60 to be built

Since the first issue of this article a photograph of an actual Indicator Type 60 installed in an aircraft has turned up showing the actual front panel layout. (See following photo) The prototype was in fact almost identical apart from the position of the handle. At the end of 2016 the project was passed over to the Duxford Radio Society which is in a better position to bring this information over to those interested in WWII electronic equipment.

One of their restoration team has already replaced the original simulator and internal oscillators with Xtal controlled ones, all on PCB,s and the following photograph shows the A, B, A ghost and C simulator signal pulses together with the bright-up A, B and C strobes.

A complete experimental Gee MK1 system was now within sight, however the already available R1324, due to many component failure over the years, could not be powered up, so modern electronics were used to simulate the receiver functions and the divider circuits that were contained within the R1324. As modern electronics are so tiny compared with the 1940’s valve equipment, all the simulator, dividers and oscillator circuits were hidden away in the newly constructed Type 60 indicator unit.

The final experimental equipment which includes the non functioning receiver, the new build indicator unit and a type 3 control unit which contains the 1500Hz static inverter looks and works the same as the original MK1 production equipment, as far as one can tell.


The story however does not stop there. What we have is the result of quite a bit guess work on the indicator unit and we are still desperate to locate original photographs or other documentation relating to Gee MK1. The pot of gold would be to find an original Indicator type 60 or a copy of the MK1 manual that also has disappeared into oblivion.

Any information you have please email me on:-

norman.groom@lineone.net

DIFFERENCES BETWEEN THE MK1 & MK2 SYSTEMS

In building up an experimental MK1 system one assumption had to be made. There must have been an overlap period when both MK’s were in operation at the same time therefore the format of the transmissions must have been the same. The MK1 receiver was a single frequency receiver that could and was easily jammed by enemy transmissions. This was a major factor in quickly bringing out the MK2 system with the interchangeable RF units in the receiver. This enabled almost instant pre-arranged frequency changes to be made by the navigator. The first operational flights were made in 1941 yet by February 1942 the first Gee MK2 flight was carried out. With such a short life span it is doubtful if many MK1 sets were made and those made may have been recycled due the shortage of components in those early years of the war, hence the rarity of the MK1 units appearing today.

GEE EQUIPMENT

The Gee system basically consists of a receiver, CRT, power supplies, timebase circuits to generate the tube traces, an onboard Crystal oscillator and divider chain to generate calibration pulses. All this had to be contained in two units, the receiver and the indicator. The MK1 packaged the receiver and the divider chain and power supplies all within the R1324 with all the rest packaged within the indicator. When MK2 came along with its interchangeable RF units the divider circuits had to be moved to the Indicator. This entailed major circuit changes to get it all in, especially as other changes were also required. I can only judge from my own observation of both displays but MK1 must have been much more difficult to operate compared to MK2. Calibration pips and other trace information on MK1 used intensity modulation of the traces and this required a lot of adjustment of the brightness controls. MK2 information was presented by deflection of the trace in the vertical direction, familiar to anyone using an oscilloscope. Some of the circuitry of MK1 was somewhat unusual as they had used a single pentode valve as cross coupled monostable or bistable circuits. Although reducing the valve count, this did involve selecting the valves for their characteristics, not a good thing when valves were in short supply in the 1940’s. The equivalent circuits in the MK2 system reverted to the normal two valve monostables or bistables. Thus Mk2 differed considerably in detail and packaging from Mk1 but the principles remained the same.

THE PRINCIPLES OF OPERATION

Note that the CRT screen views shown below are those seen on Gee MK2, Gee MK1 uses intensity modulation rather than displacing the trace vertically to show the various features.

Consider two transmitters located some eighty miles apart that alternately transmit pulses spaced exactly one millisecond apart. If the navigator on an aircraft receives those pulses and they are still exactly one millisecond apart, then the aircraft must be the same distance from each transmitter, irrespective of how far the aircraft is away from the transmitter. This can be shown on a chart as a straight line drawn from the centre of a line joining the two transmitters and at at right angles to that adjoining line. Other lines can be dawn on the chart where there is constant time difference between the two received two pulses that may be less or more than one millisecond depending whether the aircraft is nearer to one transmitter rather than the other. However in these cases the lines are no longer straight but are hyperbolic in shape and where the focal points of the two sets of hyperbolic lines are the at transmitters

If the navigator measures the time delay between these received pulses and refers to the lines drawn on his Gee map which are marked with numbers relating to the delay time, he does at least know he is somewhere along that line.

Given another pair of transmitters set up at an angle to the first pair, another set of lines are then available and another measurement can be taken. Referring to the two measurements and the two set of lines on the Gee chart, his position will be where the two lines cross.

In practice there are only three transmitters needed as one, denoted as the ‘A’ transmitter can be used for both pairs where the other two transmitters are defined as ‘B’ and ‘C’ and are given different colours on the Gee chart issued to the navigator. Four pulses are then transmitted in the sequence A, B, A, C, and in order to identify which ‘A’ pulse is which, the second ‘A’ pulse is doubled every fourth transmission and is referred to as a ghost pulse.


MEASURING THE TIME INTERVAL BETWEEN PULSES

In virtually all radar systems where the transmitter and the indicator are at the same location, the indicator unit timebase is triggered the instant the transmitter pulse occurs and the timebase is thus synchronised to the transmitter, any echo appearing stationary on the timebase (neglecting movement of the target). With the Gee system, the indicator unit is obviously remote from the transmitter and without synchronisation the pulses would occur randomly along the time base. In order to avoid this problem the time base has to be synchronised from an onboard signal source, this being a quartz crystal oscillator and divider chain capable of being set to the same pulse repetition frequency as that of the transmitter. Initially the pulses will be seen to drift either to the left or right on the CRT depending if the onboard oscillator frequency is slightly low or high compared with that of the transmitter. By use of the fine adjustment of the onboard oscillator, the drift can be stopped and importantly the ‘A’ pulse can be drifted and then stopped at the required position on the timebase.

The concept of Gee is very clever, but the concept of measuring the time difference between pair of pulses to an accuracy in the order of 0.1%, given the technology then available, is brilliant. All that was available was a six inch CRT with a usable four inch trace, a tube who’s deflection was not necessarily linear and a timebase that may also not perfectly linear. The only item with any precision was the onboard crystal controlled oscillator.

The four pulses had a repetition frequency of four milliseconds and in order to obtain greater resolution and hence accuracy the trace was divided into two. The two traces were placed one above the other on the CRT with a time of a little less than two milliseconds for each trace to allow for flyback time between traces. If the ‘A’ trace was set to the beginning of the upper trace then the ‘B’ pulse would always appear on the upper trace. The ‘A’ ghost pulse would appear immediatelt below the 'A' pulse on the lower trace together with ‘C’ pulse.


Having displayed the pulses, which may of course be moving as the aircraft is travelling in the region of 200mph, how do you measure two times down to 0.1%?

Consider a similar problem of measuring the length of a metal bar. First you need an accurate steel rule but importantly you need some form of optical magnifier to magnify the engravings on the rule and to precisely align the ends of the metal bar with the precision scale from which you take the measurement. The method of measuring the delay time with Gee is very similar, with calibration pulses on the traces taking the place of the precision engravings on the rule and a fast time base, referred to as strobe time base (STB), taking the place of the magnifying glass. As with the optical magnifier, the STB's have to be moved to point you wish to measure and in the case of Gee it’s to cover the A, B and C signals. You also need to be able to see where these STB's are on the two traces. The trigger point of the STB’s is controlled by ‘B’and ‘C’ fine controls on the indicator front panel whilst two others occur at the beginnings of the two main timebase traces and used to strobe the 'A' pulses. The 'B' and 'C'STB's are shown as negative going steps, one on the upper main timebase trace for ‘B’ and another in the lower trace for ‘C’. The navigator has to move these two using the controls to cover the ‘B’ and ‘C’ signal which appear inverted during the strobe period.


The navigator has the option of viewing either the main time base or four of the magnifying STB’s, each STB appearing vertically one above one another on the CRT. The top STB will then show the ‘A’ pulse the others in order of ‘B’ ‘A ghost’ and lastly ‘C’ but the 'B' and 'C' signal pulses, seen on their appropriate STB, not necessarily aligned one above the other.


It is very important that all four sinals are vertically aligned and by using only the fine ‘B’ and ‘C’ controls the ‘B’ and ‘C’ pulses can be moved such that they align vertically with the ‘A’ pulses. In pracice you are not actually moving the pulses but moving the STB's along the time axis.

The analogy with the mechanical method of measuring the length of a metal bar is that we now have two identical optical microscopes set up exactly at a fixed distance apart, aligned with the points we are trying to measure, i.e. the ends of the bar. At this point we can throw the bar away and just insert our precision steel rule, with the cursor lines used to take the measurement as the two microscopes are positioned exactly the same distance apart as the end of the bar.

The same applies in the case of Gee, we can use a switch on the indicator, called the Clearing switch to remove the signals and replace them with precision calibration markers derived from the crystal oscillator.


The STB view, like the microscope view, enables us the measure the small incremental distance between the main calibration markers whilst the main timebase view yields the value of the major divisions.

The received signals appear in the sequence A, B, A ghost and C. The block of four repeated every 4 milliseconds. The A and A ghost will always be 2 ms apart as they are derived from the same transmitter but the B and C times will vary depending, and this is what we are attempting to measure.

The two main traces are repeated every 2 ms thus the A and A ghost will appear on immediately above the other. When the clearing switch is depressed the calibration markers appear on both the main timebase and the STB’s. Only 15KHz markers appears on the main timebase and both 15KHz and 150KHz markers appear on the STB’s. The length of the STB timebase is preset such that just over two 15KHz markers always show. Allowing for flyback time between the main two timebase sweeps the time must be a little less than 2 ms thus the main trace will only show 25 of the 30 calibration markers, five markers being lost in the flyback time. The STB’s will of course have 10 divisions between the two 15 KHz markers due to the 150 KHz markers.

The 15 KHz divisions are referred to Gee units and lines on the Gee charts are numbered using Gee units. The ‘B’ transmitter will always have theoretical range of 0 to 2ms but the ‘C’ transmitter will be in the theoretical range of 2 to 4ms. As there are 30 Gee units per 2ms it is necessary to add 30 to the readings obtained from the ‘C’ or lower main timebase trace tie up with the readings on the Gee chart.

The greatest advantage of being able to instantly remove the actual signals and replace them with calibration markers avoids the problem of one signal shifting in time whilst the other is being measured due to the speed of the aircraft. All the navigator has to do is to note the instant he switches the clearing switch over, the equivalent time delays are then stored permanently in two CR circuits allowing time for both measurements to be taken, giving him a ‘fix’ in position and time. Repeating this procedure during the flight allows him to determines the track of the aircraft.

In the STB position the ‘A’ STB is preset to just over two Gee units long and is triggered from the start of the trace, hence the first Gee marker will appear near the centre of the ‘A’ STB. This will be the zero point and the ‘B’ and ‘C’ readings are taken immediately below this point. Examining the main time base this zero marker can also be seen as well as markers within the ‘B’ and ‘C’ strobe periods. Thus a count can be made from the zero marker to determine the value of the main Gee markers within the ‘B’ and ‘C’ strobe periods.


Reverting back to the STB these again can be seen but in an expanded mode. As the Gee markers are subdivide by ten in the STB mode it is then possible to read off the first decimal point immediately below the ‘A’ strobe maker and even estimate the time to the second decimal place. With Gee MK2 this is made even easier as an expanded STB is available using a switch on the indicator.

There is quite a difference in the way the information is presented between the MK1 and MK2 so the above description is only general and applies to Gee MK2 but it should give an idea of how the Gee system functions and roughly how the readings are taken.

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