The Radar Room

GEE AIRCRAFT NAVIGATION AID

GEE is one of three similar navigation aids devised during the 1940s then used right through to the late 1960s. The principle of GEE was quite simple. Measure the time it takes to receive signals (pulses) from two (or occasioanlly three) ground stations. These two signals ( called B and C ) are locked to a master transmitter A.

First and foremost, one needs to lock ones GEE aircraft receiver onto the pulses coming in from master transmitter A. In order to find ones location, all one does then is measure the time it takes for two more radio pulses to arrive at your aircrafts receiver from both transmitters B and C. Once these two timings are known, they are plotted onto a chart (map) overlaid with hyperbolic lattice lines. From this information once can estimate the whereabouts of your aircraft. The complexity of the circuitry within the GEE indicator allows for these timings to be quickly and accurately measured.

GEE principles

A SIMPLE OVERVIEW TO MEASURING GEE PULSES USING OUR GEE INDICATOR

Please note that we’ve deliberately left out a couple of the strobe timebase settings  to simplify the explanation. Also note that there are slight inaccuracies with the screen our GEE indicator as it doesn’t show the full 25 or 30 pulses on the main timebase.

Main TB with B and C out of phase
Another strobe TB shot with both B and C out of phase
Strobe TB with pulses out of line
Strobe TB showing pulses correctly lined up
Main TB signals correctly phased
Main TB recurrence on setting 1
Main TB recurrence on setting 2

The two trace images to the left are both showing that signals from the Master Station A and Slave stations B and C are currently being received. The A pulse appears twice. Once at the far left hand side of each of the two lines.

The second pulse on the lower line right next to the A pulse is the ‘ghost’ pulse.

The ‘ghost’ pulse identifies that the preceding A pulse belongs on the lower line. (i.e. to the C transmitter)

The B pulse is the downward spike on the top line, whereas the C pulse is the downward spike on the lower line.The two little buckets are generated in the GEE indicator and need to be lined up, or ‘phased’ with the two spikes from the B and C transmitters.

We use the four front panel fine and coarse adjustment controls on the indicator unit to shift the little buckets either to the left or right until they line up with the inner left hand edge of the buckets.

This is not to easy to do accurately on this screen, so another setting is used, the strobe timebase, which makes the adjustment far easier, quicker and more accurately.

Here is one of the strobe timebase settings with the pulses not phased (lined up). On the top line we see the A pulse, with the B pulse underneath, whereas on the lower line we again see the A pulse, with the C pulse underneath.

In this image we can see that the pulses are now lined up properly with each other.

Check back on the main timebase and we find that the B and C pulses are in their corresponding buckets.At this point we have set the indicator up with all the information we need from the receiver, so we push the clearing switch down to remove it from the display.

Whilst the A pulse is not visible to the far left of the two traces shown here on this image, it it is at the same location as found with the clearing switch up.

With the indicator screen now generating two sizes of ‘range markers’,  we can  accurately measure the position of the marker buckets with respect to the far left and the A pulse. Note that the receiver is now turned off and this display is simply showing the buckets we lined up with the received B and C pulses.

Note that as it may be necessary to expand the main timebase from the above setting, hence the display to the left now shows30 pips on each whole line as opposed to 25 on the image above. We can switch between 25 and 30 pulses per line with the recurrance switch at the top of the indicator front panel.

We now take these measurements from the indicator screen and apply it to the GEE lattice chart. A sample of which may be seen below.

The B pulse being plotted onto the corresponding RED line and the C pulse onto the appropriate GREEN line.

GEE chart segment
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