Gee-H (navigation)

Gee-H, sometimes written G-H or GEE-H, was a radio navigation system developed by Britain during World War II to aid RAF Bomber Command. The name refers to the system's use of the earlier Gee equipment, as well as its use of the "H principle" or "twin-range principle" of location determination. Its official name was AMES Type 100.

Gee-H was used to supplant the Oboe bombing system, both of which worked along similar lines. By measuring the distance to a radio station, the bomber was able to navigate along an arc in the sky, dropping their bombs when they reached a set distance from another station. The main difference between Oboe and Gee-H was the location of the equipment; Oboe used very large displays in ground stations to take very accurate measurements but could only direct one aircraft at a time. Gee-H used much smaller gear on board the aircraft and was somewhat less accurate but could direct as many as 80 aircraft at a time and was passive.

Gee-H entered service in October 1943 and first used successfully in November against the Mannesmann steel works at Düsseldorf on the night of 1/2 November when about half of the sets failed leaving only 15 aircraft to bomb the factory on Gee-H. Gee-H remained in use throughout the war, although it was subject to considerable jamming from the Germans. It also remained a standard fixture of post-war RAF aircraft like the English Electric Canberra.

Gee-H was adapted by RCA into the US wartime SHORAN system with improved accuracy. The same basic concept remains in widespread use today as the civilian DME system.

Distance measuring navigation
Determining your location in 2D space requires two measurements or angle or range - two angle measurements, two distance measurements, or one angle and one distance. Early radio navigation was typically based on angle measurements using radio direction finders, but these had limited accuracy on the order or tens of miles. The development of range-based systems had to wait until the invention of accurate time measurement of radio was possible, which came about as a result of the development of radar.

The Luftwaffe pioneered the use of distance-measuring radio navigation systems with their Y-Gerät system in 1941. Y-Gerät used a single Knickebein-like beam for steering the bomber in the proper direction, and an onboard transponder for distance measurements. A special signal was periodically sent from a ground station and on reception the transponder would send out an answering pulse after a known delay. A ground operator used an oscilloscope to measure the time between broadcast and reception, and deduced the range in a fashion similar to conventional radar systems. He then radioed this information to the bomber by voice, telling them when to release their bombs.

The downside to the "beam" system of navigation is that the beams are necessarily fan shaped, growing wider with increasing distance from the broadcaster. This means they have decreasing accuracy with increasing range. Measurements of distance are dependant only on the accuracy of the equipment, and are independent of range. When two such signals are combined the accuracy is linear with range, as opposed to the square as is the case with beam-based or hyperbolic navigation systems. However, such systems are also generally more difficult to use, as they require two range measurements to be made in quick succession, while the aircraft is moving.

Oboe
The Air Ministry developed a distance-measuring system known as Oboe which first started reaching the Pathfinder Force in late 1941 and was used experimentally in 1942. Oboe avoided the problems with two distance measurements by using only one at a time.

Before the mission the distance from one of the Oboe stations to the target was measured, and an arc of that radius drawn on a conventional navigation chart. For instance, the distance between a station near London and a target in Düsseldorf would be about 300 miles, so an arc with a 300 mile radius around the station would be drawn, passing through Düsseldorf. Now the "range" of the bombs being dropped would be calculated. Range is the distance between the point where the bombs are released and the point where they impact. For missions around 20,000 ft altitude, range is typically on the order of 1.5 miles for a high-speed aircraft like the de Havilland Mosquito. The mission planners would then calculate the location along that arc where the bombs would need to be dropped to hit the target. This calculation, carried out on the ground, could be as time consuming as required, considering winds, atmospheric pressure, even the tiny centrifugal force generated by the aircraft following the 300 mile radius curve.

During the mission, the bomber would fly itself to one end of the arc or the other, using any means of navigation including dead reckoning. When they were near the pre-arranged location, the transponder was activated, and the Oboe station would measure their current distance. The station would then send out a radio signal of either dots or dashes, allowing the pilot to adjust their path so they were at precisely the right range, where the Oboe station would send out a steady tone, the "equisignal". Operators would continue to watch the position of the aircraft, sending out correcting signals as needed so the pilot could continue to adjust their path along the arc.

Meanwhile a second station was also measuring the distance to the bomber. This station was equipped with the bomb's range value calculated earlier, and had used this to calculate the range between their station and the bomber at the point where the bomb's needed to be dropped. When this mouse station saw the bomber approaching the drop point, it sent a series of Morse code signals to inform the pilot that the drop point was approaching. At the precise instant, it would send another morse signal that would drop the bombs automatically..

The main constraint of Oboe was that it could only be used by one aircraft at a time. As it took about 10 minutes for the bomber to get itself properly positioned along the arc, this delay meant that the system could not be used for a large raid with multiple aircraft in succession. Instead, Oboe was used to guide the target marking aircraft of the pathfinder force, giving the main force of bombers an accurate aiming point in any weather. Alternately, Oboe was sometimes used for attacks on point targets by single aircraft, or a small number dropping one after the other. In tests, Oboe demonstrated accuracies greater than those of optical bombsights during daylight in good weather.

A new approach
Oboe was limited to one aircraft because the onboard transponder would send pulses every time the ground stations queried them. If more than one aircraft turned on their Oboe, the ground stations would start to receive several return pulses for every query, with no way to distinguish between them.

One solution to this problem is to have each Oboe station send out a slightly different signal, normally by changing the envelope of the signal it broadcasts to the aircraft. Similar stations with different signal modifications can be situated around the UK, so that all of them are visible to an aircraft over Germany. An aircraft that turns on its transponder will receive and re-transmit signals from all of them. Although all of the ground stations will receive all of the signals, they can pick out their own by looking for their unique signal. This change allows many Oboe stations to be operational at the same time, although it does not help the situation if more than one aircraft turns on their transponder.

Now reverse the location of the transmitters and receivers, so that the receiver is on the aircraft and the transmitter on the ground. Now every aircraft generates a different signal pattern, the operators on the aircraft can look for their own signal and ignore the others. Any number of aircraft can use the same station at the same time. As long as the ground station is equipped to quickly turn the signals around and the aircraft do not query too often, the chance of more than one aircraft querying the station at the very same time is low. This is the basic concept behind Gee-H.

Gee-H
The first radio navigation system to be operated by Bomber Command was Gee. This operated by sending out two pulses of known timing which were picked up by the aircraft and read on an oscilloscope. The time between pulses was not particularly accurate and sometimes deliberately changed, so the equipment in the bomber had a timing system that allowed it to adjust for this. When the receiver was first turned on, the pulses from the ground station would move across the display because the two time bases were not synchronized. The operator then tuned their oscillator until the pulses stopped moving, which meant that the local oscillator was now at precisely the same frequency as that in the ground station. This "froze" the signal on the screen, allowing signals from different ground stations to be seen and measured.

In order rapidly to deploy the new system, it was decided to re-use as much of the Gee equipment as possible. Gee already included the oscilloscope display and the receiver unit, so all that was needed was the broadcaster unit that would trigger the ground station transceiver. This was designed to operate on the same frequencies as Gee, so that the existing receiver and display equipment in the bombers could be used. The new transmitter sent out pulses about 100 times a second. The timing of the pulses was slightly advanced or retarded from 100 per second. This meant that every aircraft had a slightly different timing. The same signal was also sent to the Gee display unit to start the display beam moving across the face of the display. This way the received signals that did not have the same inter-pulse timing would appear to move one way or the other, exactly like a mis-tuned Gee. Only the signals originating from the aircraft's timer would line up on the display and remain motionless. This deliberate adjustment of the timing was known as "jittering".

In operation, the navigator would first set the delay of the upper trace on the Gee display to a known figure that matched the radius of the arc they wanted to fly along. This would move the "blip" from the local transmitter along the face of the display. Received signals would then be sent to the display inverted. The navigator could then direct the pilot onto the right path by giving directions until the upper and lower blips aligned. The range computed in advance, from the second station to the drop point, was dialled into the lower trace and as this range was always changing, the active blip would move along the display towards this value until they overlapped. The time taken by the transceiver to receive a pulse, send out the response and return to the receiving condition was about 100 microseconds. With a pulse timing of about 100 a second, a transceiver would be busy for 10,000 μS responding to the signals from any one aircraft in a second of time. This would leave 990,000 μS free to respond to other aircraft, giving a theoretical capacity of 100 aircraft. In practice, due to the jitter", about 70 to 80 aircraft could use a station at a time.

The system had the additional advantage that each aircraft selected its timing, which made jamming more difficult. With most pulsed navigation systems like Gee and Y-Gerät, it is relatively easy to jam the system simply by sending out additional pulses on the same frequency, cluttering up the display and making it very difficult for the operator to read the signal. The British had used this technique to great effect against Y-Gerät and the Germans returned the favour against Gee. In the case of Gee-H, each aircraft had unique timing, so unless the jammer matched the jitter, the jamming signal would simply disappear off the screen. As there were dozens of such transmitters, and a few ground stations in the case of Oboe, the magnitude of the jamming problem became considerably more difficult. As the Gee-H system used Gee equipment, turning off the transmitter turned it back into a normal Gee unit. Although Gee was susceptible to jamming, it was still useful over the UK where jamming was not an issue. Unlike Gee-H, Gee could be directly read on a map and was extremely useful for navigating home after a mission.

Gee-H's main fault was also a side-effect of the use of Gee equipment; using a higher frequency would allow a tighter envelope, which would allow more accurate timing measurements and thus improve accuracy. As the system used Gee's small oscilloscope for measurements, it did not have the same visual accuracy as Oboe, which used 12-inch oscilloscopes developed for this use. Gee-H achieved accuracy of about 150 yards at 300 miles. As with all VHF and UHF-based systems, Gee-H was limited to distances just out of line of sight, in this case limiting it to about 300 miles. Gee-H was key to Operation Glimmer, a diversionary "attack" during Operation Overlord that distracted and pinned-down German defences at Calais while the real invasion fleet was 200 miles away at Normandy. Gee-H-equipped bombers of 218 Squadron flew low, in tight circles, dropping "Window" (chaff) over radar transponder-equipped small ships, in order to deceive the German radars that they were the main invasion fleet.

Air Ministry Designations

 * Gee-H Mk. I - Airborne Radio Installation (ARI) 5525
 * Gee-H Mk. II - ARI 5597
 * Gee-H Mk. II (Tropicalised) - ARI 5696