Gee-H (navigation) facts for kids
Gee-H, sometimes written G-H or GEE-H, was a special radio navigation system. Britain created it during World War II to help RAF Bomber Command planes find their way. The name "Gee-H" came from using older Gee equipment and a new "H principle" for finding locations. Its official name was AMES Type 100.
Gee-H was like an improved version of another bombing system called Oboe. Both systems helped bombers fly along a curved path by staying a certain distance from a radio station on the ground. Bombs were dropped when the plane reached a set distance from a second ground station.
The big difference between Oboe and Gee-H was where the main equipment was. Oboe used large, very accurate displays on the ground, but it could only guide one plane at a time. Gee-H used smaller systems inside the aircraft. It was a bit less accurate, but it could guide as many as 80 planes at once!
Gee-H started being used in October 1943. Its first successful use was in November against the Mannesmann steel factory in Düsseldorf. Gee-H was used throughout the war, even though the Germans tried hard to block its signals. It was also used on RAF planes like the English Electric Canberra after the war.
The basic idea behind Gee-H is still used today in civilian DME systems.
Contents
Finding your exact spot on a map needs two measurements. You can use two angles, two distances, or one angle and one distance. Early radio navigation often used two angle measurements. But these were not very precise, only accurate within many miles.
Measuring distance with radio signals became possible after radar was invented. Radar uses radio waves to figure out how far away objects are.
Early Distance Systems
The Luftwaffe (Germany's air force) was one of the first to use distance-measuring radio systems. Their Y-Gerät system came out in 1941. It used a radio beam to guide the bomber in the right direction. It also used a special device called a transponder on the plane to measure distance.
Here's how Y-Gerät worked: A ground station would send out a radio pulse. The transponder on the plane would receive it and immediately send back an answering pulse. A ground operator would then measure the time it took for the pulse to go to the plane and come back. This time told them the distance, just like radar. The operator would then tell the bomber crew by voice when to drop their bombs.
The problem with beam-based systems is that radio beams spread out. This means they become less accurate the farther away you get from the ground station. Distance measurements, however, depend only on how accurate the equipment is. So, their accuracy stays the same no matter how far away the plane is.
The Oboe System
The Air Ministry in Britain developed their own distance-measuring system called Oboe. It started reaching the Pathfinder Force (special planes that marked targets) in late 1941.
Oboe used only one distance measurement at a time to guide the plane. Before a mission, planners would figure out the distance from an Oboe ground station to the target. For example, if attacking Düsseldorf, the distance from a station near Walmer might be about 235 miles (378 km). They would draw a curved line (an arc) on a map, 235 miles from the station, passing through Düsseldorf.
Next, they calculated how far the bombs would travel after being dropped. For planes flying at about 20,000 feet (6,100 m), bombs might travel about 1.5 miles (2.4 km) forward. Planners would then figure out the exact spot along the arc where the bombs needed to be released to hit the target. This was all done carefully on the ground, considering things like wind and air pressure.
During the mission, the bomber crew would fly towards one end of this arc. When they were close, the plane's transponder would turn on. The Oboe ground station (called the "cat" station) would measure the plane's distance. This station would send out radio signals (dots or dashes) to the pilot. The pilot would adjust the plane's path until the signal became a steady tone, meaning they were exactly on the right arc.
A second Oboe station (the "mouse" station) also measured the bomber's distance. This station knew the bomb's travel distance. As the bomber got close to the drop point, the mouse station would send Morse code signals. At the perfect moment, it would send another Morse signal that would automatically drop the bombs.
The main problem with Oboe was that only one plane could use it at a time. It took about ten minutes for a bomber to get onto the arc. This meant Oboe couldn't be used for large raids with many planes. Instead, Oboe was mainly used by the pathfinder planes to mark targets very accurately for the main bomber force, no matter the weather.
A New Idea: Gee-H
Oboe could only guide one plane because the plane's transponder would send pulses every time the ground station asked for them. If more than one plane turned on its transponder, the ground station would get many return pulses and couldn't tell them apart.
One solution was to have each Oboe ground station send a slightly different signal. This way, many Oboe stations could work at the same time. But it still didn't solve the problem of multiple planes using the same station.
So, what if the transmitters were on the ground and the receivers were on the plane? Then, each plane could create its own unique signal pattern. The operators on the plane could look for their own signal and ignore others. This way, many planes could use the same ground station at the same time. This is the basic idea behind Gee-H.
How Gee-H Worked
The first radio navigation system used by Bomber Command was Gee. It worked by sending out two timed pulses from ground stations. Planes would pick these up and show them on a screen called an oscilloscope.
To make Gee-H quickly, designers decided to use as much of the existing Gee equipment as possible. Gee already had the oscilloscope screen and receiver unit. So, they just needed a new unit that would send signals to trigger the ground station. This new unit was designed to work on the same radio frequencies as Gee.
The new Gee-H transmitter on the plane sent out pulses about 100 times a second. The timing of these pulses was slightly different for each plane. This meant that every aircraft had its own unique timing. This unique timing was called "jittering."
The navigator on the plane would set a delay on the Gee display. This delay matched the curved path (arc) they wanted to fly along. The signals from the plane's own transmitter would then line up perfectly on the display and stay still. Signals from other planes would move around, making them easy to ignore.
The navigator would guide the pilot along the correct path by making sure the signals on the screen lined up. The same was done for a second channel, setting it to the exact distance where the bombs should be dropped. The operator only had to check the first distance sometimes, while watching the second signal slowly move towards the bomb-drop point. When they overlapped, the bombs were released.
The ground station could respond to many planes at once. It took only about 100 microseconds for a ground station to receive a pulse and send back a response. This meant a station could handle about 70 to 80 planes at the same time!
Gee-H also had another advantage: it was harder to jam. In most other pulsed navigation systems, it was easy to block the signals by sending out extra pulses. This would clutter the display and make it impossible for the operator to read. The British had done this to Germany's Y-Gerät, and the Germans did it back to Gee. By the end of the war, Gee was mostly useless for bombing.
But with Gee-H, each plane had its own unique timing. To jam a Gee-H receiver, a jammer would need to match that specific timing. Since many planes might be using different timings, and there were also fake signals, jamming Gee-H was much harder for the Germans.
Since Gee-H used Gee equipment, turning off the special Gee-H transmitter made it work like a normal Gee unit again. On a typical mission, planes would use Gee to navigate when leaving England. They would switch to Gee-H for the bombing run, and then back to Gee for the return flight to find their airbase. Gee was great for general navigation, while Gee-H was mainly for guiding planes to a specific target.
Gee-H's main weakness was also due to using Gee equipment. It used a small oscilloscope screen, which wasn't as visually accurate as Oboe's larger 12-inch screens. Gee-H could achieve accuracy of about 150 yards (137 meters) at 300 miles (483 km). Oboe was even better, accurate to about 50 yards (46 meters). Also, like all VHF and UHF radio systems, Gee-H was limited to distances just beyond the line of sight, which was about 300 miles.
Gee-H played a key role in Operation Glimmer. This was a fake "attack" during Operation Overlord (the D-Day landings). Bombers from 218 Squadron flew in tight circles, dropping "Window" (strips of metal foil) over small ships with radar transponders. This tricked German radars into thinking a huge invasion fleet was heading for Calais, while the real invasion was 200 miles away at Normandy.
See also
