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. By the late war period, Gee was generally useless for bombing and used primarily as a navigational aid when returning to England. In the case of Gee-H, each aircraft had unique timing; to jam the receiver, the jammer would also have to have similar timing. As one signal might be used by dozens of aircraft, dozens of jammers set to slightly different times would be needed. As there were dozens of transceivers as well, many unused decoy signals, the magnitude of the jamming problem was considerably more difficult. As the Gee-H system used Gee equipment, turning off the interrogation transmitter turned it back into a normal Gee unit. On a typical mission, the set would be used for Gee while leaving England and forming up into a bomber stream, for Gee-H during the mission and back to Gee on the return flight for finding its airbase. Since Gee could be directly read on a map, it was extremely useful for general navigation, while Gee-H was only practically used to navigate to one place. 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. Because 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 specifically for this purpose. Gee-H achieved accuracy of about 150 yards at 300 miles, while Oboe was good to about 50 yards. 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, to deceive the German radars that they were the main invasion fleet. Air Ministry designations Gee-H Mk. I - Airborne Radio Installation