'Glimmer' was a British part of the 'Fortitude' (and more specifically 'Bodyguard') deception plan for 'Overlord', together with the similar 'Taxable' and, farther to the west, 'Big Drum' (5/6 June 1944).

'Taxable', 'Glimmer' and 'Big Drum' were thus schemed as tactical military deceptions undertaken on 5/6 June 1944 in support of the Allied 'Overlord' landings in Normandy. The operations formed the naval and air force component of 'Bodyguard', which was a series of tactical and strategic deceptions surrounding the invasion. Small boats, along with aircraft of Air Chief Marshal Sir Arthur Harris’s RAF Bomber Command, were to be used to simulate invasion fleets approaching the Cap d’Antifer, the Pas de Calais and Normandy. 'Glimmer' and 'Taxable' were specifically created to play on the German belief, amplified by Allied deception efforts over the preceding months, that the main invasion force would land in the Calais region. 'Big Drum' was undertaken on the western flank of the real invasion force to try to confuse German forces about the scale of the landings and the width of the coast area over which it was to be launched. The operations complemented 'Titanic', which was intended to confuse the Germans about the D-Day airborne operations.

It is unclear whether or not the operations were successful, this resulting from the complexity of their execution, adverse weather and the lack of response by the German forces, but it is possible that they contributed to the overall German confusion about 'Overlord' as part of the larger 'Bodyguard' scheme.

This last had been designed to confuse the Axis high command about the Allied intentions during the lead-up to the invasion. The London Controlling Section had spent some time convincing German commanders that the fictional US 1^st Army Group (FUSAG), whose existence had been fabricated by 'Fortitude South', represented the bulk of the Allied invasion force. The Allied 'story' for FUSAG was that the army group, based in south-eastern England, would invade the Pas de Calais region several weeks after a smaller diversionary landing had been made in Normandy, whereas this latter was of course the main invasion. As D-Day drew closer, the LCS moved to the planning of tactical deceptions to help cover the progress of the real invasion forces and, as well as naval operations, developed operations involving airborne forces and ground deceptions. The latter would come into effect once landings were made but the former (involving naval, air and special forces units) were used to cover the approach of the true invasion fleet.

In preparation for the forthcoming landings, Allied scientists had worked on techniques for obscuring the size and disposition of an invasion force from the German defences, which were largely reliant on the Seetakt radar system. Scientists of the British Telecommunications Research Establishment discovered that the resolution of the Seetakt radar was about 520 yards (480 m), and in order to deceive the radar proposed dropping clouds of aluminium foil (chaff, then codenamed 'Window') at 2-mile (3.2-km) intervals. The clouds would appear on the German screens as a continuous blip, essentially similar to that which would be created by an approaching fleet. The Allies also revised radio equipment to create 'Moonshine', which was intended to spoof the Seetakt signal. The Allied high command then came to the conclusion that these measures would serve not so much to mask the approaching fleet but to alert the German defences. It was decided, therefore, to combine these techniques with small groups of boats to simulate an entire invasion fleet aimed at the Pas de Calais area.

Allied planners proposed that small boats, each towing a 'Filbert' large radar-reflecting balloon and carrying both 'Moonshine' jamming and standard radio equipment (the latter for the transmission of fake signals traffic), would move toward the French coast under a cloud of 'Window'. The chaff and other countermeasures would hide the small size of the naval force, while radio traffic would play on the FUSAG story to mislead the Germans into expecting a major landing. A third deceptive force, 'Big Drum', would use radar countermeasures on the western flank of the true invasion fleet to generate confusion in German minds as to the real extent of the landings in Normandy.

'Glimmer' and 'Taxable' were very similar in this concept and operation, and were executed in the early hours of 6 June 1944 as the 'Overlord' fleet was approaching Normandy. 'Taxable' simulated an invasion force approaching the Cap d’Antifer, and Glimmer a threat to the Pas de Calais. By dropping chaff in patterns which moved slowly forward, small numbers of British bombers were able to create the illusion on the screen of the Germans' coastal radars of the advance of a large fleet. Beneath the chaff, small boats towed radar-reflective balloons and simulated the volume of radio traffic which might be expected of a large fleet. Once the Germans had responded to the 'threat' by deploying larger forces toward the coast, it was planned that the RAF would attempt to contain them in this region, and away from Normandy, by bombing bridges and roads.

The operations required precise flying in elongated circuits, and with replacement aircraft merging seamlessly into the pattern to avoid revealing gaps. Flying very precisely at low altitude, the bombers were staged at 2-mile (3.2-km) intervals parallel to the French coast: once in position they flew toward the coast for 150 seconds and dropped chaff at 15-second intervals, then turned and headed away from the coast for 130 seconds. By repeating this circuit, the wide cloud of chaff edged toward the coast to create the image of a real seaborne fleet. Each of the aircraft involved had to be modified with a hole cut into the underside of the nose to allow the large quantities of chaff to be dropped.

The aerial component of 'Glimmer' was performed by Short Stirling heavy bombers of No. 218 Squadron, carrying G-H radio navigation equipment, under the command of Wing Commander R. M. Fenwick-Wilson. The squadron was smaller than No. 617 Squadron used in 'Taxable', so no relief aircraft were available. Instead each aircraft carried two pilots who rotated flying duties. No. 218 Squadron was directed in this undertaking by a civilian physicist, Sebastian Pease, of the Operational Research Section of RAF Bomber Command, in order to ensure that the deception was as realistic as possible. In the event the effort was so convincing that German shore batteries opened fire on the 'ghost' fleet that the aircraft created, and Generalleutnant Heinrich Freiherr von Lüttwitz’s 2^nd Panzerdivision and Generalleutnant Gerhard Graf von Schwerin’s 116^th Panzerdivision were retained in the Pas de Calais for at least 14 days after the launch of the real 'Overlord' invasion away to the west in Normandy.

The naval contingent, Lieutenant Commander W. M. Rankin’s Special Task Force B, comprised 12 harbour defence motor launches outfitted with G-H, jamming equipment, radios and radar-reflecting balloons. The task force began jamming operations at about 01.00, with radio chatter following about one hour later.

'Glimmer' elicited more response than 'Taxable' from the German forces, this response including the despatch of reconnaissance aircraft sent to investigate the supposed invasion fleet. After completing its task which, unlike that of Task Force A in 'Taxable', did not include the laying mines, Task Force B returned to port by 13.00 on 6 June.

The closely related 'Taxable', 'Glimmer' and 'Big Drum' were complicated in their execution and demanded a difficult co-ordination of air and naval forces. Undertaken in poor weather conditions, 'Taxable' appeared not to have effected the desired response and failed to elicit any significant answer from the Germans. The German reaction to 'Glimmer' was more encouraging for the Allies, for the German attacks on the bomber squadrons indicated, at least to the satisfaction of RAF Bomber Command, that the Germans believed that there was a real threat. There is no evidence that 'Big Drum' elicited any specific German response.

Intelligence intercepts suggested that the German forces in the Pas de Calais area reported an invasion fleet. In addition, there were reports of the decoys being engaged by the shore batteries of the area. In an 11 June report on the operations, Lieutenant Commander Ian Cox, commander of the deception units, indicated that German forces had been convinced by the fake radio traffic. Intercepted despatches from Hiroshi Oshima, the Japanese ambassador to Germany, referred to the naval deceptions. A despatch of 8 June referred to the Calais area and stated that an Allied squadron which had been operating there had since been withdrawn.