Stabilized Automatic Bomb Sight The Stabilized Automatic Bomb Sight , or SABS , was a Royal Air Force bombsight used in small numbers during World War II . The system worked along similar " tachometric " principles as the more famous Norden bombsight , but was somewhat simpler , lacking the Norden ' s autopilot feature . Development had begun before the War as the Automatic Bomb Sight , but early bomber operations proved that systems without automatic stabilization of the bombsight crosshairs were extremely difficult to use under operational conditions . Before a stabilizer for the ABS was complete , the simpler Mark XIV bomb sight , also stabilized , was in widespread use and the SABS was not ordered into production to replace it . The SABS briefly saw use with the Pathfinder Force before being turned over to No . 617 Squadron RAF , the famed " Dambusters " , starting in November 1943 . This squadron ' s Avro Lancaster s were undergoing conversion to dropping the Tallboy bomb as a precision weapon , and required the higher accuracy of the SABS for this mission . In this role the SABS demonstrated superb accuracy , routinely placing bombs within of their targets when dropped from about altitude . The system remained hand built throughout its history and was produced in small numbers . In the end , the 617 would also be the only squadron to see operational use of the SABS , using it with the Tallboy and the larger Grand Slam . Some Avro Lincoln s also featured the SABS , but saw no service use . Development . Vector bombsights . The basic problem in bombing is the calculation of the trajectory of the bomb after it leaves the aircraft . Due to the effects of air drag , wind and gravity , bombs follow a complex path that changes over time - the path of a bomb dropped from 100 meters looks different from the one when dropped from 5 , 000 meters . The path was too complex for early systems to calculate directly , and was instead measured experimentally at a bombing range by measuring the distance the bomb traveled forward during its fall , a value known as the " range " . Using simple trigonometry , this distance can be converted into an angle as seen from the bomber . This angle is measured by setting iron sight s to this angle , known as the " range angle " or " drop angle " . The bomb aimer drops the bombs when the target passes through the crosshairs . Such systems were common during World War I . A basic system like this is missing one important factor , the effect of winds on the speed and course of the aircraft . The bombing range numbers are taken in still air , but in a wind these numbers are no longer correct and the bombs will fall off - target . For instance , a wind on the nose will reduce the ground speed of the aircraft , and cause bombs to fall short of the target . Some early bombsights had adjustments that could account for wind directly on the nose or tail , but this seriously hampered operational use . One that could indicate the correct course to fly to account for any wind would offer a major advantage . Solving for the general windage is a common problem in air navigation and was semi - automated in the Course Setting Bomb Sight of late WWI vintage . To use such a " vector bombsight " , the bomb aimer first requires an accurate measurement of the speed and direction of the wind . This was taken through a variety of methods , often using the bombsight itself as a reference . When these figures were dialled into the system , the calculator moved the sights fore or aft to account for the wind , as well as side - to - side to indicate the proper approach angle . The accuracy of such systems was limited by the time taken to measure the wind in advance of the bomb run , and the care taken to calculate the results . Both were time consuming and error - prone . Moreover , if the measurement was incorrect or the wind changed , it was not obvious during the approach what should be adjusted to correct for this - changes to either speed or direction would have similar visual effects but only one would place the bombs correctly . Generally any inaccuracies had to be left dialled in , as attempts to adjust it using the multi - step calculation procedure generally made matters worse . Even without such problems , a long bomb run was needed to ensure the aircraft was approaching along the correct line as indicated by the sights , often several miles long . Tachometric designs . During the 1930s , advances in mechanical computer s introduced an entirely new way to solve the bombsight problem . These sorts of computers were initially introduced for naval uses around the turn of the 20th century , later examples being the Admiralty Fire Control Table , Rangekeeper and Torpedo Data Computer . Fed a variety of inputs , such as the angle to the target and its estimated speed , these systems calculated the future position of the target , the time that the ordnance would take to reach it , and finally the angles to aim the guns in order to hit the target . They used a system of iterative improvements for the estimated values to calculate any measure that could not be made directly . For instance , it is not possible to measure the speed of the target directly . For this , an initial estimate was entered along with the measured location of the target relative to your own ship . The calculator then output the estimated position of the target based on the motion from this initial location . After some time the position of the target was re - measured , and any error between the calculated and measured value was corrected for by updating the estimated speed . After a few such adjustments the positions no longer diverged over time , and the target ' s speed was accurately revealed . This system of progressive estimation is what made the concept adaptable to the bombsight role . In this case the target was in a fixed location and the unknown measurements were the windspeed and heading . Using the same basic concept by locating the target and estimating the unknown values , the sight was initially pointed at the target and the calculator estimated the future location relative to the bomber . If the two diverged over time , or " drifted " , the estimates for windspeed and direction were updated until the drift was eliminated . Naval versions of these calculators were typically centralized , with the operators calling out information to the various weapons and sighting stations . In contrast , in the bombing role the calculator , sighting and bomb aiming were implemented in a single instrument and its output drove the sight directly . If the initial estimates were incorrect , the sight would slowly drift away from the target , which the bomb aimer corrected for by updating these values and driving the sight back onto the target . This approach to measuring the wind had two significant advantages . One was that the measurement was taken while on the approach to the target , which eliminated any problems with the winds changing after the measurement . Another advantage , perhaps more important , was that the measurement was made simply by aligning a sight on an object on the ground though a small telescope or reflector sight . All of the complicated calculations and setup of the vector designs were eliminated . These " tachometric " or " synchronous " bombsights were an area of considerable research during the 1930s . Norden . The US Navy began a bombsight development program with Carl Norden during the 1920s , initially focused on a gyroscopically stabilized sight of otherwise unadvanced design . The Navy had found that bombsights were almost always used with the sights not properly levelled with respect to the ground , so any angles measured through the sight were wrong . An error of only a few degrees represents an error of hundreds of feet when bombing from high altitudes . Stabilization , which automatically levels the sight , was found to roughly double overall accuracy . Norden ' s original design was a conventional system known as an " equal distance sight " that was attached to his gyroscopic stabilizer system . The Navy asked him to replace the bombsight with a tachometric design on the same stabilizer . He initially refused , but eventually took a sabbatical in Europe and returned with a workable design that was delivered for testing in 1931 . The Norden bombsight demonstrated itself able to drop bombs within a few yards of its targets from altitudes between . The Navy saw this as a way to attack ships from level bombers at altitudes outside the effective range of the ship - borne anti - aircraft gun s . The US Army Air Corps would go on to develop an entire strategic bombing concept based on using the Norden to attack factories , shipyards and other high - value targets . News of the Norden filtered to the Air Ministry in 1938 , shortly after they had began development of their own Automatic Bomb Sight ( ABS ) . The ABS was similar in concept to the Norden and offered similar accuracy , but it lacked the stabilization system and was not expected to be available before 1940 . Concerted efforts to purchase the Norden ran into continual problems and increased frustrations between the two erstwhile allies . These negotiations were still ongoing , without result , when the war began a year later . Mk . XIV . In early operations , RAF Bomber Command concluded that their existing bombsights - updated versions of the WWI - era CSBS ' s - were hopelessly outdated in modern combat . During low - level attacks the bombers had only moments to spot the target and then maneuver for attack , and often had to dodge fire all the while . When the bomber was turning , the bombsight , fixed to the frame of the aircraft , faced the wrong direction and couldn ' t be used to adjust the approach . On 22 December 1939 , at a pre - arranged meeting on bombsight policy , Air Chief Marshal Sir Edgar Ludlow - Hewitt stated flatly that the CSBS did not meet RAF requirements and asked for a bombsight that would allow the bomber to take any sort of evasive action throughout the bomb run . This , in effect , demanded the use of stabilization in order to allow the bomb aimer to continue making adjustments while the bomber maneuvered . At that time the ABS was still at least a year away from production , and lacked stabilization . Although this feature could be added , it would push the in - service date back even further . The Norden was considered a good solution , but the US Navy still refused to license it or sell it for RAF use . Both offered more accuracy than was really needed , and as both were not going to be available immediately , in 1939 the Royal Aircraft Establishment started examining a simpler solution under the direction of P . A . M . Blackett . These efforts produced the Mark XIV bomb sight . The Mk . XIV moved the calculator off of the bombsight itself to a separate box , which also included instruments that automatically input altitude , airspeed and heading , eliminating manual setting of these values . In general use , the bomb aimer simply dialled in estimates for the wind direction and speed , and set a dial to select the type of bomb being used , and everything from that point on was entirely automated . Although relatively complex to build , production was started in both the UK and US , and the new design quickly equipped most of Bomber Command by the time of the large raids starting in 1942 . Although it was a great improvement over the earlier CSBS ' s , it was by no means a precision sighting system - later being referred to as an " area sight " . SABS . Although the Mk . XIV served the RAF ' s basic needs , the requirement for a more accurate sight remained . This need became more pressing as the earthquake bomb concept was pushed forward , a system that demanded more accuracy than the XIV could provide . In 1942 the Norden was still not available for license , in spite of it being used on US bombers arriving in the UK , so a new design was needed . In response , earlier concepts of mating the ABS to a new stabilizer platform were carried out to produce the SABS . Like the Norden , the stabilizer was separate from the bombsight proper , although in the case of SABS the stabilizer moved the entire ABS sighting system , rather than just the sighting system as in the Norden . Unlike Norden , the SABS ' s stabilizer did not serve double - duty as an autopilot , as RAF bombers were already equipped with one . Instead , directional corrections from the bomb aimer were sent to a pilot direction indicator in the cockpit , similar to the original Norden models . Operational use . Small numbers of SABS became available in early 1943 and were initially sent to No . 8 Group RAF , the " Pathfinder Force " . They used them only briefly before turning their examples over to No . 617 Squadron RAF , who were in the process of converting to the earthquake bomb and required higher accuracy that the Mk . XIV could provide . SABS was used operationally for the first time by No . 617 on the night of 11 / 12 November 1943 for their attack on the Anthéor railway viaduct at Saint - Raphaël , Var in southern France . No hits on the viaduct were recorded by any of the ten Blockbuster bomb s . SABS was used both for direct aiming during daylight missions , as well as referencing on target indicator s dropped by other aircraft flying at much lower levels . In these cases the accuracy of the drops was dependant on the accuracy of the marking , which varied . For instance , during attacks on the V weapon launch site at Abbeville on 16 / 17 December 1943 , Tallboys were dropped with a circular error probable of only , a superb result , but the markers were from the target . Better results followed ; on the night of 8 / 9 February 1944 , Wing Commander Leonard Cheshire visually dropped markers on the Gnome et Rhône factory in downtown Limoges , 11 Lancasters then dropped 10 Tallboys directly on the factory with the last falling in the river beside it . The factory was knocked out of the war , with few or no civilian casualties . General accuracy improved dramatically as the crews gained proficiency with the system . Between June and August 1944 , 617 recorded an average accuracy of from , a typical bombing altitude , down to at . Between February and March 1945 this had further improved to , while Air Marshal Harris puts it at only from . Two other precision bombing squadrons formed up during this period , but used the Mk . XIV . These squadrons were able to achieve , an excellent result that offered performance roughly equal to the early SABS attempts , and far outperformed the average by the more famous Norden . The SABS ' best known role was in the sinking of the German battleship on 12 November 1944 , by a combined force from 617 and No . 9 Squadron RAF . Known officially as Operation Catechism , 30 Lancasters attacked the " Tirpitz " at altitudes from . At least two bombs from 617 hit the Tirpitz , causing it to capsize in the fjord it was hiding in . Another celebrated attack was made during daylight on 14 June 1944 against the E - boat pens at Le Havre . One bomb penetrated the roof of the heavily guarded base , knocking it out of the war . Tiger Force . As the war in Europe wound down , plans were made to start a strategic bombing campaign against Japan as Tiger Force . Requiring long range , Tiger Force planned on using the new Avro Lincoln bombers , along with other designs who ' s range would be extended using aerial refuelling . As less than 1 , 000 SABS had been delivered , supplies for the new force were hard to come by . A great debate broke out in the RAF about the relative merits of the two bombsights ; although the SABS was more accurate , the Mk . XIV was generally easier to use and offered greater tactical flexibility . In the end the point was moot , as the war ended before Tiger Force was deployed . Those Lincolns that were equipped with SABS , including those of 9 and 44 Squadron , continued use in the post - war era . These exited service along with those bombers , with their replacement by the English Electric Canberra and other types . These were fitted with the Mk . XIV instead of the SABS , if they had an optical sight at all . Comparison with the Norden . Operationally the SABS far outperformed the Norden . In 1945 , the 8th Air Force was demonstrating accuracy of about circular error probable , nowhere near the performance of the SABS - equipped 617th , which reached during this same period . However , the 8th was also flying at higher altitudes on missions deep within Germany , facing considerably stronger defences and bombing en - mass . Moreover , the 617 was a crack unit of veteran crews . Description . The SABS consisted of three primary parts , the bombsight itself , also known as the " range unit " , the stabilizing system , and the " bombing directional indicator " for the pilot and other indicators . Range unit . The range unit was the heart of the SABS , and the earlier ABS . This was a mechanical calculator with three internal functions . The first calculated the angular rate of motion of a stationary location on the ground , and output this to a reflector sight mounted on the left side of the bombsight . The key component of this system , and other tachometric designs , was the ball - and - disk integrator . This is a form of continuously variable transmission that allowed an output shaft to be driven at a controlled speed relative to an input . The input was normally attached to some sort of value to be measured , say the height of water in a sluice , and as it moved , the output disk sped or slowed . The total number of turns of the output shaft was an integrated version of the input . The SABS version of the integrator worked with two values , one for the height over the ground , and the second for the airspeed . Both used a ball - and - disk system , the output of the height disk feeding the input of the airspeed , both driven from a common electrical motor . The range control wheel was fed into the speed calculator , adjusting it in a similar fashion . The two other calculations concerned the ballistics of the bombs . To account for the effects of terminal velocity and thus the actual time it took for the bombs to reach the ground , the " bomb class " input moved a pointer over the altitude gauge . Selecting the altitude against this pointer changed the height setting to account for this portion of the ballistics problem . After bombs are released , drag causes them to fall behind the motion of the aircraft . By the time they reach the ground the aircraft is hundreds or thousands of feet in front of the impact point . This distance is known as " trail " . The SABS adjusted for trail by simply tilting the entire range unit aft on a trunnion , rather than sending adjustments into the calculator itself . If the aircraft is crabbing to adjust for any winds from the side , this also causes the trail to move to the side - the bombs are falling straight but the aircraft is actually flying sideways into the wind and imparts this velocity on the bombs . To account for this " side trail " , the sight was rotated to one side or the other . The range unit also contained the bomb release mechanism . This was an electrical contact system attached to the same output shaft as the sight , and a second contact connected to the cam - based trajectory calculator . The two contacts would approach each other as the bomber flew towards the target , and completed the release circuit at the right moment for the drop . The same system also included a set of contacts that connected earlier , turning on a red lamp on top of the bombsight and another in front of the pilot . These remained lit through the approach , for about ten minutes , and turned off the instant the bombs were released . The sight was driven electrically from the aircraft ' s 24 Vdc power supply . This powered both the sight rotation motor as well as various lamps and the electrical contacts that triggered the bombs to drop . Stabilizer . The stabilizer unit consisted of two parts , a box containing two gyroscopes , and a pneumatically powered frame that kept the range unit flat in comparison to the ground . In modern terminology this would be known as an inertial platform . One advantage of the SABS compared to similar devices like the Norden was the automatic " erection " system . Gyroscopes have no preferred direction of rotation and will hold whatever angle they initially started up in . In the Norden , adjusting the gyros to an absolute " up " required a time consuming operation that could take as long as eight minutes . The SABS solved this with a pendulum mechanism consisting of a weight on the end of an L - shaped bracket . The weight caused the bracket to be pulled vertically , and if the gyro was not level , the bracket pressed against the side of the gyro ' s shaft , forcing it in the appropriate direction . The gyros were connected to air valves on an associated supply line . This lowered or raised the pressure on one side of a servo piston , the other side being attached to the original supply without passing through the valve . Any precession of the gyros , due to movement of the aircraft , caused the pistons to move due to the differential pressure . This motion was smoothed by an oil - filled dashpot , one for each of the three servos . The entire ABS sat within the stabilized frame that was powered by the servos . The platform had fairly wide range of motion , between 20 and 25 degrees off horizontal . This allowed it to track properly through a wide range of motions . The stabilizer was powered by a 60 lb compressed air feed , fed from the same unit that also powered the automatic pilot . The system took considerable time to stabilize , the vertical gyro taking as long as 15 minutes to reach full speed . Using the SABS . Using the SABS was a relatively straightforward procedure ; although a number of steps were involved , these took place in sequence and left the bomb aimer with relatively easy tasks and low workload on the final approach . Initial setup . Prior to the mission , or early on in flight , bomb data was entered on two settings dials on the top of the range unit . These set the " trail scale " and " bomb class letter " , estimating the amount the bomb would slow in forward motion ( trail ) and how quickly it would reach the ground due to the effects of terminal velocity ( class ) . Once set , these settings could be ignored . During the approach . At least fifteen minutes before the bomber reached the target , the pilot would open valves to supply air to the bombsight . The bomb aimer would then start up the stabilizer platform , and wait as the gyros reached full speed . At this point the stabilizer platform was turned on and the bombsight was ready for use . As the bomber levelled off on its final approach , the bomb aimer would then dial in the altitude and air speed to the ground speed calculator , based on values provided by the pilot or navigator . He could also dial in approximate values for the wind speed and drift , typically provided by the navigator . Estimating these somewhat simplified the bomb run . If the bomber was releasing a " stick " of bombs , the bomb aimer was instructed to use the " false height " method to control the timing of the drop . This was nothing more than the deliberate mis - entry of the altitude in order to drop early . During the run . At some point the target would become visible to the bomb aimer , and he would use the range control wheel to rotate the reflector sight to point towards the target . Two range wheels were connected to the same shaft , a large one for fine movements , and a much smaller one that could be rapidly spun for this initial target pickup . Once the target was roughly centred in the sight , the change - over switch was thrown and the sight started rotating to track the target . This started the official bomb run . As the bomber approached the target , any mis - estimate of the wind would cause the sight to drift past or under the target . Further adjustments of the fine - gained range control wheel would bring the sight back in - line with the target , as well as update the estimated windspeed . Typically only a few adjustments like this were needed to cancel out any range drift . If the bomber was to one side of the target , or drifting away from the proper approach , the line control wheel was used to rotate the entire sight to place the crosshair back on the target . Simply flying at that angle will not bring the bomber back along the proper approach , it will cause the bomber to fly " parallel " to the correct line . In order to re - capture the approach , the bomber has to turn " past " the correct heading and erase the accumulated error , then turn back onto the proper line . To accomplish this , the SABS multiplied the error angle by four times before sending it to the pilot ' s display . By chasing the dial , the pilot automatically overcorrected the heading , bringing the aircraft back towards the proper approach . As the bomb aimer updated measurements to the drift angle , it would reduce this error back to zero . As in the range case , only a few adjustments were needed to cancel out any sideways drift . During and after the drop . At this point the bombsight now has an accurate measurement of the true motion of the aircraft . This does not imply that it is accurately measuring the wind , as the initial inputs for airspeed or altitude might have been wrong . But this makes no difference in terms of the drop ; as long as the sight ' s crosshairs remain on target , the motion over the ground is correctly measured and the bombsight will operate correctly . Setting the bomb type and trail moves a cam within the unit to a fixed angle . This cam carries several electrical contacts . As the bomber approaches the target , a metal ridge attached to the sight rotation shaft depresses the first contact , turning on the drop timing lights . Further motion causes the bombs to release . A final stop turns off the motor when the sight is fully vertical , if the bomb aimer has forgotten to do so . Measuring wind . The SABS also offered a secondary function as a windage measurement tool for accurate navigation . By simply tracking any suitable object on the ground with the range and line control wheels the wind speed and direction would be returned on the range unit ' s dials . Several methods were outlined for use at different altitudes and operational conditions .