The E160 Weather Radar system was a major evolution and update of the E120 system and was to be the last 'valve based' weather radar developed and sold by the company.
Development started soon after the E120 was launched in the autumn of 1955 and concentrated on totally redesigning the 'scanner unit' (which to a certain extent on the E120 had been a carry over from the E38) to remove weight by eliminating 'full platform stabilization' and relying instead on the host aircraft gyro's for line of sight and pitch etc but retaining the original 'roll gyro'. This allowed the large 'roll ring' of the E120 to be cut in half.
The second element of the scanner redesign was to redesign the gearbox to allow the scanner dish to rotated through 90 degree's to facilitate 'contour mapping'.
The third element was the development a 'Drift Unit', which was offered as an optional extra for those customers who had requested the development of this facility.
The E160 system technical specification was substantially the same as for the E120 however due to the use of contour mapping, a new cockpit control units was needed to go with the new gearbox and the Servo/Sync was also new, thus the E160 System comprised 6 units, as follows:-
- The nose mounted Scanner unit E152
- The avionics tray mounted Transmitter/Receiver E122
- The avionics tray mounted Servo/Sync Unit E161
- The cockpit mounted Indicator Unit(s) E124
- The cockpit mounted Control Unit E162
- The avionics mounted Back Plate Junction Box E163
Additionally, for those customers who required the 'drift unit', this unit was E153.
Of interest to note is that as with the E120 system, the E160 system was designed to fit 'British Standard 'aircraft racking' since this was the standard of the day for all 'British' aircraft and it was only with the later E190 system did 'ARINC' racking become the norm (thus being able to offer weather radar to fit American aircraft).
The E152 Scanner Unit
The above photograph is the only know photograph of an E152 scanner unit, which is in the hands of a Swiss collector. Sadly this unit does not have the scanner dish. Notwithstanding this, comparison to the E121 scanner unit shows clearly the half 'roll' ring.
This scanner design used separate axes for azimuth scan and roll stabilization while pitch and tilt were combined. The result of this was that the stabilization performance of the three axis configuration was almost as good at the fully platform stabilised E121 with the advantage of only having half the weight (27Lb instead of 55Lb) despite being technically more complicated by virtue of the dish rotation gearing and had the added benefit of allowing a larger scanner dish to be used at 24 inches instead of the 18 inch on the E121 (the larger dish providing better target resolution).
The scanner dish was provided with 'mapping vanes' as shown on this E190 dish (below).
In the normal 'weather radar mode' the dish was used in this position. i.e. with the vanes vertical and transparent to the microwaves from the aerial.
In the mapping mode, the dish rotated through 90 degree's anti-clockwise so that vanes are horizontal at the top and reflect the microwaves.
Photograph of an E160 scanner dish, which was damaged by an 'in flight' radome failure of a VC-10.
'While flying over the Persian Gulf on its way to the UK, Standard VC10 G-ARVB ran into a violent storm between 5000 and 10000 feet which contained hailstones of very large size. The damage that was done to the airliner was severe, apart from dents in all the leading edges the radome covering the radar antenna of the aircraft separated from the fuselage, hitting the top of the tail. The resulting blunt front end of the aircraft created strange airflows around the nose section, rendering the pitot tubes which are mounted just behind the cockpit on the fuselage sides completely useless. Normally the airflow into these tubes provides the airspeed information, but in this case the airspeed indicators were useless. Obviously Captain C. Ditmas radioed for assistance. To get the airliner down safely a 208 Sqn. Hunter from Muharraq was scrambled to escort the BOAC aircraft. Piloted by Flt. Lt. Ray Taylor it flew alongside the airliner, providing airspeed information over the radio, and thus enabling the VC10 to make a safe landing at Bahrain
Photograph and story reproduced from www.VC10.net
Contour mapping briefly explained
With contour mapping selected and the dish rotated, the beam is shaped and directed downwards by the vanes so that a slant angled beam scans the ground ahead as shown in the illustration below. The beam shape is such that the power returned is independent of the range and is known as a Cosecant squared beam.
Since the beam is directed down, the 'slant range' is less than the normal scanning range due to the diffuse nature of the beam (although the radiated power is the same as for the normal conical beam) and the return picture is slightly distorted particularly on short-range targets typically on the 20Nm range.
In simple terms, the beam and the reflected beam can be likened to walking out at night and shining a torch beam downward and seeing objects protruding from the ground, which are shown up in relief whereas objects flat on the ground do not show up (water shows up black).
Contour mapping was (is) a good aid to navigation since it shows up recognisable features, which can be identified by reference to a map as shown by the example below. (Remember this was the 1950's and modern navigation systems such as inertial navigation and GPS had not yet been invented for use on aircraft.)
This screen may look jumbled and not make a lot of sense in isolation, however with the aid of a map, it becomes easily recognisable.
The aircraft is flying westerly towards Southern Ireland (approximately towards Waterford) and is just north of Lynmouth in North Devon. Approximately 15 degree's to port and 15 Nm range can be seen Lundy Island and to starboard can be seen Carmarthen Bay and the Pembroke peninsular. The small white blobs are shipping.
The Drift Unit briefly explained
In addition to contour mapping as an aid to navigation, the effect of wind on the aircraft also needed to be understood and allowed for so that the aircraft was not blown off course.
During WW2, typically observing known landmarks and calculating actual to required course gave a calculated drift angle. Over sea, this was done by dropping a 'smoke marker' and observing the wind direction – both of which required sight of the ground otherwise the navigator had to rely on 'dead reckoning', which sometimes resulting in large errors (100 Nm off course was not unknown).
In the 1950's there were radio direction beacons (typically Loran) to aid the navigator (all large aircraft carried a navigator in those days), however there were still large areas of the world particularly in Africa and India where beacons were few and far between and coverage sparse. Where this occurred, the navigators had to rely on little better than WW2 techniques.
The EKCO 'drift unit' was designed to materially assist navigators overcome these shortcomings by giving them a drift angle measurement taken from the radar system.
The principle of operation used the 'doppler' information contained within the radar echo's from the ground and in simple terms the drift unit measured the 'doppler shift.
Without going into the maths, assuming there was no drift on the aircraft, the frequency of a radar return from a ground target dead ahead will have no Doppler shift however if there is a sideways wind action (known as 'crabbing') pushing the aircraft off course, then the return signal will have a frequency shift (Doppler shift) to that transmitted.
E153 Drift Unit
On the display on the drift unit, which is slaved to the main azimuth control of the scanner, the Doppler effect shows up as a spike on the small 'A' scope and builds in intensity unit the maximum angle of drift is reached.
In 'Drift mode', the scanner azimuth motor is turned off and, as the scanner is manually turned in azimuth towards the line of drift, the spike falls and when it reaches minimum (which indicates that the scanner is looking down the actual track line), the drift angle can be read off of the azimuth dial.
While the aircraft and customers of the E160 is not known, this set remained in production until the transistorised E190 was introduced and it is known that many of the customers who had the E120, upgraded to the E160 system.
1957 newspaper advert for Farnborough Air Show extolling E160
|
