Ground Radar,
Height finding equipments

The earliest method of height finding (elevation finding E/F) which is used in GH and GCI is unsatisfactory for several reasons. It uses the wave reflected from the ground or sea in front of the Station and is therefore dependent on the site, it is not highly accurate and (in CH) requires troublesome and costly calibrations. Further, the method is restricted in practice to elevation limits below or above which it yields ambiguous readings. Consequently there has existed, from the first, a requirement for a method of measuring height independent of ground reflections.

This in effect demands intense beaming in the vertical plane with the ability to move the beam through a range of elevations. Consequently the realisation of the method waited upon the development of radar practice on the shorter wavelengths of 50 centimetres and 10 centimetres although an early equipment VEB (Variable Elevation Beam) used a wavelength of 1½ metres.

These height finding equipments are designed for use as ancillary devices to other systems such as CHL, GCI and CHEL. They avoid the disadvantages of the original (ground reflection) method. The principal equipments are described in the following paragraphs.
Type
Function Variable Elevation Beam
RAF designation
Year of issue
Description Intended for use with CH stations on sites unsuitable for the reflection method of height finding.  It employed a vertical stack of 56 pairs of end-fed half-wave aerials, supported on a CH receiving tower.  A fan-like polar diagram, beamed vertically, was produced and this could be swung to different elevations by introducing predetermined phase shifts in the signals fed to the elements of the array.
Frequency 200MHz
Pulse recurrence frequency
Transmitter power
(peak pulse)
Main items
Remarks
Type Ames type 13
Function Centimetre Height - CMH
RAF designation AMES type 13 Mk II (MGRI-5555), A mobile equipment used as an ancillary to any plan position equipment (CHL, GCI. etc.). It comprises an aerial vehicle and an operations vehicle containing the display unit type 58 (range-elevation scan (H/R tube) together with a PPI display on a second tube).

Ames type 13 Mk III and IV, Similar to Mk II but with a display unit type 5, which is a universal display unit for use with all beamed radar sets. It comprises two PPI tubes and an H/R tube.

Ames type 13 Mk V, In this version the horn-fed cheese aerial of the earlier marks is replaced by a cylindrical parabola (20ft. x 5ft.6in.) fed by a linear array of shunt coupled slots cut in the narrow face of the 3in. x 1in. wave guide.
Year of issue
Description

The aerial system is the same as that of the Type 14 equipment rotated through a right angle. Thus, the rectangular aperture of the paraboloid is set with its long dimension (20 feet) in the vertical plane.  Since a pair of cheese aerials are used and are clamped together to form a single unit the horizontal dimension is 3 feet 1 inch. The beam widths, to first zero, are 1¾° and 6½°, vertically and horizontally respectively. The aerial may be turned in azimuth as required by the associated GCI or CHL, and sweeps in elevation between -1° and +20°.

Signals are displayed, by intensity modulation of a cathode ray tube (H/R tube). Constant height curves which are almost straight lines are drawn on the tube face so that the height of the aircraft may be read directly from the tube with an accuracy of +/-500 feet up to 60 miles range The maximum operational range on a Mosquito aircraft is 70 miles. Height finding is obtained from 0° (approx.) to 20° of elevation.
Frequency 3GHz
Pulse recurrence frequency
Transmitter power
(peak pulse)
Main items
Remarks
Type Ames type 20
Function Decimetre Height - DMH
RAF designation Ames type 20 Mk I (FGRI-5537), fixed equipment to provide accurate height finding at the GCI stations.
Ames type 20 Mk II, mobile version
Year of issue
Description Working on a frequency band 500-600 Mc/s and em­ployed a large paraboloid 30 feet in height by 12 feet broad which could be rotated about a vertical axis and tided about a horizontal axis. The beam of radiation, 9° broad horizontally, and 4½°  vertically, could thus be directed on to an aircraft whose height is obtained from the elevation of the beam at signal maximum, and from the radar range. As the aerial system is inconveniently large the equipment has been superseded by CMH equipment.
Frequency 590 MHz
Pulse recurrence frequency
Transmitter power
(peak pulse)
Main items
Remarks Mk I, only one buildt?
Type Ames type 24
Function Centimetre Height - CMH
RAF designation AMES type 14 (FGRI-5595). A fixed long-range S-band height finding equipment which is ancillary to a Fighter Direction Station (AMES Type 16)
Year of issue
Description
Frequency 3 GHz
Pulse recurrence frequency
Transmitter power
(peak pulse)
Main items
Remarks
Type Ames type 28
Function Centimetre Height - CMH
RAF designation AMES type 28 Mk I (ATRI-5635). An air-transportable and tropicalised version of the CMH which can also be used as a GCI. It is suitable for use as a GCI in mountainous country and in the Pacific theatre.
Year of issue
Description
Frequency 3,35 GHz
Pulse recurrence frequency
Transmitter power
(peak pulse)
Main items
Remarks
Type Ames type 66
Function
RAF designation
US designation SCR-615 = Ames type 66 Mk I
SCR-615 A = Ames type 66 Mk II
SCR-615 A = Ames type 66 Mk II
Year of issue
Description An American S-Band height finding equipment.
Frequency 3 GHz
Pulse recurrence frequency
Transmitter power
(peak pulse)
Main items RF unit BC 1241
Modulator BC 1242
Local oscillator BC 1243
Indicator BC 1244, 1245, 1249
Receiver BC 1246, 1358
Pre-amplifier BC 1247
Control unit BC 1248
Range error unit BC 1250
Synchronizer BC 1251
Console BC 1252
Short gate receiver control BC 1279
Error indicator oscilloscope BC 1295
Transmitter BC 1347
Indicator BC 1356, 1359
RC 245, RC 248
Remarks