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AN/SPS-40 Surface Search Radar

Description of the radar set, tactical-technical characteristics

Figure 1: Lightweigt antenna of the AN/SPS-40
on Bord of Lütjens class destroyer
(© 2013 www.kriegsschiffe.net)

Figure 1: Lightweigt antenna of the AN/SPS-40 on Bord of
“Charles F. Adams” – class destroyer
(© 2013 www.kriegsschiffe.net)

frequency: 402.5 … 447.5 MHz
pulse repetition time (PRT):
pulse repetition frequency (PRF): ø 257 Hz (staggered)
300 Hz (non-staggered)
pulsewidth (τ): 60 µs (long-range mode)
 3 µs (short range mode)
compressed to 1 µs (or 0.6 µs)
receiving time:
dead time:
peak power: 200 - 255 kW
average power: 2 kW
instrumented range: 370 km
range resolution: 90 m
beamwidth: β=10.5°, ε=19°
hits per scan:
antenna rotation: 7.5 or 15 rpm
(6 rpm in SPS-40 and -40A)

AN/SPS-40 Surface Search Radar

The AN/SPS-40 is the primary shipboard long-range, high-powered, two-dimensional (2D), surface and air search radar for detection of targets at long and medium ranges. It provides 10-channel operation, moving target indicator (mti), pulse compression, and high data short range mode (SRM) for detecting small, low-altitude, close-in targets. The AN/SPS-40B baseline (which includes the B, C and D radars) is designed to provide optimum performance capabilities with minimum operator interface. Special features of the AN/SPS-40B include long-range resolution and accuracy, light weight and flexible packaging for easy shipboard installation, field proven high reliability, maintainability and availability. The UHF(B) band operating frequency provides freedom from weather clutter and low vulnerability to anti-radiation missiles. The system’s digital moving target indicator provides excellent subclutter visibility and has solid-state receiver, power supplies and controls. The receivers sensitivity (minimum discernible signal) is −115 dBm with a noise figure of 4.2

The antenna reflector is a truncated paraboloid reflector of open lattice work construction, covered with a wire screen to reduce weight and wind resistance. The dual feed includes the primary radar section and an integral identification friend-or-foe antenna. The primary feed is a slot type, it has a tuned cavity and flared shape to ensure proper illumination of the reflector. The reflector then forms the RF energy into a fan shaped beam with a 19° vertical beamwidth and 10.5° horizontal beamwidth. The antenna has a gain of 21 dB at a sidelobe attenuation of 27 dB.

The AN/SPS-40 solid-state transmitter is replacing the Tetrode tube transmitter of the surveillance radar, and the new version is designated AN/SPS-40E. The nominal 250 kW output of the transmitter is achieved by combining in parallel 112 power amplifier modules arranged in two groups, 56 each. The stripline approach is used in the design of the large output 56:1 combiners. When compared with their tube counterparts, the AN/SPS-40 solid-state transmitters provide improved performance and superior reliability, availability, and maintainability. (The older tube version was in practice extremely sensitive to the vibrations caused by the ship’s artillery.)

The solid-state transmitter architecture is highly redundant. It is predicted to have a 90 per cent probability of maintenance-free operation for 90 days with no more than 11 per cent projected reduction in radar range performance. The 112 transmitter modules are identical and interchangeable, as also are the power supplies. In the event of component failure, the system undergoes a gradual and graceful degradation in transmitter output. It remains fully operational and capable of detecting targets.

The transmitter solid-state technology offers inherent tactical flexibility. For example, output power is adjustable. As a result, ships can reduce their susceptibility to detection while maintaining substantial air surveillance capability. If the tactical situation requires emission control conditions, the solid-state transmitter will respond instantly. Similarly the transmitter will immediately radiate at full power with just the touch of a push-button. Pulse-to-pulse frequency diversity is also provided. A unique automatic leveling control system greatly reduces the need for maintenance actions. This system automatically senses and compensates for degradations in transmitter module performance.

The AN/SPS-40 is operational e.g. on Bangladesh Navy’s ship Somudra Joy (Hamilton-class). (However, the antenna is no longer visible in pictures taken after 2012.) Most of the AN/SPS-40 radars were replaced by the AN/SPS-49(V) radars in the late 1980s and early 1990s.

Versions and improvements: