AN/FPS-117

A key problem for radar systems of the 1950s was the introduction of the carcinotron, a microwave-producing tube that could be rapidly tuned across a wide bandwidth. Scanning rapidly, it appeared to be a constant radiator across an entire band, creating a powerful form of barrage jamming. To overcome this form of jamming, radars of the era were extremely powerful; the AMES Type 85 of the Royal Air Force sent out pulses of at least 8 MW in an effort to overcome the jammer's signal. Systems of such power have a number of practical downsides; cooling such a system is not a small endeavour, and the physical size of the transmitter tubes precludes it from being even partially mobile.

During the 1950s, variations on the array antenna were being actively explored by many designers. In these systems, a large number of small antennas work together to produce a single output beam. By introducing small delays, using devices known as phase shifters, the output of the beam could be steered electronically. This offered the possibility of rapid scanning without mechanical movement, which made the systems much easier to produce in a mobile form. The original Marconi Martello offered the same detection capability as the Type 85 from a "transportable" design using six prime movers.

Most early phased array systems used a single transmitter tube, but experiments where every antenna elements had their own transmitter, were underway. In these "active array" systems, one could use the individual transmitters to produce multiple beams pointing in different directions, which would allow, for instance, some beams to continually track targets while others continued to scan the sky. However, such systems were extremely expensive until the introduction of solid-state transmitter modules.

While solid-state systems reduced the price of an antenna array, they did not offer nearly the same power output, even in aggregate. In previous designs, radars typically sent out extremely powerful but very short pulses of signal. The signals were short in time in order to provide reasonable range resolution. Given that the solid-state systems could not reach these power levels, longer pulses would have to be used so the total energy reflected from the target was similar. To regain range resolution, the relatively new technique of pulse compression was widely introduced.

By the late 1970s all of these technologies were maturing, and the possibility of fully active radars at reasonable price points emerged for the first time.

The DEW line system across northern Canada and Alaska was built in the 1950s using 1950s-era AN/FPS-19 radars. These used two 500 kW magnetrons on huge always-turning parabolic antenna systems and rooms filled with tube-based electronics to drive them. The systems required constant maintenance by on-site staff and was enormously expensive to operate.

Desiring a much simpler, and less-costly, system, in 1977 the Rome Air Development Center (RADC) began the "Seek Igloo" project to find a replacement for the FPS-19 that would require less power and would run for extended times without maintenance. In 1980, General Electric won the contest with their GE-592 design, and the final design was accepted by RADC on 30 September 1983 and passed acceptance tests that year.

Seek Igloo was officially concerned only with the radars in the Alaska area, while Seek Frost addressed the rest of the DEW line. However, the term is widely used to describe the entire development project. Seek Frost also included the shorter-ranged AN/FPS-124 as a gap filler, which was not needed in the Alaska area.

Conversations among NORAD commands about the DEW line had been ongoing for some time, and the Canadian Air Force was aware of the plans to convert to the new radars. As part of the 24-hour whirlwind Shamrock Summit in 1984, Canadian Prime Minister Brian Mulroney and US President Ronald Reagan signed an agreement to create the North Warning System to replace DEW. Implementation of the North Warning System has resulted in a reduction in operations and maintenance (O&M) spending by up to 50% compared to DEW. Shortly thereafter, the Air Force purchased another FPS-117 to replace the aging AN/FPS-67 radar at Berlin Tempelhof Airport.

During this time, the Royal Air Force had grown concerned about the vulnerability of their Linesman/Mediator radar network. Designed in the era of the hydrogen bomb, the system was entirely unhardened as it was believed such efforts would be futile against multi-megaton attacks. As the strategic balanced changed and conventional attacks became more likely, Linesman appeared trivially easy to defeat. The RAF planned to replace Linesman with the IUKADGE network using the Marconi Martello radars, but as this system dragged on they eventually purchased two AN/FPS-117 as well.

Further sales soon followed, and the system remains in production as of 2020. Over 120 examples have been produced and are operated by 15 countries.

AN/TPS-59. The individual transmitter/receiver modules are easily seen in this image.

As the first FPS-117 systems were being tested, the US Marine Corps sent out a tender for a air-warning radar with long range and good jamming rejection. In contrast to the Air Force, the Marines required that the system be "transportable", that is, capable of being moved between locations. GE won the contract with a modified version of the FPS-117, the TPS-59.

The TPS-59 was essentially a cut-down version of the FPS-117 split up into several components. The main antenna was mounted on a custom trailer and offloaded at the operational site and then raised and leveled using hydraulic jacks. The remainder of the system was packed into a series of ISO containers that could be carried by any semi-trailer. The first example entered service in 1985.

In the 1990s, the Marines sent out another contract for upgrades to their MIM-23 Hawk missile systems to allow them to attack short-range ballistic missiles. The TPS-59(V)3 modified the existing TPS-59 radar sets to provide much higher altitude coverage, up to 500,000 ft.[3]

All U.S. TPS-59 radars were decommissioned in September 2020.

A further version of the series was introduced as the TPS-117, soon renamed TPS-77. This is a further cut-down of the original design, producing a smaller antenna. Combined with modern electronics, the system is now transportable by a single custom prime mover vehicle. This system has replaced most other radars in the UK's network.