Against smaller targets, the need to receive a strong reflected radar signal made it difficult to achieve lock-on at the missile's effective range.

Additionally, early models were only effective against targets at roughly the same or higher altitudes, below which reflections from the ground became a problem.

In the early 1970s, the RAF developed the Skyflash version with an inverse monopulse seeker[citation needed] and improved motor, while the Italian Air Force introduced the similar Aspide.

Both could be fired at targets below the launching fighter ("look-down, shoot-down"), were more resistant to countermeasures, and were much more accurate in the terminal phase.

Fired at low altitude and flying directly at its target, though, the range of the missile in this role is greatly reduced because of the higher air density of the lower atmosphere.

The Sparrow emerged from a late-1940s United States Navy program to develop a guided rocket weapon for air-to-air use.

[5] After a protracted development cycle the initial AAM-N-2 Sparrow entered limited operational service in 1954 with specially modified Douglas F3D Skyknight all-weather carrier night fighters.

Compared to the modern versions, the Sparrow I was more streamlined and featured a bullet-shaped airframe with a long pointed nose.

It was later selected, with some controversy, to be the primary weapon for the Canadian Avro Arrow supersonic interceptor, along with the new Astra fire-control system.

Considered a "dogfight Sparrow", the AIM-7E-2 was intended to be used at shorter ranges where the missile was still travelling at high speeds, and in the head-on aspect, making it much more useful in the visual limitations imposed on the engagements.

Its worst tendency was to detonate prematurely about 1,000 feet ahead of the launching aircraft, but it also had many motor failures, erratic flights, and fuzing problems.

The AIM-7F, which entered service in 1976, had a dual-stage rocket motor for longer range, solid-state electronics for greatly improved reliability, and a larger warhead.

The most common version of the Sparrow today, the AIM-7M, entered service in 1982 and featured a new inverse monopulse seeker (matching the capabilities of Skyflash), active radar proximity fuse, digital controls, improved ECM resistance, and better low-altitude performance.

[citation needed] Plans initially called for all M versions to be upgraded, but currently P's are being issued as required to replace M's lost or removed from the inventory.

[citation needed] The final version of the missile was to have been the AIM-7R, which added an infrared homing seeker to an otherwise unchanged AIM-7P Block II.

][citation needed] As part of the Avro Canada CF-105 Arrow program, Canadair (now Bombardier) partnered with Douglas Aircraft Company in the development of the Sparrow II (AAM-N-3/AIM-7B).

Despite the significant advantages of this design over SARH guidance, all subsequent models of the Sparrow use semi-active radar homing.

[18] The Italian company Finmeccanica (now Leonardo S.p.A.), Alenia Difesa licensed the AIM-7E Sparrow technology from the US, and produced its own version.

Compared to the AIM-7E, it received an improved new monopulse guidance system that allowed for a better hit ratio and easier targeting of enemies at low altitude with ground-clutter confusion.

The missile did not enter production as the R-23 was thought to have better versatility, range, signal processing logic, and immunity to interference.

The receiver also senses the guidance radar to enable comparisons that enhance the missile's resistance to passive jamming.

In 1950s radars, these were single-target tracking devices using a nutating horn as part of the antenna, thereby sweeping the beam in a small cone.