By Mahmud Ali BBC South Asia analyst

The Russian Ilyushin-76 houses the radar system

The two countries have been negotiating the purchase of the system for several years but until recently US restrictions prevented a sale.

Last month, Washington endorsed an Israeli transfer of the highly sophisticated technology and equipment to India.

The Phalcon is a radar system installed in the nose of a Russian-built Ilyushin-76 cargo aircraft.

The aircraft acts as the hub of a communications system that spots enemy planes and missiles, and then co-ordinates defence against them.

Unlike the US Awacs plane, the Phalcon does not need a rotating radar dome on top of the fuselage, which gives it increased manoeuvrability.

It can detect enemy aircraft and missiles within a range of hundreds of miles, at high or low altitudes, by day and night and in all weather conditions.

The Phalcon also carries equipment that monitors, analyses and decodes enemy radio transmissions.

Pakistani delegation

One outcome of the sale, when and if it goes through, would be to give India a greater conventional edge with its rivals, China and Pakistan.

Although China boasts a very much larger armed force north of the Himalayas, where India suffered a humiliating defeat in 1962, the Phalcon would provide a more robust defence.

But it is against Pakistan that the Phalcon might transform Indian superiority in numbers into a technological supremacy.

Therein lies the catch. Washington earlier blocked the sale on the grounds that it could ignite an arms race and worsen the regional security environment.

Given a sale, Pakistan might be expected to take measures to counter the advantages afforded to Delhi.