According to the latest interview if Dr. Saraswat, The Indian BDM system requires 4 interceptors (2 PAD's and 2AAD's) for a kill probability of ~99.5 % for intermediate missiles of upto 2000 km range. It is quite clear that the armed forces (IAF) and DRDO have mutually agreed on the requirement for a new exo-atmospheric interceptor to tide over the limitations of the PAD (as it was a modified prithvi missile with inherent limitations). Therefore the phase 1 operationalization of Indian BMD is still a few years away. However, we can expect AAD's to be operationalized as an interim first step towards a robust Pakistan specific BMD. From what I understand, a single launcher carries 6 AAD interceptors and a battery should have about 4 to 6 launchers (as is the norm with air defence units). If we assume 2 to 4 batteries for a city like Delhi, we are talking about > 96 AAD interceptors. Once PDV is inducted (sometime after 2015) and phase 1 becomes operational, the system should have commensurate number of PDV's as per the operational scenarios. This translates to the ability to target more than 20 incoming intermediate range missiles overall. Therefore Pakistan would have to launch atleast 20 to have a reasonable certainty of getting through Delhi's defences.
However, looking at India's plans for its BMD, it seems that is will be based on a two tiered system with phase 2 planned for testing from next year. If that's the case, it would make sense for India to have short range (current) LRTR's and associated missile batteries deployed to counter the Pakistani threat while the longer range LRTR (currently under development) and phase 2 batteries deployed against the Chinese threats. While the radars will in all likelihood be placed closer to the borders, India it seems has a long way to go with phase 1 deployment for major cities (atleast 2018), phase 2 deployment of major cities (atleast 2024) with an integrated air defence network comprising newly inducted air defense radars (both aerial and ground based). Pakistani planners have atleast 6 years to develop adequate countermeasures to try and blunt the Indian BMD. The best way is to increase missile range so as to have DT missile which have a higher speed and also fly at relatively low altitudes ( < 50 km) to make interception difficult. Another method to look into are MIRV's ofcourse with dummy warheads which would seriously blunt Indian BMD. I am sure Pakistan would be already developing such systems though from the available pictures and some videos of Pakistani missile launches, Pakistan is yet to test MIRV capability.
