While the THAAD system can provide South Korea additional protection from the North’s missile attacks, it cannot cover key northern parts of the country, including Seoul. According to a report produced by the US Department of Defense in 1999, a mixed system of upper-tier — similar to the THAAD system — and lower-tier batteries could protect the country ‘beyond the immediate reach of very short-range ballistic missiles’. Seoul and its environs would be covered by a low-tier system.