NASA is currently dependent on a costly range radar infrastructure to support range safety requirements. Less expensive alternatives being investigated include Passive Coherent Location (PCL).

Silent Sentry is a PCL technology developed by Lockheed Martin for all-weather surveillance and tracking of aircraft and missiles. Unlike conventional radar, PCL does not mechanically scan a volume of space or actively radiate a radio frequency signal. Instead, it passively listens for reflections of electromagnetic signals already in the environment from sources such as commercial television and FM radio.

PCL is capable of tracking multiple objects above a threshold detection size with accuracies purportedly comparable to C-band radar. Although PCL requires high-speed parallel processing to extract the targets from the received signals, its advantages are reduced operational and maintenance costs because of its mechanical simplicity and no interference with payload electronics or environmental impact because of its passive approach.

An onsite demonstration during the launch of STS-103 in December 1999 was the first phase of a NASA plan to assess the application and feasibility of implementing PCL technology as a primary or supplementary tracking and surveillance source for future and existing space launch vehicles.

A followup phase consisted of software and modeling modifications based on lessons learned from the onsite testing.