The classified Heritage (Radiant Agate) package is a survivable and reliable satellite-borne system that uses infrared (IR) detectors to sense heat from missile plumes against the Earth's background, to detect and report in real-time missile launches, space launches and nuclear detonations.
Heritage satellites have been the spaceborne segment of NORAD's Tactical Warning and Attack Assessment System since 1970. The satellites feed warning data, via communications links, to NORAD and US Space Command early warning centers within Cheyenne Mountain. These centers immediately forward data to various agencies and areas of operations around the world. Members of the Air Force Space Command 50th Space Wing's 1st Space Operations Squadron provide command and control support for the satellite.
Hughes Aircraft Corporation is the contractor for Heritage. Three Heritage units have been placed on each Hughes' Satellite Data Systems - 2 (SDS-2) satellite. Heritage is similar to the Geostationary Environmental Monitor (GEM) infrared imaging sensor shown here on the left. The sensor's purpose is to detect, locate, and identify targets of interest that are intense sources of IR radiation.
By the middle of 1996, 3 SDS-2 spacecraft were operational. The first launch was on the 8th of August 1989 via STS 28. The second SDS-2 launch, via STS-53, occurred on the 2nd of December 1992. The most recent launch was on the 2nd of July 1996, using the "no upper stage" version of the Titan IV.
The SDS-2 has a mass of 5,150 pounds when fueled. The satellite, which is flown on the same body of LEASAT, most likely has a similar lifespan of about 7 years. LEASAT's solar arrays generate 1.5 kilowatts for the first ten years of its life, and reduces to 1.2 kilowatts thereafter, and thus the same conditions probably hold true for the SDS-2. The Heritage sensors on the SDS-2 are similar to the three that appear on the Hughes illustration on the right. The SDS-2, however, has a cylindrical body with four dish antennas and one telemetry and command antenna, which is used as a back-up.
Recent technological improvements in sensor design includes above-the-horizon capability for full hemispheric coverage and improved resolution. Increased on-board signal-processing capability improves clutter rejection enhancing reliability and survivability.
The sensor and the spacecraft, which together comprise the satellite, are placed in a highly elliptical orbit with a high apogee and low perigee, so communications are possible for the US Air Force in the North polar region. Two SDS-2 satellites must be in orbit for the communications to be constant. Its telescope is pointed toward the earth and rotated at six revolutions per minute. To provide a scanning motion for the IR sensor, the satellite is spun about its Earth-pointing axis. The axis of the satellite's rotation is normal to the earth's surface. A prime requirement of the spacecraft is to provide attitude control to maintain the pointing direction accurately. Satellite-spin momentum is reduced to a nominal value of zero by introducing an equal and opposite momentum achieved through operation of a Reaction Wheel. The resulting "zero momentum" satellite is attitude controlled by gas thrusters. A side view of the GEM sensor's infrared detector array, which is similar to Heritage, is shown above on the right.
The basic functions of the spacecraft are to: provide a spin-controlled, stable, Earth pointing vehicle for the mission data sensing and processing equipment; furnish the on-board functions required to position, control, and maintain the satellite in its proper Earth orbit; furnish, condition, and control the electrical power for all satellite requirements; provide secure downlink capabilities to transmit mission data, State-of-Health (SOH), and other relevant information to the ground for final processing; and provide a secure uplink command receiving, processing, and distribution capability for both spacecraft and sensor ground-generated commands. The spacecraft consists of the following principal systems: structure; Communication and Command and Mission Data Message; Electrical Power and Distribution; Propulsion; Attitude Control; and Thermal systems.