The Miniature Sensor Technology Integration (MSTI) Program consisted of a series of 3 spacecraft. They were designed, built, launched and operated over a span of 7 years by the Phillips Lab at Edwards AFB.
Figure 1: Summary of the MSTI Series
MSTI’s purpose was to study the Earth in medium-wave and short-wave infrared. The MSTI3 payload contained a two degree-of-freedom gimbaled mirror that could rotate to photograph any place that was line-of-sight visible.
Figure 2: MSTI3 backup payload
A second purpose of the MSTI program was to demonstrate that a spacecraft of a high level of complexity and capability could be designed and built in a comaratively short time, and on a small budget.
The 3 MSTIs each were in a Sun Synchronous, low Earth orbit. This is a polar orbit, where the vehicle passes overhead only at sunrise or sunset. They orbited the Earth once every 93 minutes, and flew at 7.6 kilometers per second.
MSTI3’s attitude control was provided by the following components:
Three Reaction Wheels: Flywheels that spin up or down, and reorient the vehicle through the principle of “Conservation of Momentum.” This type of attitude control doesn’t consume any propellant, and can exert only a pure torque (no resultant force) on the vehicle.
Hydrazine Thrusters: These are small rockets that exert both force and torque. They had two purposes: first, to despin the wheels when their speeds became too large, and, second, to occasionally readjust the vehicle’s orbital trajectory.
Star Tracker: A camera, containing its own embedded computer, that tracked up to 5 stars. Its purpose was to provide a very accurate (.01 degree) estimate of attitude (which way the vehicle is oriented). It worked by comparing the stars in its field-of-view with star locations stored in an on-board catalog. The attitude control software also had the ability to first initialize its attitude, by taking a pattern of 5 unknown stars from the star tracker, and searching the catalog to identify which 5 stars they were.
Sun Sensor: A passive device that measured the location of the Sun, and helped to orient the vehicle correctly.
GPS Receiver: A radio that provided a measurement of the vehicle’s location, that is, where it was along its orbital path.
Figure 3: MSTI3 attitude control components
Figure 4: MSTI3 without solar array
MSTI3 was expected to last one year. It in fact lasted a year and a half, and could have gone much longer. It satisfied, and in most cases exceeded, its design requirements, and produced mountains of infra-red Earth images. Sadly, due to funding limitations, MSTI3 finally had to be deorbited (burned up in the atmosphere), and so lived its last moments as a shooting star somewhere over the Pacific Ocean.
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