Inertial Measurement Unit (SIGI)

ABOUT INERTIAL MEASUREMENT UNIT (SIGI)

The Space Integrated GPS/INS, or SIGI, is a single space-qualified box that helps a launch vehicle or spacecraft answer three questions at every moment of flight: where am I, which way am I pointing, and how fast am I moving?

Quick facts

• Maker: Honeywell International.
• Type: A “strapdown” inertial navigation unit (INU) that combines an inertial measurement unit (IMU) with a GPS receiver. “Strapdown” means the sensors are bolted rigidly to the vehicle rather than mounted on a swiveling frame.
• Core sensors: Three ring laser gyroscopes (devices that measure rotation) and three Honeywell QA3000 quartz-flexure accelerometers (devices that measure acceleration), one of each for the vehicle’s three body axes.
• Outputs: Three navigation solutions computed at the same time — inertial-only, GPS-only, and a blended GPS/INS solution — plus rotation-rate data for the autopilot.
• QA3000 accelerometer (top grade): bias repeatability better than 40 micro-g, scale-factor repeatability under 80 parts per million, and a measuring range up to about plus or minus 60 g.
• Interfaces: 28-volt DC avionics power, a MIL-STD-1553 data bus, output rates supporting control loops up to about 50 Hz, and radiation-hardened electronics.
• Heritage: NASA development contract awarded in late 1996; more than 300 units delivered.

What it is and how it works

An IMU is a cluster of motion sensors. SIGI’s three ring laser gyroscopes measure how fast the vehicle is turning around each of its three axes (an axis is simply one of the three directions a craft can rotate about). Its three QA3000 accelerometers measure the acceleration — the push, or “specific force” — felt along each of those axes. An onboard computer adds up these readings over time, a process called integration, to keep a running estimate of the vehicle’s orientation, velocity, and position.

This running estimate is “dead reckoning”: you start from a known point and track every twist and shove to figure out where you must be now. It is precise from one instant to the next, but tiny errors slowly pile up, so the estimate drifts over time. To fix that, SIGI carries a built-in GPS receiver that periodically supplies an exact position and velocity. A Kalman filter — a mathematical tool that intelligently blends two imperfect data sources into one better answer — merges the two streams. The IMU smooths the motion and fills the gaps between GPS updates, while GPS reins in the long-term drift. Think of pacing out a dark room step by step, then briefly flicking on a light to correct your count.

Because SIGI runs all three solutions in parallel, the flight computer can switch to inertial-only navigation if GPS is lost — for example when antennas are shadowed during a hard maneuver, or during the plasma blackout of reentry — and resume blending the moment GPS returns. SIGI also feeds rotation-rate data straight to the vehicle’s flight-control loop to keep the craft steady.

Why it matters

Knowing position, attitude (which way the vehicle points), and motion is mission-critical: the flight computer needs all three to steer the engines and hold the craft stable. SIGI packs an inertial-grade IMU, a GPS receiver, and the blending filter into one compact, lower-power, lighter, lower-maintenance, radiation-hardened unit, replacing bulkier older navigators. Running three solutions at once gives “graceful degradation” — losing GPS does not blind the vehicle, and drift is corrected whenever GPS is available. That reliability is exactly what flight needs across very different phases: ascent, on-orbit operations, rendezvous and docking, reentry, and landing. By deriving SIGI from high-volume military and commercial product lines — navigators used on tactical aircraft, self-propelled howitzers, and mining equipment — Honeywell delivered a flight-proven design at reduced cost.

Where it is used

• Space Shuttle: flew experimentally on three missions to Russia’s Mir station, most recently STS-89 in January 1998, validating GPS/INS navigation in spaceflight.
• International Space Station: SIGI units serve as a navigation and attitude reference aboard the station.
• Boeing X-37 and the earlier X-40: these reusable autonomous spaceplanes use SIGI through orbit, reentry, and autonomous landing.
• Boeing CST-100 Starliner: the commercial crew capsule uses SIGI for rendezvous and docking with the ISS.
• Japan’s HTV (Kounotori): the uncrewed cargo spacecraft used SIGI to navigate toward the ISS. SIGI was also proposed as the primary navigation system for NASA’s Orion / Crew Return Vehicle.

OPERATING PRINCIPLE

VEHICLES USING THIS SYSTEM

SPECIFICATIONS

DETAILED SPECS