Honeywell HG1700 IMU

ABOUT HONEYWELL HG1700 IMU

The Honeywell HG1700 is a small, rugged sensor box that knows which way a vehicle is pointing and how fast it is moving — entirely on its own, without help from satellites or radio signals. It is one of the most widely produced guidance sensors in the history of flight.

Quick facts

• Type: Tactical-grade Inertial Measurement Unit (IMU) — a self-contained package that measures a vehicle’s motion to work out its orientation, speed, and position.
• Sensors: 3 ring laser gyroscopes (RLGs) plus 3 quartz resonating-beam accelerometers, sealed in an aluminum housing.
• Data rate: up to 100 Hz for guidance (100 measurements per second) and 600 Hz for control, sent over a standard RS-422 serial connection.
• Size and weight: a compact box about 73 mm (7.3 cm) tall, weighing under 0.9 kg (about 2 pounds). A slightly smaller variant weighs under 0.7 kg.
• Power: less than 5 watts, with no external heat sink or cooling required.
• Operating temperature: -54 C to +85 C.
• Introduced: 1994. Honeywell has delivered over 350,000 units (more recent counts exceed 600,000) — more than ten times its nearest competitor.

What it is and how it works

An IMU senses motion from the inside rather than receiving it from outside. To do this, the HG1700 uses two kinds of sensor working together.

The first kind is a gyroscope, a device that measures rotation — how fast the vehicle is turning. The HG1700’s three ring laser gyroscopes each watch one axis of rotation. Inside a sealed cavity, two laser beams travel in opposite directions around the same loop. When the unit rotates, one beam’s path becomes effectively longer than the other (an effect called the Sagnac effect), which shows up as a tiny, measurable frequency shift that is proportional to the turn rate. There is no spinning wheel to wear out.

The second kind is an accelerometer, which measures linear acceleration — speeding up, slowing down, or sideways shoves. The three accelerometers cover the three directions of space.

Onboard electronics read all six sensors many times a second. A flight computer then adds up those rotation and acceleration readings over time to continuously track attitude (which way it points), velocity, and position. This running tally is called dead reckoning — like tracking your position in a dark room by counting your steps and turns rather than looking out a window.

Why it matters

The HG1700’s big achievement was making dependable inertial guidance both affordable and compact enough to put on small vehicles and even one-use weapons, not just large aircraft. Because it carries no moving gyro parts, draws little power, needs no external cooling, and speaks over a simple serial link, it is easy to build into a system.

Its most important quality is independence. A satellite navigation signal (GPS/GNSS) can be jammed, blocked, or simply lost — but the HG1700 keeps producing a navigation solution no matter what, because it relies only on its own internal sensors. When GPS is available, it is used to gently correct the slow “drift” that all inertial sensors accumulate over time. That combination of self-reliance plus correction is why it became a default choice for precision weapons and for many commercial navigation systems.

Where it is used

• JDAM (Joint Direct Attack Munition): the HG1700 is the inertial heart of the JDAM tail kit, measuring position, velocity, and acceleration and combining with GPS to turn unguided bombs into precision-guided weapons.
• Missiles and precision munitions: originally built in 1994 for low-cost guidance of air-to-air and air-to-surface weapons, and adopted across U.S. forces.
• Uncrewed aircraft: a common guidance and navigation IMU on UAVs and target drones.
• Commercial GNSS/INS systems: packaged by NovAtel into SPAN products (such as the UIMU-HG1700 and HG1700 AG58/AG62) for surveying and mapping.
• Launch vehicles and stabilized platforms: serves as the inertial reference that lets a guidance computer steer a vehicle and verify its trajectory, and is also used for pointing stabilization and underwater navigation where GPS cannot reach.

The trade-offs

The HG1700 is “tactical grade,” one tier below the “navigation grade” units used for long, GPS-denied airline and strategic flights. Its gyro drift of roughly 1 degree per hour (in its best grade) means position error slowly builds up, so it is usually paired with GPS for sustained accuracy — a perfect fit for the short flight times of munitions and launch vehicles. The unit is highly configurable, offered in two variants, five performance grades, and about 30 off-the-shelf configurations, so designers can trade range and accuracy to suit the job. Exact figures vary by grade, so the chosen variant’s datasheet is the final word. (One note for the curious: NovAtel’s larger SPAN navigation boxes that bundle the HG1700 with a GPS receiver are a good deal heavier and bigger than the bare IMU described here.)

OPERATING PRINCIPLE

VEHICLES USING THIS SYSTEM

SPECIFICATIONS

DETAILED SPECS