Over the past 15 years, the Global Positioning System (GPS) has made high accuracy vehicle navigation available for a wide variety of applications reducing the market for inertial navigation systems (INS). However, the need for more reliable navigation solutions coupled with advances in inertial navigation technology have started a resurgence in interest in inertial navigation, especially for integrated GPS/INS systems. By integrating these two navigation system technologies, we are able to capitalize on the strengths of both while minimizing their weaknesses.

Complementary Navigation Technologies

As seen in the table below, GPS and INS represent complimentary navigation technologies.
 

INS Characteristics	GPS Characteristics
Self-contained Provides accurate position and velocity over short time periods but slowly drifts over time More expensive and heavier than GPS	Relies on GPS satellites: susceptible to jamming, RF interference, multipath and integrity problems Provides accurate position and velocity over longer time periods but has high frequency noise

By combining the outputs of a GPS and an INS using an extended Kalman filter (EKF), the performance issues of both systems can be remedied. By adding GPS data to an INS, lighter and lower cost gyros and accelerometers can be used to obtain the same navigation performance as heavier and more expensive systems. By adding INS data to GPS, the GPS signal tracking loops can be tightened to better reject multipath and interfering signals and improve resistance to jamming. Also, the INS data allows the GPS to more quickly re-acquire the GPS satellites after a loss of lock on the signal.

Fundamental Concepts of Inertial Navigation

An INS uses the inertial properties of onboard sensors to determine a vehicle's position and velocity. This is accomplished by processing data obtained from specific force and angular velocity measurements.

All inertial navigation systems perform the following functions:

Instruments a reference frame, usually accomplished by the use of gyroscopes.

Measures specific force, usually accomplished by the use of accelerometers.

Determines the local gravity vector using a gravity model.

Integrates the specific force data over time to obtain vehicle velocity and position, accomplished by an onboard digital computer.

where,

Applications of Lightweight, Low Cost Integrated GPS/INS

Our research focuses on the use of lightweight, low cost inertial sensors including fiber optic and MEMS gyroscopes and MEMS accelerometers tightly integrated with GPS to provide accurate and reliable navigation solutions for the following types of applications:

International Space Station (ISS). Relative navigation between an orbiting spacecraft and the ISS during rendezvous and proximity operations. As spacecraft approach the ISS from below, the GPS signals can be blocked by the large ISS structure. The INS allows accurate navigation with less than four satellites, helps reject multipath and aids the GPS in re-acquiring the GPS signals.

Spacecraft Maneuver Estimation. When spacecraft perform orbital maneuvers, the GPS receiver can lose lock on the GPS satellites. An integrated GPS/INS navigation solution allows the effect of the thrusters to be measured and calibrated, which improves the accuracy of future maneuvers.

Crew Return Vehicle (CRV). There is about 12 minutes during re-entry when the GPS signals cannot be received due to ionization. An integrated GPS/INS enables accurate navigation during this ionization blackout.

Interplanetary Navigation. Navigating on the surface of Mars will be accomplished using a sparse constellation of satellites which will provide GPS-like navigation data periodically. This data will be used to update inertial navigation systems onboard various land vehicles on Mars.

Urban Canyon Navigation. The GPS signals can be blocked by tall buildings making GPS only navigation systems less effective in large urban cities. Low cost inertial sensors will allow ground vehicles to navigate accurately in these urban canyons.