Chapter 9.2.4: INERTIAL SYSTEMS TECHNOLOGIES: INS and GNSS

9.2.4 INS and GNSS

9.2.4.1 Advantages of INS The main advantages of inertial navigation over other forms of navigation are as follows:

It is autonomous and does not rely on any external aids or on visibility conditions. It can operate in tunnels or underwater as well as anywhere else.
It is inherently well suited for integrated navigation, guidance, and control of the host vehicle. Its IMU measures the derivatives of the variables to be controlled (e.g., position, velocity, attitude).
It is immune to jamming and inherently stealthy. It neither receives nor emits detectable radiation and requires no external antenna that might be detectable by radar.

9.2.4.2 Disadvantages of INS These include the following:

Mean-squared navigation errors increase with time.
Cost, including
1. Acquisition cost, which can be an order of magnitude (or more) higher than that of GNSS receivers.
2. Operations cost, including the crew actions and time required for initializing position and attitude. Time required for initializing INS attitude by gyrocompass alignment is measured in minutes. Time to first fix for GNSS receivers is measured in seconds.
3. Maintenance cost. Electromechanical avionics systems (e.g., INS) tend to have higher failure rates and repair costs than do purely electronic avionics systems (e.g., GPS).
Size and weight, which have been shrinking:
1. Earlier INS systems weighed tens to hundreds of kilograms.
2. Later "mesoscale" INSs for integration with GPS weighed 1-10 kgms.
3. Developing MEMS sensors are targeted for gram-size systems. INS weight has a multiplying effect on vehicle system design, because it requires increased structure and propulsion weight as well.
Power requirements, which have been shrinking along with size and weight but are still higher than those for GPS receivers.
Temperature control and heat dissipation, which is proportional to (and shrinking with) power consumption.

9.2.4.3 Competition from GPS Since the 1970s, U.S. commercial air carriers have been required by FAA regulations to carry two INS systems on all flights over water. The cost of these two systems is on the order of 10⁵ U.S. dollars. The relatively high cost of INS was one of the factors leading to the development of GPS. After deployment of GPS in the 1980s, the few remaining applications for "standalone" (i.e., unaided) INS include submarines, which cannot receive GPS signals while submerged, and intercontinental ballistic missiles, which cannot rely on GPS availability in time of war.

9.2.4.4 Synergism with GNSS GNSS integration has not only made inertial navigation perform better, it has made it cost less. Sensor errors that were unacceptable for stand-alone INS operation became acceptable for integrated operation, and the manufacturing and calibration costs for removing these errors could be eliminated. Also, new low-cost MEMS manufacturing methods could be applied to meet the less stringent sensor requirements for integrated operation.

The use of integrated GNSS/INS for mapping the gravitational field near the earth s surface has also enhanced INS performance by providing more detailed and accurate gravitational models.

Inertial navigation also benefits GNSS performance by carrying the navigation solution during loss of GNSS signals and allowing rapid reacquisition when signals become available.

Integrated systems have found applications that neither GNSS nor INS could perform alone. These include low-cost systems for precise autonomous control of vehicles operating at the surface of the earth, including automatic landing systems for aircraft and autonomous control of surface mining equipment, surface grading equipment, and farm equipment.

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Chapter 9.2.4: INERTIAL SYSTEMS TECHNOLOGIES: INS and GNSS

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