Integrated INS/GPS Systems

The reasoning behind any integrated system is to combine two complementary items to create a solution that is greater than is capable with either item. GPS and INS both produce position and velocity information through differing methods. The integration process attempts to provide the advantages of both systems while minimising the errors that each system suffer. The integration of the two systems can be done to varying levels of interaction. All of the varying levels are dependent upon a common filtering system, called the Kalman filter, used to combine the systems.

 INS has many features that would ideally be maintained throughout the integration, these include:

                High measurement output rate

                High position and velocity accuracy over the short term

                Accurate attitude information

                No signal outages and resistance to jamming

There are also problems with INS that need to be addressed in the integration including:

                Accuracy decreasing over time

                Sensitive to gravity

                Unbounded errors

                Dependence on alignment

                Quality of sensor and system needed for long term stability

GPS also has properties that can be beneficial to the integration with INS, including:

                High position and velocity accuracy over the long term

                Uniform accuracy, independent of time

                Not sensitive to gravity

There are also problems associated GPS that need to be addressed in the integration, including:

                Low measurement output rate

                Non-autonomous

                Dependency on multiple satellite visibility

Cycle slip and loss of lock

Ideally the integrated system will exhibit the following properties:

                High position and velocity accuracy over the long term

                Accurate attitude information

                Uniform accuracy, independent of time

                No sensitivity to gravity

                High measurement output rate

                No dependence on GPS signal

                Resistance to jamming

                Comparable solution for all quality of systems

The list above provides the ideal solution of the integration of the two systems, however only some parts of the list are needed to achieve a result better than the individual components.

It was stated earlier that the ideal solution for an integrated system will exhibit the following properties:

                High position and velocity accuracy over the long term

                Accurate attitude information

                Uniform accuracy, independent of time

                No sensitivity to gravity

                High measurement output rate

                No dependence on GPS signal

                Resistance to jamming

                Comparable solution for all quality of systems

The ability to meet all these tasks creates contradictions in the list. It is impossible to have a system that has no dependence on constant GPS signal when it is the only way of reducing the sensitivity to gravity. The tightly-coupled system is successful in meeting most of the targets listed for the anticipated ideal combination of the systems. The properties that the tightly-integrated system exhibits:

                High position and velocity accuracy over the long term

                Accurate attitude information over the long term

                Uniform accuracy, independent of time

                Reduced sensitivity to gravity

                High measurement output rate

                Reduced dependence on GPS signal

                Short term resistance to jamming

                Comparable solution for most INS qualities

The achievements of the integrated system are a substantial improvement over both individual systems. There is still dependence on GPS signals because without these the integrated system reverts to being an INS and the errors that were mitigated through the integration will return.

 The performance of this integration can be shown in the comparison of the result of the tightly-integrated system to the individual components. A lower quality INS through integration can maintain a better estimation of position that a higher quality independent INS system over the medium to long term by numerous factors. The error growth of the integrated system will be estimated continually and adjusted accordingly whereas the independent system will continue to grow in error. The integrated system will also be less affected by the erroneous computations of the value of gravity that is applied as the system is corrected. The ability of a lower quality INS to out perform a more accurate system through the use of integration demonstrates the performance that can be achieved through the integration of INS and GPS. The same comparison can be done for a GPS receiver and the tightly-integrated system. The most pronounced benefit of the integrated system is the ability to maintain an estimation of position using data from fewer satellites than are needed to compute a full GPS solution. The GPS receiver is unable to compute a position while the integrated system can maintain a reasonable estimate of position in the short term. This shows the ability of the integrated system to provide a more reliable and precise solution to that of either component.

Top of PageReturn to Diagram


Home
IMU Components
IMU
INS
GPS
Integration Methods
Integrated Systems