Abstract
The gravity disturbance vector is one of the major error sources in high-precision and long-term inertial navigation applications. Specific to the inertial navigation systems (INSs) with high-order horizontal damping networks, analyses of the error propagation show that the gravity-induced errors exist almost exclusively in the horizontal channels and are mostly caused by deflections of the vertical (DOV). Low-frequency components of the DOV propagate into the latitude and longitude errors at a ratio of 1:1 and time-varying fluctuations in the DOV excite Schuler oscillation. This paper presents two gravity compensation methods using the Earth Gravitational Model 2008 (EGM2008), namely, interpolation from the off-line database and computing gravity vectors directly using the spherical harmonic model. Particular attention is given to the error contribution of the gravity update interval and computing time delay. It is recommended for the marine navigation that a gravity vector should be calculated within 1 s and updated every 100 s at most. To meet this demand, the time duration of calculating the current gravity vector using EGM2008 has been reduced to less than 1 s by optimizing the calculation procedure. A few off-line experiments were conducted using the data of a shipborne INS collected during an actual sea test. With the aid of EGM2008, most of the low-frequency components of the position errors caused by the gravity disturbance vector have been removed and the Schuler oscillation has been attenuated effectively. In the rugged terrain, the horizontal position error could be reduced at best 48.85% of its regional maximum. The experimental results match with the theoretical analysis and indicate that EGM2008 is suitable for gravity compensation of the high-precision and long-term INSs.
Highlights
In inertial navigation systems (INSs), the accelerometer-sensed specific force consists of the kinematic acceleration and the gravitational acceleration
INSs discussed in this paper adopt a kind a kind of high-order damping network, which is designed based on the complementary filtering to of high-order damping network, which is designed based on the complementary filtering to obtain obtain 40 dB/10 dec or higher attenuation rate to both low-frequency and high-frequency reference
Gravity calculation is triggered at a constant interval tm, and the gravity vector will be maintained at its current value until the gravity calculation process outputs a new one after a time delay tc
Summary
In inertial navigation systems (INSs), the accelerometer-sensed specific force consists of the kinematic acceleration and the gravitational acceleration. According to Kwon and Jekeli’s research, with ground data gridded with 2 arc-minutes resolution and accurate to better than 3 mGal, the error in gravity compensation contributes less than 5 m to the position error after one hour of free-inertial navigation for a typical flight trajectory at 5 km altitude and 300 km/h speed [13]. As the errors of free-inertial navigation diverge over time, most INSs utilize a kind of feedback loop with external altitude and velocity references to restrain the divergence in the vertical channel loop with external altitude and velocity references to restrain the divergence in the vertical channel and the Schuler oscillation in the horizontal channels, namely the damping network.
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