Dynamics performance of carrier and code tracking loops in ultra-tight
GPS/INS/PL integration
(2005)Abstract
Integrating the correlator measurements, I (in-phase) and Q (quadrature),
with position, velocity and attitude from INS in a Kalman filter
characterizes the ultra-tight GPS/INS integrated system. The Doppler
feedback derived from the corrected INS is fed back to the carrier-tracking
loop to remove the dynamics from the GPS/PL ranging signals, thereby
reducing the carrier tracking bandwidth. This reduction in bandwidth
results in better code and carrier measurements accuracy. Therefore
accurate estimation of the Doppler frequency from the INS becomes
paramount in leveraging the benefits of ultra-tightly coupled systems.
Although the code loops are not directly aided by the INS derived
Doppler in ultra-tightly coupled system, nevertheless, an accurate
knowledge of Doppler information from INS results in better code
thresholds. This improvement in threshold further increases the accuracy
of raw measurements and also jamming immunity. In unaided mode, the
carrier and code tracking loop bandwidths are about 15 Hz and 3 Hz
respectively, whereas in ultra-tight integration mode the bandwidths
can be reduced to 3 Hz and 1 Hz respectively. In this paper, the
performance of the carrier and code tracking loops are analyzed.
A Software GPS receiv er is used to perform the analysis. A Costas
Loop is used for carrier tracking with an arctan discriminator function,
while a narrowband DLL with a E-L/E+L discriminator is used for the
code tracking loops. A dynamic trajectory was chosen to analyze the
performance of both carrier and code tracking loops, and the results
show substantial improvement in the performance of the loops in ultratight
aided configuration.