C/A Code Cross Correlation Error with Carrier Smoothing - the Choice of Time Constant: 30 s vs. 100 s

摘要

The Gold sequences used in the GPS C/A code family arernnot completely orthogonal to each other. The C/A coderncross correlation can be as high as 23.9 dB below the autorncorrelation. It has been shown that cross correlation canrnintroduce errors in range and Carrier-to-Noise Ratiorn(C/N0) estimates. The errors can be significant when thernreceived signal power is substantially different betweenrnthe two satellites. Safety-critical systems that rely onrnpseudorange measurements are especially sensitive torncross correlation errors. For instance, cross correlation hasrnto be taken into consideration in the monitoring of lowrnsignal power, which is an important component of thernGround-Based Augmentation System (GBAS). Crossrncorrelation can potentially impact the accuracy, integrityrnand continuity of GBAS. It has been suggested thatrnpseudorange error induced by cross correlation can bernmodeled using an approach similar to that of multipathrnerrors. In addition, cross correlation was also found to berna function of range, Doppler, Doppler rate of change, datarnbit alignment, code chip alignment, received power levelrnand tracking loop parameters. The Doppler frequencyrnshift between the two signals results in an oscillatory errorrnpattern, which resembles the fading of multipath errors.rnCross correlation can be mitigated by a low-pass filter orrncarrier smoothing. A time constant that is substantiallyrngreater than the period of oscillation is very effective inrnreducing the error magnitude. In fact, for most cases, 100-rnsecond carrier smoothing has been found sufficient tornbring the pseudorange errors down to an acceptable level.rnUnfortunately, 100 second carrier smoothing cannot bernuniversally used for all applications. To better deal withrnother error sources such as ionospheric gradient, it hasrnbeen proposed that 30 second carrier smoothing should bernused in the ground subsystem of GBAS for GBASrnApproach Service Type D landing operations. This workrninvestigates the efficacy of carrier smoothing on crossrncorrelation errors, using time constant of 30 seconds andrn100 seconds. Depending on the system error budget, crossrncorrelation errors greater than a certain threshold must berndetected. Extreme scenarios exist, in which thernpseudorange errors are still greater than the threshold afterrn30 or 100 second carrier smoothing. The maximumrnresidual errors are calculated for both time constants. Arndetailed analysis of the error model makes it possible tornpredict or detect the conditions, under which the crossrncorrelation errors may exceed the threshold. Thernlikelihood of experiencing these errors in a practical setuprncan also be estimated for both time constants, which, inrnturn, can be used to evaluate the impact of crossrncorrelation on system continuity.