TABLE OF CONTENTS In addition to its interference rejection properties, DSSS is particularly appropriate for distance measurement because it provides a systematic manner, through a closed loop control mechanism, of achieving high resolution from signal bandwidths that are relatively low. For example, the short GPS C/A code can provide an accuracy of 10m with a chip length of 976 ns, equivalent to a distance of 293m. Accuracy is obtained at the expense of processing time, however. While a high-speed system clock is not required for high-accuracy time difference measurements, one should be aware of the fact that TOF or TDOA measurements do require a precise measurement of the beginning of the phase difference process in the DSSS receiver in order to utilize the high-resolution code phase difference estimation.
Noise and multipath are ultimate factors in the accuracy of the range estimation. Noise can be countered by low DLL loop bandwidth, with the penalty of increased measurement time and susceptibility to disturbance by system dynamics and phase noise. Multipath interference is alleviated by using high spreading code rates and statistical estimation when location is determined by spatially separated base stations.
