TABLE OF CONTENTS DSSS is used for high-resolution ranging due to its ability to measure to a fraction of a chip the amount of correction required to line up a received sequence with a locally produced replica. A prime advantage of the sliding correlator procedure described in this chapter is that the potential precision obtainable is much higher than the resolution of the system clock. This precision is obtained, however, by a trade-off with measurement time. TOF measuring precision is limited by several factors. Noise and multipath propagation limit the ultimate ranging accuracy that can be achieved. Performance degradation due to noise is reduced by reducing the bandwidth in the distance measuring system, again at the expense of measuring time. Combating multipath interference generally requires increasing predetection bandwidth and chip rates as echo path time differences decrease, as in indoor environments.
A fundamental limitation to the accuracy that can be attained in measuring distance is due to random noise. While the tracking discriminator may have a capability of high resolution, the distance precision limitation is determined by the signal power to noise density ratio, C/ N 0, the bandwidth of the tracking loop BL, and the early to late correlator spacing in fraction of a chip, d. It also depends on the type of tracking phase discrimator. The variance of the tracking error, 
, in units of chips squared, for a coherent tracking loop, is [8]:
| (3.28) |  |
Discriminator types may...
