How can pseudorange measurements be generated from code tracking?

Every GNSS receiver processes the received signal to obtain an estimate of the propagation time of the single components from the satellites to the receiver. These propagation times are then expressed in meters to solve a triangulation problem. Since they are not only related to the distance between the receiver antenna and the satellites, i.e. the range, but also to an imperfect alignment of the local time scale with the GPS time scale, they are called pseudo-ranges. Once the acquisition stage has aligned the received and the locally generated code within less than a half chip period, a fine synchronization takes over and keeps the two codes aligned, by means of closed loop operations. Generally, the tracking system in GNSS receivers consists of a Delay Lock Loop (DLL) for code tracking and a Phase Lock Loop (PLL) for carrier phase tracking. Both the measurements produced by the DLL and the PLL are useful to determine, respectively, code and carrier-smoothed pseudoranges. In this context, we will only consider code measurements, that are also necessary where carrier-smoothing must be performed. Considering Galileo E1 or GPS L1, the code itself is characterized by the so-called integer ambiguity problem. This means that every single period of a code (1ms for L1, 4ms for E1) cannot be distinguished from the other ones and, when computing travel time differences among the single tracked channels, these differences can be computed modulo 1ms [1]. Fortunately, in the case of code measurement this integer ambiguity can be easily solved exploiting the navigation message structure. Once the receiver is effectively tracking a signal, it also decode the navigation message and achieves frame synchronization. A counter is associated to every single channel, so that each sample is associated to a chip, bit and subframe, and it is then possible to relate these counters to the propagation times. In the following, we will consider the two main techniques that can be adopted to obtain travel times on the basis of counter values.

RAO Marco;  FALCO Gianluca; 

2017-09-12

Inside GNSS Media & Research LLC

JRC67274