Global Positioning System Reference
In-Depth Information
corresponding to the transmission of the subframe. Through the process used for navigation
data decoding, the receiver is able to understand which subframe and word a certain bit
belongs to. In this way, the receiver can have an exact, precise and real-time
“comprehension“ of each sample/bit broadcast by the satellite. This aspect will be
fundamental in the computation of the pseudoranges as it will be explained in the next
section.
(a) (b)
Fig. 6. Galileo I/NAV navigation message structure (a) and I/NAV nominal page with bits
allocation (b)
4. Performing range measurements using GNSS signals
In this section we focus on the measurements of the pseudorange, describing some methods
commonly used to estimate the distance between the satellite and the user's receiver.
So far, we have explained how the detection of a preamble is an effective way to recognize
the beginning of a subframe (a page in case of Galileo) and the starting point for decoding
the navigation message. Here and in the following we want to introduce how GNSS
receivers use the detection of a preamble to compute a valid pseudorange and estimate the
user's position and velocity. According to (Borre et al., 2006), the pseudorange estimations
can be divided into two sets of computations: the first is devoted to find the initial set of
pseudoranges, the second keeps track of the pseudoranges after the first set is estimated.
4.1 Computation of the first set of pseudoranges
Before proceeding with the explanation of the pseudorange computation, it is useful to
recall some hypothesis, that will be taken as true from now on.
All the clocks on-board of the satellites are assumed perfectly synchronized to a reference
GNSS time-scale. In other words, we assume that the first chip of a definite subframe/page
leaves the satellites at the same instant
GNSS
tx
t
. In addition:
Search WWH ::
Custom Search