GNSS Receiver Operation Overview - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 66

GNSS Receiver Operation Overview

5.1 Receiver Channels

The signal processing for satellite navigation systems is based on a channelized
structure. This is true for both GPS and Galileo. This chapter provides an overview
of the concept of a receiver channel and the processing that occurs. In later chap-
ters the specifics of the signal and data processing are outlined.
Figure 5.1 gives an overview of a channel. Before allocating a channel to a
satellite, the receiver must know which satellites are currently visible. There are
two common ways of finding the initially visible satellites. One is referred to as
warm start and the other is referred to as cold start .

Warm start In a warm start, the receiver combines information in the stored al-
manac data and the last position computed by the receiver. The almanac
data is used to compute coarse positions of all satellites at the actual time.
These positions are then combined with the receiver position in an algo-
rithm computing which satellites should be visible. The warm start has at
least two downsides. If the receiver has been moved far away since it was
turned off (e.g., to another continent), the receiver position cannot be trusted
and the found satellites do not match the actual visible satellites. Another
case is that the almanac data can be outdated, so they cannot provide good
satellite positions. In either case, the receiver has to make a cold start.

Cold start In a cold start, the receiver does not rely on any stored information. In-
stead it starts from scratch searching for satellites. The method of searching
is referred to as acquisition and it is described in the following section.