Global Positioning System Reference
In-Depth Information
5
Estimation of Satellite-User Ranges
Through GNSS Code Phase Measurements
Marco Pini
1
, Gianluca Falco
1
and Letizia Lo Presti
2
1
Istituto Superiore Mario Boella
2
Politecnico di Torino
Italy
1. Introduction
A Global Navigation Satellite System (GNSS) receiver is able to compute the user position
through a trilateration procedure, which includes the measure of the distance between the
receiver and a set of satellites. Two different approaches are tipically used and implemented
in commercial receivers. The former relies on code tracking, the latter leverages carrier
phase measurements performed during carrier tracking.
This chapter focuses on the first approach and discusses the procedures that GNSS receivers
perform to finely estimate satellite-user ranges. First, in section 2 we introduce the concept
of pseudorange and in section 3 we give some fundamentals on primary signal processing
blocks of every GNSS receiver: signal acquisition, tracking and data demodulation. In
section 4, two common methods used to estimate the user-satellite range, on the basis of
code phase measurements are presented. Finally, section 5 completes the chapter, providing
an example of combined Position, Velocity and Time (PVT) computation for a GPS/Galileo
receiver.
2. Theory and methods
Let us start with a simple example to introduce the concepts we will describe in the next
sections.
John usually bikes to school following a straight path, keeping a constant speed. John wants
to measure the distance between his house and the school and decides to compute such a
distance by measuring the time it takes to go to school. He uses the following formula:
xvt
= ⋅
(1)
where:
•
x
is the distance estimated by John;
•
v
is the average speed, read on the bike speedometer;
•
t
is the difference between the time instant when John arrives at school and the time
instant when he leaves home. In both cases, John reads the time on his digital watch.
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