Introduction to GNSS - GNSS Remote Sensing: Theory, Methods and Applications - page 6

Chapter 1
Introduction to GNSS

The Global Navigation Satellite Systems (GNSS), including the US's Global
Positioning System (GPS), Russia's GLONASS, EU's Galileo and China Beidou
(also called COMPASS) as well as several regional navigation satellite systems,
can be characterized as a highly precise, continuous, all-weather and near-real-time
microwave (L-band) technique with signals through the Earth's atmosphere. These
characteristics of GNSS imply more and wider applications and potentials (Jin et al.
2010a , b ). Each GNSS satellite continuously broadcasts radio signals in two or
more frequencies in L-band (1-2 GHz) with wavelength around 20 cm, the direct
signals are be used for navigation, positioning and timing. The refracted signals
from GNSS Radio Occultation satellites together with ground GNSS observations
can provide the high-resolution tropospheric water vapor, temperature and pressure,
tropopause parameters and ionospheric total electron content (TEC) and electron
density profile as well. The GNSS reflected signals from the ocean and land
surface could determine the ocean height, wind speed and wind direction of ocean
surface, soil moisture, ice and snow thickness. Therefore, the refracted and reflected
GPS signals can image the Earth's surface environments as a new, highly precise,
continuous, all-weather and near-real-time remote sensing tool, which will play a
key role in various atmospheric sounding, ocean remote sensing and land/hydrology
mapping. In the following sections, the GNSS history, system, signals, theory and
applications are introduced (Fig. 1.1 ).

1.1 GNSS History

1.1.1 GPS

In the early 1960s, several U.S. governmental organizations were interested in
developing satellite systems for position determination, e.g., the Department of
Defense (DOD), the National Aeronautics and Space Administration (NASA),