Introduction - Understanding GPS: Principles and Applications - page 14

determination [22]. Furthermore, pictures from NASA's LANDSAT of the Yucatan
peninsula, coupled with a GPS-equipped airborne survey enabled a National Geo-
graphic expedition to find ruins of several heretofore unknown Mayan cities.

1.9.4 Maritime
GNSS has been embraced by both the commercial and recreational maritime com-
munities. Navigation is enhanced on all bodies of waters, from oceanic travel to
riverways, especially in inclement weather. Large pleasure craft and commercial
ships may employ integrated navigation systems that include a digital compass,
depth sounder, radar, and GPS. The integrated navigation solution is presented on a
digital chart plotter as current ship position and intended route. For smaller vessels
such as kayaks and canoes, handheld, waterproof, floatable units are available from
paddle shops or the Internet. Maritime units can usually be augmented by WAAS,
EGNOS, or maritime DGPS (MDGPS). MDGPS is a coastal network designed to
broadcast DGPS corrections over coastal or waterway radiobeacons to suitably
equipped users. MDGPS networks are employed in many countries, including Rus-
sia. Russian beacons transmit both DGPS and differential GLONASS corrections.
The EGNOS Terrestrial Regional Augmentation Network (TRAN) is investigating
the use of ground-based communications systems to rebroadcast EGNOS data to
those maritime users with limited visibility to EGNOS geostationary satellites. Visi-
bility may be limited for several reasons, including the location of the user at a lati-
tude greater than that covered by the EGNOS satellites and the location of the user
in a fjord where the receiver does not have line of sight to the satellite due to obscur-
ing terrain [23]. Wide area differential GPS has been utilized by the offshore oil
exploration community for several years. Also, highly accurate DGPS techniques
are used in marine construction. Real-time kinematic (RTK) DGPS systems that pro-
duce centimeter-level accuracies for structure and vessel positioning are available.
Chapter 8 contains descriptions of WAAS, EGNOS, MDGPS, and RTK.

1.10 Organization of the Topic

This topic is structured to first familiarize the reader with the fundamentals of PVT
determination using GPS. Once this groundwork has been established, a description
of the GPS system architecture is presented. Next, the discussion focuses on satellite
signal characteristics and their generation. Received signal acquisition and tracking,
as well as range and velocity measurement processes, are then examined. Signal
acquisition and tracking is also analyzed in the presence of interference, multipath,
and ionospheric scintillation. GPS performance (accuracy, availability, integrity,
and continuity) is then assessed. A discussion of GPS differential techniques follows.
Sensor-aiding techniques, including Intelligent Transport Systems (ITS) automotive
applications and network-assisted GPS, are presented. These topics are followed by
a comprehensive treatment of GALILEO. Details of GLONASS, BeiDou, and the
Japanese Quasi-Zenith Satellite System (QZSS) are then provided. Finally, informa-
tion on GNSS applications and their corresponding market projections is presented.
Highlights of each chapter are summarized next.