Introduction - Understanding GPS: Principles and Applications - page 9

Figure 1.6 GLONASS-K satellite.

GPS/GALILEO interoperability, key elements to achieving interoperability are
compatible signal structure, geodetic coordinate reference frame, and time reference
system.

1.7 Chinese BeiDou System

The Chinese BeiDou system is a multistage satellite navigation program designed to
provide positioning, fleet-management, and precision-time dissemination to Chi-
nese military and civil users. Currently, BeiDou is in a semi-operational phase with
three satellites deployed in geostationary orbit over China. The official Chinese
press has designated the constellation as the BeiDou Navigation Test System
(BNTS). The BNTS provides a radio determination satellite service (RDSS). Unlike
GPS, GALILEO and GLONASS, which employ one-way TOA measurements, the
RDSS requires two-way range measurements. That is, a system operations center
sends out a polling signal through one of the BeiDou satellites to a subset of users.
These users respond to this signal by transmitting a signal through at least two of
the system's three geostationary satellites. The travel time is measured as the naviga-
tion signals loop from operations center to the satellite, to the receiver on the user
platform, and back around. With this time-lapse information, the known locations
of the two satellites, and an estimate of the user altitude, the user's location can be
determined by the operations center. Once calculated, the operations center trans-
mits the positioning information to the user. Since the operations center must calcu-
late the positions for all subscribers to the system, BeiDou can also be used for fleet
management and communications [14, 15].
Current plans call for the BNTS to also provide integrity and wide area differen-
tial corrections via a satellite-based augmentation system (SBAS) service. (SBAS is
described in detail in Chapter 8.) At present, the RDSS capability is operational, and