Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 623

11.2.10 System Coverage and Accuracy
The BNTS RDSS service provides continuous, all-weather, day-night coverage in
the Northern Hemisphere, focused on China and the surrounding area for mid-
latitudes where the system provides optimal satellite geometry for a position fix.
The coverage footprints depend on type of service used. For navigation and posi-
tioning, two satellites are required to be in view in order to obtain a position fix,
thus limiting coverage to the overlapping footprints of the satellites. The Chinese
advertise the coverage as being between 5ºN and 55ºN latitude and 70ºE and 140ºE
longitude. In this area, the BNTS should provide approximately up to 20m. The
accuracy typically degrades as much as several factors of ten for southern latitudes
within BNTS coverage area or in rugged terrain. For text messaging and broadcast
of GPS and GLONASS corrections, only one of the satellites is required to be in
view in order to communicate, thus expanding the coverage to the composite of the
satellite footprints [43, 44].
The Chinese advertise the BeiDou RDSS accuracy ranging from 20m to 100m.
However, this level of performance can only be obtained in areas where calibration
sites called benchmarks are deployed. With other RDSS implementations, altitude is
determined at the master control station by using digital maps in the course of calcu-
lating the user's location. Any errors in the altitude estimated would result in addi-
tional corresponding errors in the user's position [43, 44].
11.2.11 Future Developments
11.2.11.1 BeiDou-1 SBAS Service
In addition to BNTS RDSS service, the Chinese are in the process of establishing an
indigenous SBAS using the L-band transponder carried on the third BeiDou-1 satel-
lite (110ºE longitude). At the time of writing, little information was available on the
service's status or Chinese future plans. What is known is that the Chinese filed
radio frequency registrations with the International Telecommunication Union
(ITU) for L1 (1,559-1,610 MHz) and L2 (1,215-1,260 MHz) in 1999 for an SBAS
service. Subsequent filings indicated that the Chinese planned for the SBAS payload
to be nadir pointing and provide coverage in both the Northern and Southern
Hemispheres. In addition, from 2000-2002, the Chinese purchased SBAS ground
monitoring equipment from Canadian company Novatel, the supplier of ground
equipment for the U.S. WAAS, the EGNOS, and Japanese MSAS. The SBAS service
will likely share the GPS monitoring structure currently transmitting corrections via
the S-band signals. Potentially, the SBAS will augment both GPS and GLONASS. At
the time of this writing, the Chinese had not released an ICD [51-59].
11.2.11.2 BeiDou-2
Beyond the current BeiDou-1 system, the Chinese are in the late stages of designing a
follow-on system designated BeiDou-2. The Chinese announced 2005 as the target
date to complete research and finalize the design. Based on the history of the BeiDou-1
program, the Chinese will likely take about 5 years to begin launching the BeiDou-2
satellites and will take 3 to 6 years to fully populate the full constellation [60].