Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 618

2.0m, and twin solararrays with a span of 18.1m. The twin solar
arrays are composed of three panels each and are designed to generate 2,049W of
power at the start of the mission and 1,688W EOL power. During periods of eclipse,
two 45-Ah nickel hydrogen (NiH 2 ) batteries provide power. DJS-1 carries an apogee
kick motor for postlaunch orbit-transfer maneuvers. The spacecraft has an on-sta-
tion BOL mass of 1,145 kg and a dry mass of 745 kg. Thus, the mass of the fuel is
400 kg. The DJS-1 has a minimum design life of 8 years, but too few spacecraft have
been launched to independently determine the expected on-orbit lifetime [48-50].
The DJS-1 spacecraft bus carries an apogee kick motor for orbital transition and
station keeping for north/south station-keeping maneuvers. The motor is used ini-
tially during launch to finalize the spacecraft orbit. The thrust output is 490 N. The
propellant is monomethyl hydrazine and has a mass of 200 kg [49].
The DJS-1 spacecraft bus carries a three-axis attitude control system (ACS) com-
prised of Earth sensors, momentum wheels, and 14 ACS thrusters. During the
on-station operation, the ACS maintains the satellite pointing toward China with
both a northward and inward angle. The satellites are maintained to within 0.10°
beam pointing error [50].
During the preoperational mode, from launch vehicle separation through Earth
acquisition, the satellite is spin-stabilized. Data from the horizon and Sun sensors is
telemetered to the ground for determination of the satellite attitude. The satellite
transitions from spin-stabilized to three-axis stabilized with a dual-spin turn
maneuver [48].
In the normal mission mode, the satellite is in an off-nadir Earth-oriented atti-
tude with the momentum wheels. The satellite is three-axis stabilized, with an Earth
sensor providing the attitude reference, a momentum wheel providing the required
gyroscopic stiffness as well as momentum exchange for pitch control, and autono-
mous magnetic torquing providing roll and yaw control. Thrusters provide the
backup three-axis control during station-keeping maneuvers and momentum
unloading [49].

2.2m × 1.72m ×

11.2.7 RDSS Service Infrastructure
The BNTS is designed to provide regional two-dimensional communications, posi-
tioning, and fleet management services over China and Taiwan using a RDSS tech-
nique. Under the RDSS concept, the Payload Operations Center (POC) sends out a
navigation or polling signal through one of the BeiDou satellites. Subsets of users
respond to this signal via both satellites. The travel time is measured as the naviga-
tion signals loop from POC to the satellite, to the receiver on the user platform, and
back around. With this time-lapse information, the known locations of the two sat-
ellites and an estimate of the user altitude, the user's location can be determined by
the POC. Once calculated, the POC transmits the positioning information to the
user. Since the POC must calculate the positions for all subscribers to the system,
BeiDou can also be used for fleet management and communications [43, 44].
To support the RDSS positioning, the Chinese BNTS is composed of:

• A two or three geostationary satellite constellation;