IEEE Geoscience and Remote Sensing Magazine - September 2020 - 11

The ZY3-03 is designed to provide high-resolution stereo images, multispectral images, and auxiliary data over
China's territory in a long-term, stable, and efficient manner. The dual-satellite operation mode will be enabled after
the launch of the ZY3-03, reducing the revisit cycle time to
fewer than three days, compared with five days for one operational satellite. In addition, the global coverage time will
be reduced significantly.
THE ZY3-03 PLATFORM
The ZY3-03 is a three-axis-stabilized Earth observation satellite that is being manufactured by the China Academy of
Space Technology (Figure 3). The platform, although based
on upgraded avionics, retains the architecture of the ZY301 and ZY3-02.
The satellite consists of payloads and service systems.
The payloads mainly include a three-line-array camera, a
multispectral camera, a laser altimeter, a digital-transmission subsystem, digital antennas, and an image-recording
subsystem. The service systems comprise the power supply,
control, propulsion, and data and thermal management
subsystems, which provide support services, such as installation, power, control, and temperature maintenance for
the payloads.
The ZY3-03 runs in a sun-synchronous orbit at an altitude of approximately 505 km and is capable of producing
seamless imagery covering Earth's surface from latitudes
of 84° south to 84° north. The spacecraft designed for the
ZY3-03 mission is a three-axis-stabilized satellite directly placed into Earth orbit by the launcher. The platform provides high pointing accuracy (approximately 0.1°
on each axis), and the weight of the spacecraft is approximately 2,650 kg.
The main characteristics of the ZY3-03 spacecraft are
shown in Table 1. It uses a mature ZY3 satellite platform
with a stable structure. To fulfill the demands of the mapping mission, the ZY3-03 service system fully inherits the
fine state of the ZY3 series platform, which is designed with
improvements in precise attitude determination (PAD),
precise orbit determination, and time synchronization. The
main technical specifications of the service system are
shown in Table 2.
PRECISE ATTITUDE DETERMINATION
To enhance attitude accuracy and stability, star sensors
and gyroscopes are always equipped with state-of-the-art
satellites. Star sensors are used to determine the absolute
inertial attitude without drift at low frequencies to reach
the highest accuracy possible, and gyroscopes are used to
measure the angular velocity at high frequencies.
The PAD system (PADS) is designed to fulfill 1:50,000-scale
surveying and mapping requirements. The attitude determination accuracy of the ZY3-03 is 1 arc-second, surpassing other satellites with similar specifications. The ZY3-03
is equipped with three star trackers and four groups of gyroscopes to acquire the attitude data. The raw data of the
SEPTEMBER 2020

IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

star trackers and gyroscopes are processed onboard with
an extend Kalman filter to realize real-time attitude determination, followed by a forward-backward Kalman filter
for high-precision attitude postprocessing. To ensure that
the time is consistent and uniform in the satellite, time
synchronization of all subsystems is controlled by the
timing system.

FIGURE 3. An artist's interpretation of the ZY3-03.

TABLE 1. THE ZY3-03 SPACECRAFT CHARACTERISTICS.
FEATURES

SPECIFICATIONS

Orbit

Sun-synchronous at 505 km
59-day repeat cycle

Mean local solar time

10:30 a.m.

Orbital period

97 min

Attitude stabilization

Three-axis stabilized

Weight

2,650 kg

Mission duration

Eight years

Swath width

51 km

TABLE 2. THE ZY3-03 SERVICE SYSTEM CHARACTERISTICS.
FEATURES

SPECIFICATIONS

Attitude control system

Pointing accuracy: ≤0.02° (three-axis, 3 v)
Stability: ≤5 × 10 -4°/s (three-axis, 3 v)
Drift angle correction: 0.02° (3 v, calculated based on nadir-view camera)

Time-synchronization
accuracy

≤50 µs

Time reference system

Universal time coordinated, Beijing

Satellite laser ranging

Corner reflector equipped on platform

Remote observe and
control

Method: USB
Mode: plain code transmission

Star tracker

Inertial attitude determination accuracy:
≤5" (xy-plane, 3v)
Sampling frequency: 4 Hz

Gyroscope

Drift error: 0.02°/h
Sampling frequency: 4 Hz

GPS signal receiver

Type: dual band
Location accuracy: 10 m (real time, 1 v)

IEEE Geoscience and Remote Sensing Magazine - September 2020

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