The EP satellite is a 3-axis stabilized, inertial-pointed platform to explore all-sky transient X-ray sources and constantly various X-ray sources. In the all-sky survey mode the satellite observes the night-side sky and makes a coverage of the half sky in every 3 orbits. In the day-side orbit the satellite acquires power energy with the solar illumination. In the night-side orbit, the satellite observes different regions of the night-side sky by in-orbit manoeuvring. At both of the west and east side of the earth, the satellite manoeuvres 60 degree west or east off the solar-earth line respectively, as shown in Figure 1. In the sequence of every 3 orbits, the orientation of the satellite is set to 60 degree upper off the earth equator plane, parallel to the earth equator plane and 60 degree down off the earth equator plane, as shown in Figure 2.
Figure 1 Illustration of the step-and-stare survey strategy for the EP/WXT in one orbit. There are 3 pointings with ~20min exposure each and only the night-sky is monitored. During part of the orbital day time no observations are made. All-sky patrolling observation
Figure 2 Directios of WXT pointings in three successive orbits in the normal survey mode. In such a way the whole night sky is covered in 3 orbits. With the motion of the Sun on the sky the whole sky can be covered in half a year time.
2 Orbit requirements
A low-earth circular orbit with a low inclination in an altitude of 550 to 650 km is preferred to be designed for the EP satellite, which should mainly satisfy the requirements of observing operations, near-real-time data downlink/uplink, and minimizing irradiation interference by energetic particles mostly concentrated in the radiation belts and South Atlantic Anomaly (SAA) areas. The lifetime of the EP satellite is required to be 3 years (5 years as a goal). The launch site will be in Xichang Satellite Launch Centre, which allows direct orbit injection with inclination lower than 30 degree.
Table1 Operating Orbit Parameters
e ≤ 0.003（3σ）
3 Spacecraft design overview
3.1 Satellite platform
The platform uses an extension of the product that had been successfully used for the missions of Beidou, Tan and SVOM satellites, which has been proven high stability and high technical ready maturity. The launching status of the whole satellite is:
Power: 825 Watts in average
Attitude: 3-axis stabilizing, inertial orientation during observation
Manoeuvring capacity: better than 60 degree in 4 minutes
Pointing accuracy: better than 0.05 degree
Stability: better than 0.0005 degree/s
3.2 Scientific payloads alignment
The satellite aligns the 12 WXT models in a 1/9 spherical surface according to their combined FoV of 3600 square degree, with the FXT in the central position. The FXT optic axis is set to be a critical baseline for all the 12 WXT models' orientation and the star sensors that give pivotal knowledge for the satellite manoeuvring in-orbit. A preliminary layout of the platform is shown in Figure 3
Figure 3 A preliminary layout of the platform.
3.3 Satellite operating modes
EP Satellite includes the following operating modes: all-sky survey, burst observation mode, ToO mode, calibration mode, energy acquisition mode and security mode. In order to realize the variety of operating modes above, the satellite has been designed to adapt orientating strategy of solar orientation, attitude maneuvering, and target orientation (inertial orientation).
Figure 4 Operation mode and pointing strategy
4 Fast alert downlink
Once transient astronautic objects are caught sight by WXT in orbit, scientific alert message for event report will be downlink to ground stations as soon as possible via the short-message-broadcast service of Beidou navigation system or VHF network service.
5 ToO uplink
EP satellite is able to maneuver in-orbit to set the FXT pointing to transient astronautic objects found and located by other ground-based facilities, via uplinked telecommands from ground stations. The near-real-time telecommands uplinked is realized through the short-message-broadcast service provided by Beidou navigation system, with the telemetry telecommand system as an alternative backup.