Spacecraft bus

Key technical performance features:

Mass	Platform : 300 kg, Payload: > at 300 kg
Orbits	All low orbits with an inclination > 20°, semi-major axis between 500 and 1,500 km
Launcher - Volume under the fairing	Multi-launcher
Power	About 300 W platform and > 300 W payload
Manoeuvres	120 m/s for a 500-kg satellite
Autonomy	Autonomous Survival Mode, ability to support routine working days/hours
Pointing	all kinds of pointing
Pointing accuracy	0.05° bias + 0.05° outside bias / axis
Stability	1 E-³ °/s at low frequency < 1 Hz
Exchanges with payload	16 power lines + point to point links + MIL 1553 B 160 kbps bus
Data storage	500 Mbits housekeeping and 2 Gbits payload
TM/TC	S band, CCSDS, 800 kbps in TM/QPSK, 4 kbps in TC/BPSK
Orbit determination	Autonomous by GPS: neither Doppler, nor ranging, nor angular measurement at station
Lifetime	3 years, all consumables subject to wear or radiation are dimensioned for 5 years

Industrial contributors to the Proteus bus:

The Proteus User Manual, copied from the launcher user manual, provides precise details of the payloads the bus is able to accommodate. This manual defines the carrying capacities (mass properties, power according to the orbit, etc.) as well as all the interfaces between the bus and payload.

One of the most constraining specifications of Proteus was to ensure its compatibility with an extended range of launchers to have the widest range of launch possibilities. This figure shows how Proteus is accommodated under the fairing of several launchers. The bus design also takes into account the mechanical environment generated by different launchers.

Proteus flight envelope:

The altitude limit is directly related to radiation resistance. In respect, the Jason-1 mission was at the limit of the envelope and  constitutes the baseline in terms of lessons learned.

Bus performance was demonstrated first on the ground during development:

• through a mechanical and thermal mock-up (MSTH) demonstrating the carrying capabilities of the payload and the validaity of the thermal design
• during validation of the functional chains on test benches
• during assembling, integration and testing (AIT), particularly during the satellite PVT and EMC thermal vacuum tests
• by analysis and elementary tests for other aspects (lifetime and radiation resistance for example)

Bus performance was subsequently evaluated in flight on Jason-1, particularly for the AOCS which was partially validated in AIT but which mostly can be achieved on the ground through modelling and simulations.

Main operations performed during Jason-1 in-orbit commissioning. The description variable chosen is the solar angle to the orbital plane (beta angle).

Given Jason-1's orbit at 1,336 km and inclined 66°, there was a natural rotation of the satellite of -3°/day on the ascending node relative to the Sun, meaning the satellite was able to accommodate all possible local times, as well as variable eclipse lengths.

As the solar array had only one degree of freedom, the satellite's yaw motion was used to ensure good illumination of the solar panels. This "yaw steering" motion, comparable to a sinusoid along the orbit, had an amplitude that depended directly on the solar angle to the orbital plane (beta angle).

Jason-1 really constituted a demonstration of the bus pointing agility, especially as orbit correction manoeuvres at a given point may require 180-degree turn from the routine sinusoidal movement. On the ground, the guidance instruction generator handles this difficulty and has to process all possible cases of meshing between profiles.

Besides the nozzle identification manoeuvres followed by the orbit insertion, there are large-angle attitude manoeuvres to identify by stellar measurements, biases, misalignments and gyroscope scale factors. Moreover, small roll and pitch attitude manoeuvres enable residual bias between the star tracker and the altimeter to be directly identified (as the altimeter is able to deliver a reading that is proportional to the square of the pointing). These characterizations were done for different solar angles to verify the significant absence of variations due to thermo-elastic effects.