Kepler system overview
Modern satellite navigation relies on synchronized navigation signals from a constellation of satellites in Medium Earth Orbit (MEO). Today the synchronization is achieved in a complex estimation process. The new Kepler system establishes the synchronization by direct measurements with optical links. Furthermore, a small constellation of satellites in Low Earth Orbit (LEO) enables observations without atmospheric impairments. Together with precise optical ranging between selected satellites, this provides orbit determination capabilities of unprecedented accuracy. The optical links also provide connectivity at a data rate of 50 Mbps, which ensures the fast exchange of measurements in the system.
The Kepler infrastructure thus consists of three main components: a MEO constellation of satellites; a small constellation of four LEO satellites for observation, time transfer between the MEO orbital planes and as a carrier of a few long-term stable clocks; and at least one ground station to maintain the alignment with earth rotation.
24 MEO navigation satellites (semi-major axis: 29600 km) are placed in three orbital planes, in the same orbital slots as today’s Galileo satellites. This ensures a smooth transition from Galileo to Kepler. Each MEO satellite is equipped with a cavity-stabilized laser (1550 nm) and with a direct optical link to the ahead and to the behind satellite in the same orbital plane. In the case of a failure, the optical terminals can switch to the next-neighboring satellite. A third nadir-pointing optical terminal on each MEO satellite provides a link to one LEO satellite.
The optical terminals enable two-way inter-satellite links, supporting
- Precise global synchronization of all satellites (ps-level)
- Range measurements between satellites (target accuracy: 40 µm)
- High-rate transfer of measurements and data between all satellites (50 Mbps)
All these processes take place in the optical domain. The synchronization — both in frequency and time — is transferred to the L-band by the use of a frequency comb, which maintains the stability achieved in the optical domain. The L-band signals are then generated on the MEO satellites and broadcast towards earth. The signals are backward compatible with Galileo, with some evolutions to increase the performance..
Six LEO satellites (semi-major axis: 7600 km) are placed on two near-polar orbital planes (inclination: 89.7 deg) with longitudes of the ascending nodes (RAANs) separated by 120 degrees. Each LEO satellite employs three optical terminals to establish simultaneous two-way optical links to different MEO planes.
LEO satellites are equipped with the same optical frequency reference (cavity-stabilized laser) as on the MEO satellites. This enables the global synchronization. Additionally, one or several long-term stable clocks, e.g. iodine-stabilized laser units, are included in order to implement a satellite-based time scale.
Due to the global synchronization, the on-board L-band receiver tracks actual ranges measured in vacuum. The LEO constellation provides three main capabilities:
- relays synchronization signals across MEO planes by establishing optical links to MEO satellites on different orbital planes, thus supporting the overall intra-system synchronization
- supports precise orbit determination by providing atmospheric-free observations of the transmitted navigation signals, and the whole set of inter-satellite laser ranging measurements
- maintains, or contributes to, the definition of the Kepler system time realized via an ensemble of long-term stable clocks operating at optical and/or microwave frequencies
The space segment (MEO and LEO constellations) is autonomously capable of maintaining synchronization, performing orbit determination and providing intra-system robustness. At least one ground monitoring station is necessary (and sufficient) to ensure synchronization with the Earth rotation. Furthermore, thanks to the optical inter-satellite links a single uplink station can communicate with every satellite in the system at all times.
Cavity-stabilized lasers are characterized by a frequency stability below the 10−15 [s/s] (Allan deviation) level for averaging times of up to 10 seconds. All MEO satellites can communicate with a latency of at most 795.6 ms. Thus all components in the system can be synchronized within a time interval well below the stability range of the cavity-stabilized lasers.
Each satellite locally realizes the Kepler system time by computing a composite time scale, based on the measurements of clock offsets shared across the system. The offsets of the local cavity-stabilized lasers with respect to the implicit ensemble mean are compensated during the generation of the L-band signals on the MEO satellites. As a consequence, the clock offsets in the navigation message can all be set to zero.