Kepler � Satellite Navigation without Clocks and Ground Infrastructure

Abstract

Global Navigation Satellite Systems receivers use pseudorange measurements for positioning and time determination. The control system uses the same measurements for estimating the satellite orbits, clock offsets and signal biases in a complex estimation process, which additionally involves the determination of atmospheric delays. In current systems, the separation of these different parameters imposes strong requirements on the stability of the satellite clocks and on the ground infrastructure. Several hundred ground monitoring stations are additionally used in high accuracy applications. The Kepler proposal follows a different approach. It re-uses the Galileo constellation of Medium Earth Orbiting (MEO) satellites, as well as its signals but complements them by a small constellation of Low Earth Orbiting (LEO) satellites and by selected optical Inter-Satellite Links (ISL). The LEO satellites are used to observe the signals transmitted by the MEO satellites from outside the atmosphere. The optical ISL connect all satellites in a multi-hop fashion. This enables a direct synchronization of the satellites at a level not achievable today and additionally does not require that the MEO satellites are equipped with clocks. The optical ISL furthermore provide ranges rather than pseudoranges for orbit determination as well as a broadband intra-system communication network. Terrestrial infrastructures become mostly obsolete with the Kepler approach. A single monitoring and control station remains necessary to maintain the alignment with earth-rotation, the synchronization to Universal Time Coordinate (UTC), and the capability of controlling the system.

Long-term stable optical cavity for special relativity tests in space

“…The required fractional stability and the frequency range of interest are diverse and span from values close to (in units of the square root of the power spectral density, PSD) 10 −17 Hz −1/2 at short time scales in atomic clocks [1][2][3][4][5][6] to 10 −13 Hz −1/2 at long time scales in space based laser interferometers such as the future gravitational wave detector LISA (Laser Interferometer Space Antenna) [7], the LRI (Laser Ranging Interferometer) on GRACE Follow-On (Gravity Recovery and Climate Experiment Follow-On) [8,9] and the next generation of gravity field missions [10]. Applications in future GNSS (Global Navigation Satellite Systems) concepts [11,12] will also benefit from optical cavities exhibiting stability levels in the 10 −15 Hz −1/2 range at time scales of seconds. Highstability frequency references are also key elements for tests of fundamental physics such as detection of violations of Lorentz invariance [13][14][15] through Michelson-Morley (MM) [16,17] and Kennedy-Thorndike (KT) [18,19] experiments.…”

Precise Point Positioning with Kepler

“…Kepler has been proposed by [2] as next-generation GNSS. It uses optical inter-satellite range measurements, that are several orders of magnitude more accurate than current L-band measurements and that are not affected by atmospheric errors.…”

“…These estimates are derived today in global network solutions with typically more than 100 ground stations. The next generation GNSS Kepler proposed by [2] uses highly accurate optical intersatellite links, which enables a very accurate estimation of satellite position, clock offset and biases without the need of an extensive ground infrastructure. The L-band signals of Kepler are equal to those of Galileo, i.e.…”

Relativistic Modelling for Accurate Time Transfer via Optical Inter-Satellite Links

“…Typical SiSRE values in modern GNSSs are in the order of a few tens of centimeters. The DLR Institute of Communication and Navigation is currently working on a next generation satellite system (with codename Kepler) based on optical inter-satellite links (OISLs) providing autonomous synchronization with offsets below 1 ps [5,6]. OISLs allow for a better separation of space (orbits) and time (synchronization), leading to a much higher level of synchronization across the whole constellation than what is achievable in current architectures, which in turn largely enhances precise orbit determination.…”