The Kepler Satellite Navigation System

The Kepler Satellite Navigation System

Christoph Günther, DLR and TU-Münchenand the team*

Stanford PNT-Symposium 2020, October 28th

> Lecture > Author • Document > DateDLR.de • Chart 1

* members listen on the slides

Position, Orbits and Synchronization

• GNSS• measurement of L-band pseudoranges

• orbit determination capitalizes on long term stable atomic clocks: Rb, H-Maser

• estimation of the clock offsets (orbits first)biases, ionospheric delays (and more)


and on physics

Achieving High Accuracy

• Real Time Kinematic (RTK)• corrections to measurement of code and phase• local coverage (10-20 km)

• Network RTK• interpolation of the corrections at the provider or user• regional coverage (e.g. Germany)

• Precise Point Positioning (PPP)• vector corrections (orbits, biases)• long convergence, stations dist. 1000 km

• PPP-RTK• fast convergence, station dist. 100 km

• Kepler• precise orbits, sync. and biases• distributed in the navigation message• global coverage (world)


Goal: high accuracy everywhere at any time without augmentation

Sapos / Bavaria

PPP-RTKSeptentrio

The impact of Ephemeris estimation in Galileo and Kepler Initial results - Patrick Henkel, ENC 2020

• Triple frequency E1/E6/E5a, biases known, �� = 50 km/h


18 ground monitoring stations 1 ground monitoring station

Integrity

• Bound positioning errors at any time, control the tail of the distribution at the user location• Today: involves a service operator providing augmentation data and alerts

• GBAS• Single frequency measurement corrections• Uncertainty parameters, ionospheric gradients, alerts• VHF/LDACS distribution, latency 2 s.

• SBAS: WAAS, EGNOS and others• Single frequency service, vector correction parameters

(orbits, clocks, ionosphere), alerts• Geostationary and internet, latency 6 s.

• Kepler• Multi frequency service• Signal bias corrections, alerts• Navigation message, latency 3 s.


Goal: low delay integrity everywhere at any time without augmentation

Kepler System Architecture

• Reuse of the Galileo orbital slots -> migration scenario

• Stable oscillators on all satellites (no clocks needed)MEO – MEO optical two-way links within the orbital plane

• Inter plane connectivity through LEO Satellites (constellation of 6 satellites at 1209 km)

• Observation of the L-band signal from outside the atmosphere

• Mid-term stable clocks on LEOs for autonomous time keeping up to roughly 1 hour

• One ground station to preserve the alignment with earth rotation (not at the pole!) and with UTC


MEO

LEO

broadcastL-band

Dynamics of the Links Gabriele Giorgi


Synchronization and Orbits


• GNSS• measurement of L-band pseudoranges

• orbit determination capitalizes on long term stable atomic clocks: Rb, H-Maserand on physics

• estimation of the clock offsets (orbits first)biases, ionospheric delay (and more)

• Kepler

• two way optical inter-satellite links for synchronization

• synchronization first, ranges instead of pseudoranges

Optical intersatellite links

• Carrier wavelength 1064 (1550) nm

• Link distance: 22’023 - 41’686 km

• Telescope aperture: 5-7 cm

• TX power: 5 W

• Bit duration 20 ns (50 Mbps)

• Spreading factor 511 (25.55 Gcps)

• Link budget adapted to 30 m


Stability of the Time References Used

• Normalized phase increment:

• Allan Variance:

• Phase measurement: � � = ��,� � + �� - ��,� �

(FSO = Free Space Optical)


� � =1

2��

� � + � − �(�)

�

�� =

1

2 �� + � − � �

�

=1

2 2�� + � − 2� � + �(� − �) �

= Δ�� + �� + �̇�� + ��(�)

�� = ��,�

� � +3��

�

2��

1

��

Syntonization Performance - Frequency Transfer



Measurement ResultsMata-Calvo, Poliak, Surof, Wolf; ENC 2020

Synchronization Performance – Time Transfer

• Option 1 - high performance: frequency comb generates RF from optical = reference

• Option 2 - low complexity: local ultra stable RF oscillators

• Laboratory Test: H-Maser as RF reference


100 101 102 103

Sampling interval [s]

10-15

10-14

10-13

Time transfer measurement Hallway

Time transfer measurement Optics Lab

TWTT reduced interval

TWTT full data

1/

Measurement resultsMata-Calvo, Poliak, Surof, Wolf; ENC 2020

• Below ps accuracy @ 1 s, i.e. submillimeteraccuracy

• Periodic temperature variationsof the climatization at roughly 200 s.


Clock options

• Highest stability: cavity

• High TRL level: USO

• Medium term stability: Iodine clock


USO Quartz

Ultra low expansion glasNPL design

picture by Airbus

picture by Menlo

1 �� ≜ 3 ��

region of interestfor synchronization

Iodine ClockT. Schuldt, et al. ENC 2020*)


M. Oswald, T. Schuldt et al.shoe box sized of spectroscopy moduleflown on sounding rocket May 2018

*) T.Schuldt, M. Gohlke, M. Oswald, J. Sanjuan,T. Wegehaupt, T. Blomberg, J. Wüst, L. BlümerlV. Gualani, K. Abich, C. Braxmaier

Greenhall SynchronizationC. Trainotti, T. Schmidt, J. Furthner

• Implicit Ensemble Mean (IEM) computed in a first “Kalman filter” – Composite Clock

• Realization by controlling a microphase stepper to follow the IEM


Composite Clock

IEM computation

Estimation of clock

error state

Greenhall SynchronizationC. Trainotti, T. Schmidt, J. Furthner


Ensemble mean of the composite clock

� [s]

� �(�

)[s

/s]

LEOiodine

GND(UTC)H-Maser

Kepler time scale

Sat.CSL

• Time offsets to neighboring satellites are continuously measured

• All time offsets are communicated to all satellites

• Each satellite computes a composite “clock”, which uses

• the cavity stabilized lasers, • a small number of long-term stable clocks on LEOs

• selected “time biases”.or the ground, as well as

Orbit determinationby GFZ-Potsdam @ OP


• Optical intersatellite linksMEO-MEO, MEO-LEO: ranges with �� = 1 mm.

• L-band MEO-LEO links: ranges (sync. satellites) with�� = 50 cm, �� = 3 mm.

• L-band MEO-GND:pseudoranges with �� = 50 cm, �� = 5 mm.

Orbit Determination MEO+LEO – Modelling AssumptionsGrzegorz Michalak, Karl H. Neumayer, Rolf König, GFZ


Model Orbit Simulation Orbit Determination

Gravity field EIGEN-6 EIGEN-5

Solid earth tides IERS Conv. 2003 IERS Conv. 1996

Ocean loading activated deactivated

LEO air drag solar flux previous day

LEO solar rad. pressure Cannonball polygon scaling factor

LEO antenna PCO along +3, cross+4, rad.+5 cm

MEO solar rad. pressure ECOM 9 ECOM 5

MEO antenna PCO north/east +3, up +5 cm

MEO antenna thrust activated deactivated

Orbit Determination (24 MEO + 6 LEO Satellites, 1/18 Stations)Grzegorz Michalak, Karl H. Neumayer, Rolf König GFZ

• Three empirical accelerations adapted every 30 minutes for each LEO and each MEO


Next Steps

• Continue the lab experiments 30 m


• Experiments on the test rangeWeilheim-Hohenpeißenberg 10.45 km

Verification and ValidationMissions

• Compasso: verification mission on the ISS

• launch 2024

• optical terminals, USO, frequency comb,iodine clock

• satellite to ground links, technology verification

• OTTEx proposal for in MEO Orbit validation

• launch 2026-2028

• optical terminals, USO, frequency comb,iodine clock

• inter-satellite links and ground linkstechnology verification and system validation


Compasso - ISS-Bartolomeo

MEO Orbits

Selected Components


Menlo Systemssounding rockets

2015/16/18

TESAT SpacecomLEO/GEO2021/2016

Frequency CombOptical Terminal Iodine clock

Spectroscopy cellsounding rockets

2018

Ultra Stable Osc.

„Symmetricom“LRO (Moon)since 2009

Kepler - Summary

Goals

• Provide instantaneous precise point positioning

• Support global integrity with low delays

• Maintain the backward compatibility of services

• Ensure a smooth evolution of the constellation

• Minimize the extent of the ground segment

• Minimize the reliance on atomic/optical clocks

They are mainly achieved by an improved observability through measurements


Kepler constellation of MEO and LEO Satellites

Some Members of the Team


Documents

The Kepler Satellite Navigation System