distributed space platform - unsam · 2017-06-09 · distributed space platform for lateral sar...

1st IAA Latin American Symposium on Small Satellites: Advanced Technologies and Distributed Systems

IAA-LA-05-01

Distributed Space Platformfor

Lateral SAR Swath Revisit: Geometry, Guidance and Control

3/8/2017 1

Claus M. Rosito – Martín Españ[email protected] - [email protected]

March 7–10, 2017, Buenos Aires, Argentina

Objetive

To Co-Observe the lateral SAR Swath of SAOCOM

(Using a Distributed Space Platform)• Assess Overall Feasibility• Find Geometry and Constraints• Guidance & Control Implications

3/8/2017 2Distributed Space Platform for Lateral SAR Swath Revisit: Geometry, Guidance and Control

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Background

SAOCOMBACKGROUND


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J. Medina, et al, “Saocom Mission Overview,” CEOS SAR C&V Workshop, Fairbanks, Alaska, USA, 2011.

Objetive

To Co-Observe the lateral SAR Swath of SAOCOM

(Using a Distributed Space Platform)• Assess Overall Feasibility• Find Geometry and Constraints• Guidance & Control Implications


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Problem Formulation3/8/2017 5

• SAR points OFF-NADIR

Distributed Space Platform for Lateral SAR Swath Revisit: Geometry, Guidance and Control

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• SAR points OFF-NADIR• Lambert’s Cosine Law

degrades optical OFF-NADIR performance


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• SAR points OFF-NADIR• Lambert’s cosine Law degrades optical

OFF-NADIR performance

There is noÞ CENTRAL ORBIT

coincident with the SWATH

Solution: a DISTRIBUTED PLATFORM consisting of several satellites taking turns passing over

SAOCOM SWATH


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Orbits for Co-Observation3/8/2017 8

Working Hypothesis (First Approximation)1. Spherical Earth (Relaxed to WGS-84 Ellipsoid later on)2. J2: Only Kinematics3. Platform members share inclination and semi-major axis with SAOCOM

Tools:1. Observation Targets are given in (lat, long) 2. Sun synchronous orbits3.=> ECEF


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Co-Observing Orbits3/8/2017 9

Procedure:1. Obtain the points on the SWATH analytically.2. Find an orbit passing over a set of points on the SWATH3. Asses CO-OBSERVATION PERFORMANCE

THEN: Assess Guidance and Control Strategies & Challenges


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SAR pointing parameters are translated to

parameters useful for NAVIGATION

CentralAngle

1. Find Points on Swath


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Intersection:

SAR pointing parameters are translated to

parameters useful for NAVIGATION

1. Find Points on Swath


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By Hypothesis:Semi-major Axis, Inclination y Eccentricity

are fixed

3 Parameters define the Orbit

Geometry

1. Target Latitude2. Target Longitude3. Target Co-Observation Time

(or Time Difference after Main Satellite observation)

2. Find Orbit passing over Target


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Co-Observing Orbit is defined by:1. Longitude of the 1st Ascending Node2. Time of Passage over the 1st Ascending Node

2. Find Orbit passing over Target


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Sub Satellite Point of Co-Observing Satellite (lat, long) in terms of:1. Longitude of the 1st Ascending Node2. Time of Passage over the 1st Ascending Node3. Time

ThisisthepointonEarthbeingobservedbytheCo-ObservingSatellite

3. Co-Observation Performance


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Non-Spherical Earth Correction3/8/2017 15

Correction to account forWGS-84 Ellipsoid

C -> A


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Example: Target is a point in Córdoba, Argentina, and Co-Observation Delay is set to 5 min:



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Performance (Overlap Extents) Depend on Target Latitude:



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Overlap3/8/2017 19

Example over several passes. Ascending vs. Descending


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Formation Geometry Determination3/8/2017 20

Single Satellite may have low FOV


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”Virtual Chief”:

1. Doesn’t need to be close to SAOCOM

2. A formation can improve FOV around a target

3. Introduce the “Virtual Chief” as a GUIDANCE strategy


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Co-Observing orbit found,

Þ Obtain transformation:

Coordinates ofpoints near -> Relative Orbital ElementsTARGET


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Virtual Chief3/8/2017 26

Tools developed for Formation Flying …


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Tools developed for Formation Flying … can be used for AOK!


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Autonomous Orbit Keeping3/8/2017 29

Orbit Keeping with Ground in the Loop requires

1. Precise Orbit Determination (from Earth)2. Maneuver Planning (in Earth)3. Upload Maneuver (transmit from Earth to Spacecraft)

4. goto 1

Autonomous Orbit Keeping:

1. Reduces mission cost 2. Enhances co-observation precision but3. Enables larger scale deployments

Onboard Precise Orbit Determination is required


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The Dynamic Model of the Satellite

Can be separated as

CONSERVATIVE DISIPATIVE


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Force Separation


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1. “Virtual Chief” defines the Reference Trajectory

2. “Deputy” is subject to the full range of perturbations.

3. Virtual Chief is the set point for a Tracking Control Problem


VirtualChief

Deputy


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Relative Control Techniques can be used to drive the Deputy to follow the Virtual Chief

Compensates for Perturbation => Orbit Keeping


VirtualChief

Deputy


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Relative Control Techniques can be used to drive the Deputyto follow the Virtual Chief


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Error Dynamics:

Adequate choice of gain matrix elements K provide asymptotic stability.


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The Deputy is able to maintain a predetermined orbit Compensating for Perturbations

The strongest Hypothesis used is Continuous Control(Ground in the Loop is not needed)

ElectricPropulsion


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Current Work and Future Work3/8/2017 37

1. The Virtual Chief – Deputy system is being simulated to validate the Control Strategies and models.

2. Hybrid impulsive/continuous (Garulli, De Florio) control is beingtested.

3. These Control Strategies are being considered for Orbit Reconfiguration Maneuvers


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Further study must assess the implications of relaxing the hypothesis

1. Spherical Earth (up to which order. ?)

2. Eccentricity ( )

3. Inclination (while keeping Sun Synchronicity)

Current Work and Future Work3/8/2017 38Distributed Space Platform for Lateral SAR Swath Revisit:

Geometry, Guidance and Control1s

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Main Results3/8/2017 39

1. Orbits design methods to Co-Observe Lateral Swaths were obtained.

2. From Required Target Geometry to Distributed Platform Design.

3. Control Strategies for Autonomous Orbit Keeping were assessed.


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Conclusions3/8/2017 40

1. Tools to design and analyze missions of Distributed Space Platforms for Lateral Swath Co-Observation were developed.

2. All together such a mission seems feasible, challenges and lines of work have been identified and are being worked on…

3. Autonomous Obit Control requires highly reliable GNC SW (GNC Algorithms) and HW.

=> INTENSIVE ON GROUND VALIDATION IS NEEDED


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Questions3/8/2017 41

¿Questions?


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1st IAA Latin American Symposium on Small Satellites: Advanced Technologies and Distributed Systems

IAA-LA-05-01

Distributed Space Platformfor

Lateral SAR Swath Revisit: Geometry, Guidance and Control

3/8/2017 42

Claus M. Rosito – Martín Españ[email protected] - [email protected]

March 7–10, 2017, Buenos Aires, Argentina

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distributed space platform - unsam · 2017-06-09 · distributed space platform for lateral sar...

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