KOMPSAT-6 Mission & External Calibration System Design

Dochul Yang*, Horyung Jeong, Dong Kim, Donghan Lee, Jaemin Shin, and Seonho Lee

S AR Im age , 백두산 Opt ica l Im age , 백두산

• K6 Mission & System Design

• K6 External Calibration Design

• K6 Active Transponder Development

K-6 Mission & System Design

K6 Mission

Objectives: Expedite provision of the space-borne SAR standard images with sub-meter resolution required for the national demand in GIS (Geographical Information Systems), Ocean & Land management, Disaster monitoring, and ENvironment monitoring

Mission Application: “GOLDEN”

• GIS : Acquisition of independent high resolution images

• Ocean & Land Management : Survey of natural resources

• Disaster & ENvironment Monitoring : Surveillance of large scale disasters and its countermeasure

Expected Launch Date: Year 2020

Life Time: 5 years

System Architecture

Orbital Characteristics

Sun-Synchronous Dusk-Dawn Orbit

• 505 km mean altitude and 97.422 deg inclination

• 6:00 PM MLTAN (Mean Local Time of Ascending Node)

• Orbits per day of 15+2/11

• 11 number of days in the repeat ground track cycle

• Ground speed of 7.05 km/s, Nodal period of 94.85 minutes

Ground Tracks for 11 Days

• Total number of orbits: 167

• Distance between adjacent ground tracks at equator: 240 km

SAR Payload

General Characteristics

• Space-borne Synthetic Aperture Radar

• X-band radar with an active phased array antenna

• Implemented by four SAR modes

High Resolution A/B Modes : Sliding Spot

Standard Mode : Stripmap

Wide Swath Mode : TOPS

• Electrical steering capability in both azimuth and elevation planes

• Coherent Dual Polarization (HH+HV, VV+VH)

• Quad Polarization & ATI/GMTI as Experimental Modes

Imaging Operation Performance

Observation Mode

• High Resolution-A Mode (Spotlight) : 0.5 m resolution / 5 km swath

• High Resolution-B Mode (Spotlight) : 1 m resolution / 10 km swath

• Standard Mode (Stripmap) : 3 m resolution / 30 km swath

• Wide Swath Mode (TOPS) : 20 m resolution / 100 km swath

Incidence Angle Range

• 20 ~ 55 deg (nominal)

• 55 ~ 60 deg (extended)

Imaging Time

• Continuously 150 sec per orbit

Target Revisit Time

• 53 hours on average

Ground Segment

KOMPSAT-6 Ground Segment (KGS) Responsibility

• Mission Planning and Scheduling

• Command and Control

• Telemetry Reception and Processing

• Flight Dynamics Operations

• Tracking and Orbit Determination

• Ground Track Maintenance Maneuver

• Payload Data Collection, Processing, and Product Generation

• Calibration and Maintenance of Long-term System Performance

Ground Segment is composed of three elements

• MCE (Mission Control Element)

• IRPE (Image Reception and Processing Element)

• CE (Calibration Element)

KGS Architecture

K-6 External

Calibration Design

CE Overview (1/2)

CE Functions

• Range & Azimuth Antenna Pointing Offset Measurement

• Geometric Range & Azimuth Offset Measurement using Point Target

• Product Coverage Location Error Measurement

• Antenna Pattern Verification

• Absolute Radiometric Calibration Factor Evaluation

• Polarimetric Effects (Channel Imbalance & Cross-Talk) Compensation

• Long-Term & Daily Calibration Image Scheduling

CE Subsystems

• PCS: Pointing Calibration Subsystem

• GCS: Geometric Calibration Subsystem

• RCS: Radiometric Calibration Subsystem

• MCS: Multi-polarimetric Calibration Subsystem

• CSS: Calibration Scheduling Subsystem

CE Overview (2/2)

CE Architecture

MCE IRPE CSS

Image Collection

Plan

RCS MCS GCS PCS

Level Product

IRPE

Ground CAL Target ( Amazon , AT, and CR .)

K 6 CE

Download SAR Science

Data (Image + Ancillary) Upload

CMD

Level

Product

Image Collection

Request

ATT & CMD

Time Offset

APLUT

AZCUT [TBD]

CALCO

Channel

Imbalance &

Cross-talk Azimuth & Range

Time Offset

Channel

Imbalance &

Cross-talk

APLUT

AZCUT [TBD]

CALCO

Operational Flow

Calibration Element

IRPE CSS PCS GCS RCS MCS

Orbit Data

Level Product

CAL Image

Scheduling

Pointing

Determination

Calibration Image Request

Calibration Parameter (ATT & CMD Time Offset)

Level Product Geometric

Time Offset

Evaluation Calibration Parameter (Azimuth & Range Time Offset)

Level Product

Absolute CALCO Evaluation

Calibration Parameter (APLUT, AZCUT [TBD])

Level Product

Calibration Parameter (Distortion Matrix)

Channel Imbalance &

Cross-Talk Evaluation

Calibration Image Confirmation

RCS

Antenna Pattern Verification

Peak-to-Peak Gain Measure

Level Product

Calibration Parameter (CALCO)

PCS (Pointing Calibration Subsystem)

• Range Antenna Pointing Offset Determination Use SCS image on homogeneous area (Amazon) imaged with range notch pattern

Extract range gamma profile from image

Compare generated gamma profile to reference antenna pattern

• Azimuth Antenna Pointing Offset Determination Record antenna TX signal strength with active transponder

Convert recorded signal strength to antenna pattern along azimuth angle using satellite-transponder geometry and time information

Compare estimated azimuth pattern to reference antenna notch pattern

• Satellite Attitude Offset Determination using Doppler Centroid (Additional) Estimate Doppler Centroid (DC) from Raw image

Calculate satellite yaw/pitch offset & CMD time offset from the estimated DC

Antenna Pointing Example (TerraSAR-X)

GCS (Geometric Calibration Subsystem)

• Objective

Perform pixel localization calibration and product coverage location measurement in range and azimuth direction

• Pixel Localization Calibration

Provide end-to-end range & azimuth time offset using point target detected on SCS image

Calibrate systematic range delay and azimuth shift

Calculate the difference between the IRF response and mapped pixel location of point target

Utilize SAR imaging geometry with precise satellite position

• Product Coverage Location Measurement

Measure the difference in position & coverage between acquired SCS image and planned acquisition

KOMPSAT-5 Geo-location Error ( 1σ < 1 m )

Range Geo-location Error (m) of ES Mode Azimuth Geo-location Error (m) of ES Mode

RCS (Radiometric Calibration Subsystem) (1/2)

• RCS Objective

Provide a factor that converts digital number (DN) in SAR image to physical value (ex. sigma naught)

• Antenna Pattern Verification

Verify the reference antenna pattern with estimated antenna pattern from image

Consisting of three sub-functions:

− Range antenna pattern verification from homogeneous area SCS images

− Peak-to-peak gain offset measurement from WS homogeneous area SCS images

− Azimuth antenna pattern verification via active transponder

• Absolute Radiometric Factor Evaluation

Generate Calibration Constant that can converts DN to sigma naught

Measure Absolute Radiometric Accuracy with CR & AT

KOMPSAT-5 Absolute Radiometric Calibration Example

Beam No.

dB

Radiometric Accuracy of K5 EH mode < 1.0 dB

RCS (Radiometric Calibration Subsystem) (2/2)

• Internal Calibration Data Monitoring

Monitoring antenna TR module temperature

Monitoring of amplitude and phase drift of internal calibration pulses due to thermal effect

Antenna TR module health status check

Monitoring of calibration parameters estimated by CE subsystems

Monitoring of key parameters provided from payload

Internal Monitoring GUI Prototype (Kompsat-6)

MCS (Multi-polarimetric Calibration Subsystem)

• Determination of Polarimetric Characteristics of Antenna Beam Pattern

Measure cross-talk and channel imbalance from SCS images of Amazon, Corner Reflector (CR), and Active Transponder (AT) in addition to antenna information

Use covariance matrix of Amazon to calculate cross-talk

Use antenna path length information and images of CR/AT to calculate phase difference of channel imbalance

Use images of CR/AT to calculate amplitude of channel imbalance

• Provision of antenna distortion information for SAR image correction

Use determined cross-talk & channel imbalance terms

Provide composed DM to IRPE (SARP) for compensating polarimetric distortion

Distortion TX Distortion RX

Distortion Matrix (Quad. Pol. Case)

Cross-talk: 𝛿1 , 𝛿2, 𝛿3, 𝛿4

Channel Imbalance: 𝑓1, 𝑓2

CSS (Calibration Scheduling Subsystem)

• Generation of Calibration Image Collection Schedule

Schedule long-term image collection plan optimized for calibration activity

Perform orbit propagation using various propagation methods (TLE, J2, etc.)

Generate image collection request file based on Database of Corner Reflector, Active Transponder, and Homogeneous Area

Calculate SAR imaging command parameters

• Management of Calibration Imaging Status

Manage and monitor calibration imaging status for calibration activity

Image Collection Plan Example (Kompsat-5)

IQA (Image Quality Assessment Function)

• Statistical Analysis of Calibration Results

Access to Database to extract calibration results

Search the calibration results by periods and conditions

Provide Necessary statistical and graphic tools

• Composition of Calibration Report

Provide basic templates and editing tools

Prototype of Report Function of IQA (Kompsat-6 CE)

K-6 Active Transponder Development

Active Transponder Development (1/2)

Active Transponder Development Strategy

• Comply the requirement assigned by system for radiometric calibration

• Develop H/W considering enhanced payload parameters (compared to K5)

• Develop functions to support radiometric and multi-polarimetric calibration activity

• Consider “experiences and lessons” learned from K5 active transponder operation during development

Functions to support radiometric calibration

• Transfer stable power to satellite continuously during imaging operation

Operates as point target having stable RCS on ground

• Receive and record satellite TX signal versus time accurately

Supports azimuth antenna verification performed by RCS

Functions to support multi-polarimetric calibration

• All polarization (HH+HV+VH+VV) signal transferring

• Precise scattering property of AT are measured

Active Transponder Development (2/2)

Performance Requirements

• RCS Value: 60 dBsm (TBD)

• RCS Accuracy: 0.2 dB (RF system), 0.4 dB (all AT system)

• Full Polarimetric support

• Recording of azimuth antenna

• Portable (by 2 persons)

• Weather protection, operating temperature (-20 ~50 °C)

Prototype Design of Kompsat-6 AT

Active Transponder Design

Active transponder block diagram

• Satellite echo will be amplified by setting value provided by „RF Control‟

• All data received from external sensor will be recorded in „RF Control‟

• Operator can control active transponder by M&C S/W GUI

Amplification

Rx Polarization

(H, V, Slant)

Tx Polarization

(H, V, Slant)

Pulse Detection GPS ReceiverRF Control

M&C Software

Ethernet

GPS Time

Information

Signal

Strength

RF Signal

M&C

Corner Reflector (Mongolia)

KOMPSAT-5 CR (Reuse)

• Number of CR: ST (20), HR (32)

ST (45dBm2)

HR (35dBm2)

200km

THANKS FOR YOUR ATTENTIONS

KOMPSAT-6