Satellite Application Dept., Korea Aerospace Research Institute

P.O. Box 113 Yuseong Taejon, 305-600, Rep. of KOREA

1. KOMPSAT-1 OVERVIEW

1.1 INTRODUCTION

Base on Korea National Space Program, Korea Aerospace Research Institute (KARI) is developing a Korea Multi-Purpose Satellite-I (KOMPSAT-I) which accommodates Electro-Optical Camera (EOC), Ocean Scanning Multi-spectral Imager (OSMI), and Space Physics Sensor (SPS). The satellite has the weight of about 500kg and will be operated on the 10:50 AM sun-synchronized orbit with the altitude of 685 km. The satellite will be launched in 1999 and its lifetime is expected to be over 3 years. The main mission of EOC is the cartography to provide the images from a remote earth view for the production of 1/25000-scale maps of Korean territory. EOC collects 510~730 nm panchromatic imagery with the ground sample distance (GSD) of 6.6 m and the swath width of 17 km by push broom scanning. EOC also can scan ±45 degree across the ground track using body pointing method. Though the main mission of EOC is for Korean peninsula, it can be extended to the worldwide mission through direct oversea data reception or using 2.5 Gbit Solid State Recorder at end of life. The primary mission of OSMI is worldwide ocean color monitoring for the study of biological oceanography. It will generate 6 band ocean color images with 800 km swath width and 1km GSD by whiskbroom scanning. OSMI is designed to provide on-orbit spectral band selectability in the spectral range from 400 nm to 900 nm through ground command. This flexibility in band selection can be used for various applications and will provide research opportunities to support the next generation sensor design. SPS consists of High Energy Particle Detector (HEPD) and Ionosphere Measurement Sensor (IMS). HEPD has missions to characterize the low altitude high-energy particle environment and to study the effects of radiation environment on microelectronics. IMS measures densities and temperature of electrons in the ionosphere and monitors the ionospheric irregularities at the KOMPSAT-1 orbit.

1.2 MISSION OBJECTIVE

The main mission of KOMPSAT-1 is earth remote sensing. Though main sensing area is for Korean territory, it can be used to the worldwide remote sensing.

• EOC

EOC is the primary payload for KOMPSAT-1. It has cartography mission to provide images for the production of scale maps, including digital elevation models, of the Korea territory from a remote earth view in the KOMPSAT orbit. EOC collects panchromatic imagery with the ground sample distance (GSD) of 6.6 m and the swath width of 17 km at nadir through the visible spectral band of 510 nm ~ 730 nm. EOC scans the ground track of 800 km per orbit by push-broom and body pointing method. The image data taken at specific date shall be downlinked within 24 hours. In contingency state, the satellite shall have on-board fault diagnosis capability and shall be capable of record the state of the telemetry. Spacecraft shall provide ±45 degree tilting capability for in the cross-track direction. When in operation, the system shall be operated autonomously by KOMPSAT Ground Station (KSG) located in Taejon, Korea. The system shall allow the realization of 6.6 m high-resolution images and the composition of printed maps and digitized maps for domestic territories.

• OSMI

OSMI mission is worldwide ocean color monitoring for the study of biological oceanography. OSMI image data can be used for ocean ecological observation, ocean resource management, and ocean-atmosphere environment analyses. OSMI is a multi-spectral imager generating 6 color ocean images with 800 km swath width and less than 1km GSD by whisk-broom scanning method. OSMI is designed to provide on-orbit spectral band selectability in the spectral range from 400nm to 900nm for flexible ocean observation. The color images are collected through 6 primary spectral bands centered at 412, 443, 490, 555, 765, 865nm or 6 spectral bands selected in the spectral range via ground commands after launch

• SPS

Besides the above missions, KOMPSAT-1 shall perform some scientific experiments in which the payload shall be provided by Korea Advanced Institute of Science & Technology (KAIST). SPS consists of High Energy Particle Detector (HEPD) and Ionosphere Measurement Sensor (IMS). HEPD has missions to characterize the low altitude high energy charged particle environment and to study the effects of radiation environment on microelectronics such as single event upset, total dose effect and RAM test. IMS measures densities and temperature of electrons in the ionosphere and monitors the ionospheric irregularities in KOMPSAT orbit.

EOC provides 6.6 meter resolution cartography data for developing stereo images of Korean territory which can be used for 1/25000 scale mapping for land use and planning purposes. OSMI provides multi-spectral image data with 6 band which can be utilized for observing ocean in order to estimate global marine resources, and to monitor ocean environment. SPS measures the high energy cosmic particles, the ionospheric irregularities, the densities and temperature of electrons in KOMPSAT orbit for characterizing space environment.

1. 3 OPERATIONAL CONCEPT

The operational concept of KOMPSAT-I is illustrated in the following Figure 1.1 for the mission of cartography, ocean color monitoring, and space environment monitoring.

Figure 1.1 The operational concept of KOMPSAT-1

Revisit Time

The system will offer a minimum revisit time of 2-3 day (2days by 45 degree tilt, 3 days by 30 degree tilt) to any location in Korea and any place in the world with 6.6 m high-resolution panchromatic images. The spacecraft can be tilted of ±45 degree in the cross-track direction to meet the revisit requirements.

Image Acquisition and Downlink Capacity

The system will transmit in real time or in playback with the data storage system (8 Gbit at beginning of life) to KGS. The downlink data capacity shall be maintained to accomplish the image acquisition requirements as specified in Figure 1.2. The Ground System Concept is as follows.

Figure 1.2 KOMPSAT data downlink and routing operation

The followings are the KOMPSAT day in the life and month in the life respectively.

Figure 1.3 KOMPSAT Day in the Life

Figure 1.4 KOMPSAT Month in the Life

System Availability

The redundancy of all system functions necessary for the realization of the mission mentioned above shall be considered. The same day transmission of the corresponding imaging data should be implemented in the KOMPSAT-1 system. The period of the satellite autonomy without ground station support shall be of at least one month.

User Interface

The satellite imaging work plan of the KOMPSAT-1 system will be specified on a daily basis. The resulting image products, according to the requested products quality standard, shall be made available within one (1) day after the satellite passes over KGS, with the minimum system operator intervention.

1. 4 KOMPSAT-1 SPACECRAFT BUS

The KOMPSAT satellite consists of a Payload Module, a Spacecraft Bus, a Spacecraft Adapter, and a Separation System as primary elements. The Payload Module includes the Payload Instruments and the Payload Module structure, which interfaces at the Spacecraft Bus nadir platform. The Spacecraft Bus consists of an Equipment Module (including two solar array wings), and a Propulsion Module. The Spacecraft Bus integrates all of the required spacecraft subsystems and flight software in support of the instrument interface requirements.

The Spacecraft Adapter provides the interface between the Spacecraft Bus and the Separation System, which includes the upper and lower interface rings and the separation clamp band. Integration of the Payload Module with the Spacecraft Bus, Spacecraft Adapter, and Separation System forms the KOMPSAT Satellite. The deployed satellite configuration is depicted in Figure 1.5.

Figure 1.5 KOMPSAT Deployed Satellite

The spacecraft bus is designed to support planned system operations and instruments by providing a stable orbiting platform for the mission. It also provides the necessary on-orbit command, control, data handling, communications, power, and other support functions for the duration of the mission life. The spacecraft bus provides the mechanical, electrical, and data interfaces to the launch vehicle, and three-axis stabilization with zero momentum bias system provides precise pointing during all operational modes and maximum flexibility for various missions. Silicon cell solar array with single axis solar array driving mechanism provides efficient power generation, and super NiCd battery supports mission requirements during eclipse. Nadir-pointing platform configuration provides unobstructed payloads field-of-view. Space proven hardware and fully redundant system for critical components maximize robust design and on-orbit performances.

The spacecraft bus provides a communication interface with the KGS. It provides two RF links to the KGS, one at S-band and one at X-band. The S-band link receives commands and transmits satellite SOH data and low-rate science instrument data. The S-band link also provides a ranging capability. The X-band link transmits high-rate image telemetry and provides a parallel path for playback of stored SOH and low-rate science instrument data. The RF equipment provides communications capability with other compatible ground stations within duty cycle constraints.

The KOMPSAT spacecraft bus is designed to detect abnormal earth pointing and/or loss of communications with the KGS. Upon detection of abnormal operation, the satellite automatically enters a safe-hold mode. The safe-hold mode is capable of maintaining the satellite in a power and thermally safe orientation for at least 30 days. Basic satellite SOH data and command capability are available in the safe-hold mode. The satellite is capable of normal operation without uplink commands, i.e., with stored commands, for up to 24 hours.

KOMPSAT top-level requirement is driven by cartography mission and the other key requirements such as orbit selection, attitude control scheme, communication architecture, mission planning and operation are determined. Table 1.1 shows the KOMPSAT Spacecraft Bus requirements, mainly driven by mission, expanded or tranlated into specifications of engineering parameters, and these spacecraft bus level requirements are flowed down to appropriate subsystems.

Table 1.1 KOMPSAT Spacecraft Bus Requirement

Requriement	Values
Multi-mission bus	Modular structure and compatible with small launch vehicle Utilization of MIL-STD-1553B data bus Support of sun-synchronous orbit in a 600 to 800 km range
Mission orbit	Altitude of 685.13 km ± 1 km Eccentricity of 0 to 0.001 Inclination of 98.13 ± 0.05
Attitude Stabilization	Three-axis stabilization
Pointing budeget	Positioning of swath width £ 0.1 deg(2s) Localization £ 0.08 deg(2s)
Spacecraft bus power	At least 500W at the end of 3 years
Spacecraft bus dry mass	£ 310 kg
Payload support	Up to 200 kg
Reliability	0.9 at 3 years
Link compatibility	NASA/STDN, ESA Standard
Link margin	A minimum 3 dB on the X- and S-band link
Data rate	Uplink: 2 kbps / Downlink: 2 kbps (real time) 1.5 Mbps (playback)
Data fornat	CCSDS
Bit error rate (BER)	£ 10 -6 for the image data telemetry £ 10 -6 for the S-band command uplink £ 10 -5 for the SOH telemetry

The functional relationships between the spacecraft bus subsystems are depicted in the KOMPSAT system block diagram of Figure 1.6.

The Structure and Mechanisms Subsystem (SMS) provides the structural support, alignment, and stiffness required by the satellite bus, its subsystems, components, and the instruments. It also provides for assembly to the launch vehicle for launch and separation, the deployment mechanisms for the solar arrays, and support of electrical, mechanical, and thermal integration hardware. The structure and mechanisms subsystem shall include the following basic elements: a payload module structure, an equipment module structure including equipment panels and solar array wings, a propulsion module structure, a spacecraft adapter, and a separation system.

The Thermal Subsystem (TS) is designed to maintain and control satellite temperatures and instrument interface temperatures and boundary conditions within specified limits under all potential external environments. The thermal design shall rely on passive techniques (i.e., no moving parts) as much as possible. Typical passive elements include surface finishes, multi-layer insulation (MLI), and conduction isolators. Heater control is performed by either temperature sensors and spacecraft processors with a ground-commandable override capability or by thermostats.

Figure 1.6 KOMPSAT System Block Diagram

Attitude and Orbit Control Subsystem (AOCS) provides the necessary attitude and orbit control, attitude determination, and SOH function for the KOMPSAT mission operation phase. A three-axis stabilization method with zero momentum bias closed-loop system is used for attitude control of the satellite. Fine horizon and sun sensors provides precise pointing knowledge, while gyros perform attitude propagation between updates. Magnetic torquers and magnetometer are used for momentum unloading.

Electrical Power Subsystem (EPS) generates, stores, regulates, and distributes electrical power for the payloads and the spacecraft bus. Aluminum flat pack and silicon cell solar array provides enough power at the EOL. The solar array regulator operates as series regulators in conjunction with EPS control unit to control battery charging. 22-cell, 21Ah super NiCd battery provides electrical energy during eclipse. The power control unit controls and distributes primary and secondary power as required by the various spacecraft bus and instruments loads.

Propulsion Subsystem (PS) is all welded monopropellant hydrazine blowdown system and impulse is provided by catalytic decomposition of hydrazine for on-orbit drag make-up and attitude control. Propulsion components consists of Four (4) dual thruster modules (DTMs), each with primary and redundant thrusters, one diaphragm propellant tank, propellant/pressurant fill and drain valves, one propellant filter, one pressure transducer, and two latching isolation valves.

The Telemetry, Command and Ranging Subsystem (TC&RS) provides RF communications and ranging capability with S-band omni antennas, an RF assembly, and S-band transponders. Command storage, processing and distribution, telemetry input, formating and storage, data processing for the spacecraft are managed by On-board Computer. On-board computer provides the bus controller for data management using a Mil-Std-1553B data Bus and 1 Gbit mass memory storage for Satellite SOH and science data recording. GPS receiver communication provides for position, velocity and time reference. Table 1.2 shows the summary of the KOMPSAT satellite weight and power budget.

Table 1.2 KOMPSAT satellite weight and power budget

System overhead includes battery charging, regulator loss, harness loss and peak power tracking.

2.1 EOC Mission

The main mission of EOC is cartography to build up the digital map of Korean territory including Digital Terrain Elevation Map (DTEM). But its mission can be extended to the worldwide high resolution earth observation.

2.2 EOC Design

The orbit of EOC is a sun synchronous with the altitude of 685km. EOC collects ground image through 510~30nm panchromatic spectral band by push-broom scanning and spacecraft body pointing. At KGS, stereo image can be generated with images which are obtained from different body tilt angles.

Figure 2.1 Opto-mechanical structure performance

Figure 2.2 EOC electronics subsystem block diagram

The EOC system performances are as follows:

Duty cycle for image collection : 2 minutes/orbit

(800 km image/orbit)

Ground Sample Distance : 6.6 meters

at 685 km in Nadir view

Swath width : 17 km

at 685 km in Nadir view

Mission life time : 3 years

Reliability : 0.94

MTF : 10% at Nyquist frequency

SNR : 50 over entire filed of view

FPA : 2592 pixels

Digitization : 8bits

Total weight: 35 kg

Maximum power consumption: 46 Watt

Image data transmission rate : 25 Mbps . Figure 2.3. EOC configuration.

The on-board analog signal processing module of EOC has the function of programmable gain and offset to take the various conditions of ground reflected radiation into account.

The EOC (Figure 2.1) physically consists of two assemblies, the EOC Sensor Assembly (ESA) and the EOC Electronics Assembly (EEA), and each assembly is mounted on the payload platform of spacecraft. The ESA is thermally isolated from the payload platform and EEA is thermally coupled to that.

2.3 EOC Operation

For EOC mission, S-band communication link is used to transmit command from the ground station and to receive the SOH data of EOC. The On-Board Computer (OBC) controls S-band data with the MIL-STD-1553B interface. EOC transmits the image data to ground station via X-band channel.

The EOC collects image for 2 minutes during 98 minutes of orbit cycle, which covers about 800 km along ground track. The EOC image can be transmitted to KGS in real time during Korean territory observation or be stored in the Solid State Recorder (SSR) of PDTS out of the KGS reception area. The stored image data can be transmitted when the data reception connection is available later. It is possible to observe earth globally using the image storage scheme or direct reception in the oversea ground station.

KGS performs the radiometric and geometric correction on the received image data, and performs several image processing steps with ancillary data. For calibration, observed reference area such as desert or Ground Control Point (GCP) is used. Spacecraft provides ancillary data such as image time, position and attitude of spacecraft for EOC image processing. The EOC has four (4) operation modes as follows:

Safe-Hold mode: power-on for survival only

Imaging mode: All power-on, Image collection

Figure 2.4 illustrates the EOC operation concept of image collection and stereo image collection. The EOC collects stereo images of a target area from opposite sides on different passes by roll-tilting of spacecraft, then ground station can make DTEM with stereo image. It is possible to collect image data 39 times in the daytime during the 28 days of revisit cycle by roll-tilting of up to ±45 degree. For cartography, up to ±30 degree roll-tilting is used in practice. KGS can obtain EOC images 20 times out of 39 times in this case.

Figure 2.4. EOC Operational Concept.

3.1 OSMI Mission

OSMI mission objective is to provide ocean color measurements for biological oceanography. The OSMI image data is collected for various researches and applications in the fields of worldwide ocean resources management and ocean environment monitoring.

3.2 OSMI Operation Description

The KOMPSAT-1 OSMI is designed to provide worldwide ocean color data from a 685km sun synchronous orbit. The orbit crossing time is 10:50 AM and the inclination is 98.13 degrees. The OSMI instrument performs whisky-broom scan imaging operation with a cross-track ground swath of 800 km. The ground track re-visit time is 28 days. The OSMI instrument is designed to perform imaging operation for 20% per orbit and a planned on-board solid state recorder will support worldwide imaging operation by providing data archive and downlink to the KGS.

Figure 3.1. OSMI Operation Concept.

3.3 OSMI Spectral Band Selection Capabilities

The OSMI is a multi-spectral instrument covering the visible spectrum from 400 nm to 900 nm. In the OSMI design there is also built-in flexibility to provide band center and bandwidth selection capability via ground station command. Any 6 spectral band can be selected in the spectral range from 400 nm to 900 nm. There are always 6 bands without overlap among the bands. The band centers of the 6 data channels can be varied with the accuracy of 2.6 nm and the bandwidth for each of the 6 data channels can be changed from the minimum value of 5.2 nm to the maximum value of 166.4 nm by the step of 2.6 nm. The KOMPSAT-1 OSMI instrument uses 6 primary spectral bands in the 400 nm to 900 nm range to perform ocean color monitoring. The OSMI spectral band centers and bandwidths are as shown in Table 3.1.

Table 3.1 OSMI ocean color monitoring spectral channels

The OSMI ocean color spectral channels B1 through B4 provide ocean color data while channel B5 and B6 provide information for atmospheric (aerosol) corrections.

The OSMI's on-orbit spectral band center and bandwidth selectability provide great flexibility in ocean color monitoring. For example, the 412nm band, which is of interest in the Yellow Sea near the Korean peninsula, can be monitored without adding instrument complexity. In addition, this flexibility in band selection provides research opportunities to support the next generation of sensor design.

1. OSMI Sensor Assembly
   
   
   
   
   

(b) OSMI Electronics

Figure 3.2 OSMI Configuration.

3.4 OSMI Instrument Design

OSMI is designed to provide 6 spectral image data with GSD less than 1km over 800 km swath width by whisk-broom scanning method at the altitude of 685km.

The OSMI instrument optics consists of the image scanner, the objective lens group and the multi-channel spectrometer (Figure 3.3). The scanner consists of a scan mirror driven by a servomotor. The scanner has a ±30 degree scan angle with respect to nadir resulting in an 800 km swath from 685 km altitude. The objective lens group forms the primary image which is dispersed into its various colors by the spectrometer. The dispersed image is then focused onto and detected by the focal plane assembly (FPA).

Figure 3.3 OSMI Instrument layout.

The OSMI electronics consists of (1) the FPA electronics section providing timing and voltage bias for the CCD FPA, (2) the analog signal processing and analog-to-digital (A/D) conversion section providing analog gain and offset signal adjustment and a 10-bit A/D signal conversion, (3) digital data processing and compression section providing 6 channels of data with the desired band centers and bandwidths, and uses data compression to reduce data volume, (4) a control processor section to provide control, communication and house keeping, (5) a motor drive section providing servo control for the scan motor and read out of the optical encoder for the position of the scan mirror, and (6) the power converter section to regulate DC power (Figure. 3.4).

For on-orbit calibration, the OSMI design employs a two level, black and white, calibration scheme. The sensor black calibration level is provided by a dark cell while the white calibration level uses attenuated solar radiance. The solar calibration is performed over the northmost part of the satellite orbit and the dark calibration is done at the beginning and end of each continuous image scan.

Physically, the OSMI instrument consists of two components (Figure 3.2), the sensor unit and the electronics unit. The OSMI instrument nominal weight is about 18Kg and the peak power consumption is about 30 watts.

3.5 OSMI Mission Parameters

OSMI has 4 operational modes; standby mode, imaging mode, dark calibration mode, solar calibration mode and mission parameters are summarized as below:

Table 3.3 OSMI mission parameters

Items	Mission parameters
- Band selection parameter	Maximum selectable band numbers : 6 band Selectable band width : 400nm ~ 900nm Band width step : 2.6nm Selectable minimum band width : 5.2nm Selectable maximum band width : 169nm To be monitored by Telemetry
- Gain and Offset control parameter	Gain control parameter : 8 step (e. g. 0.65 times ~ 3.1 times at 510 nm) Offset control parameter for 4 channel : 256 step (16 A/D counts per each step) To be monitored by Telemetry
- On-orbit calibration by Ground command	Solar calibration on north pole per orbit Normally dark calibration before and after image acquisition /solar calibration
- LRC SOH data for mission operation	Temperatures and powers status Gain and offset Selected Bands Mirror position
- Image data storage capability	4Gbit at BOL (Beginning Of Life) allocated To be stored for minimum 35 days mission

3.6 OSMI Performance

The instrument performance is measured on-ground for 8 basic spectral bands within 400 nm ~ 900 nm which are chosen as main spectral bands for the OSMI mission and include the primary bands. Some major instrument performance test results are summarized as follows:

1. Spectral Band Characteristics

Table 3.4 OSMI spectral band characteristics

(2) Boresight/FOV

• LRC boresight angle is defined as the angle between LRC Instrument Line of Sight (LOS: +Z axis) and Alignment Cube +Z axis. The positive azimuth angle is from the +Z-axis to the -Y-axis. The positive elevation angle is from the +Z-axis to the +X-axis.

• Measured scanning slit rotation angle to alignment cube: 4' 45"
• LRC FOV measurement

(3) MTF

The satellite-level MTF is predicted as shown in the Figure 3.5 based on the measured values of instrument level MTF, space environment (temperature change and vacuum) effect analysis and satellite motion (jitter & smear) effect analysis.

Figure 3.5 OSMI MTF

(4) Input Radiance, SNR and Radiometric Linearity

Table 3.5 OSMI radiometric characteristics

TBR: To Be Resolved

1. Unit of W/m²/nm/sr

(5) The polarization of OSMI signal channels is less than _±5%.

(6) The OSMI image data is compressed and digitized by 10-bit counting system.

4. SPACE PHYSICS SENSOR

Space Physics Sensor(SPS) consists of two scientific instruments, High Energy Particle Detector(HEPD) and Ionosphere Measurement Sensor(IMS). HEPD is to characterize the low altitude high energy particle environments and effects on the micro electronics due to these high energy particles. IMS measures the in-situ electron temperature and density.

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Figure 4.1 SPS Mission and Operation

4.1. High Energy Particle Detector (HEPD)

HEPD is consists of Proton and Electron Spectrometer (PES), Linear Energy Transfer Spectrometer (LET), Total Dose Monitor (TDM) and Single Event Monitor (SEM).

PES identifies the types of particles detected in the KOMPSAT orbit and measures the energy of them. It consists of 7 measurement channels shown in following table.

Table 4.1 PES Measurement Channels

Channel	Particle Type	Energy(MeV)
pE1	Proton	30 ~ 38
pE2	Proton	15 ~ 30
pE3	Proton	6.4 ~ 15
eE1	Electron	2.0
eE2	Electron	0.72 ~ 2.0
eE3	Electron	0.25 ~ 0.7
AA	Alpha Particle	15 ~ 60

The data obtained in the KOMPSAT orbit allows the computation of the particle distributions of energetic electrons and protons through the radiation belts. These distributions can be used for developing models of the both the static and dynamic radiation belt populations.

LET measures only indirect particle energies, as we call it Linear Energy Transfer. Being unable to identify the particles, however LET can give the information on how many particles transfer their energy to silicon detector. This value can be easily converted to the radiation effect on the electrical component of same module. By the comparison with TDM and SEM data, characteristics of high energy particle which can affect the electrical performance of on-board components in a spacecraft will be analyzed.

TDM measures the long-term ionizing dose of radiation (in SiO2) accumulated at the locations of SPS. The sensing elements are RADFET dosimeters. Each RADFET sensor consists of a matched pair of p-channel MOSFET's, one of which is biased during exposure (measure mode), whilst the other remains un-biased. Exposure to radiation causes the formation of trapped holes (positive charge) in the gate oxide, which in turn causes a gradual shift in the threshold voltage (Vth) with respect to the dose of accumulation. The electric field across a biased FET causes it to experience a greater dose-effect than that across an unbiased FET. Previous measurements on accumulated dose, such as the DMSP satellite, reveal that the accumulated dose is strongly dependent on the solar activity. Especially in KOMPSAT TDM, radiation shielding effect can be also calculated and modeled according to their aluminum box shielding.

Trapped particles in the radiation belts, solar flare protons, and galactic cosmic ray can cause single-event phenomena within micro-electronic devices. For memory chips, SEM performs a series of test dedicated to measure SEU(Single Event Upset) characteristics. Four chips of 4Mbit static RAM are connected to appropriate measurement circuits, and variations of their characteristics are monitored together with the total dose measurement. The SEM is designed to test non-space qualified static RAM for the future mission.

4.2. Ionosphere Measurement Sensor (IMS)

IMS consists of Electron Temperature Sensor (ETS) that measures the temperature of thermal electrons in the ionosphere and Electron Density Sensor (EDS) that measures the density of electrons. EDS is a modification of Langmuir Probe (LP). Dynamic measurement range of IMS is 10~10⁶ electrons/cm³ in electron density and 0~1eV in electron temperature.

ETS is specially designed to measure the electron temperature accurately. The circular sensor plate is swept in voltage of 30 kHz sinusoidal oscillation with three different amplitudes and the DC component of the shifted voltage of the oscillation due to the surrounding plasma is measured with the corresponding amplitude. The DC component of the voltage shift is used to derive the surrounding plasma and the current drawn from plasma is measured as a function of the probe voltage. The current-voltage relationship is used to derive both electron density and temperature which are key variables of the ionosphere plasma.

5. PAYLOAD DATA TRANSMISSION SUBSYSTEM (PDTS)

The purpose of the PDTS is to provide KOMPSAT-I with an X-band downlink capability to transmit image and OBC (On Board Computer) data (mainly S-band data) to the KGS. The PDTS consists of an internally redundant Formatter/Multiplexer Unit (FMU), separate primary & redundant X-band transmitter (modulator and RF amplifier functions) units, a RF assembly, and an X-band Antenna as described in Figure 5.1.

Figure 5.1 The functional block diagram of PDTS.

The FMU is commanded to accept image data from one or both of two imaging instrument (EOC and OSMI) and telemetry data from the On-Board Computer (OBC) which contains SOH, satellite reference information (time, ephemeris, and orientation). The FMU is able to receive the image data at the maximum input data rates of Table 5.1 (defined for the EOC and OSMI as the rate of data obtained via the interfaces).

To provide compatibility with the KGS, the image data are formatted and encoded for transmission in CCSDS Grade 2 prior to being sent to the X-Band transmitter. The formatted data from FMU are passed to X-band Transmitter for real-time information transmission. The FMU contains Solid State Recorder (SSR) to store processed EOC/OSMI image data and OBC data for later playback downlink to KGS when KOMPSAT-I is within the data reception coverage of the KGS. The storage capacity of SSR is 2.5 Gbits at the end of life. The FMU shall format a serial CADU bit stream into two-bit wide words suitable for offset quarter phase-shift-keying (OQPSK) modulation, assigning even bits (starting with the first bit of the CADU) to one bit of the word (Q-channel bit) and odd bits to the other bit of the word (I-channel bit) in accordance with CCSDS recommendation 401-B-1.

Figure 5.2 Payload data transmission link capability

Figure 5.3 PDTS link budget at 16.6 deg. of KSG elevation angle

Figure 5.4 PDTS link budget at 59.93 deg. of KSG elevation angle

The X-Band transmitter has the data transmission rate of 45 Mbps. The transmitter center frequency is 8.3 GHz and the transmitter output power is 3 watts. Transmitter output is visible to the ground station up to 1500 km from the satellite nadir point in the ground. The X-band link margin is more than 3dB with Bit Error Rate (BER) of 10^-6. These functional performances are shown in Figure 5.2 ~ Figure 5.4. The Mass of PDTS is 14.5 Kg and the peak power consumption is 63 watts.

6. KOMPSAT RECEPTION and PROCESSINF SYSTEM

6.1 KGS Mission

The mission objectives of the ground station are to monitor and control KOMPSAT, to conduct mission planning and scheduling and to receive, process, and distribute the KOMPSAT data.

6.2 Structure of KOMPSAT-1 Ground Station

The ground segment or KGS will be installed and operated in Taejon (N36.2°, E127.2°), Korea. It contains two major components or elements: the KOMPSAT Mission Control System (KMCS) and the KOMPSAT Receiving and Processing System (KRPS); see Figure 6.1.

Figure 6.1 Structure of KOMPSAT-1 receiving and processing system

The KRPS consists of a Data Acquisition Facility (DAF) which captures the downlinked telemetry stream, a Direct Ingest System (DIS) which accepts the real-time data stream, formats the data, and stores the data on a Redundant Array of Inexpensive Disks (RAID). The data can then be archived or processed immediately by the Data Processing Facility (DPF) where standard image products are generated. A Value-Added subsystem completes the processing through the generation of end-user products. The DIS, DPF, and Value-Added subsystem are all connected by a 100 Mbps Ethernet.

The objective of the KRPS is to provide useful imagery data to assist in the accurate mapping of the Korean territory and provide scientific data for atmospheric and oceanographic applications.

6.3 EOC Product Definition

The KOMPSAT data product level definitions are based upon the standard of the Committee on Earth Observation Satellites, which is identical to the NASA EODIS data convention.

Level 0: Frame formatted, unprocessed instrument/payload data at full resolution; any and all communications artifacts (e.g., synchronization frames, communications headers) removed.

Level 1A: Unprocessed instrument data at full resolution, time-referenced, and annotated with ancillary information, including radiometric and geometric calibration coefficients and georeferencing parameters (i.e. platform ephemeris) computed and appended, but not applied, to the Level 0 data.

Level 1R: level 1A data that have been radiometrically corrected.

Level 1GR: Level 1R data that have been geometrically corrected and georeferenced.

Level 1GC: Level 1R data that have been geometrically corrected and geocoded. Level 1GC may be further processed with the following options:

(Level 1GC)_P: precise geometric correction with GCP.

(Level 1GC)_D: Geometric correction with DEM

Level 4: Value added EOC products such as mosaic, DTM and maps.

6.3 Equivalent SPOT Product Definitions

The following table describes the equivalent SPOT products that would best match the KARI/EOC products:

Table 6.1 Cross-referenced Product Table

EOC	SPOT	Product Definition
Level 0	Raw	Raw data with metadata Blank line fill Direct from DIS and archived
Level 1A	Level 0	Translated into I2S Format Post pass ephemeris Ancillary data placed in header Data represented as Digital Numbers (DN)
Level 1R	Level 1A	Radiometric corrections applied -detector response normalization Removal of camera signature Data is deblurred using the Modulation Transfer Function (MTF) Data represented as radiance values
Level 1GR	Level 1B	Geometric corrections applied Earth rotation Panoramic effect Variations in earth orbital attitude
Value-Added Products
Level 1GC.P	Level 2	Rectified with ground control
Level 1GC.D	Level 3	Orthorectified with elevation data

6.4 EOC Products

Archived products are; Level 1R, Digital Elevation Models (DEMs), Level 1GC_D and Map sheets. Special order products are Level 1GR and Level 1GC_P data. The final output of EOC are 1:25000 scale ortho-photo map derived from 20 m DEM data and refined satellite model data.

6.5 OSMI Product Definition

OSMI data has the level definitions as below:

Level 0: Frame formatted, unprocessed instrument/payload data at full resolution; any and all communications

artifacts (e.g., synchronization frames, communications headers) removed

Level 1A: Unprocessed instrument data at full resolution, time-referenced, and annotated with ancillary

information, including radiometric and geometric calibration coefficients and georeferencing

parameters (i.e., platform ephemeris) computed and appended, but not applied, to the Level 0 data

Level 1B: Radiometrically corrected level 1A data that have been converted to at-satellite radiances

Level 2: Derived geophysical variables at the same resolution and location as the Level 1A data

Level 3: Geophysical variables mapped on uniform space/time grid scales

Level 4: Value added OSMI products

6.6 HEPD & IMS Product Definition

SPS (HEPD & IMS) data has the level definitions as below:

Level 0: Frame formatted, unprocessed instrument/payload data at full resolution; any and all communications artifacts (e.g., synchronization frames, communications headers) removed

· Level 1A Unprocessed instrument data at full resolution, time-referenced, and annotated with ancillary information

7. CONCLUSION

KARI is developing three instruments of Electro-Optical Camera (EOC), Ocean Scanning Multi-spectral Imager (OSMI) and Space Physics Sensor (SPS) to be accommodated onto the Korea Multi-Purpose Satellite I (KOMPSAT-I) for the mission of cartography, ocean color monitoring, and space environment monitoring respectively. The current status design budget and performance of KOMPSAT-I payloads are summarized in Table 7.1.

The flight model (FM) of EOC and OSMI had manufactured at the end of 1997 and SPS FM had been by April of 1998. The instrument is scheduled to be assembled with spacecraft bus and tested as a part of the satellite system at KARI in the third quarter of 1998. After the final check on the interfaces between the satellite, KGS, and the launch vehicle of Taurus, the satellite is to be launched in August of 1999 and it is expected to operate more than 3 years.

Table 7.1 KOMPSAT-I Payload design budget and performance summary

Payload	Performance factor	Specification
EOC	Ground Sample Distance(GSD)	6.6 ±10 % m @685km altitude, Nadir
	Life Time	3 years at least
	Spectral Band	PAN: 0.51 ~ 0.73
	Swath Width	17 km
	Image Data Rate	25 Mbps
	Duty Cycle	2 Minutes (2 %) per orbit
	Digitization	8 bits
LRC	GSD	1 km
	Swath Width	800 km
	Spectral Band	selection of 6 bands over the range of 400~900 nm
	Duty Cycle	19.6 Minutes (20 %) per orbit
	Digitization	10 bits
PDTS	BER	10-6
	SSR	8 Gbytes (Beginning of Life) & 2.5 Gbytes (End of Life)
	Modulation	QPSK(X-band)
	Error Correction	R/S(255,223) (FEC)
	Packettizing	CCSDS …
	Data Transmission Rate	45 Mbps(Constant Rate)
IMS / HEPD	Duty Cycle	100 %