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STSat-1 (Science and Technology Satellite-1, KAISTSat-4, Uribyol-4)

The STSat-1 (Science and Technology Satellite-1) mission, formerly known as KAISTSat-4 (Korea Advanced Institute of Science & Technology Satellite-4), aims to develop a high performance small satellite bus, high performance scientific payload design and research on space science and develop advanced technology for future space missions. 1) 2) 3) 4)

STSat-1 is a low-cost KAIST/SaTReC microsatellite technology demonstration mission, funded by the Ministry of Science and Technology (MOST) of Korea, a follow-up mission in the KITSAT program. The objective is to develop and test a minisatellite bus, along with a new star sensor in a three-axis attitude control subsystem, to demonstrate the performance of new science instruments, and to deploy a newly developed DCS (Data Collection System).

Spacecraft:

The S/C structure resembles a box of approximate size: 66 cm x 60 cm x 80 cm. It is three-axis stabilized. The S/C pointing requirements call for a pointing accuracy of 0.5º, an attitude knowledge of 5 arcmin, and a stability of about 5 arcmin/s. In addition, the S/C requires a complex set of attitude maneuverability in support of its mission objectives. Hence, the ADCS (Attitude Determination and Control Subsystem) consists of four fiber optic gyros (FOG), two precision star trackers (inertial reference attitude within 10-60 arcsec) referred to as NAST (Narrow Angle Star Sensor), a coarse sun sensor and two three-axis fluxgate magnetometers for attitude sensing. Magnetic torquer coils are used for momentum unloading of the four reaction wheels as well as for spin-rate control for the initial phase after spacecraft separation. A GPS receiver is used to provide S/C position, velocity and timing. Attitude determination is based on an extended Kalman filter algorithm considering the bias drift of the gyroscope. The shared bus structure employs a modified MIL-STD-1553B bus for on-board communications.

The S/C features three solar panels, one fixed and two deployable, providing a power of 150 W. The spacecraft mass is 106 kg, power = 150 W, the mission design life is two years. 5) 6) 7)

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Figure 1: Schematic view of the deployed STSat-1 spacecraft (image credit: KAIST)

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Figure 2: Exploded view of the STSat-1 spacecraft (image credit: KAIST)

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Figure 3: The STSat-1 spacecraft (image credit: KARI)

Launch: A launch of STSat-1 on a Kosmos-3M vehicle (of Polyot) from Plesetsk, Russia, took place on Sept. 27, 2003, along with the DMC (Disaster Monitoring Constellation) payloads BilSat-1, NigeriaSat-1, and BNSCSat-1, built at SSTL, Surrey, UK. And with Mozhayets-4 and Larets, both of Russia.

Orbit: Sun-synchronous near-circular orbit, altitude = 686 km, inclination = 98.2º, period of 98.5 minutes.

RF communication: TT&C communications are provided in S-band. The S-band downlink data rate is 38.4 kbit/s (uplink 9.6 kbit/s). An X-band downlink is used [8.125-8.329 GHz, output power of 33 dBm (2 W), power consumption <30 W] for payload downloads with a data rate of 3.2 Mbit/s. The on-board recorder has a capacity of 2 Gbit (SDRAM). All mission operations are performed at SaTReC, including data acquisition, distribution and archiving.

There are four operational modes of STSat-1 to achieve the scientific objectives; these are:

1) Pointed observation mode (observation of selected and extended galactic sources with FIMS during eclipse phases of the orbit)

2) Sky-survey mode (observation of the entire sky, the S/C spins around the axis parallel to the slit of FIMS)

3) Aurora observation mode (FIMS is being pointed in the nadir direction over the north and south poles)

4) Air-glow mode (FIMS is being pointed to an inertial or to a nadir direction).

 

Mission status: The STSat-1 spacecraft is still operational as of 2007. However, the regular observation mission lasted until October 2005 - when some abnormal attitude behavior of the spacecraft was detected. In the post mission phase, the spacecraft is being used as a test bed for the attitude control and communication experiments.

• The LEOP (Launch and Early Operation Phase) was completed by the end of October, 2003.

• Regular mission operations of payloads and technology verification instruments started in January 2004. Prior to mission observations the boresight mismatch of FIMS was was measured with respect to that of the star tracker.

• During the mission life time, FIMS, the main payload of STSat-1, scanned most of the sight lines to our galaxy and some objects that were scientifically interesting (about 70% of the sky). 8) 9)

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Figure 4: All-sky FUV image of our galaxy observed by STSat-1 (image credit: KARI)


Sensor complement: (FIMS, SPP, DCS/ADAM)

FIMS (FUV Imaging Spectrograph) developed in a cooperative project by KAIST, KAO (Korea Astronomy Observatory) and UCB/SSL (University of California at Berkeley/Space Sciences Laboratory), PI: J. Edelstein of UCB/SSL. Note: The FIMS instrument is also referred to as SPEAR (Spectroscopy of Plasma Evolution from Astrophysical Radiation) in the published documentation of the USA. 10) 11) 12) 13) 14) 15)

The objective of FIMS observations is to study the diffuse hot interstellar matter in the Far Ultraviolet (FUV) spectrum. The overall goals of FIMS are 1) to map the spatial distribution of hot Galactic plasmas through a one-year sky survey, 2) to determine the physical states of hot interstellar matter such as superbubbles and supernova remnants with pointed observations, and 3) to test the models presently available for the Galactic evolution.

The instrument allows the detailed mapping of the spatial distribution of the hot galactic plasmas and the determination of the physical states of hot interstellar matters as well as the detection of the various emission lines from the Earth's upper atmosphere. FIMS employs a dual bandpass (900-1175 Ä and 1335-1750 Ä), high spectral resolution (1.5 Ä and 2.5 Ä, respectively) imaging spectrograph with a 8º x 5' FOV (Field of View) and a 5 arcmin angular resolution. FIMS is sensitive to emission line fluxes that are fainter than any previous detection by an order of magnitude. The observation data permit the determination of the thermal and ionization equilibrium state in hot Galactic plasmas.

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Figure 5: Conceptual view of the FIMS instrument optics (image credit: KAIST)

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Figure 6: Illustration of the FIMS flight model (image credit: KARI)

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Figure 7: Optics diagram of FIMS (image credit: KARI)

FIMS also makes observations of Earth auroras and airglow with high spatial resolution and detailed spectral information for a space physics mission. Over the polar region, the FIMS spectral image is compared with simultaneous in-situ measurements of keV electrons of the SPP assembly. FIMS observes auroras regularly twice a day and airglow occasionally. 16)

The FIMS instrument consists of two elements, the spectrograph and the electronics unit. The spectrograph has the following subassemblies: Contamination Door Mechanism, Entrance Baffle Assembly, Filter Wheel Assembly, and Detector Assembly. The electronics unit houses circuits for experiment power, control and communication. The MCP (Microchannel Plate) detectors are located at the focal position of the FIMS optics and convert the spectral image to electrical signal which is delivered to the spacecraft. FIMS generates about 500 Mbit of data per day. The instrument mass is 20 kg, the power is 20 W.

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Figure 8: Overview of the FIMS optical configuration (image credit: KARI)

Parameter

Short wavelength band

Long wavelength band

Bandpass

900-1150 Å

1335-1750 Å

FOV (Field of View)

4º x 5'

8º x 5'

Mirror type

Off-axis parabolic cylinder

Off-axis parabolic cylinder

Focal length

12.5 cm (f/2.2)

12.5 cm (f/2.2)

Slit height

2.75 cm; 1/33 for bright target

2.75 cm; 1/33 for bright target

Slit width

150 µm

150 µm

Grating figure

Ellipse of rotation

Ellipse of rotation

Ellipse axis A

180 mm

180 mm

Ellipse axis C

242.6 mm

242.6 mm

Diffraction order

Second inside

First inside

Input angle alpha

21.9º

21.9º

Central output angle

-5.28º

-5.28º

Detector size

2.5 cm x 2.5 cm

2.5 cm x 2.5 cm

Number of detector pixels

512 x 512

512 x 512

Mirror coating

B4C

MgF2

Grating coating

B4C

MgF2

Photocathode

KBr

CsI + Grid

Fixed filter

MgF2

CaF2

Table 1: Specification of the FIMS instrument

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Figure 9: Schematic illustration and operating principle of the FIMS MCP detectors (image credit: KAIST)

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Figure 10: Schematic view of FIMS observations (image credit: KARI)

SPP (Space Physics Package), also referred to as PIP (Plasma In-situ Package). SPP consists of four instruments:

- ESA (ElectroStatic Analyzer)

- SST (Solid State Telescope)

- LP (Langmuir Probe)

- SMAG (Scientific Magnetometer).

The science objectives are to provide a fast-sampling capability of high-energy magnetospheric plasma components, of cold ionospheric plasmas, and to measure the Earth's magnetic fields. Particular objectives are:

• Detection of directly penetrating solar wind plasmas and up-flowing, cold ionospheric electrons

• Investigation of sub-km scale structures of the Earth's polar regions

• Comparison of the in-situ measurements with the FIMS spectrographic images (Δλ/λ of about 500) of the Earth's aurora in the far-ultraviolet ranges.

SST measures the high-energy components of auroral particles. It permits research on particle acceleration mechanisms in the magnetosphere. In addition, plasma fluxes are studied flowing in and out of the Earth's magnetosphere.

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Figure 11: Some SSP instruments (image credit: KARI)

The four APP instruments operate simultaneously. The AOCS permits aligning the detectors with respect to the Earth's magnetic field to obtain the pitch angle information. The instruments began taking observations in January 2004 (survey mode and high-resolution data suitable for microburst studies).

- ESA has a FOV of 180º x 4.2º and a complete energy sweep (29 steps) is made in 210 ms. ESA measures in the energy range of 5 eV - 20 keV.

- There are 2 SSTs onboard, one viewing perpendicular to, and the other locking upward and parallel to, the Earth's magnetic field direction. Each SST has a FOV of 33.9º. In the high-resolution mode, the perpendicular SST measures electrons from ~190 keV to 360 keV; the parallel SST measures from ~170 keV to 330 keV. The time resolution is 50 ms with 30 energy channels, enough to resolve microburst structures.

- LP measures Ne and Te of the thermal electrons with a time resolution of 200 ms

- SMAG monitors the satellite alignment with the geomagnetic field line with a resolution of 3 nT.

DCS (Data Collection System). A DCS has been developed in a cooperative effort between SaTReC and ITR (Institute for Telecommunications Research) of the University of South Australia in Adelaide and CRCSS in conjunction with FedSat-1. In FedSat-1 terminology, the system is called ADAM (Advanced Data Acquisition and Messaging System). The objectives of the STSat-1 DCS are:

• Deployment of a satellite-based data collection system for environment monitoring, wildlife tracking and transportation monitoring

• Demonstration of LEO satellite communication technology by adopting new protocols for bi-directional messaging, on-board modem algorithms and architectures

• Establishment of bi-directional communication between the on-board DCS and the ground segment, consisting of many DCPs (Data Collection Platforms). The DCPs are also referred to as MTs (Mobile Terminals). A TDMA (Time Division Multiple Access) protocol is being used in the uplink to collect the data from various DCPs simultaneously.

• Acquisition of the combined operations between STSat-1 DCS, MTs, data receiving stations and those of FedSat-1 (Austraila).

In the STSat-1 mission, the primary application of DCS is oceanographic research associated with ARGO (Array for Real-time Geostrophic Oceanography) environmental monitoring project (ARGO is part of the World Ocean Circulation Experiment). The DCS payload consists of UHF electronics and BBP (Baseband Processor), identical to the one on FedSat-1. The BBP module provides data processing during packet data communication between the S/C and the MTs (Mobile Terminals) in the ground segment (these are ARGO buoys distributed around the Korean peninsula and around Australia - monitored by STSat-1 and FedSat-1). The buoys record ocean environmental data such as temperature and salinity down to depths of 2000 m.

Parameter

Uplink

Downlink

Frequency

313.55 MHz

400.4 MHz

Link speed

4 kbit/s

1 kbit/s

Multiple access scheme

TDMA

TDM

Modulation

QPSK

BPSK

Table 2: Specification of the DCS communication

The communication scheme requires pre-scheduled contacts by TT&C control. The S/C average pass contact time with the MTs is expected to be 15 minutes over Australia and about 10 minutes of Korean waters. Initially MTs or buoys on the sea surface wait for S/C signals during standby mode. Once a UHF communication link is established, bi-directional data transmissions are executed. The collected data are stored in the S/C mass memory system until they are forwarded to a ground station, where they are processed and distributed to end users. A portion of the data will be made available on Internet.


1) W. K. Lee, K. I. Kang, H. W. Lee, et al., Development of KAISTSAT-4 Expanding the Role of Small Satellites for Scientific Research," AIAA/USU Conference on Small Satellites, Logan UT, Aug. 13-16, 2001, SSC01-III-6

2) J. Seon, K. I. Deon, S. H. Kim, et al., "Brief Reports on KAISTSAT-4 Mission Analysis," Journal on Astronomy and Space Sciences, Vol. 17, No. 2, 2000, pp. 1-9

3) J. Seon, H. S. Kim, B. J. Kim, Y. S. Chang, K.-M. Park, et al., " Preliminary results from mission analysis on KAISTSAT-4," SaTReC paper provided by Woo-Kyung Lee

4) B.-S. Kim, E.-S. Sim, G. W. Morgenthaler, "The STSat-2 Program to Demonstrate Space Science and Technology," Proceedings of the 56th IAC 2005, Fukuoda, Japan, Oct. 17-21, 2005, IAC-05-B5.1.05

5) H.-W. Lee, B. J. Kim, M.-J. Tahk, D.-J. Park, "Attitude Determination and Control of KAISTSAT-4 Satellite," internal paper of SaTReC provided by Woo-Kyung Lee

6) H.-Y. Park, S.-W. Kwak, M. Choi, C. Lee, M. Nam, J.-T. Lim, "Attitude monitoring system for STSat-1," Proceedings of WSANE 2005 (Workshop for Space, Aeronautical and Navigational Electronics 2005), Daejeon, Korea, March 3-5, 2005, pp. 97-102

7) "Science and Technology SATellite Program of Korea," KARI, Nov. 2 2004, URL: http://www.aprsaf.org/data/p_saprsaf_data/repo_ap11cd/ss_info/4_SS_STSAT.pdf

8) Information provided by Seung-Wu Rhee of KARI (Korea Aerospace Research Institute), Daejeon, Korea

9) K. Kang, "STSat Program - Science & Technology Satellite," April 12, 2007, URL: http://csplab.kaist.ac.kr/lecture/SatRec2007/lecture_notes/EE807_070412.pdf

10) "SPEAR Images Supernova Shock Wave," http://www.spacearchive.info/news-2004-06-02-ucb.htm

11) J. Edelstein, E. J. Korpela, K. Nishikida, J. Kregenow, M. Feuerstein, J. Adolfo, K. W. Min, K. S. Ryu, W. Han, D. H. Lee, J. H. Park, "SPEAR Far UV Observations of the Interstellar Medium," AAS (American Astronomical Society) 205th Meeting, San Diego, CA, January 9-13, 2005

12) http://www.berkeley.edu/news/media/releases/2003/09/15_spear.shtml

13) http://space21.kasi.re.kr/fims/intro/en_intro.htm

14) http://satrec.kaist.ac.kr/fims/fims.htm

15) "FIMS - Payload Overview," URL: http://space21.kasi.re.kr/fims/intro/en_instrument.htm

16) K. W. Min, C. Lee, J. Edelstein, G. Parks, W Han, K. Ryu, E. Korpela, U. Nam, D. Lee, K. Nishikida, H. Jin, "Airglow Observation from STSat-1," AGU Fall Meeting 2004, San Francisco, CA, Dec. 13-17, 2004


This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.