Minimize TUBSAT

TUBSAT (Technical University of Berlin Satellite) Program

TUBSAT is a low-cost and fast-turnaround microsatellite program series defined, designed and built by the Institute of Aeronautics and Astronautics (ILR: Institut für Luft- und Raumfahrt) of the Technical University of Berlin (TUB), Germany. The objective is to explore technical capabilities in microsatellite design (in particular in the field of attitude determination) and space-related applications. The following fields of study are of particular interest: 1) 2) 3) 4)

• S&F (Store & Forward) communication techniques in conjunction with DCS (Data Collection Systems)

• Test of attitude control systems

• Test of disaster monitoring technologies

Background: TUBSAT is an educational program including the design, manufacture, testing, launching and operation of `microsatellites' (< 100 kg) that was initiated in 1985 for students at the Technical University of Berlin (TUB). Meanwhile not only students but also engineers from space agencies in Germany (DLR-TUBSAT), Morocco (MAROC-TUBSAT) and Indonesia (LAPAN-TUBSAT) participate in this program.

Satellite

Payload Class

S/C Mass

Launcher

Launch Date

Operational status

TUBSAT-A

S&F communications

35 kg

Ariane-4

July 17, 1991

Last contact in the spring of 2007

TUBSAT-B

Earth observation

45 kg

Tsyklon

Jan. 25, 1994

Failed after 39 days in orbit

TUBSAT-N, N1

S&F communications

8 kg, 3 kg

SHTIL

July 7, 1998

Mission ended (reentry)

DLR-TUBSAT

Earth observation

45 kg

PSLV

May 26, 1999

Mission ended in the spring of 2008

MAROC-TUBSAT

Earth observation

47 kg

Zenit

Dec. 10, 2001

Operational in 2012

LAPAN-TUBSAT

Earth observation

56 kg

PSLV-C7

Jan. 10, 2007

Operational in 2014

Table 1: Overview of the TUBSAT program 5)

 


 

TUBSAT-A

TUBSAT-A is a technology demonstrator mission. The objectives are to study S&F techniques and to test an attitude control system. The microsatellite has a cube structure with dimensions of 38 cm x 38 cm x 38 cm and a mass of 35 kg. It is three-axis stabilized, control is provided by three magnetorquers and a magnetometer. Attitude is sensed by star and sun sensors.

TUBSAT_Auto1A

Figure 1: Illustration of TUBSAT-A (image credit: TUB/ILR)

Launch: TUBSAT-A was launched piggyback July 17, 1991 from Kourou by Ariane V44/ASAP together with ERS-1 and three other microsatellites [UoSAT-5 (SSTL), SARA (French University), and DATA-X (OSC)].

Orbit: Sun-synchronous polar orbit, altitude = 780 km, inclination = 98.42º, equator crossing time at 10:30 AM.

RF communications: S&F (Store & Forward) communication is provided in VHF-band (143.075 MHz) at a data rate of 1200 baud. The satellite uses an ALOHA-type access system for ground communications; it transmits a set of telemetry data plus one kByte of text data every 4.5 minutes. On-board storage capacity is 32 kByte. Typical experiments are with lightweight (hand-held) ground stations such as manned expeditions in arctic and antarctic regions, tracking of drifting buoys and location experiments with animals (deer). TUBSAT is operated by a single ground station at TUB; it comprises a steerable cross-polarized yagi-antenna and standard radio amateur receive and transmit equipment.

Status of mission:

TUBSAT-A was operational until the spring of 2007 providing more than 15 1/2 years of service life.

The main emphasis of this first-generation mission was the verification of the communication link using VHF and UHF frequencies. Ultra mobile partners in extremely remote areas like researchers of the Alfred Wegener Institute in Antarctica, or adventurers like Arved Fuchs on his sailing trip around the North Pole, Prof. Trinks in Svalbard (Spitzbergen), or the Weber-Malakhov Expedition to the North Pole used this opportunity for store and forward communication with their home basis. In addition, tests with non-cooperative partners like migrating animals, or floating buoys, were successfully performed and confirmed the validity of this link for the spacecraft operation (Telemetry and Telecommand). 6)

In addition to a number of technology demonstrations, some tests were conducted; for example:

• Flight demonstration of a star sensor (STS-1000) that was developed at TUB. It confirmed the general performance of this equipment, but discovered at the same time the influence of radiation (hot spots), in particular over the South Atlantic Anomaly, an area were the magnetic field of the Earth is weakened and the cosmic radiation comes closer to the orbit.

Based on this extensive experience, a simple hot spot filter was conceived and successfully tested on Maroc-TUBSAT that was launched on Dec. 10, 2001 and is still operational as of 2009. 7)

• Long term degradation tests (over more than 15 years since this satellite is still active) on solar cells (Si and GaAs) and NiCd battery cells revealed that the solar cell degradation is significantly less than predicted and that the battery degradation due to the `memory effect' is significant in the beginning of a mission, but stabilizes to a reduced level after some years.

• Support of wideband communication tests using L-band frequencies.

 


 

TUBSAT-B

A microsatellite with dimensions of 38 cm x 38 cm x 50 cm and of 45 kg mass with the objective of Earth imaging. The S/C is three-axis stabilized with three reaction wheels. Attitude is sensed by star and sun sensors. 8)

RF communications: Transmission frequency in VHF-band (143.075 MHz) at data rates of 1200 and 2400 baud.

Launch: TUBSAT-B was launched on January 25, 1994 as a secondary payload (together with Meteor-3-7) by a Russian Tsyklon launcher from Plesetsk.

Orbit: Polar orbit, altitude = 1250 km, inclination = 82.3º.

TUBSAT_Auto19

Figure 2: Illustration of TUBSAT-B (image credit: TUB/ILR)

Sensor complement:

A video camera system in the VNIR spectral range capable of taking snapshots of the ground. System parameters: Zeiss Jena telescope with 1 m focal length (folded optics), aperture = f/5.6, CCD array: Thomson-CSF, 288 x 384 pixels. FOV = 0.35º x 0.5º; spatial resolution = 23 m x 30 m.

Status of mission:

• On March 5, 1994 contact with TUBSAT-B was lost after 39 days of successful operation.

 


 

TUBSAT-N / N1 (Technical University of Berlin Satellite-Nano)

TUBSAT-N and -N1 are two communication (data collection) nanosatellites of the Technical University of Berlin (Institute of Aeronautics and Astronautics) which were tandem-launched July 7, 1998 on a Russian SS-N-23 military launch vehicle (also referred to as SHTIL). 9) The submarine launch, from the Delta-IV class nuclear vessel `Novomoskovsk' of the Northern Fleet, took place in the western Barents Sea (in the vicinity of Murmansk). Both satellites were separated from the launch vehicle's payload capsule after orbit injection. The launch represents the world's first underwater/space launch of a satellite into Earth orbit on the basis of a commercial service. Both satellites are used for mobile communications with the following objectives and service provision (a consistent extension of the TUBSAT-A package): 10) 11)

• Demonstration of low-cost access to space without performance reduction

• Bidirectional data transfer between autonomous environmental stations and the satellite (e.g., drifting buoys in the ocean, arctic and antarctic meteorological stations, etc.)

• Tracking of medium-sized and large mammals

• Worldwide positioning and deactivation services of stolen cars and support of very mobile communications.

TUBSAT_Auto18

Figure 3: Illustration of TUBSAT-N and TUBSAT-N1 (image credit: TUB/ILR)

 

Spacecraft:

TUBSAT-N has the shape of a small box with dimensions of 320 mm x 320 mm x 104 mm; its mass is 8.5 kg. A carbon fiber structure was chosen for both satellites for stiffness and lightweight design (passive thermal stabilization). Implementation of a single-board design, functional integration of modules/subsystems (e.g., TT&C with communication module), there is no physical separation between platform and payload. The ACS (Attitude Control Subsystem) consists of a CCD star sensor, a 3-D magnetic compass, two magnetic coils and a reaction wheel. Note: ACS on TUBSAT-N is simply flown as an experiment to space qualify the star sensor and the reaction wheel (mobile communication with omnidirectional antennas does not require such attitude control). Electrical power is provided by 9 NiCd battery cells of 5 Ah. They are charged by two strings of solar cells (34 cells per string) with cell dimensions of 6 cm x 4 cm. The solar cells are mounted on the top and bottom surface of the box. TUBSAT-N has an on-board storage capacity of over 6000 messages at 134 bytes in length. The messages are transferred several times per day to the TUB ground station.

TUBSAT-N1 is of same design but somewhat smaller (dimension: 320 mm x 320 mm x 34 mm; mass = 3 kg; 9 NiCd battery cells of 2.8 Ah). The design life for each S/C is seven years. The solar generator consists of two parallel strings with 36 cells on each side with cell dimensions of 4 cm x 2 cm. Nine NiCD cells of 2.8 Ah provide battery power.

TUBSAT_Auto17

Figure 4: Schematic view of TUBSAT-N with electronic components shown from top (image credit: TUB/ILR)

Launch: The tandem launch July 7, 1998 on a Russian SS-N-23 military launch vehicle (referred to as SHTIL) of the two satellites represents a demonstration of the launch and separation technology of the SS-N-23 launch system. In addition, the communication coverage of the overall system is improved and redundancy provided by the two-satellite configuration.

Orbits: Elliptical orbits, both satellites are in the same orbital plane, apogee = 770 km, perigee = 404 km, inclination = 78.9º.

 

Status of mission:

Due to relatively low orbital perigees the satellites ended their service lives; TUBSAT-N1 after about one year of operation (reentry on Oct. 20, 2000), and N after more than two years of operation (reentry on April 22, 2002 into the Pacific Ocean). The lower mass of N1 (3.0 kg) was more affected by the drag forces than the N spacecraft (8.5 kg) with more than double the mass of N1.

 

Communication configuration for data collection:

TUBSAT-N uses four independent communication channels (two VHF, two UHF), all with FFSK modulation (optional GMSK). TUBSAT-N1 uses two UHF communication channels, all with FFSK modulation (optional GMSK). Data collection function: Access to the satellites is via MP-TDMA (Modified Preassigned - Time Division Multiple Access). The satellite antenna radiation pattern is sufficiently omnidirectional to guarantee a link budget in any direction. The S/C UHF and VHF transmitter power is 10 W.

Each communication module consist of:

• Electronic fuse with integrated watchdog

• 16 Bit Hitachi H8/536 CPU

• 512 kB SRAM, 64 kB ROM, 128 kB EEPROM

• Real-time clock

• 32 analog telemetry channels

• FFSK (Fast Frequency Shift Keying) or GMSK (Gaussian Minimum Shift Keying) modulation

• VHF or UHF receiver with a sensitivity of better than -120 dBm

• VHF of UHF transmitter, max. 10 W of transmit power.

TUBSAT_Auto16

Figure 5: Block diagram of the communication module (image credit: TUB/ILR)

A mobile communications terminal in the ground segment consists of the following components: 16-bit CPU, RAM and FLASH memory, real-time clock, DC/AC converter for GPS receiver, FFSK or GMSK ASIC and a UHF/VHF transceiver, based on YAESU (manufacturer) chip sets . Several I/O pins, a serial communications interface and eight analog channels provide connection to external devices (e.g., LCDs, keyboards, measurement devices, etc.). With the knowledge of position, time and orbital parameters of the satellite, provided at each contact, the mobile terminal is able to calculate the upcoming contact times with the space segment for elevations above 5º. Each terminal has predefined time slots for communicating with the satellites (including update orbital parameters). The data collection rates between S/C and terminal are either 1200 or 2400 baud. An additional downlink transmitter with 9.6 kbaud and GMSK modulation is used to transmit the collected messages to the central ground station.

The two nanosatellites collect also data from a number of Lagrangian buoys that are automatically submerged to depths of 2500 m to measure salinity and other parameters, they only rise to the ocean surface after predefined measurement periods for the purpose of data transmission.

 


 

DLR-TUBSAT

DLR-TUBSAT is a microsatellite developed in a joint venture between TUB/ILR and DLR (German Aerospace Center), Berlin Adlershof. TUB was responsible for the satellite bus and DLR for the payload. The prime objective of the mission is to test a newly developed attitude control system (S/C attitude recovery from hibernation); a secondary objective is to test a TV camera system to demonstrate disaster monitoring. The goal of the latter item involves the introduction of an interactive Earth observation concept, where the target is not identified in advance, a search action may be involved, or a particular target region has to be followed visually from orbit.

Note: Initially, the DLR-TUBSAT project was known as TUBSAT-C; the renaming to DLR-TUBSAT took place in late 1998 (only the spacecraft bus remained as TUBSAT-C). 12) 13)

TUBSAT_Auto15

Figure 6: Isometric drawing of the DLR-TUBSAT microsatellite (image credit: TUB/ILR)

Spacecraft:

DLR-TUBSAT is subdivided into a payload, a housekeeping, and an ACS (Attitude Control Subsystem) module. The last two modules represent the TUBSAT-C bus (Figure 8). The microsatellite is three-axis stabilized, it has dimensions of 32 cm x 32 cm x 32 cm and a mass of 45 kg. 14) 15) 16) 17)

Housekeeping module: The housekeeping module contains the batteries, the PCU (Power Control Unit), the air coil and the two TTU (Telemetry Telecommand Units) in VHF and UHF band, as well as the two antennae. Four duplex NiH2battery cells, each with a capacity of 12 Ah, support an unregulated 10 V bus which is charged by four identical solar panels, each containing a single string of 34 silicon cells. The PCU contains the DC/DC-converter and the power distribution device. It is capable of switching different loads simultaneously, while constantly monitoring current levels and providing protection against short circuit. The UHF/VHF-TTU receives and transmits data via FFSK modulation at a rate of 1200 baud. Both transceivers nominally operate parallel in a listening mode. As long as no telecommand is received from the ground, the satellite is in hibernation.

ACS Module: The ACS module contains three reaction wheels, three fiber optic gyros, the magnetorquer electronics, the OBDH (On Board Data Handling System), the S-band transmitter as well as the S-band antenna.

Attitude is sensed by the camera assembly; the actuators consist of three momentum/reaction wheels (TUB design) and three fiber optic laser gyros providing a pointing knowledge accuracy of ±1 arcsecond. The agile spacecraft is pointable into any direction and able to stabilize itself on short notice within a few seconds. Tracking modes for inertial pointing (S/C attitude) and/or target pointing (Earth observation) are supported. In its normal standby (hibernation) mode, the S/C tumbles in its orbit at rates of about 0.1 rpm. An acquisition sequence starts with a rate reduction command, followed by a coarse sun acquisition maneuver. This may be followed by a TV transmission.

TUBSAT_Auto14

Figure 7: Illustration of the reaction wheel and gyro units (image credit: TUB/ILR)

The core of the attitude control system of the latest TUBSAT models contains three independent control loops, each consisting of a momentum/reaction wheel (IRE) and a fiber optic laser gyro (built by LITEF). The central processor of the spacecraft sets the targets for the loops and is then free for other tasks. The following modes of operation are available:

• Current control of the wheel

• Speed control of the wheel

• Torque control of the wheel

• Rate control of the loop

• Integrated rate (angle) control of the loop.

The wheels can be operated as momentum wheels or reaction wheels depending on the mission requirement and availability of additional sensors. On DLR-TUBSAT, however, the outer control loop is closed only with a manual interaction in the control loop. This is accomplished by quickly analyzing the results of the satellite imagery and the subsequent uplinking of control commands in a joy-stick mode. 18)

TUBSAT_Auto13

Figure 8: Exploded view showing the modular design concept of the DLR-TUBSAT (image credit: TUB/ILR)

The lower compartment contains the UHF-TTU and the iron coil, which is mounted in the direction of the z-axis of the satellite. A single string of solar cells is attached at the surface in the +z-axis and is used for the sun acquisition maneuver.

TUBSAT_Auto12

Figure 9: System architecture of DLR-TUBSAT (image credit: TUB/ILR)

TUBSAT_Auto11

Figure 10: Photo of the DLR-TUBSAT microsatellite (image credit: TUB/ILR)

Spacecraft mass, size

45 kg, 32 cm x 32 cm x 32 cm

Structure

4 compartments made of aluminum

Thermal control

Passive

Power

- 4 solar panels with a single string of 34 silicon cells
- 4 NiH2 battery cells (Eagle-Picher), 12 Ah, 10 V nominal

RF communications

- VHF/ UHF-band, 1200 baud, 3.5-5 W
- S-band, 125 kbaud BPL coded or video transmission, 3.5 W

Attitude control

3 wheel/gyro units ACS 202 (TUB), magnetorquer

Payload

f= 16 mm (D= 1/0.95): pixel resolution = 375 m
f= 50 mm (D= 1/1.80): pixel resolution = 120 m
f= 1000 mm (D= 1/11.0): pixel resolution = 6 m

Table 2: Overview of DLR-TUBSAT parameters

Launch: A DLR-TUBSAT passenger launch took place on an ISRO PSLV launcher from the SHAR launch site, India, on May 26, 1999 (along with IRS-P4 of ISRO as the primary payload and KITSAT-3 of KAIST as another secondary payload).

Orbit: Sun-synchronous polar orbit, altitude = 720 km, inclination = 98.28º, period = 99.34 min, equator crossing time at 12:00 hours on a descending node.

 

Status of mission:

• The mission was retired in the spring of 2009 due to a battery failure.

• DLR-TUBSAT is operating nominally in early 2008 (providing more than 8 years of operations).

 

Sensor complement:

Camera Assembly. The system (a Sony CCD array system with Nikon optics) consists of three commercially available TV cameras:

• Wide-angle camera with 16 mm focal length with a black-and-white chip

• Standard-angle optic with 50 mm focal length and color CCD chip

• Narrow-angle camera (a tele lens) with 1000 mm focal lens and black-and-white chip.

The objective is to provide a snapshot imaging capability of medium or high-resolution ground scenes of locations stricken by disaster (hazardous or unexpected environmental events such as wild fires, floods, and earthquakes). Each camera head of the camera assembly can be selected as the active one.

All cameras are aimed in the same direction (nadir direction by default), but offer different fields of view (image sizes). One of the three cameras may provide imagery at any time (16 mm camera by default). The camera system automatically adjusts its gain to prevailing illumination conditions. A body-pointing capability is provided by ACS of the S/C, allowing the camera assembly to look at targets anywhere up to the horizon. The camera assembly has a video output as well as a digital output. Both of them can be selected to feed the S-band transmitter. The analog signal is coded as a standard FM PAL signal which may be received by a 3-m antenna dish and standard equipment. The digital data is bi-phase level encoded with a transfer data rate of 125 kbit/s. A 1.2 m antenna dish is sufficient for reception.

Parameter

16 mm camera

50 mm camera

1000 mm camera

CCD array size

750 x 580 pixels

750 x 580 pixels

750 x 580 pixels

Pixel size

8 μm x 8 μm

8 μm x 8 μm

8 μm x 8 μm

IFOV

 

9.2 x 10-3 degrees

0.46 x 10-3 degrees

FOV

 

6.9º x 5.336º

0.345º x 0.267º

Nadir image size (1000 km altitude)

110 km x

120 km x 92.8 km

6 km x 4.6 km

Nadir ground resolution (IFOV)

370 m x 370 m

120 m x 120 m

6 m x 6 m

Focal length, f/number

16 mm

50 mm, f/1.8

1000 mm, f/11

Spectral range

Panchromatic

Panchromatic

Panchromatic

Table 3: Definition of Camera Assembly

Spacecraft operation: The S/C is normally operated from a single ground station at TU Berlin (TT&C operations in UHF-band, the UHF transceiver receives and transmits data at a rate of 1200 baud, FFSK modulation). Most of the time (certainly over 95%) the satellite is orbiting in standby mode, slowly tumbling (a few rotations per orbit); all on-board systems are in hibernation (including ACS) to save power; only the UHF receiver is listening for potential commands. In case of an event, the S/C attitude may be restituted within a few minutes. However, this wake-up capability requires the S/C to be in view of a commanding UHF station. TUB has portable UHF ground stations from its TUBSAT-A experience to handle such events; it is also relying on the use of existing infrastructures of the remote sensing community.

There is no on-board storage capability for video data from the Camera Assembly. Real-time transmission of the video data is in S-band (frequency = 2.206 GHz, bandwidth = 8 MHz, data rate at 128 kbaud using analog video, the beamwidth of the S-band antenna is 70º) to a ground station in the vicinity of the event. Video transmission beyond the TU Berlin ground station has to be negotiated on a case-by-case basis. A possible alternative for video data reception coverage may also be found in the widespread use of portable commercial S-band transmitter systems and their corresponding receiver systems. The S-band transmitter system on DLR-TUBSAT happens to be of the same type as those regularly used in TV coverage of sports events, such as in the helmet of a downhill skier.

 


 

Maroc-TUBSAT

Maroc-TUBSAT is a cooperative microsatellite project between CRTS (Centre Royal Teledetection Spatiales) of Rabat, Morocco, and the Institute of Aeronautics and Astronautics at the Technical University of Berlin (TUB/ILR). In this joint venture, the Moroccan side is responsible for the development of the payload and launch of the satellite while the German side provides the satellite bus. The overall objectives of the mission are in Earth remote sensing (in particular with regard to vegetation detection at medium-scale resolutions), and in the field of store-and-forward communications for mobile localization. A further goal is to develop attitude control strategies for high-resolution Earth observations. 19) 20) 21) 22)

TUBSAT_Auto10

Figure 11: Exploded view of the Maroc-TUBSAT microsatellite (image credit: TUB/ILR)

Spacecraft:

The project selected an adaptation of the existing TUBSAT-C bus, a three-axis stabilized box structure of size: 320 mm x 340 mm x 362 mm. The main elements of the bus are: the attitude control module, the power module, and the payload module. The modular design provides functional freedom and flexibility during all phases of development. S/C power is provided by four surface-mounted solar cells (60 W max) and by 4 NiH2 batteries with a capacity of 12 Ah.

The attitude control subsystem (ACS) employs a star sensor (inertial attitude determination), a three-axis magnetic field sensor, and three fiberoptics gyros for attitude sensing. In addition, single solar cells on the surface-mounted solar panels are used for coarse sun direction determination. Attitude actuation is provided by three reaction wheels and magnetic coils. The S/C mass is 47 kg.

The attitude control of Maroc-TUBSAT is based on a momentum biased system where at least one wheel is maintained in a high speed control mode. During hibernation all the other wheels and gyros can be disabled. The direction of the momentum vector is controlled in very long intervals (once per day or less) by the onboard star sensor. However, due to special disturbance torque compensation techniques, the drift rate of the momentum vector could be reduced < 1º/day.

Steps in an attitude control acquisition sequence:

1) Hibernation mode: In this default mode, the spacecraft rotates around it's z-axis (pointing toward Earth) with ~ 1º/s, with the momentum bias vector perpendicular to the orbital plane

2) First attitude acquisition:

• Momentum bias vector is split up between the z and x-axes (primary to x). This defines the east/west position of image center

• The spacecraft rotates with 1º/s around x; while the y control circuit is nutation-damping

3) Second attitude acquisition:

• If the satellite reaches its target solar cell current (rising or falling flank) the RW (Reaction Wheels) reduce the rotation to 0.5º/s (rpm mode)

• After one half in the nutation period the RW reduce the rotation to 0º/s

• Angle step back to the target position by using RW-gyro pair

• Fine adjustment every 5 s until the time for first image is reached

• Taking of 24 or 28 snapshots and send the data down or taking one picture and store it

• After completing the snapshot sequence the spacecraft goes back into the default hibernation mode.

Advantages of Maroc-TUBSAT:

• Autonomous operations are based on a worldwide scale

• The provision of an S-band and TT&C station within the target region is not needed

• The hibernation mode is easier to control than that of its predecessor (DLR-TUBSAT)

Some disadvantages of Maroc-TUBSAT operations:

• Target position must be known exactly

• Its onboard storage capacity is still too small.

TUBSAT_AutoF

Figure 12: Illustration of the Maroc-TUBSAT spacecraft (image credit: TUB/ILR)

Spacecraft mass, size

47 kg, 320 mm x 340 mm x 362 mm

Power

- 4 solar panels, 320 mm x 320 mm, 34 cells in series, max. 14 W each
- 4 NiH2 batteries, 10 V nominal voltage, 12 Ah capacity
- PCU for power control, conditioning and distribution

ACS (Attitude Control Subsystem)

- 3 wheel / gyro couples in orthogonal axes, each wheel with integrated WDE (Wheel Drive Electronics)
- Star sensor for 3 axes inertial attitude determination (developed at TUB)
- 3 axes magnetic field sensor
- Magnetic coil in X axes
- Single solar cells for sun determination (emergency/coarse mode)

RF communications

- S-Band transmitter with patch antenna, modulation = BPSK, FEC, max 250 kbaud, 2.0 W RF output
- TTC1 in UHF, modulation FFSK, 1200/2400 Baud, 3.5 W RF output
- TTC2 in VHF, modulation FFSK, 1200/2400 Baud, 5.0 W RF output

Data handling and processing

- OBDH with 256 kB static RAM, 128 kB EEPROM, 64kB ROM
- Model calculator with 512 kB RAM, 256 kB EEPROM, 64kB ROM
- SEU protection

Payload data handling

- Frame data handling system
- Messaging system for store and forward communications

Table 4: Performance characteristics of the Maroc-TUBSAT spacecraft

TUBSAT_AutoE

Figure 13: Integration of the secondary payloads at the Baikonur Cosmodrome (image credit: TUB/ILR)

 

Launch: A launch of Maroc-TUBSAT took place on Dec. 10, 2001 on a Zenit-2 launch vehicle from the Baikonur Cosmodrome, Kazakhstan. The prime payload is Meteor-3M-1. Secondary payloads on the flight are the Badr-B satellite of Pakistan, Maroc-TUBSAT of CRTS, the Compass spacecraft of IZMIRAN, Moscow, and REFLECTOR (US/Russia).

Orbit: Sun-synchronous circular polar orbit, mean altitude = 830 km, inclination = 98.85º, period = 102 minutes; local time of ascending node is 9:15 AM.

RF communications: Store-and-forward communications of TT&C data in UHF and VHF (AMSAT standard) are provided at data rates of 1.2 kbit/s (uplink and downlink, FFSK modulation). The RF output power is 3.5 W for UHF and 5.0 W for VHF. The payload data is transmitted in S-band (BPSK modulation, FEC, 2 W RF output) at data rates of up to 250 kbit/s. Spacecraft operations are conducted at (TUB, CRTS), ground stations are at TUB and at CRTS.

Spacecraft operations: The mission scenario of spacecraft activity/operations is event-driven as practiced on DLR-TUBSAT. This implies that Maroc-TUBSAT spends most of its time in hibernation mode. Event recognition (time) or a command for a ground station cause the spacecraft to return to observation mode. - The following general start-up and acquisition sequence describes the steps involved in S/C recovery/initiation from the hibernation mode (ACS is off) into an operations mode for recording specific areas of the Earth by the payload camera:

• At a preprogrammed event time turns the ACS on. This causes a reduction in tumbling motions by a rate damping process, provided by the closed-loop system of gyros (3) and reaction wheels (3). The residual motion amounts to <0.5º/s.

• In the event that the star sensor is blinded by Earth or the sun, a slew maneuver is enacted to align the sensor to a star (information is provided by the solar cells and by orbit model inputs). The star sensor is now capable to determine the inertial attitude of the system.

• Meanwhile, the ground station software compares the current S/C position with the intended acquisition time. It seeks a minimum solution for the three different slew maneuvers (controlled by the gyros).

• The predefined attitude of the S/C is reached after slew maneuver completion. The pointing accuracy in this mode is better than ±0.05º (limited only by the star sensor performance).

• The hibernation mode is turned on again by the operator after observation and data transmission of the pass are completed. The batteries are recharged in hibernation mode.

 

Status of mission:

• Maroc-TUBSAT is operating nominally in 2012 (Ref. 23).

• Maroc-TUBSAT is operating nominally in 2011. 23)

 

Sensor complement:

EIC (Earth Imaging Camera), developed at RAL (Rutherford Appleton Laboratory), UK [same camera design as flown on BADR-B of SUPARCO (Pakistan Space and Upper Atmosphere Research Commission)]. The instrument optics have a focal length of 72 mm with f/6 number. Observations are performed in the visible/near-infrared range. A filter is used for the near-infrared range. The FOV is ±8.5º x ±8º, providing an image size of about 190 km x 144 km at a resolution of 250 m. A CCD matrix array of 770 x 576 pixels is used (of EEV, Chelmsford, UK, pixel size of 32.5 μm) with frame transfer of imagery. The nominal integration time for a snapshot is 28 ms. A 12 bit data quantization is used. The EIC instrument mass is 2.5 kg.

Instrument optics

Focal length = 72 mm, f/number = 6

FOV (Field of View)

±8.5º x ±8º

IFOV (resolution)

200-250 m

Filter

used for the near-infrared range

Detector

CCD matrix array of 770 x 576 pixel, pixelsize = 32.5 µm

Integration time

28 ms

Data quantization

12 bit

Instrument mass

2.5 kg

Table 5: Overview of instrument parameters

The challenge in designing and building this instrument has been the need to maintain performance with restrictions on the available space, mass, power and schedule. Snapshot imagery can be provided from any part of the globe, stored onboard and later transmitted to the ground. 24)

TUBSAT_AutoD

Figure 14: Illustration of the EIC camera head and the compact electronics control unit (image credit: RAL)

Star Sensor (of ACS). The instrument consists of the following elements: A CCD array camera, optics with 16 mm focal length and f/0.95, analog electronics and a microcontroller for star tracking, a microcontroller for star recognition (relative and inertial attitude determination), a RS422 serial interface to the host computer, a mechanical interface for mounting and heat transfer, and a power supply. The CCD array is of size: 288 x 384 pixels with frame-transfer capability (8 bit data quantization). The microcontroller uses a star catalog of 6000 stars and a pattern recognition database. The FOV of the sensor is 21º x 31º, an inertial pointing accuracy of ±0.02º is provided. The instrument has a mass of 0.68 kg, power = 5 W, data rate = 38.4 kbit/s.

The autonomous star sensor (Figure 15) on board was an improved design of the TUBSAT-A device. Crucial to the hotspot problem is the observation that hot spots are static objects. They obviously have a fixed position and a stable brightness for a given exposure time and stable sensor temperature. The simple solution taken was the use of a reference image. The reference image is just a normal star sensor image taken in orbit (close to operation). If this image is subtracted from the actual star sensor images all hot spots will cancel since they are static objects. Stars, on the other hand, will in general not cancel since they differ in image position all the time due to rotations of the spacecraft. This procedure was tested extensively with the Maroc-TUBSAT. A weekly update of the reference image was quite sufficient to operate the star sensor for several years (Ref. 7).

TUBSAT_AutoC

Figure 15: Illustration of the star sensor (image credit: TUB/ILR)

 


 

LAPAN-TUBSAT

LAPAN-TUBSAT is a cooperative microsatellite project between TUB (Technical University of Berlin) and LAPAN (Lembaga Penerbangan dan Antariksa Nasional), or the Indonesian National Institute of Aeronautics and Space, Jakarta. A MOU (Memorandum of Understanding) was signed in 2003, involving a training program of LAPAN engineers at TUB and at DLR, along with all the development stages of the spacecraft and its instruments. The objective of LAPAN-TUBSAT is to fly a remote sensing and store-and-forward (S&F) communications payload. 25) 26) 27) 28) 29)

Background: Indonesia is one of the largest archipelagos in the world, with around 17,000 islands, and using satellites is an efficient way to connect them. The history of Indonesian satellite applications began with the launch of the Palapa communication satellite series (Palapa-A1 was built by Hughes Aircraft Co and launched successfully on July 8, 1976 followed by a second satellite, Palapa A2, on March 10, 1977). The successful launch of Palapa encouraged several private companies to have their own satellite for communication applications.

Spacecraft:

LAPAN-TUBSAT is the first research satellite of Indonesia based on the existing TUBSAT program heritage. The S/C structure is an Al alloy box of size 45 cm x 45 cm x 27 cm and a mass of 56 kg. The various subsystems are mounted onto two shelves. The lower shelf contains ACS (Attitude Control Subsystem), TT&C, and the camera, while the upper shelf contains the battery, power control, and the data handling subsystem (see Figure 18, D and C, respectively).

The operational system design of the mission employs the proven hibernation support mode concept that was introduced with the DLR-TUBSAT and Maroc-TUBSAT missions (and subsequent missions of TUB) in 1999. In this support scheme, the ACS design supports the hibernation mode operations as a way of life (i.e., as its default mode) in regular S/C operations. The hibernation mode saves power, a convincing argument for rather limited power generation capabilities of the microsatellite.

The spacecraft is three-axis stabilized. The ACS employs a 3 wheel/gyro pair assembly: RW (reaction wheel) + WDE (Wheel Drive Electronics), fiber optical gyros; a CMOS star sensor (referred to as STS), 3 magnetic coils, and coarse sun sensors at all 6 outer sides of the S/C. The reaction wheels, RW204, developed by TUB, have an external WDE with a micro controller to provide operation modes such as current control, wheel speed control, angular velocity control, angle control and torque control.

The electric power subsystem consists of 4 surface-mounted solar panels (432 mm x 243 mm panel size). A total of 35 solar cells in series provides a power of 14 W (max). Five NiH2 batteries at a nominal voltage of 12.5 V have a capacity of 8 Ah. The house keeping system contains the batteries, the Power Control Data Handling unit (PCDH), the air coil and the two TTC (Telemetry, Tracking & Command) unit in the UHF band, as well as the two antennas. The PCDH contains the AC/DC converter and the power distribution device. It is capable of switching different loads simultaneously, while constantly monitoring current levels and providing protection against short circuit. Onboard data handling is provided by the OBC with the following rating: 524 kB external and 4 kB internal RAM, 524 kB EEPROM, 16 kB ROM, 38.4 kbit/s SCI (Serial Communication Interface). The S/C design life is 2 years.

TUBSAT_AutoB

Figure 16: Illustration of the LAPAN-TUBSAT spacecraft (image credit: TUB/ILR)

TUBSAT_AutoA

Figure 17: Top (left) and bottom view (right) of LAPAN-TUBSAT (image credit: TUB/ILR)

RF communications: Store-and-forward communications of TT&C data in UHF (436.325 MHz, AMSAT radio standard) are provided at data rates of 1.2 kbit/s (uplink and downlink, FFSK modulation). The RF output power is 3.5 W for UHF. - The payload imaging data are transmitted in S-band at 125 kbit/s (2220 MHz, FM video modulation), the S-band transmitter has an output power of 5 W.

The store and forward data communication mode utilizes the TT&C communication subsystem as well as the PCDH flash memory. The data rate to any ground station is limited to 1200 bit/s and a memory space of 512 kB. Hence, data can only be received in concise format.

TUBSAT_Auto9

Figure 18: Illustration of LAPAN-TUBSAT elements (image credit: TUB/ILR)

Legend of Figure 18: A = bottom view of the spacecraft with launch adapter, B = top view of the spacecraft, C = view into the upper shelf, D = view into the lower shelf.

Spacecraft mass, size

56 kg, 450 mm x 450 mm x 270 mm

Power system

- 4 solar panels, 432x243mm, 35 cells in series, max. 14 W
- 5 NiH2 batteries, 14 V nominal voltage, 12 Ah capacity

ACS (Attitude Control Subsystem)

- 3 wheel/gyro pairs (RW 203 wheels + WDE, fiber optical gyros)
- Star sensor

RF communications and data handling

- 2 TTC links, UHF, 1200 bit/s, 3.5 W RF, frequency of 437,325 MHz with FFSK modulation
- S-band payload communication, 2.220 GHz
- OBDH: 524 kB external and 4 kB internal RAM, 524 kB EEPROM, 16 kB ROM
- 38.4 kbit/s SCI interfaces

Payload

- 3 CCD color video camera with ~6 m ground resolution
- CCD color video camera with ~200 m ground resolution

Table 6: Performance parameters of LAPAN-TUBSAT

Launch: A launch of LAPAN-TUBSAT took place on Jan. 10, 2007 as a secondary payload on a PSLV launch vehicle of ISRO (Indian Space Research Organization) from SDSC (Satish Dhawan Space Center), Sriharikota Range, India. - The primary payload on this flight was CartoSat-2 of ISRO (680 kg). Further secondary payloads on this flight were: SRE-1 of ISRO and PehuenSat-1, an educational nanosatellite (6 kg) of Argentina.

Orbit: Sun-synchronous circular orbit, altitude = 630 km, inclination = 97.9º, period of 97.04 minutes (14.5 orbits/day).

 

Status of mission:

• The LAPAN-TUBSAT microsatellite is still operational in 2014 (more than 7 years on orbit). However, a half battery cell was lost for power storage, and the communication system (TM/TC) is to some extent handicapped. From the German side (TUB), operations support is now mainly conducted via the Svalbard complex due to massive RFI (Radio Frequency Interference) problems in the Berlin vicinity. 30)

• The microsatellite is operating nominally in 2012 (Ref. 31).

• The microsatellite is operating nominally in 2011. 31) 32)

LAPAN-TUBSAT is being operated via the ground stations in Biak (Papua, east of Indonesia) and Rumpin (near Jakarta, Indonesia). The spacecraft is in particular being used for disaster monitoring - such as floods and the volcanos Merapi and Bromo of East Java, Indonesia. In addition, the spacecraft is also being used for vegetation monitoring (Ref. 31).

 

Sensor complement:

High-resolution video camera. The CCD video camera (based on a Sony camera) is provided with a color splitter prism to split the image into 3 CCD matrixes. The objective is to improve the image location accuracy significantly. Each CCD matrix array has an effective image size of 752 x 582 pixels. A Cassegrain objective (focal length of 1000 mm, aperture of 50 mm) is integrated into the camera. The spatial ground resolution is 5 m on a swath width of 3.5 km. The camera observes in the visible spectrum, it has a power consumption of 7.6 W and a mass of 0.630 kg. This camera is equipped with an in-orbit refocus mechanism (developed by TUB) because of the high focal length and the thermal shift of the focal plane.

Low-resolution video camera. This is a wide-angle CCD video color camera (based on a Kappa camera) with a focal length of 50 mm: It comes with a CCD array of 752 x 582 elements and it provides a coarse ground resolution of 200 m on a swath of 81 km. The camera has a power consumption of 3 W and a mass of 0.200 kg.

TUBSAT_Auto8

Figure 19: Illustration of the video camera platform (image credit: TUB/ILR)

TUBSAT_Auto7

Figure 20: Photo into the lower shelf of the S/C with the video camera at the bottom (image credit: TUB/ILR)

The intent of the low-resolution video camera is to spot the general location of interest (say a coastline or a specific target region). The high-resolution video camera is then used to zoom in on the actual target of interest.

 

Attitude control strategy:

The microsatellite features the capability of interactive spacecraft pointing for event monitoring support. All activities start in general from the default hibernation mode in which the spacecraft is tumbling freely (all subsystems are switched off except the onboard data handling and the TT&C). An observation request starts with an uplink command to the spacecraft and the restitution of the spacecraft attitude. 33) 34)

Three operational modes are supported for the imaging mission:

1) Momentum-biased hibernation mode: Here, the satellite's angular momentum vector is maintained to be perpendicular to the flight direction. The maximum moment of inertia is designed to be very close to the Y-axis. In this setup, the Y-axis reaction wheel is set to absorb 90% of the angular momentum so that the satellite will rotate in the same direction as the wheel. Hence, the power consumed is only to run 1 wheel in about 80% of its performance.

2) Nadir pointing mode: The satellite's rotation in the pitch axis is terminated, and then the +Z-axis of the satellite (which is the camera side) is pointed to nadir.

3) Off-nadir pointing mode: The satellite's Z-axis is pointed off-nadir, either in the cross-track or in the along-track direction.

TUBSAT_Auto6

Figure 21: Target viewing capability of the LAPAN-TUBSAT spacecraft (image credit: LAPAN, TUB)

TUBSAT_Auto5

Figure 22: Off-nadir imaging scheme in cross-track (image credit: LAPAN, TUB)

Applications: The spacecraft may be used to support a number of imaging services including environmental monitoring. One of the planned applications is to establish a volcanic eruption warning system between LAPAN and the University of Gadjah Mada (UGM), Yogyakarta Indonesia. In this scenario, UGM acts as a ground station acquiring data of the Merapi volcano activity from the LAPAN-TUBSAT satellite.

 


 

Ground segment of LAPAN-TUBSAT:

The project ground stations are located at LAPAN and at TUB. LAPAN operates in effect two ground stations, namely the Rumpin ground station located near Jakarta, and the Biak ground station in Papua, East of Indonesia. The ground station locations are chosen so that the coverage area is large enough to cover the nation's archipelago. The LAPAN SCC (Satellite Control Center) is collocated with the Rumpin ground station. 35) 36)

TUBSAT_Auto4

Figure 23: Data acquisition at the Rumpin SCC and interconnection with the other ground stations in the network (image credit: LAPAN)

TUBSAT_Auto3

Figure 24: System architecture of the Rumpin SCC (image credit: LAPAN)

TUBSAT_Auto2

Figure 25: The volcano Merapi, Indonesia, as seen from LAPAN-TUBSAT on May 24, 2007 (image credit: LAPAN, Ref. 32)

The continuous observation of volcano activities is rather vital in particular in Indonesia because it is closely interlinked with other seismic activities underwater and on land. Figure 25 shows one of the periodic eruptions of the rather active Merapi.

TUBSAT_Auto1

Figure 26: LAPAN-TUBSAT image of Nov. 5, 2010 when thick ash is rising from Merapi (image credit: LAPAN, Ref. 32)

TUBSAT_Auto0

Figure 27: LAPAN-TUBSAT capture of Surabaya (East Java) shipyard in 2008 (image credit: LAPAN)


1) Information provided by Udo Renner of TUB

2) Udo Renner, “Small Satellites at the Technical University of Berlin,” IAA 2nd International Symposium on Small Satellites for Earth Observation, Berlin, April 12-16, 1999, pp. 253-256

3) http://www.tubsat.com/

4) H. Kayal, K. Brieß, U. Renner, “Small Satellites from Berlin,” Proceedings of the Asian Space Conference 2007, Nanyang Technological University (NTU), Singapore, March 21-23, 2007

5) http://www.raumfahrttechnik.tu-berlin.de/tubsat

6) Udo Renner, “Flight Experiment with TUBSAT,” Proceedings of the 4S Symposium: `Small Satellite Systems and Services,' Chia Laguna Sardinia, Italy, Sept. 25-29, 2006, ESA SP-618

7) M. Buhl, U. Renner, “Star Sensor Development Based On The TUBSAT Experience,” Proceedings of the 7th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, May 4-7, 2009, IAA-B7-0701

8) U. Renner, B. Lubke-Ossenbeck, P. Butz, “TUBSAT, Low cost access to space technology,” 7th Annual AIAA/USU Conference on small satellites, September 13-16, 1993, Logan, Utah

9) Note: SS-N-23 is the NATO designation for the Russian RCM 54 missile, built by the Makeyew State Rocket Center of Miass (a town in the Ural Mountains).

10) Robert Schulte, “TUBSAT-N, A Global Communication Satellite System, Based on Nanosatellites,” Proceedings of the 4th International Symposium on “Small Satellites Systems and Services,” Sept. 14-18, 1998, Antibes Juan les Pins, France

11) Robert Schulte, “TUBSAT-N, an ultra low cost global communication nanosatellite system,” 2000, URL: http://www.vectronic-aerospace.com/files/Air_and_Space.PDF

12) U. Renner, “Earth Observation with TUBSAT-C,” 4th International Symposium: Small Satellites Systems and Services, Antibes, France, Sept. 14-18, 1998

13) P. Butz, U. Renner, “TUBSAT-C, A Microsat Bus for Earth Observation Payloads,” 3rd International Symposium on Small Satellites Systems and Services, Annecy, France, June 24-28, 1996, Annecy, France, URL: http://microsat.sm.bmstu.ru/e-library/Missions/Brief/tubsats/tubsat-c.pdf

14) M. Steckling, U. Renner, H. P. R?ser, “DLR-TUBSAT, a Multipurpose Microsatellite for Varying Earth Observation Applications,” IAA 2nd International Symposium on Small Satellites for Earth Observation, Berlin, April 12-16, 1999, pp. 347-350

15) S. Roemer, U. Renner, “Flight Experiences with DLR-TUBSAT,” Proceedings of the 3rd International Symposium of IAA, Berlin, April 2-6, 2001, pp. 75-78

16) Stefan Schulz, Udo Renner, “DLR-TUBSAT, A Microsatellite for Interactive Earth Observation,” Small Satellite Systems and Services, 5th International Symposium, La Baule, France, June 19-23, 2000, URL: http://server02.fb12.tu-berlin.de/ILR/RFA/sat/dlr/paper/labaule.pdf

17) S. Roemer, U. Renner, “Flight Experiences with DLR-TUBSAT,” Acta Astronautica, Vol. 52, No 9, May 2003, pp. 733-737

18) U. Renner, J. Bleif, S. Roemer, “The TUBSAT Attitude Control System: Flight Experience with DLR-TUBSAT and MAROC-TUBSAT,” 4th IAA Symposium, Berlin, Germany, April 7-11, 2003, IAA-B4-1201, URL: http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv4/IAA-B4-1201.pdf

19) “MAROC-TUBSAT, A Microsatellite for Earth Observation,” TU-Berlin, Ref. 2 EB-MT-R01

20) Stephan Roemer, Udo Renner, “Flight Experience with the Microsatellite MAROC-TUBSAT,” 54th IAF Congress, Bremen, Germany, Sept. 29-Oct. 3, 2003, IAC-03-IAA.11.2.07

21) http://www.vectronic-aerospace.com/html/maroc-tubsat.html

22) http://www.crts.gov.ma/static.php?file=english.html

23) Information provided by Udo Renner (retired), formerly of TU Berlin, Germany

24) http://www.ssd.rl.ac.uk/ssd/ccdtg/BADR-B.htm

25) R. H. Triharjanto, W. Hasbi, A. Widipaminto, M. Mukhayadi, U. Renner, “LAPAN-TUBSAT: Micro-Satellite Platform for Surveillance & Remote Sensing,” Proceedings of the 4S Symposium: Small Satellites, Systems and Services, Sept. 20-24, 2004, La Rochelle, France, URL: http://server02.fb12.tu-berlin.de/ILR/RFA/sat/lapan/paper/triharj1.pdf

26) H. Djojodihardjo, A. Said, F. Alaudin, S. K. Adnan, “Vision, Development and Experience in Small Satellites for Remote Sensing - A Vehicle for Capacity Building: Experience in Indonesia and Malaysia,” Proceedings of the 57th IAC/IAF/IAA (International Astronautical Congress), Valencia, Spain, Oct. 2-6, 2006, IAC-06-B5.1.05

27) Information provided by Stephan Roemer of TUB, Berlin, Germany

28) Udo Renner, Matthias Buhl, “High Precision Interactive Earth Observation with LAPAN-TUBSAT,” Proceedings of the IAA Symposium on Small Satellite Systems and Services (4S), Rhodes, Greece, May 26-30, 2008, ESA SP-660, August 2008 URL: http://server02.fb12.tu-berlin.de/ILR/RFA/sat/lapan/paper/2008_05_15_renner_buhl_rhodos_final_4s.pdf

29) http://server02.fb12.tu-berlin.de/ILR/RFA/sat/lapan/content.htm

30) Information provided by Stephan Roemer of AFW (Astro Feinwerktechnik GmbH), Berlin Adlershof.

31) Information provided by Udo Renner (retired) of TU Berlin, Germany

32) Bambang S. Tejasukmana, “Indonesian Country Report,” APRSAF-17 (17th Session of the Asia-Pacific Regional Space Agency Forum), Melbourne, Australia, Nov. 23-26, 2010, URL: http://www.aprsaf.org/data/aprsaf17_data/D3-1100_B_Tejasukmana.pdf

33) S. Hardhienata, A. Nuryanto, R. H. Triharjanto, U. Renner, “Technical Aspects and Attitude Control Strategy of LAPAN-TUBSAT Microsatellite,” Proceedings of the 5th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2005, URL: http://server02.fb12.tu-berlin.de/ILR/RFA/sat/lapan/paper/hardhienata.pdf

34) T. M. Kadri, Adi S. Salatun, “Indonesian LAPAN-TUBSAT Micro-Satellite Development,” Asia-Pacific Regional Space Agency Forum (APRSAF-14) Main Theme: Space for Human Empowerment, Nov. 21-23, 2007, Bangalore, India, URL: http://www.aprsaf.org/data/aprsaf14_data/day2/P07_LAPAN%20APRSAF-14%20Plenary1c.pdf

35) W. Hasbi, E. Nasser, A. Rahman, “Spacecraft Control Center Of Lapan-Tubsat Micro Satellite,” URL: http://www.aprsaf.org/data/feature/PAPER_ASC2007-WHASBI-LAPAN-2.pdf

36) W. Hasbi, R. Widyastuti, “The use of LAPAN-TUBSAT Satellite Video Data for Earth Observation,” Proceedings of the International Workshop on Earth Observation Small Satellites for Remote Sensing Applications (EOSS 2007), Kuala Lumpur, Malaysia, Nov. 20-23, 2007


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.