PROPOSAL FOR A NEW HYPER SPECTRAL
IMAGING MICRO SATELLITE:
SVALBIRD 1 University Courses on Svalbard (UNIS), Box 156, N-9171 Longyearbyen,
Fred Sigernes 1, Udo Renner 2, Stephan Roemer 2, Jörn-Hendrik Bleif 2, Dag Arne Lorentzen 1, Stefan Claes 1, Reidar Nordheim 3, Frank Johannessen 3, Veronika Ekstrand 3, and Sten-Christian Pedersen 3
2 Technical University of Berlin, Institute of Aerospace, Marchstr. 12, D-10587 Berlin, Germany.
3 Kongsberg Satellite Services, Svalbard Station (SVALSAT), Box 458, N-9171 Longyearbyen, Norway.
1 University Courses on Svalbard (UNIS), Box 156, N-9171 Longyearbyen,Norway.
This document describes a proposal to construct and launch a new generation of polar orbiting microsatellites for Earth observation. The project is a joint venture with the Technical University of Berlin (TUB), University Centre on Svalbard (UNIS) and SVALSAT. The proposed microsatellite is designed for interactive Earth observation using a hyper spectral imager. The mission objective, the final configuration of the satellite, the ground segment, the operation, and the total cost of the project is presented.
The development of microsatellites has during the last decades evolved to be a cost efficient alternative in remote sensing. Due to the advances in the fields of electronics and optics they can in many cases replace the large, heavy, and expensive commercial satellites. Microsatellites provides platforms that can be configured and launched for a very specific mission within a short time frame. The most advanced microsatellites are also attitude controlled. This enables the ground user to stabilize and point the satellite in any direction during flight. As a consequence, not only simple telecommunication devices, but more advanced payloads involving optical instruments have been installed onboard these satellites. The user interactive concept of the microsatellites are used to monitor processes which change fast (within one day), for search actions or to track any target at ground level. The microsatellites have become true platforms for remote Earth observation.
The Technical University of Berlin (TUB) has developed expertise in construction and launch of 6 microsatellites in polar orbit with special emphasise on attitude control and environmental / house keeping aspects (The TUBSAT series). TUB is currently working on the 7'th satellite called LAPAN-TUBSAT with the Indonesian National Institute for Aeronautics and Space (LAPAN). The attitude is controlled by using fibre optical gyros, re-action wheels, magnetic torque coils and a star sensor. Their most advanced satellite is capable of tracking a ground based object in real time at a resolution of 6 m at ground level for approximately 5 minutes (max.10 min). The payloads have so far been monochrome video cameras with rather limited use when it comes to number of possible applications.
The TUBSAT's ability to lock on a ground target and sweep slowly over the terrain, makes it ideal for a line scanning instrument such as a hyper spectral imager. The University Centre on Svalbard (UNIS) has developed small airborne hyper spectral imagers that image objects with both high spatial and spectral resolution. These instruments have proven to be a successful tool on a large number of applications in remote sensing. We therefore propose to go one step further by adding a spectral imager to the proposed satellite.
The partners of this proposal have already finished the ground segment including an installed and operative command station at the roof of UNIS and a data downlink from SVALSAT. In addition, the fibre cable to Norway has also proven to be useful for real-time acquisition of the TUBSAT satellites presently in orbit. It is therefore natural for us to continue the co-operation and progress by constructing a new satellite.
Norway has no research satellites in orbit as of today, and the present proposal shows a cost efficient way of designing and constructing a satellite platform and payload to our own specifications. The proposal includes everything needed to get the satellite into in-orbit operation. Despite the high technical level of expertise in space physics in Norway, no research group has so far been able to agree or to construct a remote-sensing research satellite. This proposal is therefore important in order to achieve goals that Norway should have been able to reach a long time ago.
2. THE GROUND SEGMENT
TUB, UNIS and SVALSAT started a joint project in 2002 called the "THE DLR-TUBSAT-SVALSAT-UNIS EXPERIMENT". The purpose of this project was to establish a control station and use SVALSAT to download data of high quality from the DLR-TUBSAT satellite. Fig. 1 shows an image gallery of the key elements of the ground segment.
Figure 1. The Ground Segment. Panel (A): The SVALSAT ground station close to Longyearbyen, Svalbard (78N, 15 E). Panel (B): The tracking Yagi-antenna mounted at the roof of UNIS. Panel (C): The University Centre on Svalbard (UNIS).
DLR-TUBSAT is a co-operation with the Technical University in Berlin (TUB) and the German Aerospace Center (DLR). The satellite has successfully operated since it was launched as a piggy-back on 26 May 1999 from Shiharikota with the Indian Polar Spacecraft Launch Vehicle (PSLV). Furthermore, one more satellite MAROC-TUBSAT have been launched in December 2001, a co-operation between TUB and the Royal Center for Remote Sensing in Morocco. The ground segment, command station at UNIS and the SVALSAT downlink, can also be used for this satellite. We plan to use the same ground segment for the new satellite described herein.
Figure 2. Ground station architecture
The ground segment consists of three parts and is shown in Fig.2 Two satellite control centres at UNIS and TUB are the main ground stations which are responsible for mission control and health monitoring of satellite. This is done by the transceiver units in the VHF/UHF-band and the Yagi-antennas. The tracking antenna at UNIS is also equipped with a frequency modulator in order to keep track of any Doppler shifts of the transmitted signals from the satellite.
The video stream or digital pictures of the payload are received via S-Band with the 13 m-antenna at SVALSAT. The data is then either grabbed by a video server and stored on video tapes, or send by FTP directly down to UNIS. All units at UNIS are controlled via INTERNET in real time from Berlin. The images can also be downloaded by a 3 m S-band antenna in Berlin.
In general, a request from UNIS and TUB is send by e-mail to the Tromsø Network Operations Centre (TNOC) to find out if the 13m antenna is available (free) to track a selected pass. The TNOC operator then programs the S-band antenna to track the satellite. TUB or UNIS can then prepare the satellite for its mission by sending commands in advance or in real time to the satellite. The high speed of the internet through the fibre optical connection between Svalbard and Norway makes these operations possible. The ground segment has been operating successfully since May 2003.
Figure 3. Overview of the new hyper spectral microsatellite (SVALBIRD). Each side is labelled according to axis configuration: z - yaw axis, x - flight direction (roll), and y - pitch axis, respectively.
3. THE SATELLITE BUS
This section consist of a system description of the satellite bus - the Housekeeping and the Attitude Control. The payload will be described in section 4. The system is based on the LAPAN-TUBSAT architecture.
3.1 - Housekeeping
The configuration of the space segment is shown in Fig. 3 and 4. The cube shaped satellite measures 45 x 45 x 28 cm3 and weighs 50 kg. The cube is made out of 5 mm aluminium plates. This rather heavy solution is chosen to keep the inside temperature variations as small as possible and to protect the instruments from radiation. The S-band antenna is physically located on the same side as the front optics of the payload. It does not obstruct the field of view of the payload sensors. The S-band transmitter is located close to the antenna. Analog video transmission is performed within a bandwidth of 8 MHz, the transmission of single pictures occurs at 125 kbaud. The beam width of the antenna is 70 degree.
The house keeping system contains the batteries, the Power Control Data Handling unit (PCDH), the air coil and the two Telemetry Telecommand units (TTC's) in the UHF band, as well as the two antennae. Five duplex NiH2 battery cells from Eagle-Picher 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 short circuit current of each panel is 980 mA. The PCDH 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-TTC 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 silent.
Figure 4. A 3D colour view of the satellite (SVALBIRD). Each side is labelled by numbers. STS is Star Tracker Sensor. XYZ-coils are the magnetic torque coils, RW - Reaction Wheels, TX - Transmitter (S-Band), TTC - Telemetry Telecommand Control units, PCDH - Power Control Data Handling, and WDE - Wheel Drive Electronics. Axis configuration: z - yaw axis, x - flight direction (roll), and y - pitch axis of satellite.
3.2 Attitude Control
Three Reaction Wheels (RW) are mounted orthogonal to each other on the inside walls of the satellite. They are in-turn controlled by three fibre optical gyros. The magnet torques are used for the reduction of the angular momentum of the satellite. The reaction wheels RW203 , developed by TUB, have an external wheel drive electronic (WDE) with a micro controller to provide operation modes such as current control, wheel speed control and torque control. Furthermore, one fibre optic gyro mFORS – 6u, built by Litef, is connected to the WDE of one reaction wheel via serial communication interface and both are mounted in one body axis of the satellite. The micro controller of the WDE receives an angular increment from the gyro and calculates with this data an accumulated angle and an angular velocity. Each of the three wheel/gyro-unit ACS203 (see Fig. 5) provides operation modes which control the angular velocity or the angle in one axis of the satellite. With three of them in each body axis of the satellite the spacecraft is three axis stabilized. The highly integrated wheel/gyro-unit was designed especially for microsatellites with requirements in the field of low power consumption (wheel: 1 W steady state, gyro: 2 W), low mass (wheel: 1,2 kg, gyro: 0.15 kg), small volume (wheel: 808070 mm3, gyro: 1006520 mm3 ) and simple interfaces (electrical: 5 V & 12/15 V, data: 8 N 1). The operation modes and the main data of the performance are also shown in Fig. 5.
Figure 5. SVALBIRD Wheel/WDE/Gyro-Unit ACS203.
The satellite is equipped with a CMOS star sensor STT03, as a inertial sensor. This sensor, the signals from the solar panels and the three ACS203 allows a high accurate attitude acquisition before the visible satellite pass starts. That means that nearly the complete visible time of the satellite (typically 12 to 15 min) is free for earth observation operations.
Figure 6. SVALBIRD CMOS Star Sensor STT03
There are two different operation modes for earth observation. First is a complete automatic mode, without any interference from ground, second is interactive control from the ground (see section 4). In both cases the attitude acquisition is pre-programmed some hours before and done automatically before the pass begins. After that a known target can be observed automatically, by using a rotation profile around one or two axes that is pre-programmed, or by an interactive control from a ground based operator.
A more detailed description of the data flow and the electrical system is found in the appendix. The above description [cf. Schulz/Roemer and Renner] is based on components that are well tested on the TUBSAT series.
4. INTERACTIVE EARTH OBERVATION
Interactive earth observation means that an operator located at one of the control stations (UNIS or TUB) receives real-time video images from SVALSAT. The video server at SVALSAT and the optical fibre between Svalbard and Norway provides high speed data transmission through the internet. The operator is then able send commands to the satellite. He / She can steer the pointing direction of the camera platform, interactively via the command stations keyboard, to any interesting event on the ground. (see Fig. 7).
Figure 7. Interactive Earth observation configuration. ACS is the Attitude Control System. It response to the angular velocities issued by the operator at the control station. Rotations in the counter clockwise direction around the x-axis is roll. Correspondingly, around the y- and z-axis are pitch and yaw, respectively.
This is appreciated in applications where e.g. the target has not been identified clearly in advance, a search action is involved or a target has to be visually followed for a while.
The above principle is tested successfully. Fig. 8 shows how the DLR-TUBSAT can be used. A video camera with a 50 mm objective is first used to get an overview of the ground. The island Corsica is identified. Next a camera with a telescope lens is used to zoom in, and a more detailed scene can be studied. Note that the images in Fig. 8 are taken over a period of 1 second. They clearly demonstrates the high precision and speed of the Attitude Control System (ACS) on board the satellite. This type of interactive use is ideal for push broom hyper spectral imagers (see payload section below).
Figure 8. Image gallery from DLR-TUBSAT over Corsica. The red frame is taken with the camera using a 50 mm lens (120 m resolution). The green frames (1 - 25) are over a period of 1 second using the telescope lens (focal length 1000 mm; 6 m resolution).
The orbits of the TUBSAT family are typical sun synchronous with inclinations close to 90 degree (polar orbit), altitudes of approximately 700 km, and speeds up to 25 Mach (~100 minutes period of revolution around the Earth).
Another example of interactive use is shown in Fig. 9. The MAROC-TUBSAT satellite was in this case pre-programmed to take digital pictures. The satellite was configured and setup in advance by the control station in Berlin and at UNIS. During the next orbit SVALSAT recorded the data. The image sequence shows North-West of Svalbard, Norway. The images have a spatial resolution of approximately 250 m.
Our satellite will be programmed in a similar manner. First the target area will be selected with a request to TNOC about SVALSAT availability / down-link support. Secondly, the pre-programming of the satellite is performed making it ready for the selected orbit. During the pass, SVALSAT records the real time image sequence. If the hyper spectral imager is used, then the image sequence consists of spectrograms. Finally, post-processing of the spectrograms result in multi wavelength band images of the target.
5. THE PAYLOAD
The payload contains one fore field camera (50 mm focal length objective) with medium resolution (120 m resolution at ground level) and a high resolution telescope (6 m resolution) with a focal length of 1 m as front optics to the spectral imager. The detectors used on both instruments are Charge Couple Devices (CCD's). Each CCD-chip will be able to transmit video images in the CCIR-standard or digital pictures.
Figure 9. MAROC-TUBSAT image sequence of Svalbard, Norway, 28.05.2004.
Figure 10. A 3 dimensional optical diagram illustrating the main principle of hyper spectral imaging. L1 is front lens, S1 entrance slit, L2 collector lens, G grating, P prism, L3 camera lens and CCD imaging detector. The optical axis is parallel with the X-axis of the XYZ-coordinate system. The slit is located parallel to the Y-axis. q is the angle between the mirror's normal and the optical axis. The detector is located in the lS - plane which is parallel to the ZY- plane (Note: static object setup).
We aim to use a colour CCD on the 50 mm camera. As seen above, the use of monochrome (black and white) CCD detectors have been operating well on both DLR- and MAROC-TUBSAT. In order to be safe, we could use the same detectors (SONY 1/2" CCD with liquid capacitors replaced with solid ones). However, we will first see how the new 3-chip colour CCD performs on the LAPAN-TUBSAT before a final decision is made. The LAPAN-TUBSAT will be launched next year. Nevertheless, the technology and the know-how of the fore field camera is well established.
The challenge will be the spectral imager. Below is a description of the principle behind spectral imaging, the design of the proto-type instrument and samples from airborne campaigns conducted by the Geophysical department at UNIS.
5.1 Main principle of spectral imaging
The main idea behind spectral imaging of objects may be described as follows: Firstly, light from the object must be focused by a lens or mirror to form an image at the spectrographs entrance slit plane. The resulting spectrogram is the intensity distribution as a function of wavelength and position along the slit. The diffracted slit image contains both spectral and spatial information along a thin track of the object (See Fig. 10).
Secondly, in order to obtain the object's full spatial extent, it is necessary to sample the whole object. This requires the use of a high resolution rotary element. The whole idea is to record spectrograms for each track of the object as the image at the entrance plane is moved across the slit. A front surface mirror in front of the focusing element may be used in connection to a high resolution stepper motor.
Figure 11. Airborne spectral imager generation VI (Airspex VI). H is a Holographic aim point. L1 is front lens, S1 entrance slit house, L2 collector lens, GP Grating/Prism holder, L3 camera lens and CCD imaging detector (video). The labels are according to Fig. 10. The spectral range for this instrument is 450 - 830 nm and the bandpass is 5 nm.
Another approach would simply be to rotate/move the whole instrument itself or the object on a conveyor belt etc. In fact, the use of a mirror enables us to sample target objects that are static or moving relatively to the instrument by rotating or keeping the mirror fixed, respectively. In our case the scanning mirror will be removed since the satellite is able to scan in any desired pointing direction due to its high precision attitude control system.
Fig. 10 shows a typical choice of experimental design. Please note that the use of lenses instead of mirrors is no limitation in implementation of the above principle. The use of a transmitting grating combined with a prism increases the resolution especially in the blue part of the spectrum. In addition, each optical element is aligned along the optical axis. Consequently, the imager is easy to assemble.
The intensity at each pixel of the spectrogram, the output image of the CCD, can be defined as
Jq= Jq ( l , h ) , [mW/m2Å] (1)
where l is the wavelength and h represents the slit height at the detector surface. Both slit S1 and the CCD detector are positioned parallel to the YZ plane with surface normal along the X-axis, Subscript q denotes the angle between the mirror normal and the optical axis, which is parallel to the X-axis of Fig. 10. On the satellite, q is the angle that the satellite rotate by to lock on a target on the surface of the Earth (interactive process to reduce the relative speed between the sensors and the ground level as it moves over the target).
The object or target image (one row of the image matrix) is
constructed by integrating over the desired wavelength region:
where the integration occurs in the wavelength interval Dl = l 1 - l 2 . l c is then defined as the centre of the wavelength region from l 1 to l 2.