Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
PROPOSAL FOR A NEW HYPER SPECTRAL IMAGING
MICRO SATELLITE: SVALBIRD
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
1
University Courses on Svalbard (UNIS), Box 156, N-9171 Longyearbyen, Norway.
Phone: +47 790 23 335, Fax: +47 790 23 301, freds@unis.no
2
Technical University of Berlin, Institute of Aerospace, Berlin, Germany.
3
Kongsberg Satellite Services, Svalbard Station (SVALSAT), Longyearbyen, Norway.
ABSTRACT
This document describes a proposal to construct and launch a new generation of polar
orbiting micro satellites 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 micro satellite is designed for interactive Earth observation
using a hyper spectral imager. The mission objective, the final configuration of the
satellite, the ground segment, the payload and the operation is described.
1. INTRODUCTION
The development of micro satellites 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. Micro satellites provide platforms that can be configured and launched for a
very specific mission within a short time frame. The most advanced micro satellites 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 micro satellites are used to
monitor processes which change fast (within one day), for search actions or to track any
target at ground level. The micro satellites have become true platforms for remote Earth
observation.
The Technical University of Berlin (TUB) has developed expertise in construction and
launch of 6 micro satellites in polar orbit with special emphasise on attitude control and
environmental / house keeping aspects (the TUBSAT series) [1, 2]. The attitude is
controlled by using fibre optical gyros, reaction 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.
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
Figure 1. The Ground Segment. Left 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). Right panel shows the ground station architecture.
2. THE GROUND SEGMENT
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. Fig.
1 shows an image gallery of the key elements together with the architecture of the ground
segment. 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 ground segment is tested on the TUBSAT A, DLR-TUBSAT and MAROCTUBSAT presently in orbit. The video stream or digital pictures of the payloads are
received via S-Band with the 13 m-antenna at SVALSAT. The data is then either
grabbed by a video server or 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 3m 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.
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
Figure 2. A 3D colour view of the satellite (SVALBIRD). Each side is labelled by letters. STS is Star
Tracker Sensor. XYZ-coils are the magnetic torque coils, RW - Reaction Wheels, TX - Transmitter (SBand), 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. THE SVALBIRD SATELLITE BUS
This section consists of a system description of the SVALBIRD 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. 2. The cube shaped satellite
measures 45 x 45 x 28 cm3 and weighs 57 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.
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
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.
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, developed by TUB, have external wheel drive electronics (WDEs)
with micro controllers to provide operation modes such as current control, wheel speeds
and torque control. The fibre optic gyros built by Litef are connected to the WDEs and
the reaction wheels via serial communication interfaces. The micro controllers of the
WDEs receive angular increments from the gyros and calculate accumulated angles and
angular velocities. Each of the three wheel/gyro-units 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 satellite is also equipped with a CMOS star sensor, as an inertial sensor. The star
sensor, the signals from the solar panels and the three gyro units 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.
4. THE PAYLOAD
The payload contains one fore field camera (50 mm focal length objective) with
medium resolution (120 m at ground level) and a high resolution telescope with a focal
length of 1/2 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 or digital pictures.
We aim to use a colour CCD on the 50 mm camera. The monochrome (black and
white) CCD detectors have been used successfully on both DLR- and MAROCTUBSAT. 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 3chip 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 [1, 2].
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
Figure 3. Basics of hyperspectral imaging. Panel (A): A 3 dimensional optical diagram illustrating the
main principle. Each optical component is labeled with letters along the Z – axis of the XYZ – coordinate
system. L1 is front lens, S entrance slit, L2 collector lens, P grism, L3 camera lens and CCD imaging
detector. The optical axis is parallel with the Z - axis. The slit is located parallel to the Y - axis. The
detector is leveled in the XY - plane. SW is swath width. Panel (B): Assembled prototype instrument
with identical optical components.
The challenge will be the spectral imager. Below are a description of the principle
behind spectral imaging [4, 5] and the design of the proto-type instrument.
4.1 Principle of hyperspectral imaging
Fig.3 panel (A) shows the optical components of the spectrograph. The essential parts
are one entrance slit (S), a collector lens (L2), a dispersive element (P) – the grating /
prism (grism), a camera lens (L3) and the detector (CCD). The main purpose of a
spectrograph is that it generates images of the entrance slit as a function of color. The
number of colored slit images on the CCD depends on the width of the entrance slit and
the dispersive element’s ability to spread – diffract the colors, and is directly connected
to the spectral resolution of the instrument [3].
Focusing light from a target by a front lens (L1) onto the slit - plane, forces the
instrument to accept structure of the target along the slit. The resulting image
(spectrogram) recorded by the CCD is the intensity distribution as a function of
wavelength (color) and position along the slit. The spectrogram contains both spectral
and spatial information along a thin track of the target object. In our case the target is
the ground surface.
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
Next, in order to obtain the object's full spatial extent, it is necessary to sample the
whole object. 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. The movement is created by
the satellite itself. As a result, an image of the target may be generated at any selected
wavelength by stacking the average slit response side by side for each track. The
procedure is repeated for the next wavelength region of interest.
4.1 The prototype design
A prototype instrument is shown in Fig. 3 panel (B). The camera head made by the
company Watec LCL contains a 795 x 596 pixel CCD detector. The size of the CCD is
6.837 x 4.9468 mm2 with 8.6x8.3m sized pixels. The spectral range is 400-700 nm and
the sensitivity is 0.00015 lux. The dynamic range is 8 bits per pixel. The camera head
has both auto and manual gain settings. Taking into account the pixel format
convention of a typical digital video recorder, the effective number of pixels for each
frame is 320 x 240. At 25 frames per second, the exposure time is 8.33 ms and the
readout time is 31.67 ms (CCIR standard). The spectral bandpass is then 2 nm [3].
A standard C-mount 25 mm focal length objective (L3) is used to focus the diffracted
and dispersed light from the grating/prism (P) onto the CCD. The lens has manual iris
control used to detect the background level of the CCD. The front of the lens is
connected to a 110 mm long barrel, which is 30 mm in diameter with threads on both
ends to accept components. A handheld spectroscope made by Paton Hawksley
Electronics Ltd. is fitted into the barrel. It features a 30 degree Littrow dispersive prism
(P), a 600 grooves / mm transmitting holographic grating , a 42 mm focal length lens
(L2), and a fixed size slit (S). These components are housed in a stainless steel tube.
The lens is 12 mm in diameter. The slit is 4.5 mm high and 25 m wide. The grating is
fixed parallel to the prism. The prism and grating normal coincide.
The field lens (L1) or the front lens of the spectroscope is directly connected to the 110
mm barrel. The barrel has standard inside C-threads. The spectroscope's entrance slit is
located 17.5 mm into the barrel - positioned in the focal plane of the lens. The lens in
Fig. 3 is a 500 mm focal length telescope. All mounts and adapters are found in the Mix
and Match assemblies of the company Edmund Scientific (ES).
Using a sun synchronous orbit with an altitude of 700 km, the ground resolution will be
approximately x = 35.5 m and y = 48.5 m, assuming a relative ground speed of 200
km / hr. The swath width will be SW = 6.3 km.
The use of a transmitting grating combined with a prism (grism) increases the
resolution especially in the blue part of the spectrum. In addition, each optical element
is aligned along the optical axis (on –axis), reducing geometrical aberrations such as
astigmatism and coma. This is the main reason why this type of instrument is one of the
most popular designs in the field of hyperspectral imaging [4, 5, 6, 7, 8].
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
Figure 4. Image from DLR-TUBSAT over the Croatian coast. Top panel (A) shows the scan line
direction of the satellite together with the geo-corrected video frame of interest, panel (B). Panel (C)
shows the image that is generated as results of using the satellite as a line scanner. The ground resolution
is 120 m.
5. INTERACTIVE OBERVATIONS VERSUS SPECTRAL IMAGING
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. 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.
Fig. 4 demonstrates the DLR-TUBSAT capability as a liner scanner. The satellite is
tracking along a straight line the Croatian coast - an ideal situation for a push broom
spectral imager. Stacking rows from each frame side by side a few seconds, results in
an image that is 0.63 times smaller in the scan direction compared to individual shots
taken by the video camera itself. This under sampling can easily be reduced by
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
decreasing the relative ground speed, increasing or decreasing the rotation of the
satellite towards the pointing direction of the target area. Note that a spectral imager
would in any case not under sample the target area due to the fact that the slit width is
about three times wider than one single pixel.
The above example demonstrates that the Attitude Control System of the TUBSAT
series is capable of handling a push broom hyperspectral imager.
Figure 5. Airborne hyper spectral imaging of Longyearbyen, Norway, 12.04.2002.
6. HYPERSPECTRAL SAMPLES
Data from our airborne campaigns in the arctic are reported by [4]. Fig. 5 shows a
typical example of hyperspectral data from Longyearbyen (78oN, 15oE), Norway. The
two Gray scaled image strips to the left are generated with a centre wavelength of 546.1
nm. The bandpass is 5 nm. The altitude is 1698 m and the view angle is 45 degrees to
nadir. The ground speed is 184 km/hr. The spatial resolution is 1.6 x 0.9 m2 using a 50
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
mm front lens. Panel (A) shows the 546.1 nm image of the cleaning facility for coal
owned by the local coal company. A successful unsupervised classification [9] with 10
classes is presented in panel (B). The GPS position of the plane is found in the upper
left corner of the high resolution camera image in panel (C).
Figure 6. Long distance hyperspectral imaging of mountain range north - west of Longyearbyen,
29.03.2005.
The instrument has been tested with the 500 mm focal length telescope as front optics
using a rotary table. Fig. 6 shows the resulting spectral image sequence from the blue
(435 nm) to the red (675 nm) part of the visible spectrum. The images were generated
with a spectral bandpass of 15 nm. The false colour image is obtained by making a
composite RGB image from the 635 nm, 555 nm and 495 nm images.
The distance to the mountain range is close to 50 km and above. The corresponding
spatial resolution is 2.5 x 3.5 m2. At such long range, a noticeable effect is clearly seen.
The red images are better focused or less blurry than the blue images. The effect is due
to atmospheric Rayleigh scattering, which is inversely proportional to the fourth of the
wavelength.
Small Satellites for Earth Observation, ISBN – 10: 3-11-018851-1, 134 – 144, 2005
Even though the light gathering power of the telescope is low (f/value = 8), the
sensitivity of the detector compensate quite good provided daylight conditions. A
discussion on sensitivity versus front optics and spatial resolution is necessary in order
to optimize the throughput of the instrument. A high resolution camera combined with
a medium or low resolution spectral imager might be the solution depending on the
application.
Provided funding, we aim to make observations covering a number of applications
including, but not limited to:
1.
2.
3.
4.
Ocean colour and algae’s
Vegetation and rocks
Sea ice and leads
Snow and cloud cover.
7. CONCLUDING REMARK
Based on experiments both with airborne hyper spectral imagers and the highly
sophisticated attitude control systems on the TUBSAT series, it is believed that these
micro satellites will have multi-wavelength band sensors capability very soon.
8. REFERENCES
[1] J. -H. Bleif, S. Roemer and U. Renner, The TUBSAT Attitude Control System: Flight Experience
with DLR-TUBSAT and MAROC-TUBSAT, IAA-B4-1201, Technical University of Berlin,
Institute of Aerospace, Berlin, Germany (2003)
[2] S. Roemer and U. Renner, Flight Experiences with the Micro-Satellite MAROC-TUBSAT, IAC-03
IAA.11.2.07, Technical University of Berlin, Institute of Aerospace, Berlin, Germany (2004)
[3] C. Palmer, Diffraction Gratings Handbook, 5th ed., pp. 29 – 30, Thermo RGL, Richardson Grating
Laboratory, New York (2002)
[4] F. Sigernes, D. A. Lorentzen, K. Heia and T. Svenøe, A multi-purpose spectral imager, (font cover
issue) Applied Optics, 39, No. 18, 3143-3153 (2000)
[5] W. L. Wolfe, Introduction to Imaging Spectrometers, Vol. TT25 of Tutorial Text Series, pp. 1-147,
SPIE Press, Bellingham, Washington (1997)
[6] E. Herrala and J. Okkonen, Imaging spectrograph and camera solutions for industrial applications,
International Journal of Pattern Recognition and Artificial Intelligence, 10 (1), 43-54 (1996)
[7] F. Sigernes, K. Heia, H. Nilsen and T. Svenøe, Imaging spectroscopy applied in the fish industry?,
Norwegian Society for Image Processing and Pattern Recognition, 2, 16-24 (1998)
[8] H. Nilsen, M. Esaiassen, K. Heia and F. Sigernes, VIS/NIR spectroscopy - a new tool for the
evaluation of fish freshness?, Journal of Food Science, 67, No. 5, 1821 – 1826 (2002)
[9] W. Niblack, An Introduction to digital image processing, Vol. 1, pp. 168-181, Prentice / Hall
international Ltd., UK (1986)