BAYERNSAT – HOW TO UTILIZE RELAY
SATELLITES FOR REAL-TIME DATA ACQUISITION ON
SMALL SATELLITES
Matthias Raif, Jürgen Letschnik, Kristian Pauly, Ulrich Walter
Technische Universitaet Muenchen
Institute of Astronautics, Boltzmannstr 15, Garching, 85748 Germany
Phone: +49 (89) 289-16012, E-mail: [email protected]
ABSTRACT
Low Earth orbit satellites such as Earth observation satellites usually use the store and
forward approach to dump their data to the user during overflight over a ground station.
This method relies on a large number of different ground stations or on the patience of the
user. If the data is needed on time or if long-lasting real time access to the satellite itself is
essential, the implementation of a relay satellite is reasonable. This paper introduces
BayernSat, a micro-satellite which demonstrates the technology necessary for telepresence
in space. BayernSat will use a commercial available geostationary relay satellite to send
live video footage to the user on ground. Furthermore the satellite can be controlled in realtime from ground using the return link also via the relay satellite. Besides the
demonstration of the telepresence technology, BayernSat will demonstrate a newly
developed high performance onboard computer system, a color video camera for space
application and a steerable low profile high gain S-band antenna. BayernSat also acts as a
tool to inspire the public for space. The live video footage will be presented on the Internet
and selected users will be able to steer the satellite via Internet.
1.
INTRODUCTION
Earth observation satellites with high spatial resolution have to be placed in low earth orbit.
Unfortunately, the strongly restricted coverage in this altitude permits only sporadic
downlinks with short
contact time to dump
the data. A work around
is either to have several
ground stations with
overlapping coverage
areas or to store the
acquired data onboard
the satellite and forward
it to a ground station
with high data rate
during overflight.
In
military
and
Figure 1: Comparison of access times
national
applications
another work around is to employ geostationary relay satellites like DRTS or Artemis for
half-orbit or TDRS for even full-orbit coverage.
This paper presents the micro-satellite BayernSat, which uses a geostationary satellite as
communication relay for commercial applications. BayernSat shall demonstrate this relay
technology by the application of a modular, simple, and low-cost design. BayernSat has
three further mission goals: The satellite shall proof new space technology in orbit. In
addition, it shall arouse public interest for spaceflight. Finally, BayernSat shall enable the
Institute of Astronautics at the Technical University at Munich to educate its students with
hands-on experience in a real space mission.
2.
TELEPRESENCE
BayernSat should demonstrate the technologies enabling telepresence in space.
Telepresence means in our case that the latency between the ground facility and the Earth
observation satellite and the same way back (operator – teleoperator, i.e. BayernSat –
operator) should be less than one second. Telepresence should not only be possible during
coverage of a ground station but also in an extended timeframe. The approach is to have a
real-time access to the satellite via a geostationary relay satellite. In view of upcoming
telepresence applications such as on-board servicing and robotics in space it is the goal of
BayernSat to arrive at a delay time of less than 0.8 seconds. The physical delay time caused
by the far distance between the ground station or the low Earth orbit satellite and the
geostationary relay satellite is about 0.5 seconds. Currently the delay time in actual
missions with a geostationary relay is at best 4 seconds (e.g. ROTEX on D-2 mission) [1].
Relay-link technology is favorable whenever time-critical interaction is needed to access
satellite subsystems. For example stranded satellites can be repaired or refueled by docking
servicing satellites with telepresence techniques. Disaster management can be undertaken
in real-time, and many other space applications are feasible by supporting or replacing
astronauts in space by robonaut-controllers on the ground. Our ultimate goal is to develop a
modular telepresence module, which allows other space missions to readily utilize this
telepresence technology.
3.
MISSION AND TECHNOLOGY
BayernSat is designed as a micro-satellite. The main characteristics are:
ƒ Size
approx. 50 cm x 50 cm x 50 cm
ƒ Mass
approx. 50 kg
ƒ Max. power
120 W
ƒ Life time
min. 9 months
ƒ Attitude control
3-axis stabilized
The satellite will be launched as auxiliary payload. In order to be flexible, BayernSat can be
lifted into orbit by a variety of launchers. Also an orbit with a altitude between 400 km and
780 km and an inclination between 51.6° and sun-synchronous can be handled by the
satellite’s layout. The overall layout can be seen in Figure 2.
To enable telepresence on BayernSat, a bunch
of new technology will be demonstrated. A
newly developed color video camera system
records the video footage of planet earth in the
visible spectrum. This video footage will be
compressed aboard the satellite by a new high
performance data handling system (including a
PowerPC), and will be transmitted via an intersatellite link to the ground facility.
3.1.
Camera
Figure 2: BayernSat Layout
The camera system of BayernSat consists of
three separate independent color cameras each
with a different lens. The field of view (FOV) of the camera is 82°, 14.1° or 2.5°
respectively. The best ground resolution with a FOV of 2.5° will be 22 m per pixel from a
400 km orbit.
Each of the cameras possesses four CCD chips. Each CCD chip is equipped with its own
lens system including a filter to let pass only one wavelength. The filters are adjusted to let
pass read, green and blue color respectively as well as one integral channel. All four CCDs
will be multiplexed to a RGB signal. If one of the CCDs should fail, the color video signal
can be fully recovered from the remaining three CCD signals (R+G+B=I).
The interface to the onboard computer system is done by a space wire connection. The
camera electronics is able to handover the video signal as raw, multiplexed signal or
preprocessed by an internal hardwired compression algorithm.
3.2.
High Performance Modular Onboard Computer
All necessary satellites functions of BayernSat will be controlled by a new developed
modular function unit named
ICDS (Integrated Control and
Data System). The ICDS is
structured
as
a
modular
assembly of already available
components and functions as
well as modified parts and new
developments.
Figure 3 shows the
layout of the electronics part of
BayernSat’s ICDS. This onboard
computer takes care of all
functions of BayernSat like data
handling, command and control,
ADCS and payload data
processing.
Figure 3: Layout of the ICDS of BayernSat
Core of this system is the
high performance processor unit (HPE). The HPE is a compound of a commercial standard
processor (PowerPC) and an array processor (XPP). Standard functions of the satellite like
telemetry/telecommand, power distribution, sensor and actuator electronic will be handled
by already space proven concepts.
Also a space proven processor module (ERC32) is included into the ICDS electronics to
ensure the control of the satellite in off-nominal situations inside the HPE unit.
4.
PUBLIC OUTREACH
From the ground facility the acquired video data will be distributed to the user, in this
case to the public via Internet. On the return-link selected users in the Internet will be able
to steer the cameras onboard BayernSat. Thereby the relay link will not only enable
telepresence (operator – teleoperator – operator) in orbit, but also telepresence with very
long contact times. However, delay times are critical for telepresence applications, in
particular for remote camera steering in the case of BayernSat the Internet user must be
able to react to the visible real-time video stream to control the satellite’s camera. This
implies delay times of less than one second.
5.
CONCLUSION
The micro satellite BayernSat will demonstrate the technology necessary for long duration
telepresence in space through a color video camera system steerable by users from the
internet and a modular high performance onboard computer. The future application for such
telepresence can range from controlling docking maneuvers in orbit to on-orbit servicing of
damaged satellites and ground controlled robonauts working in space instead of vulnerable
astronauts.
Furthermore BayernSat will involve the public by publishing the acquired video data in real
time to the Internet. Through the interactive commanding of the satellite these Internet
users will enthuse about space.
The whole project is an excellent means to educate students at the Institute of
Astronautics in all areas of satellite development. The integration of the local industries will
also strengthen this area of expertise.
The intended launch date of BayernSat is foreseen for the beginning of 2008.
REFERENCES
[1] G. Hirzinger, K. Landzettel, J. Heindl and B. Brunner, ROTEX – Die Telerobotik-Konzepte des ersten
Roboters im Weltraum, Proc. Virtual Reality94 Forum, February 1994