Trends in satellite communications and the role of optical free-space communications

The communication needs of Earth observation satellites is steadily increasing. Within a few
years, the data rate of such satellites will exceed 1 Gbps, the angular resolution of sensors will be
less than 1 μrad, and the memory size of onboard data recorders will be beyond 1 Tbytes.
Compared to radio frequency links, optical communications in space offer various advantages
such as smaller and lighter equipment, higher data rates, limited risk of interference with other
communications systems, and the effective use of frequency resources. This paper describes and
compares the major features of radio and optical frequency communications systems in space and
predicts the needs of future satellite communications.
OCIS codes: 010.0010, 010.1330, 010.3310, 060.4510, 350.6090.
1. Introduction
Images sent from the National Aeronautics and Space Administration’s (NASA) rover Opportunity
indicate the former existence of a salty sea on Mars, which is essential information when investigating the
origin of the planet [1]. Today, images like those from Opportunity, as well as other types of data, are
routinely conveyed in real time to points all over the world. Radio frequencies (RF) are usually used for
such long-distance links in space. However, the recent progress in optics and laser technologies,
especially in fiber optics, is ushering in an era of inter-orbit communications using laser beams. The
European Space Agency (ESA), in its Semiconductor Laser Intersatellite Link Experiment (SILEX), has
routinely used a 50-Mbps optical communication page link twice a day between a low earth orbit (LEO)
satellite and a geostationary earth orbit (GEO) satellite since 2003 [2]. The Optical Inter-orbit
Communications Engineering Test Satellite (OICETS) developed by the Japan Aerospace Exploration
Agency (JAXA) will be launched in summer of 2005 and will feature a laser communication page link with the
SILEX terminal [3]. The SmartSat-1 project of the National Institute of Information and Communications
Technology (NICT) in Japan plans to demonstrate in-orbit verification of a small optical terminal on
board twin satellites in 2007 [4]. NASA, in its Mars Telecommunications Orbiter project, plans to
establish a laser communication page link between Mars and Earth in 2010, which will be able to transmit
information at a data rate of 1~30 Mbps [5].
Both RF and optical waves are electromagnetic waves; however, there are many advantages to using
optical waves in space. These include reduced mass, power, and volume of equipment, higher data rates,
no tariffs and no regulatory restrictions as experienced for RF bands [6]. These assets are a consequence
of the high frequency of optical waves. This paper is organized as follows. The next section describes
trends in satellite communications and the future use of optical communications. The main features of RF
and optical communications systems in a LEO-GEO scenario are discussed in Section 3. Section 4
compares LEO-GEO, LEO-LEO, GEO-GEO, and deep space communication scenarios. Section 5
classifies the RF and optical communications systems based on their beam divergence and data rate.
2. Trends in satellite communications
2.A. Support for manned space activity

Recently, the first private manned spacecraft exceeded an altitude of 100 km twice within a 14 day period
[7]. In the near future, people who have not undergone astronaut training will be able to travel into space
in space planes. High-speed Internet access should therefore be available in a space plane as well. In a
manned space station like the International Space Station (ISS), the leisure available to the astronauts
should reflect that available on the ground. For instance, to relieve stress, popular movies, audio, and
multimedia contents should be available to astronauts in the ISS. A 1-Gbps optical communication link
would enable us to send, e.g., the latest movies to the ISS within one minute. Another, may be more
important aspect for the ISS, is data transmission of the many scientific missions to be performed. They
produce massive scientific experimental data which, in many cases, should be downloaded
instantaneously to a ground station. An optical communication page link is the proper medium for such
infrastructure in space.
2.B. Data transmission from observation satellites
Many Earth observation satellites have been developed for weather forecasting and for probing our
environment. For more accurate measurements, higher resolution will be required from onboard sensors
and the frequency and area of the observations will increase. Figures 1 and 2 show trends in the data rate
and resolution of the sensors for non-military Earth observation satellites. The trends cover optical
sensors and RF synthetic aperture radar (SAR) systems via intersatellite and direct communication links
to ground stations [8]. The data rates seem to drop with passing time for GEO. However, the data relay
satellites and commercial communication satellites are not included in the figure. As recently launched
commercial satellites at GEO have the total transponder bandwidth of about 1 GHz even in GEO, the
communication capacity does not drop [9]. Figures 3 and 4 show the trends in data storage capacity and
the number of bits per pixel of stored image data. For 2010, one can easily make the following
predictions: The data rate of some satellites will increase to several gigabits per second; the angular
resolution of some satellites will be approximately 0.1 μrad, corresponding to a resolution of several ten
centimeters on Earth; the data storage capacity of onboard data recorders will be several terabytes; and
the number of bits per pixel will be larger than 13. The acquired information will drastically increase with
monitoring frequency, observation area and the resolution of the images. Monitoring from satellites will
not only be done for special area but real-time observations of the entire world will take place.
Gigabit-per-second-class direct links to ground stations will be necessary during the short download time
of the direct communication page link from a LEO satellite. Optical communication systems are preferable for
this increasing communication demand.

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