future satellite communication
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This paper outlines some of the techniques being developed to provide affordable, reliable satellite communications suitable for a wide range of military aircraft, from agile platforms such as jets and helicopters to surveillance, tanker and transport aircraft. It also gives an overview of airborne SHF (Super High Frequency) and also EHF (Extremely High Frequency) satcom techniques. Although presently used UHF (Ultra High Frequency) satellite communication are relatively simple to install and comparatively inexpensive, suffer from very limited capacity and are prone to multipath and unintentional interference due to their poor antenna selectivity. Whereas, the SHF satcoms offer significantly increased bandwidth for high data rates or increased use of spread-spectrum techniques, together with localized (spot) coverage and adaptive antenna techniques â€œ for nulling unwanted signals or interference.
The beginning of new millennium sees an important milestone in military aviation communication with the introduction of the first Super-High-Frequency (SHF) airborne satellite communication (Satcom) terminals, which are due to enter service on Nimrod maritime reconnaissance aircraft (MRA4). Satcom terminals using the Ultra-High-Frequency (UHF) band have been fitted to larger aircrafts for a number of years. Although relatively simple to install and comparatively inexpensive, UHF satcoms(240-270 & 290-320 MHz bands) suffers from very limited capacity (a few 25 KHz channels per satellite) and are prone to multipath & unintentional interference due to their poor antenna selectivity. SHF satcoms (7.25 â€œ 7.75 & 7.9-8.4 GHz) offer significantly increased bandwidths (hundreds of MHz) for high data rates or increased use of spread-spectrum techniques, together with localised coverage and adaptive antenna techniques for nulling unwanted signals or interference.
For airborne platforms, the advantages of SHF satcoms come at the expense of a significant additional burden in terms of antenna siting and pointing, particularly for smaller, highly agile aircrafts. Antenna should be large enough to support the desired data rate and to provide enough directivity to minimise interference with adjascent satellites and avoid detection by hostile forces. Another feature of satcoms, unique to aircraft, is the effect of unwanted modulation from moving parts such as helicopter rotor blades, propellers and jet engines.
This paper gives an overview of development of airborne SHF and also Extremely-High-Frequency (EHF) satcom techniques, and terminal demonstrators by DERA (Defence Evaluation and Research Agency). This research is aimed at providing affordable, secure and robust satcoms to a range of military aircraft, supporting ground attack and reconnaissance roles to surveillance, transport and tanker aircraft.
The UK Ministry of Defence (MoD) currently operates a constellation of three geostationary military communication satellites collectively known as skynet. The current generation of skynet 4 satellites provides satellite communication for all types of armed services at both UHF and SHF bands. Demand for all types of military satcom is rising rapidly, due principally to the need for even more timely information to prosecute operations effectively. Another factor is the growing expectations of service personnel familiar with a world of instant global communication and rapid availability of information via internet-type services.
Satcoms can span distance, terrain and hostile forces to provide a global reach for dispersed mobile platforms such as aircraft, submarines, surface ships, vehicles and man-packs. Fig.1 illustrates schematically, the breadth of military aerial roles that satcom may be required to support in future, providing global beyond-line-sight communications between aircraft and commander in theatre. Information carried by satellite could include:
Â¢ Near-real-time command & control â€œ tasking, position, reporting etc.
Â¢ Data from reconnaissance & surveillance aircraft.
Â¢ Targeting data for stand-off weapons.
Â¢ Situation awareness.
Â¢ Transfer data from dissimilar and/or geographically separated line-of-sight (LOS) networks.
Fig. 1 Potential use of satcom by military aircraft.
In some case, direct communication with aircraft will not be necessary. For eg: air superiority fighters operates in the vicinity of an E3-D Sentry air-borne early-warning (AEW) aircraft with which they are in LOS communication. For these fighter aircrafts it would be more efficient to communicate via their LOS link to the E3-D and then via satcom from E3-D, with the transfer between the two links operating seamlessly.
Typical rates for some of these military roles are given in Table-1. At the other end of the scale, unprocessed reconnaissance data demand large amount of bandwidths - the amount increasing year-on-year as the resolution of imaging and radar sensor technologies improves.
Scenario Data type Typical data rates
Tasking and position reporting
Receive-only direct broadcast short messages
video/multimedia 50 â€œ 300 bit/s
2.4 â€œ 4.8 kbit/s
16 kbit/s â€œ 20 Mbit/s
2 â€œ 20 Mbit/s
Table 1: Typical data rates for various air missions
USE OF CIVILLIAN SATELLITE SYSTEMS
The use of non-military satellite systems, principally INMARSAT, by the military to supplement capacity for non-mission-critical communications is already common. Civilian mobile satcom is a $ 2.5 billion global industry and can provide significant additional capacity using the latest state-of-art technology.
Use of commercial satellite systems is bound by international treaties which typically place severe constraints on the use of these systems by military. Commercial satellites also have no specific built in resiliance to jamming and limited protection against information warfare.
Until recently INMARSAT was the sole provider of commercial aeronoutical satellite services, using 10MHz of spectrum in L-band (~ 1.6 GHz). Iridium system developed by Motorola provides truly global coverage using 66 satellites in polar orbit with 48 beams per satellite. This revolutionary system uses L band for communication with mobiles and Ka-band (20/30 GHz) for links to ground stations. Iridium is the first commercial satellite system to use on-board switching with inter satellite links (in the 23 GHz water vapour absorption band) so that links between mobiles need not use terrestrial gateways. Although Iridium system is primarily intended to support personnel communications aeronautical terminals with one to eight channels are being developed for the Iridium system by AlliedSignal Aerospace. Other Low Earth Orbit (LEO) personal communication systems (PCSs) such as Global star and Medium Earth Orbit (MEO) systems such as ICO are due to come on-line in near future.
Fig. 2(a) AlliedSignal Iridium terminal currently undergoing trials
(b) An RAF Hercules aircraft.
THE AIRFRAME â€œ ANTENNA PLACEMENT
One of the first problems encountered when considering an airborne satcom terminal is where to site the antenna on the airframe so as to gain an uninterrupted path to the satellite. Helicopters in particular present a unique challenge due to their rotor and body shape, which make it difficult to find an uninterrupted view of the satellite for all orientations. Fig. 3 shows the blockage for two hypothetical antenna location on the Apache attack helicopter. The blockage has been computed by project and implimentationing the silhouette of the airframe and main rotor, as illuminated from the antenna site, onto a hemisphere. Most apparent from these diagrams is the extent of the shadowing by the main rotor blade, which causes unwanted amplitude and frequency modulation. With the antenna mounted on the helicopter spine just behind the engine, the silhouettes of the cockpit (fore) and tail (aft) are very apparent. Moving the antenna to one of the two stubby `wingsâ„¢ which support the armament provides better fore and aft coverage but suffers blockage on one side due to the fuselage. Two antenna, on each wing would provide the necessary hemispherical coverage. Surprisingly, the best location for a satcom antenna on the Apache is above the centre of the main rotor on a de-spun mast.
In addition to engine/rotor modulation, the modem (modulator/demodulator) in an airborne satcom terminal has to be designed to accommodate the effect of Doppler shift of the RF signal due to the aircrafts speed and acceleration. This speed could be upto 600m/s relative to satellite. The amount of Doppler shift increases with increasing radial velocity and RF carrier frequency.
Data rate C/N
Sensitivity Mode 1
Sensitivity Mode 2
Max. Doppler offset
Max. Doppler rate of change
Max. rotor modulation degradation in C/No 50 bit/s 23 dB HZ
75 bit/s 25 dB HZ
300 bit/s 31 dB HZ
2.4 kbit/s 40 dB HZ
4.8 kbit/s 43 dB HZ
Table 2: Required SHF modem performance for large and rotary wing aircraft
An airborne tactical modem has the following requirements:
Â¢ To be efficient interms of carrier-to-noise density (C/No) for a
given bit error ratio (BER).
Â¢ To operate under high Doppler offsets & rates of changes due to
Â¢ To operate with minimal degradation in the presence of
multipath propagation & occasional signal loss due to airframe
blockage & antenna switching.
Â¢ Rapid acquisition of communications following signal loss.
Â¢ To operate with minimal degradation in the presence of rotor
Â¢ To comply with international regulation on power spectral
density & adjacent satellite interference.
Â¢ To implement electronic protection measures as required.
To illustrate the combined effects of Doppler, rotor modulation and multipath, we show Fig.4, the scattering function for communication to a medium-sized helicopter. Here the satcom antenna is mounted behind the main engines on the central fuselage below the main rotor.
The scattering function is based upon the cross-ambiguity analysis, a tool commonly used in radar. It is essentially the output of a `filterâ„¢ that is matched to the uncorrupted transmitted signal (Sref). The signal filtering and processing incorporated in a satcom receiver is normally a very close approximation to that required to perform matched filtering of a received signal from the satellite. The magnitude of the scattering function illustrated in Fig.4, is a measure of the interference present in the received signal (S), which comprises the vector sum of several signal paths, each simultaneously offset in frequency (fD) and delayed in time (). Ideally the matched filter output for zero delay (multipath) and frequency shift (Doppler) would be a `spikeâ„¢ centered upon zero Doppler and zero delay (broadened by the finite time-bandwidth product). Significant signal loss arises when the received signal scattering function is `smearedâ„¢ in time and frequency by Doppler and multipath smearing is non-symmetric due to aircrafts relative motion.
The received signal undergoes both amplitude and phase modulation as a consequence of rotor modulation. Periodic fading results from rotor blade obscuration, and deep nulls arises due to complex coupling of phase and Doppler shift. The resultant scattering function in this case depends upon many factors, including the aspect angle to the rotors and aircraft from terminal and receiver, the number of rotor blades and structural composition, pitch angle, flexing and hub rotation rates. Rotor modulation thus poses serious problems for conventional satcom link acquisition, RF carrier tracking and link retention. With correct signal processing, the effect of rotor modulation can be reduced to <1dB power penalty.
After determining the required communication data rate and the available satellite power to support it, the aircraft antenna (aperture) has to be determined. Antenna requirements for use with geostationary satellite fall into three categories:
Â¢ Low gain for position reporting & messaging.
Â¢ Medium gain for voice & low-data-rate communication.
Â¢ High gain for applications such as file-transfer, surveillance/reconnoissance data etc.
The requirements for low-gain and medium-gain antennas are summarised in Table 3.
Parameter Low-gain antenna Medium-gain antenna
Packaging 6 dBi
Rx7.25 â€œ 7.75 GHZ
Tx 7.9 â€œ 8.4 GHZ
N/A (different antennas)
aircraft blade radome
1 Tx and 1 Rx blade 16 dBi
7.25 â€œ 8.4 GHZ
aperture less than 10 cm
fully contained within 12cm
Table 3: Required SHF antenna performance (R (L) HCP = right (left) handed circular polarisation)
For satcom applications, the size, performance (aperture efficiency noise temperature) and cost of current phased-array technologies severely limit the usefulness of `holy grailâ„¢ of airborne antenna which is thin, conformal phased array which take the shape of the aircrafts skin. To reduce the cost, DERA has developed compact proof-of-concept low-gain and medium-gain airborne SHF satcom antennas using more conventional technologies.
Low Gain Antenna
A number of approaches have been investigated for a low gain antenna with the additional goal of reducing the complexity of the antenna pointing method to further minimise the cost. In particular the use of a number of static, switched-beam antennas with azimuth-independent gain has been pursued. The minimum number of independent beams required to cover a given solid angle can be inferred from the gain required for a single beam and Table 4, gives the theoretical directivity of switched beam antennas for various number of beams. In practice, the antenna gain will be lower than the directivity due to feed losses. The poor directivity of low gain antenna necessitates the need for alternative technique to reduce the unwanted interference, notably spectral spreading.
Beams per hemisphere Directivity per beam
5 3 dBi
Table 4: Minimum antenna directivity for different numbers of switched beams
A practical realisation of a switched three beam low-gain antenna is shown in Fig 5. Each beam is omni directional in azimuth with overlapping elevation coverage. The three elements (high, middle & low) are switched according to the aircrafts attitude. The preferred configuration is to have one transmit and one receive antenna, switched in tandem, each containing three beams in a so called `bladeâ„¢ housing (radome).
Fig. 5 Axial-symmetric, switched 3-beam, SHF low-gain antenna: (a) schematic diagram; (b) antenna patterns; © practical realisation
Medium Gain Antenna
The bidirectional medium-gain antenna for voice/low data-rate communication originally employed a corrugated circular horn Fig 6a, which has good cross polarisation and low sidelobe performance. The swept volume of this antenna is determined by its length and not its aperture, with the consequence that a relatively large radome is required â€œ with a resultant impact on aerodynamic drag. Alternative solution is the short `back-fireâ„¢ antenna Fig 6b, appear to offer a more compact mechanical solution. These antennas operate in X band with 15% fractional bandwidth. The back-fire antenna has a very high aperture efficiency (the ratio of antenna gain to that of a perfect antenna of the same aperture area) of around unity, partly as a result of its semi resonant structure. It also has a very small swept volume â€œ essentially limited by its aperture.
Fig. 6 Medium-gain antennas: (a) corrugated horn; (b) compact `short back-fireâ„¢ antenna
PLATFORM DYNAMICS & ANTENNA POINTING
Having sited the antenna on airframe it is necessary to ensure that it continues to maintain the transmitted and received signals over a given link within specific limits, regardless of the motion of the aircraft. Signal loss is an unwanted consequence of the inherent uncertainty in the tracking process and is affected by:
Â¢ Angular pointing offsets & drift.
Â¢ Antenna beamwidth.
Â¢ Platform stability.
Â¢ Static & dynamic loading.
Â¢ Beam squint resulting from near-field distortions & multipath.
Â¢ Attitude-sensor data accuracy & update rate.
Â¢ Time & frequency dependent collimation errors.
For a given positional accuracy, the signal loss due to antenna pointing is intrinsically proportional to the antenna beamwidth. This in turn is inversely proportional to the square root of the antenna gain. High gain antennas therefore require high accuracy pointing or tracking systems.
Most modern aircrafts incorporate an inertial navigation system (INS) or similar integrated navigation system that can potentially provide the information on the aircraft attitude to allow the antenna to be steered towards the satellite. The data obtained from the INS is not always suitable for satcom antenna pointing due to inadequate update rates or flexing of the aircraft structure between the INS sensor and the antennas location.
Here the approach is to exploit INS data where available but to concentrate on the use of autonomous attitude and heading reference system (AHRS), built into the satcom terminals as close to the antenna as is feasible. The two low-cost AHRS systems are:
The Watson C304 unit, Fig 7a is a small, lightweight strap-down AHRS based on a solid-state three axis gyrocluster, a pair of liquid angle sensors and a three axis magnetometer. The attitude is derived by integrating the signals from the triaxial gyro. These are then referenced to the earth axes via the angle sensors, which are employed as the gravitational reference and the triaxial magnetometer.
Fig. 7(a) Watson AHRS
The Trimble Advanced Navigation Sensor (TANS) vector, Fig 7b is a solid state attitude-determination and position-location system which uses the carrier phase difference of global positioning system (GPS) signals from upto six satellites, received at four separate antennas. The TANS vector requires no attitude or position initialisation.
Fig. 7(b) Trimble TANS Vector
The complete autonomous antenna pointing sub-system, Fig 7c comprises a GPS receiver, AHRS, antenna positioner and computer based antenna control circuit unit. The basic subsystem employs an open-loop pointing system, which in its simplest form predicts the antenna pointing angles based on satellite ephemeris data, air platform location and attitude.
Fig. 7 © control system schematic diagram
The accuracy and update rate of these low-cost AHRS systems with medium-gain antennas are, in fact, considered adequate for all the most demanding air platforms. Fig.8 shows the relative pointing losses associated with a range of platforms for 1 and 10 antenna (3 dB) beamwidths with both low-cost and high-grade AHRS. Pointing errors are most significant for satcoms with narrowest beamwidths on-board the most agile platforms. This impacts most markedly on the use of EHF satcoms on fast jets.
Fig. 8 Relative pointing antenna errors for various manoeuvres on different platforms: (a) 1 antenna beamwidth, high-grade AHRS; (b) 1 antenna beamwidth, low-cost AHRS; © 10 antenna beamwidth, low-cost AHRS
LOW-DATA RATE & VOICE-CAPABLE SHF SATCOM TERMINAL
DERA has developed two variants of a proof-of-concept airborne SHF satcom terminals, Fig 9 based around a modular (plug & work) approach. The terminals can be configured to operate in various modes by connecting modules to a common controller. These terminals are fully autonomous requiring only DC power from the host platform. The principal operating modes of these terminals are:
Â¢ Mode 1: World-wide position reporting & messaging.
Â¢ Mode 2: Voice communication (in satellite spot beams).
Â¢ Mode 3: Voice plus position reporting & messaging.
Fig. 9 Proof-of-concept airborne SHF satcom terminal control unit
The SHF terminal modules are:
Â¢ The attitude & heading unit, comprising the AHRS.
Â¢ The antenna unit, comprising either the switched low-gain or steerable high-gain antenna together with a low-noise amplifier (LNA), high-power amplifier (HPA) & diplexer filters.
Â¢ The terminal control unit, developed in conjunction with Delta communication which contains the RF up & down-converters, the GPS receivers, the voice coder-decoder (vocoder) & a PC 104 embedded Pentium PC.
Â¢ The remote display unit, which provides the operator interface and comprises embedded PC with a touch-sensitive LCD display.
MILLIMETER WAVE SATCOMS: THE FUTURE
The existing skynet 4 constellation is due to be replaced by 2007. The next generation of satellites, skynet 5, due to be introduced from 2005, is currently the subject of a series of `Public Finance Initiativeâ„¢ studies. One of the options being actively considered for skynet 5 is the provision of an on-board processed, Extremely-High-Frequency (EHF) payload similar to that used on US Milstar constellation.
The Milstar EHF system operates in the 20.2-21.2 GHz (down-link)/ 43.5-45.5 GHz (up link) bands and was originally developed to provide post-nuclear strike command and control facilities. Milstar was designed to provide:
Â¢ Enhanced resistance to jamming.
Â¢ Enhanced immunity to ionospheric scintillation (such as that
following a high-altitude nuclear explosion).
Â¢ Reduced probability of interception/detection.
Milstar achieves this through combination of four factors:
Â¢ Use of an EHF for earth-to-satellite links.
Â¢ Use of highly robust communication waveform.
Â¢ Use of sophisticated on-board processing.
Â¢ Use of cross-satellite links.
Recent Milstar satellites have been adapted to provide tactical users with up to 1.5 Mbits of high-data-rate communications each. The use of time division multiplexing (TDM) on the down-link contributes to increased efficiency for high-data-rate services and removes the problem of intermodulation with frequency division multiplexing (FDM). On-board regenerationswitching also allows direct communication between so-called disadvantaged users, such as aircraft, without the need to use a large ground station as a hub.
High antenna gains are often cited as a benefit of operation at EHF frequencies; however the beamwidth of satellite antennas are generally determined by the geographical coverage required, with the consequence that all antennas with similar coverage also have similar gain, regardless of frequency.
The principal appeal for the higher frequency bands is therefore largely due to the increased spectrum available. Wider spectrum allows either accommodation of more users/services or greater exploitation of spread-spectrum techniques to counter jamming and to reduce the probability of interception through the use of spread-spectrum is usually characterised interms of the coding gain â€œ the ratio of the spread bandwidth to the unspread bandwidth.
For airborne users the primary advantage of EHF frequencies is reduced likelihood of interception due to smaller antenna beamwidths for a given data rate. Deep-strike aircraft and attack helicopters can operate hundreds of kilometers behind enemy lines to disable the enemyâ„¢s infrastructure and supply lines. Low probability of detection (LPD) is vital on such missions. Increased spectrum for use by spread-spectrum techniques work in favour of the aircraft and against those trying to detect its transmission since a detector must search a larger spectral space to detect the aircraft signal.
Fig. 10 Integrated atmospheric attenuation against frequency at 90 elevation for an aircraft near sea level. The lower line is for relatively dry, rain- and cloud-free conditions; the upper line is for 12 mm/hr rain.
The most obvious disadvantage of operating at EHF frequencies is increased atmospheric attenuation in the troposphere, as shown in Fig 10. The familiar absorption peak occurs at ~60 GHz and ~118GHz due to gaseous oxygen and at ~23 GHz and ~183 GHz due to water vapour. Continuum attenuation above 10 GHz also increases significantly in the presence of rain and rain-bearing clouds as the signal wave-length becomes comparable with the size of water droplets. One advantage of the increased atmospheric attenuation at sea level is likely to be that terrestrially based interception will be more severely affected by atmospheric attenuation than airborne satcoms. At sea level, the effects of attenuation are greatest, and at about 20000ft (6096m) the attenuation is almost negligible.
Atmospheric attenuation is a major concern for users of EHF, but it is not the only problem to affect operation at these frequencies. Antenna pointing and airframe integration are key issues which are likely to become even more difficult as we move towards more agile, stealthy aircrafts such as the future offensive air system (FOAS). Providing highly robust, Milstar compatible, EHF satcoms to a highly agile, stealthy, air platform is a significant challenge. By developing and evaluating key technologies in airborne EHF terminal design, DERA is able to investigate the practical issues associated with these and other potential EHF options (such as use of military Ka band) for skynet 5 and minimize the risks associated with the procurement of the MoDâ„¢s future airborne satcom terminals.
For airborne users the primary advantage of EHF (Extremely High Frequency) is reduced likelihood of interception due to smaller antenna beamwidths for a given data rates. Increased spectrum for use by spread- spectrum techniques work in favour of the aircraft and against those trying to detect its transmission since a detector must search a larger spectral space to detect the aircraft signal. The combination of wider spread-spectrum bandwidth and narrower antenna beamwidth at EHF result in a significant improvement in aircraftâ„¢s LPD (Low Probability of Detection). Providing highly robust, Milstar compatible, EHF satcoms to a highly agile, stealthy, air-platform is a significant challenge and making it affordable is an even greater one.
1. Electronics and Communication Engineering Journal, IEE
2. Link margin for UHF airborne satellite communications operation. (FRANKE, E., and VAAL, G.) IEEE
3. An overview of aeronautical telecommunications in Europe and worldwide, IEE
4. Fixed and mobile terminal antennas, IEE
5. Rationale for future use of EHF for military SATCOMâ„¢s, IEE
6. Fundamental performance characteristics that influence EHF MILSATCOM systems, IEEE.
I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head of the Department for providing me with the guidance and facilities for the Seminar.
I express my sincere gratitude to Seminar coordinator
Mr. Manoj K, Staff in charge, for his cooperation and guidance for preparing and presenting this seminar and presentation.
I also extend my sincere thanks to all other faculty members of Electronics and Communication Department and my friends for their support and encouragement.
Â¢ INTRODUCTION 01
Â¢ THE MILITARY PERSPECTIVE 02
Â¢ USE OF CIVILIAN SATELLITE SYSTEMS 04
Â¢ THE AIRFRAME-ANTENNA PLACEMENT 06
Â¢ THE MODEM 07
Â¢ THE ANTENNA 10
Â¢ PLATFORM DYNAMICS AND ANTENNA POINTING 15
Â¢ LOW-DATA-RATE AND VOICE-CAPABLE SHF SATCOM TERMINAL 20
Â¢ MILLIMETRE-WAVE SITCOMS: THE FUTURE 21
Â¢ CONCLUSION 25
Â¢ REFERENCES 26
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Before the lecture
Try to find out more by reading:
LEOs - Low earth orbit
MEOs - Medium earth orbit
GEOs - Geostationary earth orbit
Originally proposed by Arthur C. Clarke
Circular orbits above the equator
Angular separation about 2 degrees - allows 180 satellites
Orbital height above the earth about 23000 miles/35000km
Round trip time to satellite about 0.24 seconds
GEO satellites require more power for communications
The signal to noise ratio for GEOs is worse because of the distances involved
A few GEOs can cover most of the surface of the earth
Note that polar regions cannot be seen by GEOs
Since they appear stationary, GEOs do not require tracking
GEOs are good for broadcasting to wide areas
US Navy bounced messages off the moon
ECHO 1 balloon satellite - passive
ECHO 2 - 2nd passive satellite
All subsequent satellites used active communications
Photo from NASA
4000 miles orbit
Allowed live transmission across the Atlantic
First Geosynchronous satellite
Picture from NASA
Picture from NASA
Major problems for satellites
Positioning in orbit
This can be achieved by several methods
One method is to use small rocket motors
These use fuel - over half of the weight of most satellites is made up of fuel
Often it is the fuel availability which determines the lifetime of a satellite
Commercial life of a satellite typically 10-15 years
It is vital that satellites are stabilised
to ensure that solar panels are aligned properly
to ensure that communications antennae are aligned properly
Early satellites used spin stabilisation
Either this required an inefficient omni-directional aerial
Or antennae were precisely counter-rotated in order to provide stable communications
Modern satellites use reaction wheel stabilisation - a form of gyroscopic stabilisation Other methods of stabilisation are also possible
eddy currrent stabilisation
(forces act on the satellite as it moves through the earthâ„¢s magnetic field)
Reaction wheel stabilisation
Heavy wheels which rotate at high speed - often in groups of 4.
3 are orthogonal, and the 4th (spare) is a backup at an angle to the others
Driven by electric motors - as they speed up or slow down the satellite rotates
If the speed of the wheels is inappropriate, rocket motors must be used to stabilise the satellite - which uses fuel
Modern satellites use a variety of power means
Solar panels are now quite efficient, so solar power is used to generate electricity
Batteries are needed as sometimes the satellites are behind the earth - this happens about half the time for a LEO satellite
Nuclear power has been used - but not recommended
Satellite components need to be specially hardened
Circuits which work on the ground will fail very rapidly in space
Temperature is also a problem - so satellites use electric heaters to keep circuits and other vital parts warmed up - they also need to control the temperature carefully
There are a number of components which need alignment
These have to point at different parts of the sky at different times, so the problem is not trivial
A parabolic dish can be used which is pointing in the correct general direction
Different feeder horns can be used to direct outgoing beams more precisely
Similarly for incoming beams
A modern satellite should be capable of at least 50 differently directed beams
Satellite - satellite communication
It is also possible for satellites to communicate with other satellites
Communication can be by microwave or by optical laser
Low earth orbit satellites - say between 100 - 1500 miles
Signal to noise should be better with LEOs
Shorter delays - between 1 - 10 ms typical
Because LEOs move relative to the earth, they require tracking
Circular orbits are simplest
Inclined orbits are useful for coverage of equatorial regions
Elliptical orbits can be used to give quasi stationary behaviour viewed from earth
using 3 or 4 satellites
Orbit changes can be used to extend the life of satellites
Microwave band terminology
L band 800 MHz - 2 GHz
S band 2-3 GHz
C band 3-6 GHz
X band 7-9 GHz
Ku band 10-17 GHz
Ka band 18-22 GHz
Early satellite communications
Used C band in the range 3.7-4.2 GHz
Could interfere with terrestrial communications
Beamwidth is narrower with higher frequencies
More recent communications
Greater use made of Ku band
Use is now being made of Ka band
Above 10 GHz rain and other disturbances can have a severe effect on reception
This can be countered by using larger receiver dishes so moderate rain will have less effect
In severe rainstorms reception can be lost
In some countries sandstorms can also be a problem
Ku band assignments
Satellites do not just stay in their orbits
They are pushed around by various forces
They require active management
Systems of satellites
Example - Iridium
Deploy many satellites to give world wide coverage - including polar regions
So far have not proved commercially viable
Other systems coming along - Teldesic
Because Iridium has not been a commercial success the future of satellites is uncertain
Satellites still have major advantages for wide area distribution of data
1945 Arthur C. Clarke Article: "Extra-Terrestrial Relays"
1955 John R. Pierce Article: "Orbital Radio Relays"
1956 First Trans-Atlantic Telephone Cable: TAT-1
1957 Sputnik: Russia launches the first earth satellite.
1962 TELSTAR and RELAY launched
1962 Communications Satellite Act (U.S.)
1963 SYNCOM launched
1965 COMSAT's EARLY BIRD: 1st commercial communications satellite
1969 INTELSAT-III series provides global coverage
1972 ANIK: 1st Domestic Communications Satellite (Canada)
1974 WESTAR: 1st U.S. Domestic Communications Satellite
1975 RCA SATCOM: 1st operational body-stabilized comm. satellite
1976 MARISAT: 1st mobile communications satellite
1988 TAT-8: 1st Fiber-Optic Trans-Atlantic telephone cable
1994 GPS system deployed by USAF
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The State of the Global Satellite Industry.ppt (Size: 1.46 MB / Downloads: 168)
The State of the
Global Satellite Industry:
Trends, Opportunities, Challenges
Irwin Communications, Inc.
Satellite Communications Consulting
Founded ın 1985
Based in Washington, D.C.
Market Research, Business Development, Market Analysis, Capacity Brokerage
FSS, MSS, Broadcastıng, Broadband, VSATs
Chair, SATCON (Satellite & Content Delivery Conference and Expo) since 2002, next SATCON October 15,16 ın NYC
Global VSAT Forum
Non-Profit VSAT Industry Association
180+ Member Organizations in 80 Nations
Advancing Satcom Interests Worldwide Thru:
Regulatory Reform & Market Access
Education & Training
Technical Support (e.g. Type Approvals, etc)
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Market Intelligence & Commercial Leads
Satellite Industry Overview
Component and Subsytem
Satellite Services Findings
Satellite Services growth of 19% in 2006 outpaced 2005 growth of 13%
Satellite Television revenues increased 17% globally in 2006, again contributing the largest portion of overall satellite services growth
Global satellite TV subscribers grew about 10% over 2005 levels, reaching approximately 89 million worldwide
Satellite Radio continued to experience strong growth
Revenues doubled from $0.8 billion in 2005 to $1.58 billion in 2006
Subscribers grew by 50% - from 9.4 million in 2005 to 14.2 million in 2006
Overall Fixed Satellite Services revenue grew by 20% over 2005, while Mobile Satellite Services revenue increased by 18%, driven by growth in voice traffic
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02-02-2011, 10:52 AM
need the figures.........
need a complete seminar and presentation information about futere satellite communinication along with figures....
Active In SP
Joined: Feb 2011
19-02-2011, 10:07 AM
satellite 2.doc (Size: 427.5 KB / Downloads: 77)
SATELLITE DATA COMMUNICATIONS USING VSAT SYSTEMS – EXTENDING IT NETWORKS TO THE GLOBAL CONTEXT
This paper provides a review and assessment of satellite technologies for providing broadband data communications using very small aperture terminal (VSAT) network systems. In this context, “broadband” means that the application requires a data transfer rate greater than 100 kbps and should allow broadcast, multi- and uni-cast, and interactive bi-directional services to fixed locations worldwide. The applications considered are: Internet access over satellite, digital content distribution, wide area network (WAN) connectivity, video teleconferencing, distance learning, and telephony. The systems examined include digital broadcasting (e.g., DVB) with IP encapsulation, and bi-directional VSAT star networks. Detailed comparisons of various transmission parameters are provided to help evaluate currently available satellite and ground equipment capabilities. It is observed that improved forward correction is desirable, namely the turbo product codes now being introduced widely in satellite ground equipment.
1 Broadband service – a definition
A broadband data communications service is one that requires a transfer rate greater than that afforded by a dial-up telephone line using a V.92 modem. This places the minimum data transfer rate at about 100 kbps, which is typical of current high-speed access (HSA) services from Digital Subscriber Line (DSL) in its many forms, cable modems, and comparable fixed wireless and satellite HSA services. There is also the question of whether the two directions of transmission are of equal speed (symmetrical) or asymmetrical such that the inbound speed from server to user is greater than the outbound speed from user to server. From an IT perspective, broadband service supports standard office applications including email and file transfer, and major software systems like Enterprise Resource Planning and distance education. Organizations are structuring many of their IT applications for use within a standard Web browser, allowing employees and partners to access services within the Intranet and from the external Internet as well. This makes applications seem relatively similar to the network, but the detailed structure cannot be ascertained in general. HSA can provide video distribution, telephony and video conferencing, although these may not be deliverable through a browser since they require specialized user terminal devices or other appliances.
2 Role of Operating GEO satellites
The single most critical element and technology in broadband satellite communications is the satellite itself, since every link within a common footprint must pass through it. Spacecraft designed and constructed in recent years are larger in physical size and mass, and provide substantially more power than their predecessors. This results from improvements in launchers, on-board power systems, high performance components, and radio-frequency high power amplifiers. Other components used within the microwave repeater have improved as well, with benefits showing up in reduced component mass, lower signal loss, and enhancement of transmission quality. A typical satellite weighs almost 5000 kg on top of the launch vehicle, has a lifetime of 15 years, and provides between 50 and 90 channels of wideband transmission (commonly referred to as transponders) with individual power levels of up to 200 RF watts, each. A modern GEO satellite may serve relatively small antennas throughout a large area such as the entire Euro-Asia continent or the full breath of the Pacific region (illustrated in Figure 1 for the PanAmSat 2 satellite).
A typical satellite transmit (downlink) footprint of the PanAmSat 2 satellite, located at 169º East Longitude; satellite radiated EIRP values are approximate (courtesy PanAmSat Corp).
Digital communications and GEO satellites have long partnered and in fact innovations such as TDMA, CDMA, digital speech interpolation and video compression were applied to space-ground links well ahead of terrestrial networks. The Digital Video Broadcast (DVB) standard fits tightly to the satellite’s natural ability to transmit the same high-quality signal across a wide region, rendering the cost per location to an infinitesimally small number (not including the cost of the dish and set-top box installation). Moving forward, this platform provides broadband data delivery and facilitates return channel service if remote sites are suitably configured. Competition from new entrants like PanAmSat, SES and Loral has impacted satellite operation such that quasi-governmental operators like Intelsat and Eutelsat have become commercial enterprises. Suitable GEO satellite capacity is now available throughout Asia-Pacific for networks that serve most any location.
3 Application Interface Standards
A summary of applications and interface standards is provided in Table 1. For digitized content, quality is set at the source encoder and transmission only introduces time delay. For a satellite hop, this delay is small compared to that of compression/decompression. The terrestrial interface concerned with DVB and digital TV in general is called ASI, a high speed serial connection used primarily on the uplink side. Telephone service is usually delivered on an analog basis (2-wire or 4-wire). A properly engineered satellite voice circuit meets the currently accepted standard of 400 ms total delay, including the added delay for speech processing (e.g., compression and decompression, if applied), routing and switching. Importantly, such a satellite circuit will sound better to subscribers than casual Voice over IP connections through the Internet.
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19-02-2011, 10:13 AM
satellite.ppt (Size: 200 KB / Downloads: 166)
Basics of Satellites
Types of Satellites
Basics: How do Satellites Work
Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.
The two stations can use a satellite as a relay station for their communication
One Earth Station sends a transmission to the satellite. This is called a Uplink.
The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.
Basics: Advantages of Satellites
The advantages of satellite communication over terrestrial communication are:
The coverage area of a satellite greatly exceeds that of a terrestrial system.
Transmission cost of a satellite is independent of the distance from the center of the coverage area.
Satellite to Satellite communication is very precise.
Higher Bandwidths are available for use.
Basics: Disadvantages of Satellites
The disadvantages of satellite communication:
Launching satellites into orbit is costly.
Satellite bandwidth is gradually becoming used up.
There is a larger propagation delay in satellite communication than in terrestrial communication
Basics: Factors in satellite communication
Elevation Angle: The angle of the horizontal of the earth surface to the center line of the satellite transmission beam.
This effects the satellites coverage area. Ideally, you want a elevation angle of 0 degrees, so the transmission beam reaches the horizon visible to the satellite in all directions.
However, because of environmental factors like objects blocking the transmission, atmospheric attenuation, and the earth electrical background noise, there is a minimum elevation angle of earth stations.
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15-02-2012, 02:56 PM
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18-02-2012, 05:04 PM
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) Earth orbits.
For fixed (point-to-point) services, communications satellites provide a microwave radio relay technology complementary to that of communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, and for TV and radio broadcasting, for which application of other technologies, such as cable television, is impractical or impossible.
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03-03-2012, 10:46 AM
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