Satellite communication systems consist of two main segments, the space segment and the earth or ground station. The ground station system coordinates the communication process with satellites in space. A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth.
Satellite communications involve four steps: An uplink Earth station or other ground equipment transmits the desired signal to the satellite; The satellite amplifies the incoming signal and changes the frequency; The satellite transmits the signal back to Earth, and The ground equipment receives the signal. Satellite communications networks consist of user terminals, satellites, and a ground network that provides control and interface functions.
Satellite communications systems use various frequencies for signal propagation depending on the purpose, nature, and regulations in the region of operation. Examples of frequency bands are Very High Frequency (VHF) ranging from 30 to 300MHz, Ultra High Frequency (UHF) ranging from 0.3 to 1.12GHz, L-band ranging from 1.12-2.6 GHz, S-band ranging from 2.6 to 3.95 GHz, C-band ranging from 3.95 to 8.2 GHz, X-band ranging from 8.2 to 12.4 GHz, Ku- band ranging from 12.4 to 18 GHz, K- band ranging from 18.0 to 26.5 and Ka-band ranging from 26.5 to 40 GHz. However, communications above 60 GHz are generally challenging because of the high power needed and equipment cost.
Geostationary satellite networks utilize a smaller number of satellites, and each satellite provides satellite coverage to a fixed area of the Earth. Geostationary satellites are generally located above the equator and follow the Earth’s orbit which means each Geostationary satellite stays in the same place relative to the Earth’s surface. Geostationary satellites generally have superior data bandwidth, but since the satellites are around 36,000 km above the Earth will experience greater latency or voice delay than LEO satellites.
Low Earth Orbit (LEO) satellite networks are made up of a constellation of small satellites that orbit the Earth in a series of planes. The orbiting pattern of LEO satellite networks means that the individual satellites in the constellation are continually moving relative to the Earth’s surface. LEO satellite networks are well suited to mobile applications, where you need to use the service while on the move.
LEO satellites are also much closer to the Earth (800-1,400 km) than Geostationary satellites (approximately 36,000 km), so latency or voice delay in the network can be considerably less. Pivotel products that use LEO satellite networks include all Iridium, inReach, SPOT and Globalstar satellite solutions.
Satellite Ground Station
Ground station facilities are used for satellite tracking, controls and provision of Telemetry and Command (T&C) services . Ground stations are also responsible for planning and allocation of satellite resources to each gateway in mobile satellite communications.
Ground station consists of an antenna subsystem, with an associated tracking system, a transmitting section and a receiving section. It includes an antenna system for the transmission and reception of signals. Low noise block down converter, High Power Amplifier (HPA) transmitter with power from a few watts to hundred kilowatts depending on capacity and regulations, Up and Down converters, modem, encoders, multiplexers, control and tracking systems, interfaces for user terminals.
The antenna is generally common to transmission and reception for reasons of cost and bulk. Separation of transmission and reception is achieved by means of a diplexer. The tracking system keeps the antenna pointing in the direction of the satellite in spite of the relative movement of the satellite and the station.
The node configuration depends on the size and required services. It ranges from large ground station use as gateways in a telecommunication network to Very Small Aperture Terminals (VSAT) that deliver data communication applications to remote region.
The figure of merit (G/T) of Earth station is defined at the station receiver input as the ratio of the composite receiving gain G to the system noise temperature T of the earth station. The gain of an antenna is maximum in the direction of maximum radiation (the electromagnetic axis of the antenna, also called the boresight) therefore it is necessary to point the antenna such that the satellite is on its foresight.
Large earth station antennas are expensive to construct and to maintain so that there is a premium in obtaining the maximum efficiency from the system. Typical values are 40.7dBK−1 for an INTELSAT A, 30 metre diameter antenna operating at 4/6GHz
The minimum receivable signal level is set by inherent noise in the system. Earth stations are required to detect small signals so the control of the noise parameters is important. . For an earth station, the noise acquired by the antenna originates from the sky and surrounding ground radiation. It depends on the frequency, the elevation angle and the atmospheric conditions (clear sky or rain). The system noise temperature T is a function of the antenna noise temperature TA, the feeder losses LFRX, the thermodynamic temperature TF of this feeder and the effective noise temperature TeRX of the receiver.
Satellite Earth Station antennas
Despite more micro and small satellite missions are moving into the higher frequencies of S-band and X-band, satellite operators are confronted with challenging restrictions on available antenna gains in the space segment, RF output power and Signal-to-Noise-Ratio (SNR). In general, larger G/S antennas of more than 3 m diameter are one solution to assure the radio link for space missions with a low satellite antenna gain or for missions beyond Low-Earth-Orbit (LEO).
The antennas can be divided into three types:
- 1 Large antennas required for transmit and receive on the INTELSAT type global networks with gains of 60 to 65dBi (15 to 30 metres diameter).
- 2 Medium-sized antennas for cable head (TVRO) or data receive only terminals (3-7 metres diameter)
- 3 Small antennas for direct broadcast reception (0.5-2 metres diameter).
Professional G/Ss operated by Space Agencies or service providers, e.g., DLR GSOC G/Ss in Weilheim, Germany features usually antennas with a diameter of 3 to 15 m for Telemetry and Telecommand (TM/TC) in S- and X-band – whereas smaller operators of satellites usually use antenna sizes smaller than 3 m.
Most of the power is radiated (or acquired) in the major lobe. However, a non-negligible amount of power is dispersed by the side lobes. The side lobes of an earth station antenna determine the level of interference with other orbiting satellites.
Types 1 and 2 have to satisfy stringent specifications imposed by regulatory bodies. When the recommended spacing of satellites in the geostationary arc was 3 degrees, the pattern envelope was specified by 32 – 35 logθ. This could be met with a symmetric reflector antenna. With the new spacing of 2 degrees, the pattern spec has been improved to 29 – 25 log θ. This can best be met with low sidelobe, offset reflector designs.
Earth station antennas are at the earth end of satellite links. The characteristics required for an earth station antenna are as follows:
- high directivity, in the direction of the nominal satellite position (for useful signals);
- low directivity in other directions, in particular that of nearby satellites to limit interference with other systems;
- antenna efficiency as high as possible for both frequency bands (uplinks and downlinks) on which the antenna operates;
- high isolation between orthogonal polarization;
- the lowest possible antenna noise temperature;
- continuous pointing in the direction of the satellite with the required accuracy;
- limitation, as far as possible, of the effect of local meteorological conditions (such as wind, temperature, etc.) on the overall performance.
The antenna gain arises directly in the expressions for the effective isotropic radiated power (EIRP) and the figure of merit (G/T) of the station. The antenna beamwidth determines the type of tracking system used in accordance with the particular characteristics of the satellite orbit.
The value of polarisation isolation determines the ability of an antenna to operate in a system with frequency re-use by orthogonal polarisation. Assuming that the carrier powers of orthogonal polarisations are the same, the interference introduced by the antenna from one carrier to the other is equal to the polarisation isolation which must, therefore, be greater than a specified value. By way of example, INTELSAT advocates, for certain standards and applications, a value less than 1.06 for the axial ratio (AR) in the direction of a satellite with new antennas. This corresponds to a carrier power-to-interference power ratio (C/NI) greater than 30.7 dB.
One type of antenna is “RF hamdesign,” an aluminum rip structure bestride by a 2.8 mm aluminum mesh and kept in position by rivets. The mesh reflector allows for usage of up to 11 GHz, while significantly reducing mass and wind loads compared to a solid re-flector. The antenna comes with a focal length to diameter ratio (F/D) of 0.45, resulting in a focal length of 202.5 cm.
Mountings to permit antenna pointing and tracking
Azimuth–elevation mounting: An azimuth–elevation mounting corresponds to a vertical fixed primary axis and a horizontal secondary axis constrained to rotate about the vertical axis. Rotation of the antenna support about the vertical axis enables the azimuth angle A to be adjusted and rotation of the antenna about the associated horizontal axis of the support then permits the elevation angle E to be adjusted.
This is the mounting most commonly used for antennas of steerable earth stations. Azimuth–elevation mounting has the disadvantage of leading to high angular velocities when tracking a satellite passing through the vicinity of the zenith. The elevation angle then reaches 90 which generally corresponds to a mechanical stop to prevent overtravel of the antenna about the secondary axis. To track the satellite, the antenna must thus perform a rapid rotation of 180 about the primary axis.
An X–Y mounting has a fixed horizontal primary axis and a dependent secondary axis which rotates about the primary axis and is orthogonal to it . This mounting does not have the disadvantage of the azimuth–elevation mounting when the satellite passes through the zenith. (a high speed of rotation about the primary axis). X–Y mounting is thus useful for satellites in low orbits rather than for geostationary satellites and stations mounted on mobiles.
Polar mounting or equatorial mounting corresponds to a primary axis (the hour axis) parallel to the axis of rotation of the earth and a secondary axis (the declination axis) perpendicular to the former. This mounting is used for telescopes since it permits tracking of the apparent movement of stars by rotation only about the hour axis which thus compensates for the rotation of the earth about its line of poles. This mounting is useful for links with geostationary satellites since it is possible to point the antenna at several satellites successively by rotation about the hour axis. However, the fact that the satellites are not at infinity necessitates, in principle, slight adjustments of orientation about the declination axis.
Tracking consists of maintaining the axis of the antenna beam in the direction of the satellite in spite of the movement of the satellite or station. Even in the case of a geostationary satellite, orbital perturbations cause apparent displacements of the satellite which are, however, limited to the ‘station-keeping box’. Furthermore, the station can be installed on a mobile vehicle, the location, and direction of which vary with time.
The performance required of the tracking system varies in accordance with the characteristics of the antenna beam and the satellite orbit. For small antennas, the tracking system can be eliminated (fixed mounting) and this enables costs to be reduced.
Several types of tracking are possible and are characterized by their tracking error (pointing angle error). Choice of the type of tracking depends on the antenna beamwidth and the magnitude of the apparent movement of the satellite. Decisions relating to antenna installation and tracking procedure depend on the beamwidth in relation to the magnitude of apparent movement of the satellite; the determining criterion is the variation of antenna gain with depointing.
Another antenna characteristic that is associated with its diameter and directly affects the performance of orientating devices is its mass. For small antennas, the mass of the parabolic reflector ranges from a few tens to several hundreds of kilograms. For large antennas, it is several tons.
With programmed tracking, antenna pointing is achieved by providing the antenna orientation control system with the corresponding values of azimuth and elevation angles at each instant. Pointing is then performed in open loop without determination of the pointing error between the actual direction of the satellite and the aiming direction at each instant. Programmed tracking is mainly used for earth station antennas with having a large enough beamwidth that high pointing accuracy is not required.
If high pointing accuracy is required , programmed tracking is used with non-geostationary satellites to pre-position the antenna in an area of the sky where the satellite will appear in such a way as to ensure acquisition by a closed-loop tracking system operating on the satellite beacon.
This system is a variant of programmed tracking and is well suited to tracking geostationary satellites for antennas having an intermediate value of beamwidth which does not justify the use of closed-loop beacon tracking. With computed tracking, a computer incorporated in the pointing system evaluates the antenna orientation control parameters. The computer uses the orbit parameters (inclination, semi-major axis, eccentricity, right ascension of the ascendant node, argument of the perigee, anomaly) and, if necessary, a model of their progression. The data in memory are, if necessary, refreshed periodically (after a few days). The system can also extrapolate the progression of the orbit parameters from daily satellite displacements which are stored in memory.
Closed-loop automatic tracking
With antennas having a small angular antenna beamwidth with respect to the apparent magnitude of satellite movement, precise tracking of the satellite is obtained by continuously aligning the antenna direction to that of a beacon located on the satellite. In addition to an accuracy that can be very high (the tracking error can be less than 0.005 degrees with a monopulse system), an advantage of this procedure is its autonomy since tracking information does not come from the ground.
Moreover, it is the only conceivable system for mobile stations whose antenna movement cannot be known a priori (if the itinerary of the mobile is known, programmed tracking could conceivably be used). Two techniques are used for beacon tracking—tracking by sequential amplitude detection and monopulse tracking
Sequential amplitude detection tracking systems make use of variations in received signal level as a consequence of commanded displacement of the antenna pointing axis. The level variations generated in this way enable the direction of maximum gain, which corresponds to the highest received signal level, to be determined. Various procedures are used: conical scanning, step-by step tracking and electronic tracking.
Step-by-step tracking. Antenna pointing is achieved by searching for the maximum received beacon signal. This proceeds by successive displacements (steps) of the antenna about each of the axes of rotation (the method is also known as step-track or hill-climbing). The direction of the subsequent displacement is determined by comparing the received signal level before and after the step. If the signal increases, the displacement is made in the same direction. If the signal decreases, the direction of displacement is reversed.
There are several limits to the accuracy of tracking. The gain of the antenna (and hence the level of the received signal) about the direction of maximum gain varies slowly with depointing angle (the lobe has a flat top). Determination of the direction of maximum gain is thus less precise than determination of the pronounced null of the gain characteristic of monopulse systems.
Electronic tracking. This recent technique is comparable with step-by-step tracking. The difference lies in the technique used for successive displacement of the beam in the four cardinal directions, since this is realised electronically. Depointing by a given angle is obtained by varying the impedance of four microwave devices coupled to the source waveguide; these devices are located symmetrically on each side of the waveguide in two perpendicular planes
The monopulse technique uses an Excitation of an antenna pattern which is specifically intended for tracking and contains a zero on the axis permits the antenna to be orientated in such a way as to cancel the received signal. The error angle measurement signals are provided either by comparison of the waves received from four sources located around the electromagnetic axis of the antenna (multiple source monopulse) or by detection of the higher order modes generated by depointing of the antenna in the waveguide coupled to the primary source (mode extraction).
Multimode monopulse Low-Earth-Orbit (LEO) satellites
In the case of Low-Earth-Orbit (LEO) satellites, that appears for the 10-15 minutes time window to earth station antenna, it’s necessary to communicate with satellite effectively in that short duration of time and receive or download all necessary data, that is stored in satellite computer to earth station server.
Typically, the tracking system on the satellite utilizes a monopulse-tracking configuration in which a plurality of antennas, feeding a reflector system, is employed to develop three tracking signals indicative of the pointing accuracy of the satellite antenna. These three tracking signals are the azimuth difference signal, elevation difference signal, and the sum signal.
One disadvantage of conventional monopulse tracking systems is that such systems are designed to operate with cumbersome antenna arrays (4 horns are 5 horns). In such arrays, the plurality of antennas is used to develop the sum and difference signals needed to provide the receiver with the means for developing the elevation and azimuth angle error signals for controlling the tracking system. Such cumbersome plural antenna arrays tend to be larger and heavier than desirable at high frequencies.
The Monopulse multimode tracking feed eliminates all these issues. The constraints of the requirement of single dual-mode tracking feed with communication channel necessitate the development of this new type multi-mode tracking feed. In a multimode monopulse system, higher order modes of a circular waveguide are used for tracking. In this system, when an antenna receives an incident wave, the output level of the communications signal is maximum when the antenna points directly toward a point signal source. On the other hand, higher order modes are excited in the waveguide when the boresight axis of the antenna feed is not in line with the point source.
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