Satellite communications involves 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.
The Satellite user terminal typically consists of an antenna subsystem, with an associated tracking system, a transmitting section and a receiving section. 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.
A very small-aperture terminal (VSAT) is a two-way satellite ground station with a dish antenna that is smaller than 3.8 meters. The majority of VSAT antennas range from 75 cm to 1.2 m. VSATs usually operate in the C, Ku, and Ka frequency bands with data rates between 4 Kbps to 16 Mbps. VSATs access satellites in geosynchronous orbit or geostationary orbit to relay data from small remote Earth stations (terminals) to other terminals (in mesh topology) or master Earth station “hubs” (in star topology).
An everyday example of VSAT terminals are the dish antennas used for D2H (Direct to Home) satellite television broadcast. They are also widely used by businesses for enterprise resource management and media networks to provide live coverage.
VSATs are used to transmit narrowband data (e.g., point-of-sale transactions using credit cards, polling or RFID data, or SCADA), or broadband data (for the provision of satellite Internet access to remote locations, VoIP or video).
VSAT consists of the following elements: an antenna, orthomode transducer (OMT), block up-converter (BUC), lownoise block down-converter (LNB), interfacility link cable (IFL), indoor unit (IDU). IDU is a VSAT modem, also called a baseband data processing subsystem with a modulator and demodulator or simply – a baseband subsystem.
During transmission, the indoor unit sends the signal to BUC which upconverts and amplifies it and then sends it out to the satellite via the dish antenna. When receiving signals, the VSAT Antenna receives a signal, sends it to a Low Noise Block (LNB) which filters, amplifies and down-converts the signal and then sends it to the Satellite modem for further processing. It is to be noted that this Up/Down conversion is required because signals need to be transmitted at high frequencies through air while the indoor unit process signals at considerably lower frequencies.
All the outdoor parts on the dish are collectively called the ODU (Outdoor Unit), i.e., OMT to split signal between BUC and LNB. The IDU is effectively a modem, usually with ethernet port and 2 x F-connectors for the coax to BUC (Transmit) and from LNB (Receive).
The Astra2Connect has an all-in-one OMT/BUC/LNA that looks like a Quad LNB in shape and size which mounts on a regular TV satellite mount. As a consequence it is only 500 mW compared with the normal 2W, thus is poorer in rain. Skylogic’s Tooway system also uses an integrated OMT/BUC/LNB assembly called a transmit and receive integrated assembly (TRIA), which is 3W.
VSATs are also used for transportable, on-the-move (utilizing phased array antennas) or mobile maritime communications. A maritime VSAT has features that allow it to be operated on a ship at sea. A ship that is underway is in continuous motion in all axes. The antenna part of a marine VSAT system must be stabilized with respect to the horizon and true north as the ship moves beneath it. Motors and sensors are used to keep the antenna pointed accurately at the satellite. This enables it to transmit to and receive from the satellite while minimising losses and interference with adjacent satellites. New technology is emerging that will allow a solid-state device (flat panel) to steer an antenna electronically without moving parts.
Satellite Earth Station
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.
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.
Earth Station measurements and testing
Earth stations operating with GEO satellites require a fairly simple pointing and tracking system due to the fact that the system TT&C center maintains satellite orbit within a tight angular position. On the other hand, non-GEO satellites can require complex tracking systems in order to follow the satellites’ orbit. For GEO satellites, antenna pointing can be optimized by tracking a beacon signal transmitted by the satellite.
With a new satellite installation, antenna pointing can be manually accomplished using a spectrum analyzer connected to a monitor port along the received signal path. As the earth station antenna is moved across a small angular displacement, the measured signal level will increase and decrease as the antenna’s boresight direction moves across the direction of the satellite.
Because geosynchronous satellite orbital slots are specified at every 2° of longitude, earth station antennas must have high gain and low sidelobe levels. If the gains of the antenna sidelobes are too high, the earth station antenna will produce high levels of interference to the adjacent satellites, especially when those systems share the same frequency spectrum and antenna polarization. For example, a single-beam earth station reflector antenna having a diameter greater than 100 wavelengths should have a sidelobe level determined by the following equation 29 – 25 log θ.
One way to measure the relative sidelobe level of an earth station antenna is to transmit a signal from the desired antenna and record the received signal level at another earth station as the desired antenna is rotated over a narrow angular displacement. The displacement should cover the main beam and the peaks of the two adjacent sidelobe levels. The same technique can be applied to capture the receive antenna pattern by using a fixed transmit antenna and a rotated receive antenna. In general, the measurement of sidelobe level in the transmitter antenna is of most importance as this signal potentially creates interference to other satellite systems if the sidelobes are out of specification.
Determination of Earth Station Antenna G/T
There are two primary methods for obtaining an antenna’s G/T parameter, a direct measurement or calculating a mathematical approximation. To obtain an accurate measurement of antenna G/T a source of RF emissions is needed. Typical RF sources are either another antenna with known parameters or a celestial body such as the sun or a radio star. The use of celestial bodies, primarily the sun, is most common since these bodies are visible for extended periods of time at almost any location on Earth.
A problem arises however in geographical locations where the sun may not be visible for several weeks (i.e. extreme latitude locations). These locations are ideal for LEO tracking. This is not a problem for larger systems (> 10m) as there are several radio stars visible in each hemisphere (i.e. Cassiopeia A, Taurus A, Orion, Virgo, Omega) that have well defined flux density equations that can be used for an accurate measurement. This is not possible on smaller systems (≤ 5.0m) as the measured 𝑃𝑑 cannot be accurately determined with enough precision. This leaves the moon as the only other possible celestial source of RF emissions for smaller antenna systems.
Rx Gain and System Temperature Method
This test procedure requires the AUT to measure its Rx gain and system noise temperature, and then by subtracting the measured results obtain the G/T. Rx Gain Two direct methods of measuring the Rx gain can be used; integration of the Rx sidelobe pattern or by determination of the 3dB and 10dB beamwidths. The use of pattern integration will produce the more accurate results but would require the AUT to have a tracking system..
System Noise Temperature
The system noise temperature can be calculated from measurement of the difference in noise power when the input to the receive system is terminated in hot and cold loads. This is known as a Y-factor measurement, where Y represents the difference in the noise power. The test
configuration for measuring the system noise temperature is illustrated in Figure
In the above test configuration, a spectrum analyser could be used as a less accurate alternative to the noise gain analyser. In order to measure the system noise temperature the AUT is pointed to clear sky, and the antenna is then considered to be the cold load. An input waveguide load at ambient temepature provides the hot load. This waveguide load typically forms an integral part of the LNA redundancy switching system, and can be switched in out using the LNA controller.
The AUT switches the LNA input to the ambient load and measures the noise power exhibited due to the hot load. The AUT then switches the LNA input to the antenna and measures the noise power exhibited due to the cold load. The difference between the measurements is the Y-factor. Using the measured Y-factor, the temperature of the hot load (ambient) and the LNA temperature (manufacturer’s results) the system temperature can be calculated using the formula below.
The modem used in VSAT application is referred as VSAT modem. It is also known as satellite modem. There are different bands of VSAT modem available based on IF frequencies of use in its construction. Typically VSAT modems are available in 70+/-18 MHz or 140+/-18 MHz or L band (950 to 1450 MHz) range.
Following diagram depicts modules used in the design of L Band Satellite Modem:
• Baseband part which consists of FEC modules (Convolutional encoder & decoder), differential encoder & decoder, scrambler & descrambler etc.
• Modulator & demodulator which takes care of modulation and demodulation respectively.
• Modulator module takes information as input and produces modulated carrier as output at IF frequency.
• Demodulator module takes modulated carrier as input and produces information (voice/data) as output.
Synthesize is required in VSAT modem to select different RF carriers as required. Variable RF attenuators are used to provide different attenuation or gain values for testing as well as maintenance of VSAT network. Switches are used for various baseband loop back as well as IF loop back testing purpose in case of faulty conditions of VSAT modem. Usually VSAT modem is interfaced with Multiplexer/Demultiplexer using RS422/V.35 etc. interfaces.
VSAT modem testing
Following tests are conducted during maintenance and installation phase of VSAT modem:
• Baseband Loopback: It tests baseband part of the VSAT modem.
• IF loopback: It tests baseband as well as modulator-demodulator IC of this modem.
• RF loopback: It tests modem including RF part of the VSAT terminal.
• Bit Error Rate testing for short duration and long duration.
• Voice call flow and data flow through VSAT modem
• Functional tests under various environment tests (temperature, vibration etc.)
The bit error rate (BER) measures the performance of the demodulator by counting the number of bits in error, n, in a stream of N received bits: BER = n/N
The BER constitutes an estimate of the bit error probability (BEP). A level of confidence is associated with this estimate, as follows:
BEP = BER +/- ( k * sqrt(n) / N)
A 63% level of confidence is obtained for k= 1 and a 95% level of confidence for k = 2. For instance, if n = 100 errors are observed within a run of N = 10 exp(5) bits, the BEP is 10 exp(-3) +/- 10exp(-4) with a 63% confidence level.
VSAT modem traffic Interface testing
One of the VSAT modem also called EUT is Teledyne Paradise Datacom PD10L modem. The VSAT modems can be equipped with various traffic interfaces. Currently, Ethernet, i.e., 10/100 Base-T, is most commonly used. Due to the high popularity, most modems are also equipped with G.703 interface.
Traffic interfaces :
• standard option: Ethernet (IP traffic on RJ45),
• additional options: G.703 (balanced on EIA530 120Ω or unbalanced on BNC 75Ω female), RS422, X.21, V.35, and RS232 (on EIA530 connector, 25-pin D-type female),
• other optional interfaces: Serial LVDS (25-pin D-type female), Quad E1 G.703 (balanced on RJ45), HSSI (50-pin HD SCSI-2 connector), Eurocom (D/1,D,C,G).
This procedure is implemented in the following steps:
a) preparation of the digital analyzer of interfaces – turn on its power supply and leave it in this state for the time necessary to obtain proper stability, and then carry out its self-test;
b) set up the tested satellite terminal modem (EUT) according to its user manual;
c) check if EUT has a proper connector (port) for the analyzed traffic interface type or if EUT has been equipped with an appropriate interface converter; if YES then go to (d) point of the procedure; if NO then the verification of the analyzed interface in the tested modem is negative – in the test report, note that there is no appropriate interface connector and go to (k) point of the procedure;
d) connect EUT to the measuring instrument according to the scheme shown in Fig. 4, selecting the appropriate connector and the type of traffic interface;
e) turn on the EUT power supply and leave it in this state for the time necessary to obtain adequate stability;
f) configure the modem to work with a fixed data transmission speed (bit rate), B;
g) tune EUT to the required frequency f = 0.5 * ( fmax +fmin) where max f and min f are maximum and minimum intermediate frequencies of the modem, respectively.
h) set the output (Tx) and input (Rx) parameters for the traffic interface in the digital analyzer of interfaces;
i) measure BER for a selected transmission speed – start the test on the analyzer and then, after the measurement, note the BER result;
j) repeat steps from (f) to (i) for EUT tuning frequencies (g) point of the procedure by decreasing f1 and increasing f2 by 5%
k) repeat steps from (c) to (j) for all analyzed types of traffic interfaces.
Analysis of the obtained results of BER measurement consists in checking the condition
Subsystem test and measurement
Coaxial and waveguide components
Environmental conditions, such as rain and humidity, can affect other parts of the system especially the interconnecting transmission lines including many coaxial and waveguide components. Maintenance and troubleshooting of these transmission lines may occur more frequently and rely on measurement techniques called line sweeping and distance to-fault.
Line sweeping is a measurement of the frequency response of a long transmission line, such as a coaxial cable or waveguide, connecting a transmitter to its antenna or between an antenna and its receiver. The measurement reports the signal attenuation and return loss of the complete transmission path. DTF is a mathematical transform of the measured frequency response into the time domain.
Filters are used quite extensively in all communication systems. In earth station applications, they are typically integrated into the high-performance upconverters and downconverters. Filters are used primarily for their out-of-band rejection in both uplink and downlink paths. Filter performance is measured using a VNA that displays the scattering parameters (S-parameters) of the two-port device. The measured S-parameters are a function of frequency and are related to specifications of insertion loss, return loss, group delay, and out-of-band rejection. The bandwidth of the filter and its ripple response are measured quantities that are typically determined by using the marker functions on the VNA.
Another important characteristic of a filter is its transmission phase, and associated group delay response. In communications systems, it is important to have a linear phase response across the pass band to avoid distortion in the desired signal. Another way to specify a linear phase response is to have a flat group delay response in the pass band.
Frequency Converter Measurements
Frequency converters provide translation between a modulated signal’s intermediate frequency (IF) and the uplink, or downlink, RF frequency of the system. Block downconverter (BDC) translates a large block of frequencies captured from the satellite downlink to a lower frequency for additional signal processing and demodulation. BDCs typically use a single internal local oscillator (LO) for frequency translation. The IF from commercial BDCs is typically in the L-band frequency range, 950 MHz to 1450 or 2150 MHz.
Other types of downconverters are designed with an IF in the VHF range, typically 70 MHz or 140 MHz. These types of “frequency synthesized” downconverters will have two LOs (double-conversion) and can be used to tune to a specific communications channel. Channel bandwidths for these downconverters are typically 40 MHz or 80 MHz wide, providing a high level of dynamic range and adjacent channel rejection.
Ideally, a downconverter only changes the center frequency of the signal and does not alter or distort the signal in any way. Unfortunately, the RF/IF components along the converter path create some level of distortion to the signal and it is up to the design engineer to select components that minimize these effects. Several types of test equipment are required to fully characterize the performance of a frequency converter. For example, intermodulation distortion (IMD), harmonic and spurious levels can be measured using a spectrum analyzer whereas return loss is typically measured using a vector network analyzer for its speed and accuracy.
Other important specifications for any earth station frequency converter include conversion gain, gain flatness and gain stability over temperature. The measurement of these parameters requires test equipment capable of sweeping a signal generator across a range of input frequencies and measuring the amplitude response across the associated range of output frequencies.
Military testing of VSAT terminals
Purchase of communication equipment by armed forces from a civilian market often requires special procedures, tests, or adaptations of this equipment to specific conditions for military operations. Such equipment is usually made to order or supplied by leading global companies. Nevertheless, national regulations regarding introducing the equipment to the army require, e.g., tests confirming its relevant parameters and functionalities.
Whereas, the “IEC 60835-3” subseries, i.e. , provide methodologies for measuring typical parameters of terrestrial satellite terminals, including: parameters and characteristics of antennas, low noise and high power amplifiers, up- and down-converters, etc. One of these standards focuses on a very small aperture terminal (VSAT).