QUESTION BANK SOLUTION UNIT 1 Overview of Satellite Systems 1. Explain briefly various services provided by a satellite [ 06, May/June 2010] Large areas of the earth are visible from the satellite, thus the satellite can form the star point of a communications net linking together many users simultaneously, users who may be widely separated geographically. Provide communications links to remote communities. Remote sensing detection of pollution, weather conditions, search and rescue operations. 2. What are the frequency bands allocated to various satellite services? [ 04, Dec2010] SJBIT/Dept of ECE Page 1
UNIT 2 Orbits 1. State Keplers laws of elementary motion, with the help of a neat diagram and give necessary equations. [ 08, May/June 2010 and Dec 2010] Kepler s first law: It states that the path followed by a satellite around the primary will be an ellipse. An ellipse has two focal points shown as F1 and F2 The center of mass of the two-body system, termed the barycenter, is always centered on one of the foci. In our specific case, because of the enormous difference between the masses of the earth and the satellite, the center of mass coincides with the center of the earth, which is therefore always at one of the foci. The semimajor axis of the ellipse is denoted by a, and the semiminor axis, by b. The eccentricity e is given by For an elliptical orbit, 0 < e < 1. When e = 0, the orbit becomes circular. SJBIT/Dept of ECE Page 2
Kepler`s Second Law: Kepler s second law states that, for equal time intervals, a satellite will sweep out equal areas in its orbital plane, focused at the barycenter. Thus the farther the satellite from earth, the longer it takes to travel a given distance Kepler s Third Law: It states that the square of the periodic time of orbit is proportional to the cube of the mean distance between the two bodies. The mean distance is equal to the semimajor axis a. For the artificial satellites orbiting the earth, Kepler s third law can be written in the form a = semimajor axis (meters) n = mean motion of the satellite (radians per second) Q = earth s geocentric gravitational constant. = 3.986005 v 1014 m3/sec2 This equation applies only to ideal situation satellite rbiting a perfectly spherical earth of uniform mass, with no pertubing forces acting, such as atmospheric drag. 2. Define keplarian elemental set. [ 06, May/June 2010] Earth orbiting artificial satellites are defined by 6 orbital elements called keplerian element set: Semi major axis(a), Eccentricity (e), The mean anomaly(mo), The argument of perigee (w), Inclination(i), Right ascension of ascending node(ω). SJBIT/Dept of ECE Page 3
3. Define the terms (a) Prograde orbit (b) Apogee (c) Argument of perigee )d) Ascending node. [08, Dec2010] Prograde orbit: An orbit in which the satellite moves in the same direction as the earth s rotation. Also known as a direct orbit. The inclination of a prograde orbit always lies between 0 and 90. Apogee: The point farthest from earth. Argument of perigee :The angle from ascending node to perigee, measured in the orbital plane at the earth s center, in the direction of satellite motion. Ascending node: The point where the orbit crosses the equatorial plane going from south to north. SJBIT/Dept of ECE Page 4
UNIT 3 Propagation impairments and space link 1. Explain atmospheric losses and ionospheric losses for satellites. [08, Dec 2010] Atmospheric Losses Losses occur in the earth s atmosphere as a result of energy absorption by the atmospheric gases. These losses are treated quite separately from those which result from adverse weather conditions, which of course are also atmospheric losses. To distinguish between these, the weather-related losses are referred to as atmospheric attenuation and the absorption losses simply as atmospheric absorption. Ionospheric Effects Radio waves traveling between satellites and earth stations must pass through the ionosphere. The ionosphere has been ionized, mainly by solar radiation. The free electrons in the ionosphere are not uniformly distributed but form in layers. Clouds of electrons may travel through the ionosphere and give rise to fluctuations in the signal. The effects include scintillation, absorption, variation in the direction of arrival, propagation delay, dispersion, frequency change, and polarization rotation. Ionospheric scintillations: Are variations in the amplitude, phase, polarization, or angle of arrival of radio waves. Caused by irregularities in the ionosphere which changes with time. Effect of scintillations is fading of the signal. Severe fades may last up to several minutes. Polarization rotation: porduce rotation of the polarization of a signal (Faraday rotation) When linearly polarized wave traverses in the ionosphere, free electrons in the ionosphere are sets in motion a force is experienced, which shift the polarization of the wave. Inversely proportional to frequency squared. Not a problem for frequencies above 10 GHz. SJBIT/Dept of ECE Page 5
2. Explain combined uplink and downlink C/N ratio. [07, Dec 2010] The complete satellite circuit consists of an uplink and a downlink, as sketched in Fig (a) combined uplink and downlink (b) power flow diagram Noise will be introduced on the uplink at the satellite receiver input. PNU = noise power per unit bandwidth PRU = average carrier at the same point The carrier-to-noise ratio on the uplink is Note that power levels, and not decibels, are being used. PR = carrier power at the end of the space link = the received carrier power for the downlink. = K x the carrier power input at the satellite Where SJBIT/Dept of ECE Page 6
K = the system power gain from satellite input to earth station input. This includes the satellite transponder and transmit antenna gains, the downlink losses, and the earth station receive antenna gain and feeder losses. The noise at the satellite input also appears at the earth station input multiplied by K, and in addition, the earth station introduces its own noise, denoted by PND. Thus the end-oflink noise is KPNU + PND. The C/No ratio for the downlink alone, not counting the KPNU contribution, is PR/PND, and the combined C/No ratio at the ground receiver is PR/(KPNU + PND). The power flow diagram is shown in Fig. The combined carrier-to-noise ratio can be determined in terms of the individual link values. To show this, it is more convenient to work with the noise-to-carrier ratios rather than the carrier-to-noise ratios, and these must be expressed as power ratios, not decibels. Denoting the combined noise-to-carrier ratio value by No/C, the uplink value by (No/C)U, and the downlink value by (No/C)D then, --(4) Equation (4) shows that to obtain the combined value of C/N0, the reciprocals of the individual values must be added to obtain the N0/C ratio and then the reciprocal of this taken to get C/N0. The reason for this reciprocal of the sum of the reciprocals method is that a single signal power is being transferred through the system, while the various noise powers which are present are additive. Similar reasoning applies to the carrier-to-noise ratio, C/ N. SJBIT/Dept of ECE Page 7
3. Explain the different transmission losses in a satellite link [07, May/June 2010] The [EIRP] is the power input to one end of the transmission link, and the problem is to find the power received at the other end. Losses will occur along the way, some of which are constant. Other losses can only be estimated from statistical data, and some of these are dependent on weather conditions, especially on rainfall. The first step in the calculations is to determine the losses for clear weather, or clear-sky, conditions. These calculations take into account the losses, including those calculated on a statistical basis, which do not vary significantly with time. Losses which are weatherrelated, and other losses which fluctuate with time, are then allowed for by introducing appropriate fade margins into the transmission equation. 4. Define saturation flux density. Obtain the equation for saturation EIRP for uplink [06, May/June 2010] A key parameter in link budget calculations is the equivalent isotropic radiated power, conventionally denoted as EIRP. The Maximum power flux density at some distance r from a transmitting antenna of gain G is An isotropic radiator with an input power equal to GPS would produce the same flux density. Hence this product is referred to as the equivalent isotropic radiated power, or EIRP is often expressed in decibels relative to one watt, or dbw. Let PS be in watts; then where [PS] is also in dbw and [G] is in db. SJBIT/Dept of ECE Page 8
UNIT 4 Space Segment 1. Explain (a) the power supply subsystem (b) thermal control subsystem [08, Dec 2010] The Power Supply The primary electrical power for operating the electronic equipment is obtained from solar cells. Individual cells can generate only small amounts of power, and therefore, arrays of cells in series-parallel connection are required. For the HS376 satellite manufactured by Hughes Space and Communications Company. The spacecraft is 216 cm in diameter and 660 cm long when fully deployed in orbit. During the launch sequence, the outer cylinder is telescoped over the inner one, to reduce the overall length. Only the outer panel generates electrical power during this phase. In geostationary orbit the telescoped panel is fully extended so that both are exposed to sunlight. At the beginning of life, the panels produce 940 W dc power, which may drop to 760 W at the end of 10 years. During eclipse, power is provided by two nickelcadmium long-life batteries, which will deliver 830 W. At the end of life, battery recharge time is less than 16 h. Thermal Control Satellites are subject to large thermal gradients, receiving the sun s radiation on one side while the other side faces into space. In addition, thermal radiation from the earth and the earth s albedo, which is the fraction of the radiation falling on earth which is reflected, can be significant for low-altitude earth-orbiting satellites, although it is negligible for geostationary satellites. Equipment in the satellite also generates heat which has to be removed. The most important consideration is that the satellite s equipment should operate as nearly as possible in a stable temperature environment. often used to remove heat from the communications payload In order to maintain constant temperature conditions, heaters may be switched on (usually on command from ground) to make up for the heat reduction which occurs when transponders are switched off. In INTELSAT VI, heaters are used to maintain propulsion thrusters and line temperatures (Pilcher, 1982). 2. With the help of a neat diagram explain the operation of a power amplifier of a transponder along with its transfer characteristics. [12, Dec 2010] A transponder is the series of interconnected units which forms a single communications channel between the receive and transmit antennas in a communications satellite. Some of the units utilized by a transponder in a given channel may be common to a number of transponders. Thus, although reference may be made to a specific transponder, this must be thought of as an equipment channel rather than a single item of equipment. SJBIT/Dept of ECE Page 9
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3. With a neat diagram explain satellite altitude. Explain 3 axis methods of satellite stabilization. [04, May/June 2010] Altitude refers to orientation in space. Spinning satellite stabilization. Momentum wheel stabilization SJBIT/Dept of ECE Page 11
UNIT 5 and 6 Earth Segment 1. Explain the receive only home TV system. [13, Dec 2010] Planned broadcasting directly to home TV receivers takes place in the Ku (12-GHz) band. This service is known as direct broadcast satellite (DBS) service. There is some variation in the frequency bands assigned to different geographic regions. The comparatively large satellite receiving dishes (about 3-m diameter) which are a familiar sight around many homes are used to receive downlink TV signals at C band (4 GHz). Such downlink signals were never intended for home reception but for network relay to commercial TV outlets (VHF and UHF TV broadcast stations and cable TV head end studios). Although the practice of intercepting these signals seems to be well established at present, various technical and commercial and legal factors are combining to deter their direct reception. The major differences between the Ku-band and the C-band receive-only systems lies in the frequency of operation of the outdoor unit and the fact that satellites intended for DBS have much higher EIRP. SJBIT/Dept of ECE Page 12
2. Explain a MATV system, with a neat diagram [07, Dec 2010] A master antenna TV (MATV) system is used to provide reception of DBS TV/FM channels to a small group of users, for example, to the tenants in an apartment building. It consists of a single outdoor unit (antenna and LNA/C) feeding a number of indoor units. It is basically similar to the home system already described, but with each user having access to all the channels independently of the other users. The advantage is that only one outdoor unit is required, but as shown, separate LNA/Cs and feeder cables are required for each sense of polarization. Compared with the single-user system, a larger antenna is also required (2- to 3-m diameter) in order to maintain a good signal-to-noise ratio at all the indoor units. Where more than a few subscribers are involved, the distribution system used is similar to the CATV system described in the next section. 3. With a neat block diagram explain the outdoor and indoor unit for a analog FM TV [12 May/June 2010] The outdoor unit This consists of a low noise amplifer/converter combination. A parabolic reflector is used along with horn mounted at focus. The downlink frequency band of 12.2 to 12.7 GHz spans a range of 500 MHz, which accommodates 32 TV/FM channels, each of which is 24 MHz wide. Obviously, some overlap occurs between channels, but these are alternately polarized left-hand circular (LHC) and right- hand circular (RHC) or vertical/horizontal, to reduce interference to acceptable levels. This is referred to as polarization interleaving. A polarizer that may be switched to the desired polarization from the indoor control unit is required at the receiving horn. The receiving horn feeds into a low-noise converter (LNC) or possibly a combination unit consisting of a low-noise amplifier (LNA) followed by a converter. The combination is referred to as an LNB, for low-noise block. The LNB provides gain for the broadband 12-GHz signal and then converts the signal to a lower frequency range so that a low-cost coaxial cable can be used as feeder to the indoor unit. The standard frequency range of this downconverted signal is 950 to 1450 MHz. The coaxial cable, or an auxiliary wire pair, is used to carry dc power to the outdoor unit. Polarizationswitching control wires are also required. The low-noise amplification must be provided at the cable input in order to maintain a satisfactory signal-to-noise ratio. A low-noise amplifier at the indoor end of the cable would be of little use, because it would also amplify the cable thermal noise. Of course, having to mount the LNB outside means that it must be able to operate over a wide range of climatic conditions, and homeowners may have to contend with the added problems of vandalism and theft. SJBIT/Dept of ECE Page 13
The indoor unit for analog (FM) TV The signal fed to the indoor unit is normally a wideband signal covering the range 950 to 1450 MHz. This is amplified and passed to a tracking filter which selects the desired channel. As previously mentioned, polarization interleaving is used, and only half the 32 channels will be present at the input of the indoor unit for any one setting of the antenna polarizer. This eases the job of the tracking filter, since alternate channels are well separated in frequency. The selected channel is again down converted, this time from the 950- to 1450-MHz range to a fixed intermediate frequency, usually 70 MHz although other values in the VHF range are also used. The 70-MHz amplifier amplifies the signal up to the levels required for demodulation. A major difference between DBS TV and conventional TV is that with DBS, frequency modulation is used, whereas with conventional TV, amplitude modulation in the form of vestigial single sideband (VSSB) is used. The 70-MHz, frequency-modulated IF carrier therefore must be demodulated, and the baseband information used to generate a VSSB signal which is fed into one of the VHF/UHF channels of a standard TV set. 4. Explain spade system with a neat diagram [08, May/June 2010] [07, Dec 2010] The word Spade is a loose acronym for single-channel-per-carrier pulse-code-modulated multiple-access demand-assignment equipment. Spade was developed by Comsat for use on the INTELSAT satellites (see, e.g.,martin, 1978). However, the distributed-demand assignment facility requires a common signaling channel (CSC). The CSC bandwidth is 160 khz, and its center frequency is 18.045 MHz below the pilot frequency. To avoid interference with the CSC, voice channels 1 and 2 are left vacant, and to maintain duplex matching, the corresponding channels 1 and 2 are also left vacant. Recalling from Fig. 14.5 that channel 400 also must be left vacant, this requires that channel 800 be left vacant for duplex matching. Thus six channels are removed from the total of 800, leaving a total of 794 one-way or 397 full-duplex voice circuits, the frequencies in any pair being separated by 18.045 MHz.(An alternative arrangement is shown in Freeman, 1981). All the earth stations are permanently connected through the common signaling channel (CSC). This is shown diagrammatically in Fig. for six earth stations A, B, C, D, E, and F. Each earth station has the facility for generating any one of the 794 carrier frequencies using frequency synthesizers. Furthermore, each earth station has a memory containing a list of the frequencies currently available, and this list is continuously updated through the CSC. To illustrate the procedure, suppose that a call to station F is initiated from station C in Fig. Station C will first select a frequency pair at random from those currently available on the list and signal this information to station F through the CSC. Station F must acknowledge, through the CSC, that it can complete the circuit. Once the circuit is established, the other earth stations are instructed, through the CSC, to remove this frequency pair from the list. cities chosen at station C may be assigned to another circuit. In this event, station C will receive the information on the CSC update and will immediately choose another pair at random, even before hearing back from station F. Once a call has been completed and the circuit disconnected, the two frequencies are SJBIT/Dept of ECE Page 14
returned to the pool, the information again being transmitted through the CSC to all the earth stations. As well as establishing the connection through the satellite, the CSC passes signaling information from the calling station to the destination station, in the example above from station C to station F. Signaling information in the Spade system is routed through the CSC rather than being sent over a voice channel. Each earth station has equipment called the demand assignment signaling and switching (DASS) unit which performs the functions required by the CSC. Some type of multiple access to the CSC must be provided for all the earth stations using the Spade system. This is quite separate from the SCPC multiple access of the network s voice circuits. Time division multiple access, described in Sec. 14.7.8, is used for this purpose, allowing up to 49 earth stations to access the common signaling channel. 5. Explain possible interference nodes between satellite circuits and a terrestrial station. [07, Dec 2010] Interference may be considered as a form of noise, and as with noise, system performance is determined by the ratio of wanted to interfering powers, in this case the wanted carrier to the interfering carrier power or C/I ratio. The single most important factor controlling interference is the radiation pattern of the earth station antenna. Comparatively large-diameter reflectors can be used with earth station antennas, and hence narrow beamwidths can be achieved. For example, a 10-m antenna at 14 GHz has a 3-dB beamwidth of about 0.15. This is very much narrower than the 2 to 4 orbital spacing allocated to satellites. To relate the C/I ratio to the antenna radiation pattern, it is necessary first to define the geometry involved. The orbital separation is defined as the angle subtended at the center of the earth, known as the geocentric angle. However, from an earth station at point P the satellites would appear to subtend an angle ß. Angle ß is referred to as the topocentric angle. In all practical situations relating to satellite interference, the topocentric and geocentric angles may be assumed equal, and in fact, making this assumption leads to an overestimate of the interference (Sharp, 1983). 6. With a neat block diagram explain frame and burst formats for a TDMA system [07, May/June 2010] With time-division multiple access, only one carrier uses the transponder at any one time, and therefore, inter modulation products, which result from the nonlinear amplification of multiple carriers, are absent. This leads to one of the most significant advantages of TDMA, which is that the transponder traveling-wave tube (TWT) can be operated at maximum power output or saturation level. Because the signal information is transmitted in bursts, TDMA is only suited to digital signals. Digital data can be assembled into burst format for transmission and reassembled from the received bursts through the use of digital buffer memories. Figure 1 illustrates the basic TDMA concept, in which the SJBIT/Dept of ECE Page 15
stations transmit bursts in sequence. Burst synchronization is required, and in the system illustrated in Fig. 1, one station is assigned solely for the purpose of transmitting reference bursts to which the others can be synchronized. The time interval from the start of one reference burst to the next is termed a frame. A frame contains the reference burst R and the bursts from the other earth stations, these being shown as A, B, and C in Fig. 1. Figure 2 illustrates the basic principles of burst transmission for a single channel. Overall, the transmission appears continuous because the input and output bit rates are continuous and equal. However, within the transmission channel, input bits are temporarily stored and transmitted in bursts. SJBIT/Dept of ECE Page 16
Figure 3 shows some of the basic units in a TDMA ground station, which for discussion purposes is labeled earth station A. Terrestrial links coming into earth station A carry digital traffic addressed to destination stations, labeled B, C, X. It is assumed that the bit rate is the same for the digital traffic on each terrestrial link. In the units labeled terrestrial interface modules (TIMs), the incoming continuous-bit-rate signals are converted into the intermittent-burst-rate mode. These individual burst-mode signals are time-division multiplexed in the time- division multiplexer (MUX) so that the traffic for each destination station appears in its assigned time slot within a burst. Certain time slots at the beginning of each burst are used to carry timing and synchronizing information. These time slots collectively are referred to as the preamble. The complete burst containing the preamble and the traffic data is used to phase modulate the radiofrequency (rf) carrier. Thus the composite burst which is transmitted at rf consists of a number of time slots, as shown in Fig. 4. These will be described in more detail shortly. The received signal at an earth station consists of bursts from all transmitting stations arranged in the frame format shown in Fig. 4. The rf carrier is converted to intermediate frequency (IF), which is then demodulated. A separate preamble detector provides timing information for transmitter and receiver along with a carrier synchronizing signal for the phase demodulator, as described in the next section. In many systems, a station receives its own transmission along with the others in the frame, which can then be used for burst-timing purposes. SJBIT/Dept of ECE Page 17
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UNIT 7 and 8 DBS, Satellite mobile and specialized services 1. Explain the following (a) Transponder capacity (b) Frequencies and polarization [08, Dec 2010] Transponder Capacity The 24-MHz bandwidth of a transponder is capable of carrying one analog television channel. To be commercially viable, direct broad- cast satellite (DBS) television [also known as direct-to-home (DTH) television] requires many more channels, and this requires a move from analog to digital television. Digitizing the audio and video components of a television program allows signal compression to be applied, which greatly reduces the bandwidth required. The signal compression used in DBS is a highly complex process, and only a brief overview will be given here of the process. Before doing this, an estimate of the bit rate that can be carried in a 24-MHz transponder will be made. Frequencies and Polarization The requencies for DBS varies from region to region throughout the world. The available bandwidth (uplink and downlink) is seen to be 500 MHz. A total number of 32 transponder channels, each of bandwidth 24 MHz, can be accommodated. The bandwidth is sometimes specified as 27 MHz, but this includes a 3-MHz guard band allowance. Therefore, when calculating bit-rate capacity, the 24 MHz value is used. The total of 32 transponders requires the use of both right-hand circular polarization (RHCP) and left-hand circular frequency plan for Region 2. SJBIT/Dept of ECE Page 20
2. Explain home receiver indoor unit[12, Dec 2010] The block schematic for the indoor unit (IDU) is shown in Fig1. The transponder frequency bands shown in Fig2 are down converted to be in the range 950 to 1450 MHz, but of course, each transponder retains its 24-MHz bandwidth. The IDU must be able to receive any of the 32 transponders, although only 16 of these will be available for a single polarization. The tuner selects the desired transponder. It should be recalled that the carrier at the center frequency of the transponder is QPSK modulated by the bit stream, which itself may consist of four to eight TV programs time-division multiplexed. Following the tuner, the carrier is demodulated, the QPSK modulation being converted to a bit stream. Error correction is carried out in the decoder block labeled FEC 1. The demultiplexer following the FEC 1 block separates out the individual programs, which are then stored in buffer memories for further processing (not shown in the diagram). This further processing would include such things as conditional access, viewing history of pay per-view (PPV) usage, and connection through a modem to the service provider (for PPV billing purposes). A detailed description of the IRD will be found in Mead (2000). SJBIT/Dept of ECE Page 21