lecture 3: satellite communications 3_EEC 284 Lecture 3_2.pdf
- 1. Overview of Satellite Systems Part 3
- 2. Polar Orbiting Satellites, Argos system • The Argos data collection system (DCS) collects environmental data radioed up from platform transmitter terminals (PTT). • The transmitters can be installed on many kinds of platforms, including fixed and drifting buoys, balloons, and animals. • The physical size of the transmitters depends on the application. • The PTTs transmit automatically at preset intervals. PTTs within the 6000 km swath are received by at least two satellites. • A PTT located at the polar regions would deliver approximately 28 messages daily. • At the equator the situation is different. The equatorial radius of the earth is approximately 6378 km, which gives a circumference of about 40,074 km. • Relative to the orbital footprint, a given longitude at the equator will therefore rotate with the earth a distance of 40074 × 102/1440 = 2839 km. • This assumes a stationary orbital path, but as mentioned previously the orbit is sun synchronous, which means that it rotates eastward almost 1 ° per day, that is in the same direction as the earth’s rotation.
- 3. Polar Orbiting Satellites, Argos system • The overall result is that an equatorial PTT starting at the western edge of the footprint swath will “see” between three and four passes per day for one satellite. • Hence the equatorial passes number between six and seven per day for two satellites. • During any one pass the PTT is in contact with the satellite for 10 min on average. The messages received at the satellite are retransmitted in “real time” to one of a number of regional ground receiving stations whenever the satellite is within range. • The messages are also stored aboard the satellites on tape recorders and are “dumped” to one of three main ground receiving stations. These are located at Wallops Island, VA, USA, Fairbanks, Alaska, USA, and Lannion, France. • The Doppler shift in the frequency received at the satellite is used to determine the location of the PTT. This is discussed further in connection with the Cospas-Sarsat search and rescue satellites.
- 4. Polar Orbiting Satellites, Cospas-Sarsat system • There are (as of November 2004) 37 countries and organizations associated with the program. Canada, France, Russia and the USA provide and operate the satellites and ground- segment equipment, and other countries provide ground-segment support. • The system has now been developed to the stage where both low earth orbiting (LEO) satellites and geostationary earth orbiting (GEO) satellites are used, as shown in Fig. 1.8. • The basic system requires users to carry distress radio beacons, which transmit a carrier signal when activated. • A number of different beacons are available: emergency locator transmitter (ELT) for aviation use; emergency position indicating radio beacon (EPIRB) for maritime use; and personal locator beacon (PLB) for personal use.
- 5. Polar Orbiting Satellites, Cospas-Sarsat system • The beacons can be activated manually or automatically (e.g., by a crash sensor). • The transmitted signal is picked up by a LEO satellite, and because this satellite is moving relative to the radio beacon, a Doppler shift in frequency is observed. • In effect, if the line of sight distance between transmitter and satellite is shortened as a result of the relative motion, the wavelength of the emitted signal is also shortened. This in turn means the received frequency is increased. • If the line of sight distance is lengthened as a result of the relative motion the wavelength is lengthened and therefore the received frequency decreased. • It should be kept in mind that the radio-beacon emits a constant frequency, and the electromagnetic wave travels at constant velocity, that of light.
- 6. Polar Orbiting Satellites, Cospas-Sarsat system • Denoting the constant emitted frequency by 𝑓0 , the relative velocity between satellite and beacon, measured along the line of sight as 𝑣, and the velocity of light as 𝑐, then to a close approximation the received frequency is given by: 𝑓 = 1 + 𝑣 𝑐 𝑓0 • The relative velocity 𝑣 is a function of the satellite motion and of the earth’s rotation. • The frequency difference resulting from the relative motion is Δ𝑓 = 𝑓 − 𝑓0 = 𝑣 × 𝑓0 /𝑐 • The fractional change is Δ𝑓 𝑓0 = 𝑣 𝑐 • When v is zero, the received frequency is the same as the transmitted frequency. • When the beacon and satellite are approaching each other, 𝑣 is positive, which results in a positive value of Δ𝑓. • When the beacon and satellite are receding, 𝑣 is negative, resulting in a negative value of Δ 𝑓. • The time at which Δ 𝑓 = 0 is known as the time of closest approach.
- 7. Polar Orbiting Satellites, Cospas-Sarsat system • Figure 1.9 shows how the beacon frequency, as received at the satellite, varies for different passes. • In all cases, the received frequency goes from being higher to being lower than the transmitted value as the satellite approaches and then recedes from the beacon. • The longest record and the greatest change in frequency are obtained if the satellite passes over the site, as shown for pass no. 2. • This is so because the satellite is visible for the longest period during this pass.
- 8. Polar Orbiting Satellites, Cospas-Sarsat system • Knowing the orbital parameters for the satellite, the beacon frequency, and the Doppler shift for any one pass, the distance of the beacon relative to the projection of the orbit on the earth can be determined. • However, whether the beacon is east or west of the orbit cannot be determined easily from a single pass. • For two successive passes, the effect of the earth’s rotation on the Doppler shift can be estimated more accurately, and from this it can be determined whether the orbital path is moving closer to, or moving away from the beacon. • In this way, the ambiguity in east-west positioning is resolved.
- 9. Polar Orbiting Satellites, Cospas-Sarsat system • The satellite must of course get the information back to an earth station so that the search and rescue operation can be completed, successfully one hopes. • The SARSAT communicates on a downlink frequency of 1544.5 MHz to one of several local user terminals (LUTs) established at various locations throughout the world. • In the original Cospas-Sarsat system, the signal from the emergency radio beacons was at a frequency of 121.5 MHz. • It was found that over 98 percent of the alerts at this frequency were false, often being caused by interfering signals from other services and by inappropriate handling of the equipment. • The 121.5-MHz system relies entirely on the Dopplershift, and the carrier does not carry any identification information. • The power is low, typically a few tenths of a watt, which limits locational accuracy to about 10 to 20 km. • There are no signal storage facilities aboard the satellites for the 121.5-MHz signals, which therefore requires that the distress site (the distress beacon) and the LUT must be visible simultaneously from the satellite.
- 10. Polar Orbiting Satellites, Cospas-Sarsat system • Because of these limitations, the 121.5-MHz beacons are terminated on February 1, 2009. Newer beacons operating at a frequency of 406 MHz are being introduced. • The power has been increased to 5 W, which should permit locational accuracy to 3 to 5 km. • The 406-MHz carrier is modulated with information such as an identifying code, the last known position, and the nature of the emergency. • The satellite has the equipment for storing and forwarding the information from a continuous memory dump, providing complete worldwide coverage with 100 percent availability. The polar orbiters, however, do not provide continuous coverage. • The mean time between a distress alert being sent and the appropriate search and rescue coordination center being notified is estimated at 27 min satellite storage time plus 44 min waiting time for a total delay of 71 min.
- 11. Polar Orbiting Satellites, Cospas-Sarsat system • The status of the 121.5-MHz LEOSAR system as of November 2004 consisted of repeaters on five polar orbiters, 43 ground receiving stations (referred to as LEOSAR local user terminals, or LEOLUTs), 26 mission control centers (MCCs), and about 680,000 beacons operating at 121.5 MHz, carried mostly on aircraft and small vessels. • The MCC alerts the rescue coordination center (RCC) nearest the location where the distress signal originated, and the RCC takes the appropriate action to effect a rescue. • The status of the GEOSAR segment of the Cospas-Sarsat system is shown in Table 1.10.
- 12. Polar Orbiting Satellites, Cospas-Sarsat system • Since the geostationary satellites are by definition stationary with respect to the earth, there is no Doppler shift of the received beacon carrier. • The 406-MHz beacons for the GEOSAR component carry positional information obtained from the global navigational satellite systems such as the American GPS (see Sec. 17.5) system, the Russian global navigation satellite system (GLONASS) and Galileo (European). • These navigational systems employ medium earth orbiting (MEO) satellites, and the space agencies responsible for these navigational systems have plans to include 406-MHz repeaters on the MEO satellites. • Although the GEOSAR system provides wide area coverage it does not cover the polar regions, the antenna “footprint” being limited to latitudes of about 75 ° N and S. The coverage areas are shown in Fig. 1.10.
- 13. Sheet 1 P.n. 1/2 • Describe briefly the main advantages offered by satellite communications. Explain what is meant by a distance-insensitive communications system. • Comparisons are sometimes made between satellite and optical fiber communications systems. State briefly the areas of application for which you feel each system is best suited. • From Table 1.3, and by accessing the Intelsat web site, determine which satellites provide service to each of the regions AOR, IOR, and POR. • Referring to Table 1.4, determine the power levels, in watts, for each of the three categories listed. • From Table 1.5, determine typical orbital spacing in degrees for (a) the 6/4- GHz band and (b) the 14/12-GHz band. • Give reasons why the Ku band is used for the DBS service. • An earth station is situated at longitude 91 ° W and latitude 45 ° N. Determine the range to the Galaxy VII satellite. A spherical earth of uniform mass and mean radius 6371 km may be assumed.
- 14. • Given that the earth’s equatorial radius is 6378 km and the height of the geostationary orbit is 36,000 km, determine the intersatellite distance between the VisionStar Inc. satellite and the NetSat 28 Company L.L.C. satellite, operating in the Ka band. • Explain what is meant by a polar orbiting satellite. A NOAA polar orbiting satellite completes one revolution around the earth in 102 min. The satellite makes a north to south equatorial crossing at longitude 90 ° W. Assuming that the orbit is circular and crosses exactly over the poles, estimate the position of the subsatellite point at the following times after the equatorial crossing: (a) 0 h, 10 min; (b) 1 h, 42 min; (c) 2 h, 0 min. A spherical earth of uniform mass may be assumed. • Intelsat satellite 904 is situated at 60 ° E. Determine the land areas (markets)the satellite can service. The global EIRP is given as 31.0 up to 35.9 dBW, beam edge to beam peak. What are the equivalent values in watts? • A satellite is in a circular polar orbit at a height of 870 km, the orbital period being approximately 102 min. Assuming an average value of earth’s radius of 6371 km determine approximately the maximum period the satellite is visible from a beacon at sea level. • A satellite is in a circular polar orbit at a height of 870 km, the orbital period being approximately 102 min. The satellite orbit passes directly over a beacon at sea level. Assuming an average value of earth’s radius of 6371 km determine approximately the fractional Doppler shift at the instant the satellite is first visible from the beacon. Sheet 1 P.n. 2/2