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Satellite link project: Moon communication constellation Abstract: The aim of the project is to design a satellite constellation around the Moon, in order to collect data from research engines and to transmit mobile communication to Cape Canaveral Earth station. The communication on the Moon must be reliable 99% of the time when the moon is seen from the Earth station. During that time, it has to provide a link to the Earth of 20 Mbit/s. Moreover the system should be flexible in order to be used by other mission not again planned. Chalmers University of Technology 1/15 Frederic Noel Satellite link project: Moon communication constellation 1 SUMMARY 1 SUMMARY 2 2 INTRODUCTION 3 3 SATELLITES’ ORBITS 3 3.1 3.2 3.3 3.4 3.5 3.5.1 3.5.2 3.6 4 PROBLEMATIC THE NUMBER OF SATELLITES ORBITAL RADIUS LIMIT SATELLITES’ ALTITUDE POSITIONS OF THE SATELLITES ON THEIR ORBITS First criterion: How to communicate with the Earth Second criterion: How to communicate with the other satellites VISIBILITY DURATION FROM CAPE CANAVERAL THE COMMUNICATION SATELLITE-EARTH 4.1 REQUIREMENT ON SNR 4.1.1 Modulation 4.1.2 Signal to Noise energy ratio 4.1.3 Bandwidth 4.1.4 Signal to Noise power ratio 4.2 COMPUTATION OF THE LINK BUDGETS 4.2.1 The polarisation, frequencies and rate 4.2.2 The power transmitted 4.2.3 Channel Atmospheric attenuation Space loss 4.2.4 5 Telecommunication devices 6 7 9 9 10 10 LINK BUDGETS 11 FREQUENCIES, MODULATION AND BANDWIDTH PATH LOSS THE DOPPLER SHIFT DEPOINTING LOSSES ANTENNA LINK BUDGET SATELLITE-MOON COMMUNICATION 6.1 6.2 6.3 6.4 6.5 12 12 13 13 13 13 13 14 FREQUENCIES: BANDWIDTH: MULTIPLE ACCESS: ANTENNAS LINK BUDGET 14 14 14 14 15 CONCLUSION Chalmers University of Technology 7 7 7 7 7 8 8 8 8 9 THE COMMUNICATION BETWEEN THE SATELLITES 5.1 5.2 5.3 5.4 5.5 5.6 7 8 9 Wave guides attenuation Low Noise Amplifiers Earth station and satellite antenna temperature Antenna Gain 4.3 3 3 4 4 5 5 6 7 15 2/15 Frederic Noel Satellite link project: Moon communication constellation 2 INTRODUCTION Three decades after the first steps on the Moon, Human wants to settle the Earth satellite... In order to achieve this mission, previous data have to be collected and communication network has to be built. The issue of this project is the setting up of the needed communication devices. The transmission can be decomposed in 3 types of links: - The communication between the Earth and the gateway satellites. - The communication inside the constellation of satellites - The communication on the moon with mobile-satellite links 3 3.1 SATELLITES’ ORBITS Problematic Because of the fixed parameters given by nature, the questions related to the orbital component of the project are treated at first. However lot of other parameters remain to be chosen, therefore different solutions are possible. This problem is also the basis of the other parts of the project. 3.2 The number of satellites This is the first question we have to face. The constellation should cover all the surface of the moon and be easy to set. Which means, as it is respecting the gravitational laws, the satellites’ positions have to be easily located. First, since a sphere is a 3D object, the easiest solution would be to put this sphere in the simplest volume: a tetrahedral. However, this shape is not kept with gravitational motion. Then the next plain polyhedral has to be considered, which keeps its shape with motion thus we have to take a cube. Secondly, an approximation of the number of satellites can be computed. Rm Dsm cos1 We get: 88.45 1.54 rad Rm Dsm Thus using the formula of optimisation: 4 3 N 9 Chalmers University of Technology 2 We get: 3/15 N 3.2 N 4 Frederic Noel Satellite link project: Moon communication constellation Since N=3,5 and the tetrahedral solution is not possible, the number of satellites for covering the all surface has to be raised to 6. The simplest configuration is to have 3 satellites per plane and to use 2 planes (www.etek.chalmers.se/~em9noel/). 3.3 Orbital radius limit As the satellites orbits have to be kept by the attraction of the moon, the radius has to be smaller than the border of the attraction field which is represented by the LaGrange point L1. 1 3 , 0 L1 : Rem 1 =61.10^3 3 with km Mm Me Mm Then, the maximum radius is about 60.10^3km. Let’s compute the geostationnary position in order to get an idea of the speed and the distance. This is made by using the #2 Kepler’s law: T 2 4 2 a3 a 3 T2 4 2 With T = rotation period of the moon=2.360.580 s And µ= 4,9*10^12 m^2/s^2 It gives a=92000km, the altitude for having a geostationnary satellite around the moon, which is not possible since it would be grabbed by the Earth attraction. 3.4 Satellites’ altitude The possible footprint of one satellite antenna is located between 88°, which is the maximum in the literature and minimum of 60°. This minimum comes from the assumption that with one orbital plane, the satellites of this plane should cover the 360° circumference and the angle found is twice the angle looked for the footprint. Thus, for 6 satellites a reasonable footprint would be =70°, with a margin because the line of sight is not the horizon. For covering this angle, a horn antenna will be better than a reflector one, since we don’t want to focus the beam. In order to find the angular beamwidth, the size of the antenna has to be assumed. Taking that the diameter of the antenna is equal to D=4* of the signal. A global angular beamwidth of 3D 70 =17.5° is obtained. D Then, the altitude d can be computed by using the following formula (Eq 7.45 p270 R). Rm cosE 3D sin cosE h Rm 1 =9467km 2 Rm h sin 3 D 2 Chalmers University of Technology 4/15 Frederic Noel Satellite link project: Moon communication constellation with the elevation angle E 90 3D 2 11.25 Which gives a distance from the centre of the moon of dms=11205km and a rotating period T=29h using Kepler’s formula O E 3D Rm E h Fig 1: Footprint and altitude of the satellite 3.5 Positions of the satellites on their orbits 3.5.1 First criterion: How to communicate with the Earth Two main satellites make the communication with the Earth, so that even if the moon, because of the moon revolution around the earth, hides one, the link is kept by using the other satellite and the intra-satellite network. Since it is not easy to establish a satellite link between the planes, it is preferable to use that connection mainly for mobile communication on the Moon and for the case of one main satellite hidden. Therefore, it is reasonable to put one main satellite is each plane. Fig 2: Communication with the Earth Chalmers University of Technology 5/15 Frederic Noel Satellite link project: Moon communication constellation 3.5.2 Second criterion: How to communicate with the other satellites Since the orbital planes are rotating relatively to earth because of the motion of the moon around the earth. The moon can hide each satellite even if it is on a polar orbit. Therefore a solution is to have a satellite transmitting with the earth for each plane and to be able to communicate between the 2 planes. The most difficult part of this problem is to make satellites of different plane communicate each other. To do so, a special geometry, that minimises the distance between them and makes them visible from each other, is required. The geometry suggested is the following: Earth Figure 2: the intra-satellite communication When the satellites are rotating, the antennas are going to track each other until the configuration of 4 satellites in a same plane is found again. The periodicity of the configuration is T/6, T being the period of revolution of the satellite around the Moon. It means that a satellite needs to track the satellites of the other plane during T/6. To sum up, in each plane, there is one satellite with 6 antennas: one for the moon, one for the earth, 2 for the satellite in the same plane and 2 for the 2 closest satellites in the orthogonal plane. The other satellites in this plane do not have an antenna for communicating with the earth. In order to keep this plane configuration, a telemetry system has to be developed. The first objective is to assure an equatorial triangle shape between the 3 satellites in the same plane. Secondly, the planes have to be orthogonal. To achieve this, the tracking system measures distance and angle Chalmers University of Technology 6/15 Frederic Noel Satellite link project: Moon communication constellation between the linked satellites. Then, the differences with the ideal configuration are corrected by using engines to rotate antennas and to shift bask the satellites 3.6 Visibility duration from Cape Canaveral The time of visibility of the satellite from the earth can be approximated by the time of visibility of the moon. Since the orbital plane of the moon is leaned of 5,145° with the sun plane and the equatorial plane is also leaned of 23,4° with the sun plane, the angle between the plane composed of Cape Canaveral, the centre of the earth and the moon orbital plane is between 0,1° and 56,9°. Therefore, the visibility evolves between 8hours and 13hours. 4 4.1 THE COMMUNICATION SATELLITE-EARTH Requirement on SNR 4.1.1 Modulation The transmission of data implies a bit error probability of 106. QPSK coherent is used to code our data. This choice is relevant for its simplicity compared to differentials coding, which propagate errors, and for its efficiency compared to BPSQ. 4.1.2 Signal to Noise energy ratio By using QPSK with raised cosine pulses and Gray code, it is possible to achieve the energy ratio Ec/N0=10.5 dB. However, such communication link can not be fully reliable, that is why redundant bits are added to the code. The assumption chosen is a correction code rate (CRC) of =0.8. This gives us a ratio Eb/N0= 6,2dB. 4.1.3 Bandwidth The signal is sent over a carrier frequency with a bandwidth allowing to convey a bit rate of 20Mbits. The bandwidth takes also into account the pulse shape, here raised cosine for avoiding intersymbol interference, and the type of modulation. B PulseShape Rb 1,26 20.10 6 16, 8MHz QPSKefficiency 1,5 Downlink B=0,84MHz Uplink 4.1.4 Signal to Noise power ratio Eb C B No N Rc with the channel rate Rc Rb Thus, C/N=8,2dB. A 3 dB margin should be afford to secure the link for the signal power in the receiver at the border of the region covered by the satellite. Chalmers University of Technology 7/15 Frederic Noel Satellite link project: Moon communication constellation 4.2 Computation of the link budgets 4.2.1 The polarisation, frequencies and rate The geometry of a raindrop produces interference between two signals of same frequency even if they are in quadrate. Therefore, one frequency polarised in quadrate cannot be used to achieve a full-duplex link. Thus, a couple of frequencies have to be used and the polarisation should be circular for avoiding the phase rotation due to the ionosphere, called Faraday effect. The frequencies of carrier couple used are 4-6 GHz in C-Band for the communication EarthSatellite because it is far from water and oxygen absorption frequencies. Moreover, low frequencies have lower pass loss and avoid the interference with the frequencies used by terrestrial communication. The requirements of the project define a downlink of 20Mbit/s (data, voice, and telemetry). Besides, an uplink of 1Mbit/s is assumed for controls. 4.2.2 The power transmitted On earth, the question of available power for the communication is not relevant since there is no constraint on it. But for the satellite, the limit comes from the solar sails, which collect the energy from the sun radiation. For a normal satellite, the power available is 1kW at the beginning of the satellite’s life and 500W at the end of life evaluated to 7 years. Therefore, a power of 100W can be dedicated to communication links. 4.2.3 Channel Atmospheric attenuation The atmospheric attenuation is mainly due to the rain. The statement defines a transmission 99% of the time when Cape Canaveral is visible from the Moon. Since the communication link has to be assured 99% of the time of visibility, the power budget should consider the rain attenuation Arain at the Cape Canaveral (=28,45°). At first, the height rain has to be calculated: Hr 3 0, 028 3, 7966km Secondly, the slant path through rain can be computed: Ls (Hr Hs ) 43, 33km , sin where is the line of sight angle and Hs the station height above the sea level and Hs the altitude in km of the Earth station. From the rainfall maps, it is known that the rainfall rate is R0,01 95mm / h . Then, the path reduction factor can be calculated: Chalmers University of Technology 8/15 Frederic Noel Satellite link project: Moon communication constellation r0,01 1 0,015R0,01 8, 41km 0,16306 with L0 35e 1 cos Ls L0 Since circular polarisation is used for avoiding the ionosphere rotation effect, we get the specific attenuation by using the nomogram: for Fc=6Ghz uplink, 0,7dB/ km and for Fc=4Ghz downlink, 0,1dB / km . The attenuation is then for the uplink A0,01 * Le * Ls *r0,01 5dB and for the downlink A0,01 * Le 0, 7dB . Finally, for a rate of 1%, a link is assured 99% of the time. The attenuation becomes: for the uplink A1% A0,01 * 0,12*1% (0,5460,043log1%) 5,08dB , and for the downlink A1% 0, 725dB For the other losses (gas), although this attenuation can be neglected for those frequencies, it is preferable to take it into account. Thus, this effect has an approximate attenuation of 0,5dB with normal pressure and temperature at the frequencies of 4-6GHz. Space loss This is the main attenuation in the link budget. It represents the free space attenuation due to the dispersion of the signal through the longest link Earth-satellite d=411000km 4d SE 22 21 LFS , for the uplink L fs 1, 06.10 220dB and for the downlink LFS 4, 74.10 216, 7dB 2 4.2.4 Telecommunication devices Wave guides attenuation Inside the transmitter and the receiver, there are attenuation coming from the loss in the wires, it can be estimated at LFX 1dB with a thermodynamic temperature TFX 290K . Low Noise Amplifiers In order to be able to receive the attenuated signal, a low noise amplifier has to be placed before the receiver. The type of the LNA is a FET ambient for the satellite since it can not be cooled and a cryogenic on earth because it has the best properties. Uplink: Satellite LNA technology FET ambient, with Tlna=70K and Glna=60dB Downlink: Earth station LNA technology cryogenic, with Tlna=15K and Glna=30dB The downconverter, the IF amplifier and the demodulator are not taken into account here because they belong to the receiver. Chalmers University of Technology 9/15 Frederic Noel Satellite link project: Moon communication constellation Earth station and satellite antenna temperature The antenna noise temperature is the sum of the contribution of the beam (main lobe) temperature and the antenna environment noise temperature (side lobes). The calculation has to be computed in the worth case, i.e. in rain weather with a low elevation angle. Downlink: Tbeam = Tmoon + Tsky at low elevation=420K Tearth station =Tbeam.e/Arain + TThermodynamic(1-1/Arain).e + Tearth.=472K Uplink: Tbeam = Tearth + Tsky at low elevation=360K Tsat=Tbeam.s/Arain + TThermodynamic(1-1/Arain).s + Tmoon.’=149K With the contribution of the sidelobes evaluated to =0,9 for the antenna on the Earth, and ’=0,1 for the satellite around the moon. Antenna Gain The main limitations are coming from the satellite, because of the limited power available on-board and the dimensions of the spacecraft. Since the beams are narrow, the antennas have to be reflector antennas. For the downlink: This dimension restriction directly affects the gain of the satellite antenna and their angle. The formulae are: D = 70 / 3dB and GT = (Df/c)2. Let’s choose a diameter of 1.2 m, which is relevant for a satellite. Then the computed angle 3dB: for 4GHz: 3dB = 4.4°. This angle is reasonable because it permits to cover all the Earth even on the perigee on the orbit Moon-Earth. The transmit gain of the antenna of the satellite is thus: GT = (Df/c)2, with = 0.55 and f=4GHz, since GT = 31.42 dB. For the uplink: The size of the antenna is computed thanks to the link budget, by the same formula, it is possible to calculate the antenna diameter. Chalmers University of Technology 10/15 Frederic Noel Satellite link project: Moon communication constellation 4.3 Link budgets PR = PTX + GT – LS – LR – LA + GR - LFTX - LFRX Signal power: Noise temperature of the input of the receiver: TSyst = TA/LFRX+TF(1-1/LFRX)+TLNA Downlink: Tsyst=450K Noise power: and Uplink: Tsyst=248K PR= (C/N) x (k x T x B) Satellite GR LFRX LNA RX PR LS + LR + LA Earth Station GT TX LFTX PT Figure: Uplink representation Chalmers University of Technology 11/15 Frederic Noel Satellite link project: Moon communication constellation Data Uplink 6GHz value Earth -> Sat C/N(dB) Temperature(K) QPSK 11,2 Pnoise(dB) -144,5 PR (dB) -133,3 PTX (dB) 248 3dB margin 7,4 GT (dB) 55,9 GR (dB) 31,42 GLNA (dB) 60 LS (dB) 220 LR (dB) 5,08 LA (dB) 0,5 unlimited power 70K (FET ambient) LFTX (dB) 1 290K LFRX (dB) 1 290K C/N(dB) Achieved C/N Sat -> Earth QPSK Bandwidth=16,8MHz 8,2 11,2 Pnoise(dB) -128,9 PR (dB) -117,7 PTX (dB) Divers Bandwidth=0,84MHz 8,2 Achieved C/N Downlink 4GHz Tolerance 450 3dB margin 15 GT (dB) 31,42 Reflector=> D=1,2 m GR (dB) 55,9 Reflector => D=19,2 m GLNA (dB) 30 LS (dB) 216,7 LR (dB) 0,73 LA (dB) 0,5 15K (cryogenic) LFTX (dB) 1 290K LFRX (dB) 1 290K 5 THE COMMUNICATION BETWEEN THE SATELLITES 5.1 Frequencies, modulation and bandwidth For the intra-satellite link, the easiest is to use radio frequency instead of laser transmission. It is also relevant since this link is developed for locating satellites from different planes with Doppler shift and relative position change. A frequency of 12GHz in Ku-Band can be chosen. Chalmers University of Technology 12/15 Frederic Noel Satellite link project: Moon communication constellation Because this link is mainly realised for mobile communications and telemetry, a rate of 5MHz should be sufficient for fulfilling the objectives. The polarisation use is the QPSK with raised cosine pulses and Gray code in quadrate for achieving a full-duplex link. 5.2 Path loss The maximum distance between two communicating satellites is the distance between two satellites from the same plane R=19408km which is the length of an equatorial triangle. 5.3 The Doppler shift The velocity of a satellite is V dms 661, 5m.s 1 with =GMm=49.1013 and dms=11.103km. Then, the frequency shift can be computed to fd=fV/c=26kHz. This shift can be handled by a Phase Locked Loop at the entrance of the receiver. 5.4 Depointing losses Compare to the other link budget, the influence of the depointing losses are mostly important in the case of satellites intra-communication. Therefore, it should be taken into account in this link. It is evaluated to10dB. 5.5 Antenna The antenna efficiency for satellites is =0,55. The antennas used are reflector antennas in order to be able to focus on the other satellites without having the moon in the main beam. This is also possible because of the tracking technique, which tends to avoid depointing losses 5.6 Link budget Data Link- 12GHz C/N(dB) minimum Achieved C/N value Temperature(K) Sat- Sat QPSK Divers Bandwidth=4,2MHz 8,2 11,2 Pnoise(dB) -139,3 PR (dB) -128,1 PTX (dB) Tolerance 202K if we see the earth 3dB security margin 10 GT (dB) 35,45 Reflector => D=51cm GR (dB) 35,45 Reflector => D=51cm GLNA (dB) Lpointing losses(dB) LS (dB) 60 130K (FET ambient) 10 200 LFTX (dB) 1 290K LFRX (dB) 1 290K Chalmers University of Technology 13/15 Frederic Noel Satellite link project: Moon communication constellation 6 SATELLITE-MOON COMMUNICATION 6.1 Frequencies: To reduce the power needed for the communication between the moon and the satellite, but mainly for the mobile, a relatively low frequency of 1Ghz in L-Band has to be chosen. Moreover, for achieving a full-duplex link, the solution is to use a quadrate polarisation. 6.2 Bandwidth: According to the statement, a bit rate of 20Mbits/s is required to be sent to the earth, which means 16,8MHz with QPSK modulation using raised cosine pulse and Gray code. Since the system has to be considered as never fully saturated, the estimated rate per satellite can raised to 5Mbits/s per satellite (Bandwidth of 4,2 MHz) 6.3 Multiple access: To avoid any synchronisation problems, the TDMA is put apart from possible solutions. Besides, for an uncertain future of the mission, it is preferable to use CDMA which will allow band reuse if it is needed, instead FDMA. For transmitting voice&data, a data rate per channel of 50kbits/s is chosen. This choice san be compared to the rate achieved by other constellations link Iridium with 42kbits/s. Thus, within the bandwidth of 4,2Mhz, the number of channel is: (Rc / Rb) with Eb/No=6,2 because =0,75 , =QPSKefficiency=1,5 N max 1 (Eb / No) Let’s choose N=100 users (engines and mobiles), the channel bit rate deduced is Rc=31Mbits/s which give a bandwidth of 26MHz. The back-off between two satellite footprints is assured because of, on the one hand, the altitude of the satellite and the aperture angle of the antenna for having a full coverage of the moon even with an elevation angle of 5°, and on the other hand, the software handle this back-off procedure. This operation has to be led with the experience of terrestrial GSM. 6.4 Antennas On the on hand, in order to have a large footprint, the satellite antenna should be a horn antenna with a beam of 17,5°. On the other hand, on the Moon, power losses have to be avoided because regions are out of solar energy during 14 days. Therefore, directive antennas should be used for mobile transmissions. To achieve this, Helix antennas can be chosen with a number of 5 turns, a diameter of 10,7cm, a pitch angle of 12,8° and a gain of 11dB. Chalmers University of Technology 14/15 Frederic Noel Satellite link project: Moon communication constellation 6.5 Link budget Data Downlink 1GHz value Sat -> Moon C/N(dB) Temperature(K) CDMA & QPSK Tolerance Divers Bandwidth=26MHz 8,2 Achieved C/N 11,2 Pnoise(dB) -129,8 PR (dB) -118,6 290K If we see the earth 3dB security margin PTX (dB) 16 30Watt GT (dB) 26 Horn => D=15cm GR (dB) 11 Helix antenna LS (dB) 172 LFTX (dB) 1 LFRX (dB) 1 Uplink 1GHz Moon -> Sat C/N(dB) Achieved C/N Pnoise(dB) PR (dB) 290K 290K CDMA & QPSK Bandwidth=26MHz 8,2 11,2 -137,2 328K -126 3dB security margin PTX (dB) 9 8Watt GT (dB) 11 Helix antenna GR (dB) 26 GLNA (dB) 60 LS (dB) Horn => D=15cm 70K (FET ambient) 172 LFTX (dB) 1 290K LFRX (dB) 1 290K 7 CONCLUSION This project has been decomposed in 3 communication channels linked by transponders. The most improtant link is the one between the earth and the main satellites. Without one of the 2 main satellites, the transmission with the Earth can’t be achieved all the wanted time. But it will be almost half of it. To compare with the other satellites, if one of the secondary satellites disapeares, only one region on the moon will not be covered until its replacement. Earth satellites constellations use backup satellites for replacing defectuous satellites. It is also possible to do so on the moon. Finnally, a simulation should be done to confirm the theory and to setup the datacom part of the mission which has not been treated in this project Chalmers University of Technology 15/15 Frederic Noel