Download 6 Satellite-Moon communication

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 
  cos1 
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
cosE 
  3D 

sin 
cosE   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% (0,5460,043log1%)  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
4d 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  290K .
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