Earth-Satillite Link
 

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The RF link between Earth and Mars is a long distance connection.  Extremely high gain antennas will be used to enable a high speed connection between these two planets while using a reasonable amount of power.

The NASA Deep Space Network (DSN) will be used as the Earth terminal of the Mars network.  The DSN has multiple 34-meter antennas located around the world.  At least one of these antennas is always able to view Mars.

Two of the satellites orbiting Mars will have large parabolic dishes for communicating with Earth.  They orbit on opposites sides of Mars, so at least one of these dishes will always be able to see Earth.  Their dish will be 4.5-meters in diameter.  This is the largest diameter dish that can be launched from Earth with today's Space Shuttles.

The Earth - Mars link will be full duplex.  Per the system requirements, it will support fifty 1.5 Mbps links.  Time division multiplexing (TDM) will be used to bundle all of these channels into one large data stream for the long distance communication link.


Earth to Mars Link Budget

Transmitted Power (Earth DSN) dBW 56.99
Frequency Hz 3.45E+10
Wavelength m 0.00870
Diameter (Satillite Antenna) m 4.50
Diameter (Earth Antenna) m 34.00
Satellite Antenna Efficiency   0.80
Earth Antenna Efficiency   0.94
Gain (Earth Antenna) dBi 81.52
Gain (Satellite Antenna) dBi 63.25
Noise Temperature (blackbody) K 15
Noise Temperature (Satellite LNA) K 30
Max Distance m 4.01E+11
Loss (propagation loss) dB -2.95E+02
Bandwidth Hz 1.75E+08
Rain Fade dB 3
Design Margin dB 2
   
Signal Power Received at Satellite dBW -98.51
Noise Power at Satellite dBW -129.64
C/N dB 31.13
Required C/N (w/ Turbo Coding) dB 0.9
Extra C/N Margin dB 30.23

The table above show the various parameters used to calculate the Earth to Mars link budget.  A frequency of 34.5 GHz was chosen for this link.  As you can see, due to having such a large transmit power, the signal to noise ratio of 31 dB is excellent.  A design margin of 2 dB was calculated in all link budgets.  This accounts for any non-ideal effects in the actual hardware.  An Excel spread sheet was used to make this calculation and includes all of the formulas.

 

Mars to Earth Link Budget

Transmitted Power (Earth DSN) dBW 23.80
Frequency Hz 3.20E+10
Wavelength m 0.00938
Diameter (Satillite Antenna) m 4.50
Diameter (Earth Antenna) m 34.00
Satellite Antenna Efficiency   0.80
Earth Antenna Efficiency   0.94
Gain (Earth Antenna) dBi 80.86
Gain (Satellite Antenna) dBi 62.60
Earth Antenna System Noise Temp. K 20
Max Distance m 4.01E+11
Loss (propagation loss) dB -294.61
Bandwidth Hz 1.50E+08
Rain Fade dB 3
Design Margin dB 2
   
Signal Power Received at Earth dBW -132.35
Noise Power at Earth dBW -133.83
C/N dB 1.48
Required C/N (w/ Turbo Coding) dB 0.9
Extra C/N Margin dB 0.58

A total of 240 watts is used to transmit the signal back to Earth.  This gives a C/N of 1.48, which is adequate for a turbo coded link.  There is only a 3 dB buffer for rain fade.  This should be adequate for down times of no more than 0.3% of a year.  This is assuming that all the DSN dishes are in a relatively dry region (Region E).  During heavier rain, the bandwidth of the link can be reduced.  The link can still operate during heavier ran, but at reduced capacity.


Satellite Transceiver Design


 

 

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