communications

Question 1

why are communications essential?

Answer 1

for spacecraft operators to be able to send and receive data from the spacecraft

Question 2

describe basic communications architecture

Answer 2

ground station for up-link and down-link, and an on board comms system capable of transmitting and receiving signals to and from the ground stations

Question 3

describe the advantages and disadvantages for the LEO store and forward communications system orbit type

Answer 3

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* low launch cost and potential low cost satellite
* small antenna power
* polar coverage with inclined orbit
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* long access time transmission delay
* short access times
* satellite tracking and doppler compensation

Question 4

describe the advantages and disadvantages for the GEO geostationary communications system orbit type

Answer 4

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* no switching between satellites and ground station
* no tracking
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* high launch costs and potential high satellite costs
* need for station keeping
* no polar region coverage

Question 5

describe the advantages and disadvantages for the Molniya communications system orbit type

Answer 5

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* low launch costs
* polar coverage
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* several satellites required for continuous coverage of one hemisphere
* complex antenna pointing and satellite
* need for station keeping

Question 6

describe the advantages and disadvantages for the GEO with crosslink communications system orbit type

Answer 6

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* reduced propagation delay
* no ground stations in foreign territory = increased security, reduced cost
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* high launch costs and high satellite complexity
* need for station keeping
* no polar region coverage

Question 7

describe the advantages and disadvantages for the LEO with multi satellite with crosslink communications system orbit type

Answer 7

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* low launch cost per satellite
* potential low satellite cost
* high level of redundancy multiple paths polar coverage
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* complex link acquisition
* complex network control
* many satellites required

Question 8

name the three modulation schemes and how do they work

Answer 8

• amplitude shift keying (ASK) - varies amplitude to signify 0 or 1
• frequency shift keying (FSK) - varies frequency to signify 0 or 1
• phase shift keying (PSK) - varies phase to signify 0 or 1

Question 9

which modulation scheme is the most commonly used and why?

Answer 9

PSK - phase shift keying, because it is simpler to implement, is more bandwidth efficient and has better noise performance

Question 10

define frequency bandwidth

Answer 10

difference between upper and lower frequencies used to transmit data

Question 11

what does the link budget do?

Answer 11

considers all the gains and losses in a communications link to determine the link margin and hence the quality of the link

Question 12

what are meant by gains and losses?

Answer 12

• gains = increased power or beam focusing using antennae
• losses = energy dissipation in free space or through media or through pointing errors

Question 13

define noise

Answer 13

unwanted interference with a signal

Question 14

what does the signal to noise ratio characterise?

Answer 14

effectiveness of a signal

Question 15

define the bit error rate (BER) and state what it signifies

Answer 15

• BER = number of bit errors divided by the number of transmitted bits
• shows the signal quality, related to the signal to noise ratio

Question 16

list some sources of noise

Answer 16

• atmospheric emission, scattering from rain, sun, earth, lightning and galactic sources
• artificial sources such as cars, internal electronic / microwave devices

Question 17

what are antennae used for?

Answer 17

increase signal strength without increasing direct power from transmitter by focusing energy in a particular direction

Question 18

define an isotropic radiator

Answer 18

power is radiated with equal intensity in all directions

Question 19

what does a high gain antenna do?

Answer 19

concentrates the radiated power in a particular direction, known as the antenna bore-sight

Question 20

define antenna gain

Answer 20

ratio of power from an effective isotropic radiator (EIRP) to actual transmitted power

Question 21

what does a BER of 10^-6 mean, and what does BER depend on?

Answer 21

• 1 error for every 10^6 bits transmitted
• depends on the choice of modulation and the signal to noise ratio

Question 22

define free space losses

Answer 22

losses due to energy dissipation over a large area

Question 23

how do precipitation losses arise?

Answer 23

microwave frequencies absorbed by water vapour and precipitation in the atmosphere

Question 24

define line losses

Answer 24

losses in the electrical circuit between the transmitter and the antenna