220517 ISDP メモ - 宇宙への歩み

TT&C

Requirements

Data transmission rate:
- Consider Uplink & Downlink
  - generally downlink need more data than uplink
  - limited power available -> constraints on the data transmission rate
- Dependent on mission operations (attitude), payload
Link Margin:
- Near Earth: 6 dB
- Deep Space 3 dB
Bit Error Rate:
- Uplink: $\leq 10^{-5}$ to $10^{-6}$
- Downlink: $\leq 10^{-4}$

Uplink is more important (because commands you are sending to the satellites need to be sent accurately) , so it tends to be higher than downlink.

Constraints:
- Power
- Size (antenna)
- Frequency band (interference)
  - e.g.) wifi, bluetooth and microwave use the same 2.4 GHz --> wifi/ bluetooth doesn't work near the microwave
- Mass

Link Budget

Linke Budget:

$M = SNR - SNR_{req}$
SNR = Actual Signal-to-Noise Ratio (dB)
Signal-to-Noise Ratio: analogy) people talking around you. we are in a large room. the theater is quiet. we are standing far apart. I can talk in a normal voice. -> low noise
if the room is full of people, even if I talk to you normally as before, you can't hear my voice. I need to yell for you to hear me. -> more power to increase the signal-to-noise ratio.

- All calculation sin decibels
- Link Margin based on requirements

Required SNR Method 1 (Shannon-Hartley Theorem):

$C = B \log_2(1-SNR)$
C = Channel capacity (bit/s)

Required SNR Method 2 (Bit Error Rate):

$SNR_{req} = \frac{E_b}{N_O} + 10 \log_{10}(c)$

Bit Error Rate (BER): how often your transmission signals contain errors

Actual Signal-to-Noise Ratio (SNR):

$SNR = EIRP + G_R - FSL - L_{atm} - N$
EIRP: Effective Isotropic Radiated Power
G_R: Receiver Antenna Gain: direction of the antenna
Free Space Losses: radiated out signals are lost to free space
Atmospheric Losses: atmosphere, rain
Noise: inefficiency of noise

Effective Isotropic Radiated Power:

$EIRP = P_{Tx} + G_{Tx} - L_{Tx}$
P_Tx = Transmitter Power (dBW)

- not transmitter input power

G_Tx = Transmitting antenna gain (dB)
L_Tx = Transmitter & Antenna Losses (dB)

Isotropic: same to every directions

Free Space Losses:

$FSL = 20[\log_{10}(4\pi) + \log_{10}(r) + \log_{10}(f) - \log_{10}(c) + 12]$
r = Transmission range : farther away higher losses
f = Transmission frequency: more frequency higher losses
c = speed of light

Atmospheric Losses:
- Possible to consider atmospheric losses negligible
- Rain can increase antenna temperature (noise)

Noise:

$N = 10[\log_{10}(k) + \log_{10}(T_s) + \log_{10}(B_N) - 6$
k = Boltsmann constant (J/K)
Ts = System temperature (K) : it's not the atmosphere temperature
Bn = Noise bandwidth (MHz)

Ts: based on hardware design (approximations are OK)

Antenna Design
Antenna Gain: How much an antenna can "focus" in one direction, expressed in decibels.

- Applicable to both transmitting and receiving signals
- general trade-off: range vs. flexibility

High gain can mean much less flexibility.

Link budget analysis process

1. Determine requirements (data rate, BER, etc.)
2. Determine frequency band
3. Determine hardware (antenna, transmitter/ receiver, etc.)
4. determine ground station
5. Calculate required SNR (Shannon channel capacity)
6. Calculate actual SNR
7. Calculate link margin
8. Adjust design accordingly

Half-power Beam Width
Width of beam at which signal strength is 50% of maximum (-3 dB)
- minimum of link margin (even if the signal get weaker by 50% still we can read)
based on antenna design
may affect ADCS requirements (pointing accuracy)
one reason for Link Margin

e.g.) Hayabusa 2 has two antennas with different gains

Ground Segment

TT&C Interface
- Compatible frequency, modulation etc.
- Transmitter / Receiver Gain
Visibility/ Line of Sight
- How often do you need a line-of-sight connection to your spacecraft?
- Can also use software tools (GMAT, STK, etc.)
Availability
- Sharing of ground station resources between users
Multiple ground stations may be required.

TT&C

Requirements

Link Budget

Link budget analysis process

Other Topics

Ground Segment