COMS Mission Requirements

Note: Work by previous team headed by Ryan Laird
Correct at time of publishing and most work still relevant, for more current information see Link Budget page


Mission requirements determine the working parameters required for successful LEO satellite communication and their principal values, as well as the data rates required for uplink/downlink. As such, this section contains information defining the parameters governing the satellite communications - power, antenna gain, signal noise, data rate etc. as well as the parameters required to achieve communications which satisfy the goals outlined in the Sci-RD e.g. analysis of the link budget/hardware/data rate required.


1. INTRODUCTION

The primary goal of the communications subsystem is to provide a link to relay data to ground station and send commands to and from the CubeSat and as such will primarily consist of telemetry and command sequences that send and receive data respectively. Data is collected by the payload and sensors on the satellite in the form of analogue or digital data which must be sent to the ground station.


2. CONTROLLING PARAMETERS GOVERNING SATELLITE COMMUNICATIONS

RADIO PROPAGATION

Radio waves consist of a system of electric and magnetic fields that travel through space at a velocity of 3 x 108 ms-1. For most of their journey the signals that propagate from the satellite in essentially what is free space, traveling without change aside a change in intensity in proportion to 1/r2 as the distance r from the source increases.
In the last few kilometers, as they pass through the Earth’s atmosphere there are significant effects to the system performance.
• Propagation describes the transmission of radio waves through non-ideal media.
• Atmosphere can cause refraction, absorption and scintillation.
Refraction: for a standard atmosphere, shift, R, is given by:

degrees.
where e is the true elevation in degrees
Scintillation (like stars twinkling) is not usually a problem.

POWER

ANTENNA GAIN
Gain (directive) G is ratio of power density with directional antenna to power density with isotropic radiator with same total radiated power.

For uniformly illuminated antenna with area A at wavelength λ

(1)
\begin{equation} G=4 πA/λ2 \end{equation}

Alternatively effective aperture, Ae is given by

(2)
\begin{equation} Ae= λ 2G/4π \end{equation}

Here $η=A,,e,,/A$ is the aperture efficiency (typically 0.5-0.7)

SIGNAL NOISE
All electrical systems have noise - ultimately due to random thermal motion of electrons which appear as random voltages and currents. Each resistive element in a circuit generates a thermal noise with a power spectral density:

If $hf<<kT$ (as it will be here) this reduces to:

$P,,0,,(f)=kT$ in W/Hz

When two independent sources of noise are combined we add their powers.
For constant power spectral density P0(f)=kT, the power in a bandwidth B is

(3)
\begin{equation} P=kTB \end{equation}

in W

Each component in a telecomms system has a noise temperature. Input noise temp of T is increased by the amplifier’s noise temp Tamp

Sun: T=104-1010 K

If sun enters the FOV of a high gain antenna (HGA) communications will be disrupted.

Earth: T=290 K
Galaxy: Negligible above 1 GHz
Sky: T=30K-150 K
Atmosphere: T<50 K below 20 GHz
Rain: 275 K

Ts for groundstation: typical ~130 K, state of the art cryogenically cooled e.g. deep space missions ~30 K

DATA RATE


3. REQUIRED PARAMETERS

LINK BUDGET

Energy per bit EB(J),

(4)
\begin{equation} EB =C/R \end{equation}

where R (bps) is the data rate (remember phase/frequency shift modulation runs transmitter at max power all the time).

For noise we have:

(5)
\begin{equation} N,,0,,=kT,,S,, \end{equation}

so, for a bandwidth, B:

(6)
\begin{equation} N=N,,0,,B=kT,,S,,B \end{equation}

The required receiver (and hence noise) bandwidth depends on the type of modulation used. In the simplest case, BPSK, R=B

The bit error rate depends on $EB/N,,0,,$ which is the figure of merit for the link.

These intense calculations can be done imputing values using the Jan King Link Calculator obtained at the Amsat Colloquium.

This will give a summary of values shown. E.g.

Uplink
Parameter: Value: Units:
Ground Station:
Ground Station Transmitter Power Output: 50.0 watts
In dBW: 17.0 dBW
In dBm: 47.0 dBm
Ground Stn. Total Transmission Line Losses: 2.9 dB
Antenna Gain: 16.3 dBi
Ground Station EIRP: 30.4 dBW
Uplink Path:
Ground Station Antenna Pointing Loss: 0.3 dB
Gnd-to-S/C Antenna Polarization Losses: 1.9 dB
Path Loss: 153.3 dB
Atmospheric Losses: 2.1 dB
Ionospheric Losses: 0.4 dB
Rain Losses: 0.0 dB
Isotropic Signal Level at Spacecraft: -127.6 dBW
Spacecraft (Eb/No Method):

-— Eb/No Method -
Spacecraft Antenna Pointing Loss: 0.0 dB
Spacecraft Antenna Gain: 0.0 dBi
Spacecraft Total Transmission Line Losses: 1.5 dB
Spacecraft Effective Noise Temperature: 245 K
Spacecraft Figure of Merrit (G/T): -25.4 dB/K
S/C Signal-to-Noise Power Density (S/No): 75.6 dBHz
System Desired Data Rate: 9600 bps
In dBHz: 39.8 dBHz
Command System Eb/No: 35.8 dB

Demodulation Method Seleted: Coherent FSK
Forward Error Correction Coding Used: None

System Allowed or Specified Bit-Error-Rate: 1.0E-05

Demodulator Implementation Loss: 1.0 dB

Telemetry System Required Eb/No: 11.9 dB

Eb/No Threshold: 12.9 dB

System Link Margin: 22.9 dB

Spacecraft Alternative Signal Analysis Method (SNR Computation):
-- SNR Method --
Spacecraft Antenna Pointing Loss: 0.0 dB
Spacecraft Antenna Gain: 0.0 dBi
Spacecraft Total Transmission Line Losses: 1.5 dB
Spacecraft Effective Noise Temperature: 245 K
Spacecraft Figure of Merit (G/T): -25.4 dB/K

Signal Power at Spacecraft LNA Input: -129.1 dBW

Spacecraft Receiver Bandwidth: 1,200 Hz

Spacecraft Receiver Noise Power (Pn = kTB) -173.9 dBW

Signal-to-Noise Power Ratio at G.S. Rcvr: 44.8 dB

Analogue or Digital System Required S/N: 12.9 dB

System Link Margin 31.9 dB

Downlink
Parameter: Value: Units:
Spacecraft:
Spacecraft Transmitter Power Output: 0.9 watts
In dBW: -0.5 dBW
In dBm: 29.5 dBm
Spacecraft Total Transmission Line Losses: 2.3 dB
Spacecraft Antenna Gain: 0.0 dBi
Spacecraft EIRP: -2.8 dBW
Downlink Path:
Spacecraft Antenna Pointing Loss: 0.0 dB
S/C-to-Ground Antenna Polarization Loss: 0.2 dB
Path Loss: 153.3 dB
Atmospheric Loss: 2.1 dB
Ionospheric Loss: 0.8 dB
Rain Loss: 0.0 dB
Isotropic Signal Level at Ground Station: -159.2 dBW
Ground Station (EbNo Method):
-— Eb/No Method -
Ground Station Antenna Pointing Loss: 0.1 dB
Ground Station Antenna Gain: 16.1 dBi
Ground Station Total Transmission Line Losses: 0.5 dB
Ground Station Effective Noise Temperature: 151 K
Ground Station Figure of Merit (G/T): -6.3 dB/K
G.S. Signal-to-Noise Power Density (S/No): 63.1 dBHz
System Desired Data Rate: 9600 bps
In dBHz: 39.8 dBHz
Telemetry System Eb/No for the Downlink: 23.3 dB

Demodulation Method Selected: BPSK
Forward Error Correction Coding Used: Conv. R=1/2,K=7 & R.S. (255,223)

System Allowed or Specified Bit-Error-Rate: 1.0E-06

Demodulator Implementation Loss: 1 dB

Telemetry System Required Eb/No: 2.5 dB

Eb/No Threshold: 3.5 dB

System Link Margin: 19.8 dB

Ground Station Alternative Signal Analysis Method (SNR Computation):

-- SNR Method --
Ground Station Antenna Pointing Loss: 0.1 dB
Ground Station Antenna Gain: 16.1 dBi
Ground Station Total Transmission Line Losses: 0.5 dB
Ground Station Effective Noise Temperature: 151 K
Ground Station Figure of Merrit (G/T): -6.3 dB/K

Signal Power at Ground Station LNA Input: -143.7 dBW

Ground Station Receiver Bandwidth (B): 1,000 Hz

G.S. Receiver Noise Power (Pn = kTB) -176.8 dBW

Signal-to-Noise Power Ratio at G.S. Rcvr: 33.1 dB

Analog or Digital System Required S/N: 3.5 dB

System Link Margin 29.6 dB

HARDWARE

Adhering to requirements of the size and mass of the CubeSat, any equipment must not have dimensions larger than 10cm each side and have a mass of no more than 1kg (take structure). Only a small fraction of this volume can be occupied by the communications hardware. The equipment that is chosen must also be space-worth mainly that it can withstand strong vibrations through launch, can operate within suitable temperatures as well as able to work under bombardment with radiation.

The system must also be fairly inexpensive as well as regulate within designated frequency bands set by OfCom guidelines with regards to amateur radio communications.

DATA RATE

For reasonable communications time it is necessary that the data transfer rate is adequate. We need to work with PAY and find what kind of resolution and memory size the pictures are going to be to get an idea how long it will take to send the data. It is very likely we will compress the data before sending it to the ground station as this will allow a quicker transfer rate for the same data, but this may cause some problems with loss of data itself through the compression.

The orbit that is chosen is hugely important to the same regard as the data transfer. The orbit chosen affects the pass over time and hence the time we have available for communications. Typically it can be assumed that the satellite will take between 5 and 10 minutes to pass over the visible sky. It is fundamentally required that the satellite passes over the chosen ground station. Equipment is available to us via our team leaders who use the National Space Centre, Leicester (52°38'N 1°08'W). One needs to achieve an inclination of at least about 40-45 degrees as the satellite will never be seen above the horizon and is determined by the launch latitude.


REFERENCES:

• Dr. Nigel Bannister, University of Leicester. PA3613 - Space Mission Design - Lecture 7, Communications Subsystem
• ‘Spacecraft Systems Engineering’ Third Edition - Fortescue, P. & Stark, J.


This section can be downloaded here


"Finding Our Cubesat" by John Heath - An overview of why a beacon is required for our satellite

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