From SEDSWiki

Contents 1 Channel parameters 1.1 Equivalent Isotropic Radiated Power 1.2 Saturation flux density 1.3 Transmission losses 1.3.1 Free space transmission losses 1.3.2 Feeder losses 1.3.3 Antenna misalignment losses 1.4 The link power budget equation 1.5 System noise 1.5.1 Thermal noise 1.5.2 Antenna noise 1.5.3 Amplifier noise 1.5.4 Carrier to noise ratio 1.6 Effects of rain 1.6.1 Uplink rain fade margin 1.6.2 Downlink rain fade margin 1.7 Thermal effects

Channel parameters

We have here a brief description of the link power budget calculations and the various parameters to be considered during channel modelling for SEDSAT 2.

Equivalent Isotropic Radiated Power

The maximum flux density at some distance r from an antenna of gain G,

• ¥= GP/4∏r^2
• EIRP=GP, where P is the transmit power

Saturation flux density

This is,in simple terms defined as the minimum amount of flux required at the receiving antenna in order to produce saturation of the receiver amplifier.We use this quantity in order to estimate the EIRP of the earth station antenna.

• ¥= EIRP/4∏r^2

Transmission losses

Free space transmission losses

This is the loss resulting from the spreading of the signal during transmission. The power delivered to a matched receiver is given by

• Pr = ¥A ,

where A=effective aperture of the receiving antenna

Feeder losses

These occur in the connection between the receive antenna and the receiver proper,in the connecting waveguides, filters and couplers.

Antenna misalignment losses

This occurs when the earth station and satellite antennas are not aligned for maximum gain. It’s also known as off-axis loss.In addition to this losses may also result due to the misalignment of the polarization losses.These losses are estimated from statistical data,based on the previous recorded earth station errors and separate antenna misalignment losses for uplink and downlink must be taken into account.

The link power budget equation

The losses for clear sky conditions are

• LOSSES=FSL+RFL+AML+AA+PL

FSL=free space spreading loss, RFL=receiver feeder loss, AML=antenna misalignment loss, AA=atmospheric absorption loss, PL=polarization mismatch loss

System noise

Thermal noise

The main characteristic of thermal noise is that it has a flat frequency spectrum. The noise power per unit bandwidth is termed as noise power spectral density No

• No = kT

k= boltzmann’s constant, T = equivalent noise temperature

Antenna noise

There are basically two types of antenna noise.

• Sky noise:It is the microwave radiation which is present throughout the universe

Refer: graph on page 314 of “Satellite Communications” by dennis roddy The sky noise is less for an antenna pointing directly overhead than for an antenna pointing just above the horizon. But the SEDSAT 2 operating frequencies are not affected by the background radiation so much, as the main noise peaks are observed at frequencies above 10GHz

• Noise from antenna losses: self explanatory

Amplifier noise

This is the noise from the low noise amplifier(LNA).The output noise from the amplifier is

• No=FGkTo

F=noise factor, G=power gain, To=room temperature

There is a direct equivalence between noise factor and noise temperature

• Te=(F-1)To

Carrier to noise ratio

This ratio is the most important performance parameter of a satellite communication circuit.The C/N ratio is analyzed separately for the uplink and downlink.

• [C/N]u=[EIRP]u + [G/T]u –[LOSSES]u – [k]

The subscript u stands for the uplink.

• [C/N]d=[EIRP]d + [G/T]d –[LOSSES]d – [k]

The subscript d stands for downlink.the free space and other losses are calculated for downlink frequencies.

Effects of rain

Rainfall results in the attenuation of radio waves by scattering and by absorption of energy from the wave. The attenuation increases with increase in frequency. When a radio wave with some arbitrary polarization passes through the raindrops,the component of the electric field in the direction of the major axes of the raindrops will be different from the component along the minor axes. The wave becomes elliptically polarized. The effect is not that pronounced for single polarization but in cases of frequency reuse by orthogonal polarization, depolarizing components may need to be installed.

Uplink rain fade margin

The noise introduced by rain is not that significant because the temperature of the earth is so high that the noise introduced is minimal in comparison. Rain fade margin tables have been prepared for different areas, which describes the amount of rain attenuation for a specified time interval.

Downlink rain fade margin

The effective noise temperature of rain is given by

• Tr = Ta (1-1/A)

A=rain attenuation by absorption, Ta =apparent absorber temperature, it has different values for different regions.

Thermal effects

Satellites are subjected to intense thermal gradients,receiving the sun’s temperature on one side while the other fades into space. The earth’s albedo has to be taken into consideration for LEO satellite. Thermal blankets and shields might have to be used to provide insulation. Spinning satellites provide a suitable averaging of the overall temperature. Heaters could be controlled from the ground to make up for temperature gradients.