Ground Station Workshop A flashback

By:Deepika Thakur

Conceptual design of the Communication Subsystem

Why 437 MHz and 145 MHz

a) Availability of frequency bands b) Payload Requirementsc) Availability of modulesd) Interband Interference and noisee) Amateur frequency band

Why FSK?

a) Spectrally Efficient b) Moderate value of Eb/Noc) Modules Readily Availabled) Low intersymbol interference as compared

to FM

Why AX.25 protocol?

It is a standard protocol used by the amateur radio enthusiasts

It is robustly designed for packet data transfer

Why monopole?

a) Being Omnidirectional it reduces the constraints on attitude control

b) It produces a linearly polarized beam.c) High polarization purity can be obtained

by careful design of the antenna.d) The length of the antenna is comparable

to the satellite dimensionse) It is easy to fabricate .f) The loss in gain can be compensated by

using a directional antenna on the ground station

Why beacon is separate from OBC?

Indicator of satellite is aliveShould work even if OBC failsHence, no dependency on OBC

Why CC 1020? a) It is industrial grade.b) It is operational over a frequency band

of 405MHz to 470 MHz and is tunable.c) It’s power requirements are low.d) It supports packet data transmission.e) Maximum bitrate is 156.2 kBps which is

much higher than our requirements.f) space heritage.

Why ADF?

a) It is industrial grade.b) It is operational over a frequency band

of 128MHz to 150 MHz and is tunable.c) It’s power requirements are low.d) Maximum bitrate is 100 kBps which is

much higher than our requirements.e) It has Space heritage.

Why crossed yagi?

The onboard monopoles are transmitting linearly polarised light for satisfying payload requirements.

The signal strength of the incoming wave at the Groundstation needs to be determined in 2 perpendicular planes to get the angle of polarisation

The difference in polarisation angles is used for computation of TEC.

Why 4 antennae on GS?

The Groundstation at IITB serves the dual purpose of demodulation and Payload

The antennae to be used for demodulation cannot be used for measurement of polarization because of synchronization issues.

2 antennae are needed at each frequency ; one for demodulation and one for polarisation measurement

How many LNA’s and why?

The polarisation meaurement IC needs a signal strength level of -60 dBm.

The received Signal strength from link budget calculations is -110 dBm.

Hence we would require atleast 3 LNA’s for polarisation measurement

We would require atleast 1 LNA for CC1020 and ADF7020 to raise the strength of the incoming signal above the sensitivity levels

Why tracking above 300 elevation?

The free space losses during transmission is given by Friis formula: 22 + log (S/λ)

The slant range (S) increases with decreasing elevation angles increasing the free space losses

The attenuation also increases due to obstruction in the line of sight caused by Buildings, towers etc.

Yagi design and 4NEC2X

A yagi antenna consists of the following elements

a) Director b) Reflector c) Folded dipole or feed element The impedance of the Yagi should be

matched to 50 ohms for it to transmit effectively

This is achieved using a balun of length λ/4.

The antenna is simulated and optimised using the 4NEC2 software

Thank You