To determine why a helical antenna is used for satellite tracking, let's analyze the options given:

1. Circular Polarisation: Helical antennas are known for their ability to provide circular polarization. This is particularly useful in satellite communication as the signal can be received irrespective of the antenna's orientation. Circular polarization helps in mitigating the polarization mismatch between the transmitter and receiver.
2. Good Front to Back Ratio: This is a measure of the antenna's ability to reject signals coming from the back side. While important in certain contexts, it is not the primary reason for using helical antennas in satellite communications.
3. Tropo Scatter: Troposcatter communication involves the scattering of radio waves in the troposphere. Helical antennas are not specifically designed for troposcatter purposes.
4. Faraday Effect: The Faraday effect deals with the rotation of the plane of polarization of light waves passing through a magnetic field. Helical antennas can help combat this effect in satellite communication, ensuring the signal maintains its intended polarization.

Based on the analysis above, the correct reasons for using helical antennas in satellite tracking are their capability for circular polarisation and dealing with the Faraday effect.

Thus, the correct answer is (a) and (d).