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HD-189733b : A planet in orbit around a star in the constellation of Vulpecula

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System noise temperature calculation

Talking to Icarus

The purpose of this page is analyse the problem of communication from an exploration probe called Icarus that has hopefully reached Alpha Centauri, about 4.22 light years away. The idea is to send a mass of information back to Earth at an interstellar distance.

I've read an article in the British Interplanetary Society magazine, of August 2107. It talks about bit rates of 20 Gbit/s from Alpha Centaur to Earth, using two simultaneous 10 Gbit/s carriers on different polarisations with a pair of 1 MW amplifiers and frequency of 32 GHz. I thought I would see what results my calculation method would give. A 1 km diameter antenna at Alpha Centauri and 15km diameter at Earth are suggested. In both cases such antennas would be in orbit near the Alpha Centauri or Earth.

My assumptions are that the 10 Gbit/s carrier is supported by a hypothetical 1 MW transmit amplifier and uses QPSK modulation and 1/2 rate Viterbi FEC, so its symbol rate and noise bandwidth is about 1.1 x 20,000,000,000 Hz  and it needs a carrier to noise ratio (C/N) in excess of 6 dB

Using the above input data my calculator method below gives a range of 14 light years, which is more than required.

Icarus transmit frequency GHz
Icarus transmit antenna diameter m
Icarus transmit antenna gain dBi
Icarus transmit power W
Icarus transmit EIRP dBW
Earth receive antenna diameter m
Receive system noise temperature (antenna+LNA) K
Receive antenna gain dBi
Receive antenna G/T dB/K
Bandwidth Hz
Required overall link carrier to noise ratio C/N dB
Path (spreading) loss dB
Range km
Range AU (1AU=Earth to Sun distance)
Range light seconds
Range light minutes
Range light years


I'm a bit sceptical about antennas 1km or 15km diameter that can be constructed and maintained with surface accuracy of 1/8 wavelength at 32 GHz, but whatever; this is futuristic stuff!.

Also, I have never heard of a 10 GHz wide carrier centered on 32 GHz.

You may try other assumptions.  Enter your data into the light grey boxes (by modifying the default values shown) and then click the calculate button to obtain results in the green boxes.

Press calculate and see what is the resulting maximum range.

Repeat the range calculation several times, experimenting with different powers, bandwidths, antenna sizes etc.

Have fun and good luck.

I am interested to know if this page gives correct results.  If you agree or disagree please send details to me by e-mail Eric Johnston

Copyright (c) 2002 Eric Johnston, All Rights Reserved.

Revised:  9 July 2017. Amended: 10 June 2021.