INFINITY SPACE

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RF characteristics

The data rate is 10.4kbits/s as explained before. We plan to use a half-rate turbocode that would enable us to achieve a CNR as low as possible. This turbocode will double the datarate, which becomes 20.8kbits/s. Assuming a BPSK modulation and using a matched raised cosine filter, with roll-off factor alpha=0.5, we get an RF bandwidth of less than 32kHz.

Although the optimal frequency through the Venus atmosphere would be around 6GHz [11], we need to stay within the ITU frequency bands allocated for deep space communication. Thus, we would select a channel in the S band (2.3GHz [26]). The Doppler shift, of 250kHz (see here) is negligible.

Turbo-codes

Telemetry from space is regulated by the CCSDS (Consultative Committeefor Space Data Systems) recommendations, used worldwide by international space Agencies (NASA, ESA, CNES, DLR, JAXA, etc.). These recommendations establish common basis for all aspects of space telemetry communication links, including their physical layers, data formats and protocols [52]. Turbo-codes were chosen by the CCSDS as the new standard for telemetry coding schemes in 1999. The reason was the significant improvement in terms of power efficiency assured by turbo-codes over the old codes of the standard [52].

In the Venus Vibes mission, our company decided to choose turbo-codes. Turbo-code encoder is built using a parallel concatenation of two Recursive Systematic Convolutional codes and the associated decoder, using a feedback decoding rule, is implemented as P pipelined identical elementary decoders [51].

Receiving antenna

The gain of a 34m dish antenna at S band is at minimum 54.5 dB [25]. This results in a -3dB mispointing loss for an angle of 0.03°. Those values include atmospheric effects of a 90% cumulative distribution of clouds, but not the effects of rain itself.

Noise power and atmospheric considerations

The system noise temperature expected at the receiving antenna is 38K for a 10°elevation [25], including atmosphericand cosmic background noises. The antenna will be pointed towards Venus, so ve need to add the temperature contribution of that planet. According to [27], this value is 16K. Total noise temperature is 54K. Given the RF bandwidth, the corresponding noise power is - 166dBW.

One also needs to take into account the noise temperature increase due to the sun, when Venus passes in front of it. The angle made by Venus, the Earth and the Sun during the scientific phase of the mission goes from 45° to -15°. Thus, this approximatively results in 1.5 days during which the solar noise temperature exceeds 200K (figure 1). Given the previous system temperature, this means that the noise power increases more than 9 dB during less than 1.5 days. We decided to account for 9 dB of margin. Finally, this results in an acceptable down time, given that out total scientific phase was determined to be 91 days (see here).

Rain attenuation

According to [22], the DSN station with higher precipitation rate is the Canberra complex, with more than 50mm/hour during 0.01% of the year. This means the maximum attenuation is given by 0.00175L_eff(50)^1.308 dB, according to [23]. Assuming a pathlength under rain of L_eff = 3km, the attenuation is less than 1dB.

Venus atmosphere

The Venus atmosphere attenuation was computed according to the figure 4 here. It results in an attenuation of 1 dB at S band, assuming the elevation is at least 20°, which corresponds to our results here.

Emitter

The emitter antenna would have a 2 meters diameter and a 0.7 efficiency, which results in a 32 dB gain at S band. Although it means more transmitted power is required, we chose to keep the antenna size small for several reasons: First, a bigger antenna would take more space and the total lander size would be increased. Also, a bigger antenna means more weight, and eventually, more launching costs. But more importantly, a higher weight would mean more vibrations while steering the antenna. Because the purpose of the mission is on high quality seismometry measurement, we need to avoid antenna vibrations to be transferred to the ground. The antenna will be built with light-weight material graphite/aluminum, the same as the Magellan Mission to Venus.