Voyager 1 Spacecraft Communications System PDF

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Projecte de l'assignatura Comunicacions Aeronàutiques 1 (CA1)....

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Aeronautical Communications 1 B.Sc. in Aerospace Systems Engineering Polytechnic University of Catalonia

VOYAGER 1 SPACECRAFT COMMUNICATIONS SYSTEM

Johann Schwarze

June 2020

Contents 1 Introduction

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2 General overview

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2.1 Deep Space Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Uplink 3.1 Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.2 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Downlink 4.1 Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.2 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.

Introduction

Voyager 1 is an uncrewed spacecraft operated by NASA’s Jet Propulsion Laboratory and launched in 1977 widely known for being the first artificial object to get out of the heliosphere and reach the interstellar space. Its first mission objectives were the observation of different celestial objects along the Solar System, studying their atmosphere, rings and magnetic fields and providing detailed images of some of them.

Figure 1.1: ments.

Voyager 1 instru-

The space vehicle was equipped with several scientific instruments which besides making observation possible they also set up the propulsion and control subsystems, the electrical power supply system, the flight data subsystem, the command computer subsystem and the radio communications system. The particularities and characteristics of the aforementioned space communications system will be studied in this document, as it was designed to be able to transmit from outside the Solar System and it is still operative and expected to keep working until 2025 approximately.

2.

General overview

The figure below shows a general overview on both ground and spacecraft communications systems between the Voyager 1 space probe and the ground station, which is the so-called Deep Space Network that will be introduced in the next section.

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Figure 2.1: Spacecraft and ground communications scheme. Notice that in the spacecraft system, when uplink, a command signal will be received and demodulated, whereas when downlink, a telemtry signal will be modulated and transmitted. Within satellite links, only atmospheric and relativistic effects must be considered, being the ionospheric effects the most notorious, whereas when talking about deep space communications, plasma and planetary effects caused by solar wind among other factors must also be taken into account. Within the context of space communications, two directions will be defined: • Uplink: transmission from ground to spacecraft. • Downlink: transmission from spacecraft to ground.

2.1

Deep Space Network

NASA’s Deep Space Network (DNS) consists of three deep space communications complexes located in Madrid, Canberra and Goldstone respectively, whose principal function is providing the ground segment communications facilities 2

Figure 2.2: DNS antenna.

for interplanetary missions. It uses huge parabolic antennas in order to get reasonable power levels, as it will be seen in the following sections. The International Telecommunication Union (ITU) have designated the frequency bands for deep space and near Earth communications. These are listed in the table below. Table 2.1: Frequency bands ITU designation for space communications. Designated Deep space bands Near Earth bands Uplink Downlink Uplink Downlink band S band 2110 MHz - 2120 MHz 2290 MHz - 2300 MHz 2025 MHz - 2110 MHz 2200 MHz - 2290 MHz X band 7145 MHz - 7190 MHz 8400 MHz - 8450 MHz 7190 MHz - 7235 MHz 8450 MHz - 8500 MHz K band 25.5 GHz - 27 GHz Ka band 34.2 GHz - 34.7 GHz 31.8 GHz - 32.3 GHz -

Nevertheless, the DSN does not support the K band, so only S, X and Ka bands are used in the Voyager missions. Notice that empty cells from table 2.1 correspond to not supported or allocated frequency ranges.

3.

Uplink

3.1

Transmitter

In order to reach the spacecraft with reasonable power levels, the DSN transmits an uplink S band carrier frequency of 2114.676697 MHz at incredibly high power levels up to the order of 20 kW, which corresponds to the DSN Channel 18. This transmission uses a BPSK modulation, in particular Manchester codes, with a bit rate of 16 bps.

(a) BPSK constellation.

(b) Manchester encoding example.

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3.2

Receiver

Voyager 1 has a narrow-band super-heterodyne receiver that detects the signal from the uplink carrier through a high gain Cassegrain antenna with a 3.7 m diameter. In uplink transmission, this antenna operates in S band frequencies and presents a beamwidth of 2.3º and 36 dB gain, as it is shown in the figure below. The spacecraft is also equipped with a low gain antenna but it is no longer in operation.

Figure 3.1: Voyager 1 high gain antenna. For proper command signal acquisition and demodulation, the transponder, which includes a receiver and an exciter, must be configured in coherent mode, so knowing this, it can be said that the probability of error in the Voyager 1 demodulator stands for: Pe =

1 √ Eb erfc ( γ) with γ = N0 2

(3.1)

where γ is the signal-to-noise relation (SNR) at the demodulator input and erfc(·) denotes the complementary error function.

4.

Downlink

4.1

Transmitter

The spacecraft is also equipped with a voltage-controlled oscillator that provides a frequency reference to the exciter from the aforementioned transponder whenever it is phase locked in 4

order to generate a downlink carrier which is coherent with the uplink. This downlink carrier has a frequency of 2296.481481 MHz, which is also S band frequency corresponding to Channel 18 and has assigned a right circular polarization by the aforementioned Cassegrain antenna. The telemetry modulator, which modulates the telemetry signal that will be transmitted from Voyager 1 to Earth, also uses a BPSK and has two operative channels: • Low-rate channel: 40 bps. Only uncoded bits can be transmitted. • High-rate channel: from 10 bps to 115.2 kbps. Transmits coded symbols.

4.2

Receiver

Because of the huge distance to the Earth, the signal transmitted by Voyager 1 is received with incredibly low power levels, on the order of −120 dBm, as it can be seen in the Friis equation: PRr =

PTg GT GR λ2 (4πr)2

(4.1)

Notice that increasing the gain of any antenna, the received power is increased proportionally, so in order to maximise this effect, parabolic antennas are used in the Deep Space Network, as parabolic antennas use curved surface with parabolic reflectors and can reach very high directivity with the requirement of needing a large structure, as all the received rays are reflected in the surface and cross the focal point, as it can be observed in the figure below.

Figure 4.1: Parabolic antenna functioning.

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Bibliography [1] All About Circuits: Communicating Over Billions of Miles: Long Distance Communications in the Voyager Spacecraft, https://www.allaboutcircuits.com/news/voyager-mission-anniversary\ -celebration-long-distance-communications/ [2] California Institute of Technology. 201, Rev. B, DSN Telecommunications Link Design Handbook: Frequency and Channel Assignments . California, 2009. [3] Fortescue P, Stark J, Swinerd G. Spacecraft Systems Engineering. Fourth edition, 2011, Chichester, United Kingdom. [4] Haynes R. How We Get Pictures From Space. National Aeronautics and Space Administration, 1989. [5] Ludwig R, Taylor J. Voyager Telecommunications. JPL Deep Space Communications and Navigation Systems Center of Excellence, 2002. [6] NASA JPL: About the Deep Space Network, https://deepspace.jpl.nasa.gov/about/ [7] NASA JPL: Voyager - Mission Status, https://voyager.jpl.nasa.gov/mission/status/ [8] Wikipedia: Manchester code, https://en.wikipedia.org/wiki/Manchester_code [9] Wikipedia: NASA Deep Space Network, https://en.wikipedia.org/wiki/NASA_Deep_Space_Network [10] Wikipedia: Parabolic antenna, https://en.wikipedia.org/wiki/Parabolic_antenna [11] Wikipedia: Voyager 1, https://en.wikipedia.org/wiki/Voyager_1

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