Communication Systems

OVERVIEW

It is required to acquire detailed knowledge of Venus' seismic activity. This depends upon the ability to send large amounts of data to Earth from the surface of Venus. A two way communication system is required for this purpose that relays the data from the seismic nodes placed on the surface of Venus to the Deep Space Network (DSN) on Earth.

The communication system includes a wireless transmitter and receiver system (transceiver) attached with the seismic sensor on Venus. This transceiver establishes a wireless communication link with a satellite orbiting around Venus. The satellite then relays the information/data back to the DSN on Earth. The Earth Station (DSN) can also communicate with the sensor node by transmitting the information to the relay satellite which then relays the information to the sensor.

LANDER ON VENUS

    Sensing Unit

Seismic sensor node is limited with sensing, computation, and wireless communication capabilities. The sensor node is equipped with a power unit, communication subsystems (transmitter), processing unit (storage and processing resources), a sensing unit (an analog-to-digital converter (ADC) and sensors) as shown in the figure 1. The sensing unit consists of one through which information regarding a physical phenomenon is collected. The collected analog information is converted to digital format by the ADC, processed by the micro-controller unit (MCU), and transmitted to the processing unit [1].

Figure - 1 : Sensor Node and Communication System on Venus Lander

    Processing Unit

The processing unit on the Lander on Venus consists of a decision controller, processor, solid state hard drive, turbo encoder, QPSK modulator and demodulator and pulse shaping filter. The processing unit takes the digital raw data from the sensor node. This raw data is compressed using lossless compression scheme and then passed to Turbo Encoder.  A rate ½ Turbo Encoder is used for encoding purposes. The encoded data is then modulated using Quadrature Phase Shift Keying (QPSK). The modulated samples/pulses of data are then passed to the Root Raised Cosine (RRC) Pulse Shaping Filter.

Figure - 2 : Baseband processing of data obtained from the sensor node

The earth station can also communicate with the sensor node at the Venus. It allows the earth station to monitor the performance and remotely control the sensor node. In order for this communication to take place, the sensor nodes is equipped with a decision unit (controller) and an actuation unit in addition to a transceiver and an MCU.

The decision unit (Controller) functions as an entity that takes information from the earth station, relayed to it from the Satellite orbiting around Venus. It generate action commands as output via the actuation unit. The actuation unit includes several actuators to monitor power conditioning unit, temperature of many of the subsystems and sighting devices. The sighting devices used to maintain attitude are monitored via the telemetry link.

Figure - 3: Decision control unit employed on Lander on Venus

    Transceiver Unit

The Transceiver unit is required to transmit and receive data on the Lander. It consists of two major components i.e. a transmitter and a receiver system.

• • Transmitter

The transmitter systems consists of an up-converter block that converts the baseband signal (signal after RRC Filter) to higher frequency (L-band) signal. This signal is then amplified using a high power amplifier (HPA) and fed to the antenna for transmission to the orbiting satellite. The figure 4 shows the block diagram of the transmitting system along with the baseband processing unit for the data. 

Figure - 4: Transmitter System on Venus Lander

• • Receiver

The receiver system consists of a receive antenna, LNA and a down-converter block that converts the received high frequency RF signal (at the receive antenna) to baseband signal. This signal is then demodulated and decoded to extract the actual message for the lander at Venus. The figure 5 shows the block diagram of the receiver system. 

Figure - 5: Receiver System at Venus Lander

    ANTENNAS

The Lander unit uses two antennas for receiving and transmitting purpose.

Parabolic Grid antennas are used as high-gain antennas for point-to-point communications. Parabolic grid antennas have some of the highest gains, that is, they can produce the narrowest beamwidths, of any antenna type due to which it is used to transmit data from seismic sensor on Venus to the satellite orbiting around Venus.  A low weight and a very small package volume help to meet the design requirement of the lander. Parabolic grid antenna and its beamwidth are shown in figure 6 and 7. 

   Figure - 7 : Vertical and Horizontal Beamwidth of Parabolic Grid Antenna 

Yagi-Uda antenna is a directional antenna referred to as "beam antennas" due to their high gain used to communicate from the satellite orbiting around Venus and the lander on Venus. It also meets the design specification of the lander on Venus because of its low weight and very small volume. The antenna exhibits a directional pattern consisting of a main forward lobe and a number of spurious side lobes. The antenna can be optimized to either reduce this or produce the maximum level of forward gain. The antenna pattern of Yagi antenna and Antenna is given in figure 8 and 9.

Figure - 8: Yagi-Uda Antenna [3]

Figure - 9: Vertical and Horizontal Beamwidth of Yagi-Uda Antenna

ORBITAL SATELLITE

Communication link between Earth Station (DSN) and Venus Lander is established with the help of a satellite orbiting around Venus. The satellite is equipped with on-board communication systems to facilitate the communication from lander on Venus to the DSN on Earth. 

To ensure proper communication, there is an on-board controller that monitors and facilitates the communication between actuation unit of the satellite and the lander on Venus as shown in the figure 10. There is another on-board processor and solid state hard drive to store the data obtained from Venus locally on the hard drive. This stored data is then relayed back to the Earth Station. This helps in ensuring reliable data transfer to the Earth Station and have the ability to re-transmit the data in case of data packet failure [4].

Figure - 10: System Level diagram of communication system of satellite

The communication system is also equipped with a set of on-board transponders to relay the information from Venus to the Earth Station. Redundant set of transponders are also added to ensure communication in case of failure of any on the transponder chain. The figure 11 shows the transponder chain used in the satellite.

Figure - 11: Satellite Transponder System

The satellite also stores a localized copy of received data on an on-board solid state hard drive to ensure reliable communication in case of loss in transmitted data. The satellite is equipped with a complete set of transmitter and receiver systems other than the transponder system. The control among these components is shared using the on-board data controller. The receiver system consists of a Low Noise Amplifier (LNA) and Band-Pass Filter (BPF). The LNA amplifies the signal early in the receiver chain adding as little noise as possible. LNA is used in front of detection devices to achieve high gain [5]. Band-pass filter have a bandwidth of 20 kHz with center frequency of 915 MHz.The received signal is then is then demodulated to store the information. This is shown in the figure 12.

Figure - 12: Satellite receiver system (Venus / Satellite)

The transmitter (used in case of re-transmission when data is lost) consists of a 1/2 Turbo decoder, QPSK modulator, RRC pulse shaping filter (α = 0.25) for baseband modulation of the store data on the hard drive. It then up-converts the data to X-band (8GHz to 12 GHz) RF frequency to be transmitted back to earth station. The RF frequency up-conversion and transmission block consists of a Mixer, Local Oscillator, Band-Pass Filter, Linear Power Amplifier (LPA), and High-Power Amplifier (HPA) as shown below. 

Figure - 13: Satellite transmitter system (Satellite / Earth Communication)

The satellite communicates with the Earth Station and Venus at the same time. The link between Satellite and Earth Station is established on the X-band while the link between the Lander at Venus and Satellite is established using L-Band. 

The data from Venus is transmitted to the satellite which is relayed to the earth station via transponders. It is also saved locally on a solid state hard drive at the satellite to re-transmit in case of communication loss or link failure. The satellite discards the data when it receives information that it has reached the earth station properly. The satellite attaches its information header along with the data packets from Venus for the earth station. In this way the earth station receives the data from Venus and information of the satellite as well.

The earth station communicates with both the satellite and the lander on Venus. The information packet from earth is send to the satellite which is then relayed to the Venus lander. The information packet from earth contains a header meant for the satellite system. The on-board processor on the satellite extracts the header information and performs the required task assigned in accordance to it. This mechanism saves the cost of adding additional physical layer components on the satellite. 

Antennas used in the satellite system are Parabolic Grid (15dB gain)  for communication with Venus Lander and Parabolic (40dB) for communication with the Earth Station. Different antennas are used for transmitter and receiver to avoid inter-modulation issues.

Figure - 14: Parabolic Antenna

EARTH STATION

The NASA Deep Space Network (DSN) - is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The network also supports selected Earth-orbiting missions. The DSN currently consists of three deep-space communications facilities placed approximately 120 degrees apart around the world: at Goldstone, in California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia. This strategic placement permits constant observation of spacecraft as the Earth rotates, and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world. The dish antennas employed by the DSN are 34 meters in diameter, have a system temperature of 20 Kelvin, and an aperture efficiency of 0.94.

The DSN is used to communicate between earth and the relay satellite. The DSN supports the following frequencies:

X -band is used for communication between the orbital satellite around Venus and the DSN.

LINK BUDGET

Optimal communication requires a carrier to noise ratio of 20dB. The link budget is designed to achieve this carrier to noise ratio for all the uplink and downlink paths. The link budget is developed for the following paths.

1) Up-link from Venus to Satellite

2) Downlink from Satellite to Venus

3) Up-link from Earth to Satellite

4) Downlink from Satellite to Earth

Paths (1) and (4) hold key importance for reliable transmission of data. 

One of the key aspects in the link budget analysis is the calculation of radio-wave attenuation in the Venus atmosphere. A research publication [6] is used as a reference to estimate the attenuation of the atmospheric attenuation of Venus. The estimated / calculated values are around 20 dB to 25 dB.

25 dB attenuation is used in the analysis assuming worst case conditions.  

>>>>>>>>>>Complete link budget analysis can be downloaded below!<<<<<<<<<

>>>The hardware specifications for the communication systems can be downloaded below.<<<

DATA FLOW ESTIMATE

The following diagram shows the flow of data across different components of the Lander on Venus. 

Figure - 15: Data Flow Estimates (Lander to Orbiter)

References:

[1] Akyildiz, Ian. Advanced Texts in Communications and Networking : Wireless Sensor Networks. Wiley. 2010. Print.

[2] ”The Parabolic Grid Antenna.” EVDO Tips and Tweaks. Web. 10 Jan. 2013. http://evdotips.blogspot.com/2008/10/parabolic-grid-antenna.html

[3] Pole, Ian. “Yagi Antenna.” Radio-Electronics. Web. 10 Jan. 2013. http://www.radio-electronics.com/info/antennas/yagi/yagi.php

[4] Pratt Timothy, Charles Bostian, Jeremy Allnut. Satellite Communications, 2nd Edition. Wiley. 2003. Print.

[5] Pozar, David. Microwave Engineering: 3rd Edition. Wiley. 2005. Print.

[6] Steffes, Paul G., and Von R. Eshleman. "Laboratory measurements of the microwave opacity of sulfur dioxide and other cloud-related gases under simulated conditions for the middle atmosphere of Venus." Icarus 48.2 (1981): 180-187.