Design for a Deep Space Communications System

The communication system will comprise of a redundant dual-band transmission channel, namely an S-band system and an X-band system. The S-band system will be designed specifically for providing tracking, telemetry and control, while the X-band will be used exclusively for telemetry and scientific data. These systems will operate within their specified ranges (S-band: transmit – 2290-2300 MHz, receive – 2110-2120 MHz, X-band: transmit – 7145-7190 MHz, receive – 8400-8450 MHz[1]) as would be specified by the ____Governing body____.

The basic mission requirements and assumptions have changed since the general specifications laid out in Assignment 3, primarily that the satellite must land on, or come in contact with, the comet at some point rather than perform a fly-by. With this in mind, the individual components that will be used on the communications system are detailed in the following sections

Antennas:

There will be both an omnidirectional S-band helical antenna[2], specifically designed for telemetry and commands, as well as a 2.2 metre high-gain antenna[3], akin to the Rosetta satellite antenna. This antenna has optimal performance within both S- and X-band frequencies and both are manufactured by RUAG Space AG, based in Switzerland.

Transponder:

The system will incorporate two redundant small deep-space transponders (SDST’s) [4], developed by General Dynamics and NASA’s Jet Propulsion Laboratory. This device combines a number of communication functions – receiver, command detector, telemetry modulator, exciter, beacon generator and control functions all into one package. This transponder has Ka-band capability as well for future missions, which comprises of a second X-to-Ka band multiplier.

Envelope Size: 7.13”L x 6.55”W x 4.50”H

Mass: 7.0 lbs (3.2 kg)

Input Supply Power:

Receiver Only: 12.5 W

Receiver + X-band Exciter: 15.8 W

Amplifiers:

Two 17 W, 8.4 GHz solid-state power amplifiers[5], manufactured by General Dynamics will be implemented as smaller, lighter and less expensive alternative to the traveling-wave-tube X-band amplifier. These amplifiers are designed for use as a ‘companion unit’ to the SDST and can supply telemetry signals that can be connected directly to the SDST to make a complete transmitter/receiver with a single data interface.

Maximum dimensions: 6.85”L x 5.275”W x 1.85”H

Mass: 3.02 lbs (1.37 kg)

Data interface: MIL-STD-1553B data interface

Other components:

Other smaller components include a diplexer, attached to the high-gain amplifier, which will allow the S- and X- band transmitter to use the same antenna, as well as allowing the antenna to be used for transmissions on one band and receive on another band. The system will also require a coupler to assign the amplifiers to the respective antennas as well as a hybrid coupler between the amplifiers and the transponders to allow either transponder to drive either amplifier without requiring active switching.

Issues in Deep Space Communications:

Compared with normal satellite communications, deep-space communications present a significant challenge – specifically from the distance resulting in low signal-to-noise ratio, propagation delays, corruption as well as environmental factors such as temperature variations and electromagnetic radiation. The satellite will be passing behind the Sun for a period of time, it is important to note that communication will be masked for a substantial period of time. One possible solution is to take advantage of NASA’s STEREO (Solar TERrestrial Relations Observatory) satellites in orbit around the sun to provide a link between the satellite-comet intercept point and Earth while the satellite is obscured.