Deep Space Communication - Engineering Seminar Report

 INTRODUCTION

Deep space exploration and utilization are all along the dreams of human beings. The exploration and utilization of the deep space are all along the dreams of human beings. Since the Soviet Union began to explore the moon by using moon-1 in January 1959, there has existed drastic competition in the area of deep space exploration and utilization among the countries all over the world, especially among the United State, Russia and some countries in Europe. Besides the technologies of launching and controlling of the probe, deep space communications has played an important role in deep space exploration.

Deep Space Communication transmits the information obtained by the probe to the ground and processes and analyzes it. Deep space usually refers to the outer space more than 2 million kilometers away from the earth. And deep space communications is referred to as communication  between the earth and other planets (including the Moon, the Mars, the Jupiter  etc.). Now spacecrafts are send to the farthest planet Pluto called Newhorizons which will enter into Pluto’s orbit in 2015.  Among them, the explorations to the Mars and moon are more frequent. In recent forty years, Russia, United State and several Europe countries have made explorations to the Mars more than thirty times and sent probes to the Moon. European Space Agency (ESA), Japan, China  and India also have their own Moon exploration probes right now.      

Comparison with Normal Communication Compared with common terra and satellite communications, deep-space communications presents more challenging environment for data communications. The radio frequency channel predominantly used for communication typically operates under the following constraints.

Long Distance: A lot of planets in deep space are several hundred million kilometers away from the earth. Such long distance results in very low signal to noise ratio (SNR).

High Signal Propagation Delays: This is due to the enormous distances involved between the communicating entities and the relativistic constraint restricting signal transmissions to the speed of light. For example, one-way signal propagation delays for the Cassini mission to

Saturn are in the range of 1 hour and 8 minutes to 1 hour and 24 minutes.

High Data Corruption Rates: Extremely long distances cause the signals to be received at extremely low strengths at the receiver, and thereby increase the probability of bit-errors in the channel due to random thermal noise errors, burst errors due to solar flares, etc.

Disruption Events: Since communicating entities in deep-space tend to be in motion relative to one another, the communication channel between them is prone to disruption. A planetary probe on the surface of Saturn’s moon Titan, for example, could experience disruption due to

the rotation of Titan on its own axis (when it goes to the night side of Titan), when Titan passes under Saturn’s shadow during its revolution around the planet, and when other moons/ planets/or the Sun itself block the line of sight to the destination. Moreover, communicating with

an entity in deep-space requires expensive specialized equipment.

Complex Geography Environment: In the moon and other planets, conditions such as the temperature radiation and liberation etc are more complex than those in the earth. For example, the variation of the temperature in the moon is very high, from -183°C to 127°C. The lowest temperature is -132°C in Mars and -140°C in Jupiter. So the electronics in the spacecraft must be designed to support these extreme temperature variations.

SPACECRAFTS

spacecraft is the destination system of the deep space communication. spacecrafts are launched for scientific study, the launch cost is higher as the spacecraft become bigger so we use light weight systems and equipments so we cannot incorporate large antennas or powerful communication systems. The spacecraft's small, light communications equipment consequently transmits at very low power, typically limited to 20 watts, about the same as a refrigerator light bulb. Signal power arriving at the antenna can be as weak as one 100-millionth of one 100-billionth of a watt - 20 billion times less than the power required for a digital wristwatch. To "hear" the whisper of a signal from a spacecraft at planetary distances, receiving antennas on Earth must be very large and be equipped with highly sensitive receivers. The two main antennas in a spacecrafts are high gain antenna and a low gain antenna.

Low Gain Antenna

Low-gain antenna that sends a very simple signal that small receivers can pick them up on Earth. The "low-gain" antenna is constantly broadcasting one of four possible signals by a simple code.

• Everything is OK
• Track me when you can
• Track before a certain time
• Help! Red alert!

High Gain Antenna

Other, more important and complicated data is sent with the high-gain antenna only when NASA can be relatively sure that DSN will pick up the signal. On scheduled times, the large DSN (Deep Space Network) receivers are used to receive this "high-gain" signal with its more complete information. This signal is used to send most of the scientific data DS1 will collect.

 FUNCTIONS  OF  DSN

some of the functions of DSN are shown below

 Telemetry

The purpose of the Telemetry System is to provide the capability to acquire, process, decode and distribute deep space probe and Earth orbiter telemetry data. Telemetry data consists of science and engineering information modulated on radio signals transmitted from the spacecraft. The Telemetry System performs three main functions: Telemetry data acquisition, telemetry data conditioning and transmission to projects and telemetry system validation.two types of telemetry,  science data and engineering data. science data is the important data because the mission success depends on this data. engineering data contains the health and positional information of the spacecraft.

Spacecraft Command

The purpose of the Command System is to provide the means by which a Project controls the activities of its spacecraft. Control information (Command Data), provided by the Project, is modulated on the RF carrier and transmitted to a spacecraft by a DSN station. The Command System functions as a transfer medium between the Project Control Center and its spacecraft.

Radiometric Tracking

The purpose of the Tracking System is to provide two-way communication between Earth based equipment and spacecraft, to make measurements that will allow the state vector (position and velocity) of spacecraft to be determined.

Very Long Baseline Interferometry

The purpose of the Very Long Baseline Interferometry (VLBI) System is to provide the means of directly measuring plane-of-the-sky angular positions of radio sources (natural or spacecraft), DSN station locations, interstation time and frequency offsets, and Earth orientation parameters.

Radio Science

The field of Radio Science improves our knowledge of the solar system and the theory of general relativity through radio frequency experiments performed between spacecraft and the Deep Space Network's (DSN) Radio Science System. In the past, Radio Science has performed experiments which have allowed scientists to characterize planetary atmospheres and ionospheres, characterize planetary surfaces, characterize the planetary rings, characterize the Solar corona, confirm general relativity, characterize interplanetary plasma, search for gravitational waves, characterize planetary gravity , and determine the mass of the planets, moons, and asteroids.

Monitor and Control

The purpose of the Monitor and Control System is two-fold: to provide real time monitor data to projects which reflect the status of project support by DSN systems, and to provide monitor and control capabilities to operators of DSN systems' components.

WORKING  OF  DSN

This section includes the working and challenges of deep space communications.

The theory and challenges of deep space communications

Distance is the main problem in space communications, since the intensity of electromagnetic radiation decreases according to 1/r2, that is why signals from deep space probes are usually very weak when they reach the Earth. In order to receive the faint signal back on Earth large parabolic disc antennas are used. To collect as much as possible of the faint signal the antenna dish must be big. Since the electromagnetic radiation cannot move faster than the speed of light there are considerable time lag introduced in the communications making real time communications impossible.

It takes over 5 hours for a signal from earth to reach the orbit of Pluto in the outer part of the solar system. In order to communicate with the Earth the spacecraft must have a free line of sight to the Earth, since radio waves cannot pass through large solid objects such as planets and moons. A space probe orbiting a planet will therefore lose contact with earth every time it gets on the far side of the planet. This means that the spacecraft will not be able to communicate with the Earth at all times. Even if the probe has a free line of sight to the Earth the receiving antenna could be on the wrong side of the Earth, however by using several antennas in different places around the planet that could be solved.

The gain of an antenna is a measure of how good the antenna is at focusing the radiated energy. A low gain antenna radiates in a wide angle, while a high gain antenna radiates in a narrow beam. On spacecraft high gain antennas are used to send scientific measurements at high data rates back to earth as well as receiving steering commands from earth; these antennas are highly directional and require very accurate aiming. Spacecraft are always equipped with at least one low gain antenna often two. These low gain antennas are very important since they can intercept signals from almost any direction, this is useful if the spacecraft gets disoriented and the main high gain antenna doesn’t point towards Earth. If the spacecraft only had a high gain antenna it would then not be able to receive any more instructions from Earth. The low gain antennas are used in these kinds of situations as a backup to receive the appropriate commands that will turn the spacecraft so that the main antenna gets properly aligned to earth again. However, the low gain antenna can only handle a fraction of the data rate compared to the high gain antenna.

Frequency used by DSN

Since the signal has to pass through the Earth’s atmosphere some limitations are placed on which frequencies that could be used.

The ionosphere is almost opaque to some of the lower frequency bands so space communication mainly uses high frequency bands between 2GHz and 40GHz which are less affected by atmospheric disturbances. However at these frequencies one start to get interference from molecular excitations, there are several frequency bands that could not be used because of this e.g. water has a strong resonance frequency at 22GHz. Water is a severe problem at frequencies above 2GHz, dense clouds, rain and snow can distort and absorb large parts of a transmission. Despite that, frequencies above 2GHz are in common use for space communication, that is because higher frequencies allows for higher data rates, short wavelength radiation can carry much higher data rates than long wave radiation.

 The space industry is always looking for ways to increase the data rate between Earth and interplanetary probes; low data rate has always been limiting factor during interplanetary communications. On a common interplanetary space probe the low gain antenna usually receives/transmits in the S-band while the high gain antenna receive/transmit in the X band, however we are now in a transition phase and in the future the high gain antennas will be used with the higher Ka-band.

Coding used in DSN

Most missions employ error detecting – error correcting codes to substantially improve telemetry link performance. DSN users are reminded that their encoders should conform to the CCSDS Telemetry Channel Coding Blue Book (CCSDS 231.0-B-1, September 2003.

Acceptable codes include:

1) Convolutional r = 1/2, k = 7 only.

2) Reed-Solomon 223/255 only.

3) concatenated  Convolutional / Reed-Solomon

4) Turbo codes with rates: 1/2, 1/3, 1/4, or 1/6, block sizes: 1784, 3568, 7136, and 8920.

CCSDS File Delivery Protocol (DSN) to improve station utilization efficiency as well as reduce mission risk and costs, all DSN users should employ the CCSDS File Delivery Protocol (CFDP), to transfer data to and from a spacecraft. CFDP operates over a CCSDS conventional packet telecommand, packet telemetry, or an Advanced Orbiting System (AOS) Path service link. CFDP enables the automatic transfer of a complete set of specified files and associated information from one storage location to another replacing an expensive labor-intensive manual method. It can transfer a file from a source point to a destination site using an Automatic Repeat Queuing (ARQ) protocol.

Bandwidth Efficient Modulation (DSN)

Missions operating in the 2 and 8 GHz bands, should employ bandwidth efficient modulation methods in conformance with SFCG (space frequency coordination group) and CCSDS Recommendations.

Multiple Spacecraft Per Antenna (MSPA)

Where a multiplicity of spacecraft lie within the beam width of a single DSN antenna, it may be possible to capture data from two or more spacecraft simultaneously using the Multiple Spacecraft per Aperture (MSPA) system. MSPA decreases DSN loading and will save the project’s money. There are a few constraints.

First, only a single uplink frequency can be transmitted. Generally, this means that only one spacecraft at a time can operate in a two-way coherent mode, while the remainder must be in a one-way (i.e., non-coherent) mode.

 Second, multiple independent receivers are required at the Earth station. This sets a practical limit of two spacecraft that can be served simultaneously.

Third, ranging and two-way coherent Doppler data can only be obtained from the single spacecraft operating in a two-way coherent mode.

Approximately 30-minutes are required to transfer two-way coherent operations from one spacecraft to another irrespective of whether or not the spacecraft, which will be in the two-way coherent mode, is currently part of the MSPA cluster. When switching the uplink from one spacecraft to the next, full Aperture Fee (AF) costs apply to the new two-way coherent user at the onset of the switching operation. Transfers of two-way coherent operations require:

1)  Tuning the uplink of the spacecraft in a two-way coherent mode to its rest frequency,

2)  Setting the station uplink frequency to the next spacecraft’s and acquiring the uplink,

3)  Reconfiguring the command subsystem (if required) for the next spacecraft,

4)  Reconfiguring ranging (if required) for the next spacecraft,

5)  Reconfiguring the Monitor and Control subsystem,

6)  Relocking the Earth station’s receiver and telemetry processor following the switch.

For a Project to avail itself of the MSPA savings, the following conditions must apply:

 All spacecraft must lie within the beamwidth of the requested antenna.  Projects must accept reduced link performance from imperfect pointing. Spacecraft downlinks must operate on different frequencies. Only one spacecraft at a time can operate with an uplink in a coherent mode.

a. Commands can only be sent to the spacecraft receiving an uplink.

            b. Ranging & coherent Doppler are available from the spacecraft in a 2-way mode.            c. Remaining spacecraft transmit 1-way downlinks with telemetry only.

Data Relaying

Some missions may propose dropping probes, landers, or even rovers to explore the surface of a planet/body. Others may insert orbiters around the same body. The result can be a multiplicity of spacecraft on or around a planet/body. While Mars has been the recent focus, it is foreseeable that other planets or objects in space could be of equal interest in the future. Where several spacecraft are relatively close together and positioned far from the Earth, it makes sense to send data to and from small vehicles via a relay (Proximity Link). Typically, this has been an orbiting spacecraft carrying a special transceiver operating at UHF frequencies.

Relaying data from surface objects can save money and reduce size and power requirements of landed equipment. Proposals for landed objects in the vicinity of an orbiting spacecraft should consider whether a data relay makes sense for their application. Some Announcements of Opportunity (AOs) have required orbiting spacecraft with certain characteristics to carry Proximity Link hardware.

Critical Event Communications

Some times spacecraft needed emergency telemetry support during Critical Events. Critical Events are defined as: “spacecraft events that could result in the loss of mission if anomalies occur.” These events include launch, early orbit operations, and those listed as follows:

 Spacecraft separation

 Powered flight

 Critical Maneuvers (e.g., DSMs)

 Orbit insertion

 Entry/Descent/Landing

 Flybys

An Earth station is normally required during launch, early orbit and separation. It could be one of the DSN or NEN Earth stations if the launch trajectory permits; however, in cases where there are gaps, another Agency’s Earth station or a small portable station may be required. The costs for Critical Event support must be included in the proposal.

MANNED SPACE FLIGHT NETWORK (MSFN)

 Tracking vehicles in low Earth orbits is quite different from tracking deep space missions. Deep space missions are visible for long periods of time from a large portion of the Earth's surface, and so require few stations (the DSN uses only three). These few stations, however, need huge antennas and ultra-sensitive receivers to cope with the very weak signals. Low earth orbit missions, however, are only visible from a small fraction of the Earth' surface at a time, and the satellites move overhead very quickly. Therefore a large number of tracking stations are required, spread all over the world. The antennas do not need to be so big, but they must be able to track quickly.

These differing requirements led NASA to build a number of independent tracking networks, each optimized for its own mission. Prior to the mid 80's, when the TDRSS satellites became operational, NASA used a several networks of ground based antennas to track and communicate with earth orbiting spacecraft. For the Mercury, Gemini, and Apollo missions, these were the primary means of communication, with the DSN being assigned a supporting/backup role.

The Apollo Missions

The MSFN during the Apollo era was also called the Apollo Network. Large dish antennas with high gains, such as the 26-m paraboloids employed in the DSN, would have to be added to the MSFN to track and communicate at lunar distances. Extant MSFN stations could not properly monitor the very critical mission phases when the spacecraft was inserted into its lunar trajectory and when it plunged into the narrow reentry corridor on the return trip. The result was that the MSFN had to be extended with ships, aircraft, and additional land sites. Small paraboloidal antennas would have to be added at some MSFN sites to communicate with the Apollo spacecraft while it was still below the horizon for the 26-m dishes (below about 16,000 km) but beyond the range of the Gemini telemetry antennas. The communication traffic during the Apollo missions would be several times that planned for Gemini. NASCOM lines would have to be augmented. To meet these requirements, the MSFN used a combination of resources. A JPL system called "Unified S Band" or USB, was selected for Apollo communications. It allowed tracking, ranging, telemetry, and voice to all use the same S band transmitter. Near-earth tracking was provided by upgrading the same networks used for Mercury and Gemini. New large antennas for the lunar phase were constructed explicitly for the MSFN, with DSN large antennas used for backup and critical mission phases.

DSN Support during Apollo

Although normally tasked with tracking unmanned spacecraft, the Deep Space Network (DSN) also contributed to the communication and tracking of Apollo missions to the Moon, although primary responsibility remained with the MSFN. The DSN designed the MSFN stations for lunar communication and provided a second antenna at each MSFN site (the MSFN sites were near the DSN sites for just this reason). Two antennas at each site were needed since the beam widths of the large antennas needed were too small to encompass both the lunar orbiter and the lander at the same time. DSN also supplied some larger antennas as needed, in particular for television broadcasts from the Moon, and emergency communications such as Apollo 13.

Another critical step in the evolution of the Apollo Network came in 1965 with the advent of the DSN Wing concept. Originally, the participation of DSN 26-m antennas during an Apollo Mission was to be limited to a backup role. This was one reason why the MSFN 26-m sites were collocated with the DSN sites at Goldstone, Madrid, and Canberra. However, the presence of two, well-separated spacecraft during lunar operations stimulated the rethinking of the tracking and communication problem. One thought was to add a dual S-band RF system to each of the three 26-m MSGN antennas, leaving the nearby DSN 26-m antennas still in a backup role. Calculations showed, though, that a 26-m antenna pattern centered on the landed Lunar Module would suffer a 9-to-12 db loss at the lunar horizon, making tracking and data acquisition of the orbiting Command Service Module difficult, perhaps impossible. It made sense to use both the MSFN and DSN antennas simultaneously during the all-important lunar operations. JPL was naturally reluctant to compromise the objectives of its many unmanned spacecraft by turning three of its DSN stations over to the MSFN for long periods. How the goals of both Apollo and deep space exploration could be achieved without building a third 26-m antenna at each of the three sites or undercutting planetary science missions?

The solution came in early 1965 at a meeting at NASA Headquarters, when Eberhardt Rechtin suggested what is now known as the "wing concept". The wing approach involves constructing a new section or "wing" to the main building at each of the three involved DSN sites. The wing would include a MSFN control room and the necessary interface equipment to accomplish the following:

1. Permit tracking and two-way data transfer with either spacecraft during lunar operations.

2. Permit tracking and two-way data transfer with the combined spacecraft during the flight to the Moon

3. Provide backup for the collocated MSFN site passive track (spacecraft to ground RF links) of the Apollo spacecraft during trans-lunar and trans-earth phases.

With this arrangement, the DSN station could be quickly switched from a deep-space mission to Apollo and back again. GSFC personnel would operate the MSFN equipment completely independently of DSN personnel. Deep space missions would not be compromised nearly as much as if the entire station's equipment and personnel were turned over to Apollo for several weeks.

INDIAN DEEP SPACE NETWORK (IDSN)

The Indian Deep Space Network consists of a 18-m and a 32-m antennae that are established at the IDSNcampus, Byalalu, Bangalore. The Network is augmented with a couple of stations in the western hemisphere in addition to the 64-m antenna in Bearslake, Russia to improve the visibility duration and to provide support from the antipodal point.             The existing ISTRAC (ISRO Telemetry Tracking and Command Network) S-Band Network stations will be used to support the mission during Launch and Early Orbit Phase (LEOP) that includes Earth Transfer Orbit (ETO) up to a range of about 1,00,000 km. Although the 18-m antenna is tailored for Chandrayaan-1 mission, the 32-m antenna can also support other planetary missions. The established IDSN is a state-of-the-art system, with its base band system adhering to CCSDS (Consultative Committee for Space Data Systems) Standards, thus facilitating cross-support among other TTC agencies. The supporting network stations will ensure the adequacy of the link margin for telemetry/dwell, tracking, telecommand payload data reception. The IDSN station has the responsibility of receiving the spacecraft health data as well as the payload data in real time. Later, conditioning of the data takes place, before onward transmission of the same to Mission Operations Complex at Bangalore. The tracking data comprising Range, Doppler and Angle data will be transferred to the control center for the purpose of orbit determination. The payload data will be transmitted to the Indian Space Science Data Center (ISSDC) as and when received by the payload data acquisition system, located at the station.

The 18-m dish antenna is configured for Chandryaan-1 mission operations and payload data collection. The antenna is established at the IDSN Campus, Byalalu, situated at the outskirts of Bangalore with built in support facilities. A fibre optic/satellite link will provide the necessary  communication link between the IDSN Station and Mission Operations Complex (MOX) / Indian Space Science Data Centre (ISSDC). This antenna is capable of S-Band uplink (2 kW) and both X-Band and S-Band downlink. This system has provision to receive two downlink carriers in S-Band and one carrier in X-Band (RCP and LCP)

simultaneously, whereas, the uplink is either RCP or LCP. The system will have a G/T of 30/39.5 dB/K (45º elevation, clear sky) for S/X-Band. The base-band system will adhere to the CCSDS Standards. The station can be remotely operated from ISTRAC Network Control Centre (NCC). The figure7.1  depicts the 18-m antenna.

  The wheel and track 32-m antenna is a state-of-the-art system that will support the Chandrayaan-1 mission operations and beyond. This is co-located with 18-m antenna in the IDSN site at Byalalu. A fibre optics / satellite link will provide the necessary connectivity between the IDSN site and Spacecraft Control Centre / Network Control Centre. This antenna         is designed to provide uplink in both S-Band (20/2 kW) and X-Band (2.5 kW), either through RCP or LCP. The reception capability will be in both S-Band and X-Band (simultaneous RCP & LCP). It can receive two carriers in S-Band and one carrier in X-Band, simultaneously. The system will have a G/T of 37.5/51 dB/K (45° elevation, clear sky) for S/X-Band. The base-band will adhere to CCSDS Standards facilitating cross-support among the space agencies. The station is also equipped for remote control from the ISTRAC Network Control Centre (NCC).    

Existing S-Band ISTRAC Network     Indian lower earth orbit satellites are controlled by the ISRO Telemetry Tracking and Command (ISTRAC) Network stations. The Elevation over Azimuth 10/11/12-m dish antennae at the existing ISTRAC network stations (Bangalore, Lucknow, Mauritius, Bearslake, Biak, Brunei, Trivandrum and Port Blair) will be augmented to serve the Chandrayaan-1 mission during Earth Transfer Orbits and Lunar Transfer Trajectory up to a range of about 1,00,000 km. All these antennae are configured for two-carrier reception (RCP&LCP) and uplink, in either RCP or LCP in S-Band. The G/T of the stations is 21/23 dB/K. The base-band will adhere to CCSDS Standards, facilitating cross-support among the TTC agencies. The stations are being equipped for remote control from the ISTRAC Network Control Centre (INCC). These stations are linked to MOX by dedicated communication links.       

External network stations APL, JPL (Goldstone, Canberra, Madrid), Hawaii, Brazil (Alcantara, Cuiaba) are requisitioned in for the purpose of extended visibility of Launch and Early Orbit Phase (LEOP) operations, as well as to gain the near continuous visibility during the normal phase operations. All the external stations will ensure the required compatibility to communicate with the spacecraft.