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 Plutoâ„¢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 Saturnâ„¢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 Saturnâ„¢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 -183Ã‚Â°C to 127Ã‚Â°C. The lowest temperature is -132Ã‚Â°C in Mars and -140Ã‚Â°C in Jupiter. So the electronics in the spacecraft must be designed to support these extreme temperature variations.
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.
2.1 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.
Â¢ Everything is OK
Â¢ Track me when you can
Â¢ Track before a certain time
Â¢ Help! Red alert!
2.2 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.
3. DEEP SPACE NETWORK
The NASA Deep Space Network - or DSN - is an international network of huge antennas that allows people on the ground to communicate with satellites and other spacecraft missions, as well as providing radio and radar astronomy observations for the exploration of the solar system and the universe. It is best known for its large radio antennas. The network also supports selected Earth-orbiting missions. DSN is part of the NASA Jet Propulsion Laboratory (JPL).
3.1 History of Deep Space Network
The forerunner of the DSN was established in January, 1958, when JPL, then under contract to the U.S. Army, deployed portable radio tracking stations in Nigeria, Singapore, and California to receive telemetry and plot the orbit of the Army-launched Explorer 1, the first successful U.S. satellite. NASA was officially established on October 1, 1958, to consolidate the separately developing space-exploration programs of the Army, Navy, and Air Force into one civilian organization.
On 3 December 1958, the JPL was transferred from the Army to NASA and given responsibility for the design and execution of lunar and planetary exploration programs using remotely-controlled spacecraft. Shortly after the transfer of the JPL to NASA, NASA established the concept of the Deep Space Network as a separately managed and operated communications system that would accommodate all deep space missions, thereby avoiding the need for each flight project to acquire and operate its own specialized space communications network. The DSN was given responsibility for its own research, development, and operation in support of all of its users. Under this concept, it has become a world leader in the development of low-noise receivers; large parabolic-dish antennas; tracking, telemetry, and command systems; digital signal processing; and deep space navigation.
The largest antennas of the DSN are often called on during spacecraft emergencies. Almost all spacecraft are designed so normal operation can be conducted on the smaller (and more economical) antennas of the DSN, but during an emergency the use of the largest antennas is crucial. This is because a troubled spacecraft may be forced to use less than its normal transmitter power, attitude control problems may preclude the use of high-gain antennas, and recovering every bit of telemetry is critical to assessing the health of the spacecraft and planning the recovery. The most famous example is the Apollo 13 mission, where limited battery power and inability to use the spacecraft's high gain antennas reduced signal levels below the capability of the Manned Space Flight Network, and the use of the biggest DSN antennas (and the Australian Parkes Observatory radio telescope) was critical to saving the lives of the astronauts. Although in this case Apollo was also a USA/NASA mission, DSN also provides this same emergency service to other space agencies as well, in a spirit of inter-agency and international cooperation. For example, the recovery of the Solar and Heliospheric Observatory (SOHO) mission of the European Space Agency (ESA) would not have been possible without the use of the largest DSN facilities.
3.2 Present Condition
DSN currently consists of three deep-space communications facilities placed approximately 120 degrees apart around the world. They are:
Â¢ The Goldstone Deep Space Communications Complex outside of Barstow, California, United States;
Â¢ The Madrid Deep Space Communication Complex, 60 kilometers (37 miles) west of Madrid, Spain; and
Â¢ The Canberra Deep Space Communications Complex (CDSCC) in the Australian Capital Territory, 40 kilometers (25 miles) southwest of Canberra, Australia near the Tidbinbilla Nature Reserve.
Each facility is situated in semi-mountainous, bowl-shaped terrain to shield against radio frequency interference. 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.
NASA's scientific investigation of the Solar System is being accomplished mainly through the use of unmanned spacecraft. The DSN provides the vital two-way communications link that guides and controls these machines, and brings back the images and new scientific information they collect. All DSN steerable, high-gain, antennas are parabolic Figure 1: DSN facility around the world reflector antennas.
The antennas and data delivery systems make it possible to:
Â¢ Acquire telemetry data from spacecraft.
Â¢ Transmit commands to spacecraft.
Â¢ Track spacecraft position and velocity.
Â¢ Perform Very Long Baseline Interferometry observations.
Â¢ Measure variations in radio waves for radio science experiments.
Â¢ Gather science data.
Â¢ Monitor and control the performance of the network.
The network is a facility of the JPL and is managed and operated for NASA by the California Institute of Technology (Caltech). The Interplanetary Network Directorate (IND) manages the program within JPL.
3.3 Antennas in DSN
Each complex consists of at least four deep space terminals equipped with ultra-sensitive receiving systems and large parabolic-dish antennas.
Â¢ One 34-meter (111-ft) diameter High Efficiency antenna.
Â¢ One or more 34-meter Beam Waveguide antennas (three at the Goldstone Complex, two at the Robledo de Chavela complex (near Madrid), and one at the Canberra Complex).
Â¢ One 26-meter (85 ft) antenna.
Â¢ One 70-meter (230 ft) antenna.
Five of the 34-meter beam waveguide antennas were added to the system in the late 1990s. Three were located at Goldstone, and one each at Canberra and Madrid. A second 34-meter beam waveguide antenna (the network's sixth) was completed at the Madrid complex in 2004.
Antenna arraying combines the signals received by multiple antennas at different locations to synthesize a single large antenna. It is commonly used to improve reception of weak signals. Arraying is beneficial in deep space communications where the signal transmitted by a spacecraft becomes very weak as it travels across vast interplanetary distances. When the signal arrives at Earth, it is spread over a large area, and the ground antenna is able to receive just a small part of the signal. Arraying allows the capture of these very weak signals and enables a higher data rate.
The ability to array several antennas was incorporated to improve the data returned from the Voyager 2 Neptune encounter, and extensively used for the Galileo spacecraft, when the high gain antenna did not deploy correctly. The array electronically links the 70-meter dish antenna at the Deep Space Network complex in Goldstone, California, with an identical antenna located in Australia, in addition to two 34-meter (111 ft) antennas at the Canberra complex. The California and Australia sites were used concurrently to pick up communications with Galileo.
Arraying of antennas within the three DSN locations is also used. For example, a 70-meter dish antenna can be arrayed with a 34-meter dish. For especially-vital missions, like Voyager 2, the Canberra 70-meter dish can be arrayed with the Parkes Radio Telescope in Australia; and the Goldstone 70-meter dish can be arrayed with the Very Large Array of antennas in New Mexico. Also, two or more 34-meter dishes at one DSN location are commonly arrayed together.
All the stations are remotely operated from a centralized Signal Processing Center at each complex. These Centers house the electronic subsystems that point and control the antennas, receive and process the telemetry data, transmit commands, and generate the spacecraft navigation data.
Once the data is processed at the complexes, it is transmitted to JPL for further processing and for distribution to science teams over a modern communications network, frequently using satellite communications.
4. FUNCTIONS OF DSN
some of the functions of DSN are shown below
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.
4.2 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.
4.3 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.
4.4 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.
4.5 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.
4.6 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.
5. WORKING OF DSN
This section includes the working and challenges of deep space communications.
5.1 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 doesnâ„¢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.
5.2 Frequency used by DSN
Since the signal has to pass through the Earthâ„¢s atmosphere some limitations are placed on which frequencies that could be used.
S-Band 1.55 â€œ 5.2 GHz (2.3 GHz)
X-Band 5.2 â€œ 10.9 GHz (8.4 GHz)
Ku-Band 12 â€œ 8 GHz
Ka-Band 20 â€œ 40 GHz (32 GHz)
Table 5.1: commonly used frequency bands for space communication, most common frequencies within parenthesis
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.
5.3 Coding used in DSN
Most missions employ error detecting â€œ 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.
In an acknowledged mode, the receiver notifies the transmitter of any undelivered file segments or ancillary data so that the missing elements can be retransmitted guaranteeing delivery. An unacknowledged mode is also permitted.
5.4 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.
5.5 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 projectâ„¢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 spacecraftâ„¢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 stationâ„¢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.
5.6 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.
5.7 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:
Critical Maneuvers (e.g., DSMs)
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 Agencyâ„¢s Earth station or a small portable station may be required. The costs for Critical Event support must be included in the proposal.
5.8 Unmanned Spacecraft Tracking
One of the prime objective of DSN is telemetry and tracking of unmanned space missions.there are about 3 dozens of spacecrafts right now functioning,some of them are in the edge of our solarsystem.Let me explain the tracking and telemetry of mars exploration rovers.
5.8.1 Mars Exploration Rovers
NASA's Mars Exploration Rover (MER) Mission is an ongoing robotic mission of exploring Mars, that began in 2003 with the sending of two rovers Spirit and Opportunity to explore the Martian surface and geology.
Primary among the mission's scientific goals is to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars. The mission is part of NASA's Mars Exploration Program which includes three previous successful landers: the two Viking landers in 1976 and Pathfinder in 1997.
Two Mars rover missions will be launched by NASA in May and June of 2003, during the 2003 Mars launch opportunity. They are the Mars Exploration Rovers, MERA and MERB. The spacecrafts will enter the Mars atmosphere directly, without first going into Mars orbit. The rovers will land on the Mars surface in January and February of 2004, in a similar manner to the successful Mars Pathfinder landing in 1996.
5.8.2 EDL sequence
During the Entry, Descent and Landing (EDL) phases, it is important to maintain communications from the spacecraft to the Earth. Although this communication cannot affect the landing because the long round-trip-light time precludes real time feedback from Earth to the spacecraft, the communication could be critical to the success of future missions. This is especially true in case of a mission failure, when the diagnostic data would be very important.
As the EDL scenario of Figure 5.1 begins, the lander is enclosed in a heat shield and a backshell, all attached to the main spacecraft. The entry turn begins approximately 70-min before entry, properly orienting the heat shield. When this is completed, approximately 15-min prior to entry, the cruise stage separation occurs, leaving the lander protected by the heat shield. Entry, defined as reaching a predefined altitude above the Mars surface, occurs approximately 365-s before landing.
Figure 5.1 steps of EDL sequence of MER.
5.8.3 Communications Links
The rover has a low-gain and a high-gain antenna. The low-gain antenna is omnidirectional, and transmits data at a low rate to Deep Space Network (DSN) antennas on Earth. The high-gain antenna is directional and steerable, and can transmit data to Earth at a higher rate.
The rovers also use the low-gain antennas to communicate with spacecraft orbiting Mars, the Mars Odyssey and (before its failure) the Mars Global Surveyor. The orbiters relay data from and to Earth; most data to Earth is relayed through Odyssey. The benefits of using the orbiters are that they are closer to the rovers than the antennas on Earth, and have view of Earth for much longer than the rovers. The orbiters communicate with the rovers using UHF antennas, which have shorter range than the low and high-gain antennas. One UHF antenna is on the rover and one is on a petal of the lander to aid in gaining information during the critical landing event.
From cruise stage separation until the lander is separated from the backshell, communication is by a direct-to-Earth (DTE) X-band (8.4-GHz) link, using the backshell low-gain antenna (BLGA). After the lander separates from the backshell, the BLGA can no longer be used. From this point until landing, two methods of communication will be used: a DTE link using the rover low-gain antenna (RLGA), and a UHF relay link. The UHF link transmits the data to either the Mars Odyssey or the Mars Global Surveyor spacecraft, which then relays the data to the Earth using a standard phase-coherent X-band link. The reason that the UHF relay link is used is that sufficiently reliable communication is not possible with the DTE link, as explained later. The UHF link is the prime communication link, but it is not as reliable as desired, so the DTE link will also be used, as a backup. Although the UHF link is prime during this period, it is not discussed further in this paper, because the subject of this paper is the DTE link. After landing, the UHF link will no longer be used, and the DTE link is again the only link.
5.9 Spacecraft Emergencies On November 2, 2006, NASA planned to communicate with MGS through the Deep Space
Network (DSN) via a prescheduled 13-minute routine contact. Prior to this contact, commands had been transmitted to MGS. These commands were designed to move the position of the solar arrays away from the sun line in order to maintain thermal control. At the beginning of the contact on November 2, the spacecraft reported numerous alarms, indicating that one solar array drive had been stuck and that the spacecraft had automatically switched to the redundant drive controller. The spacecraft telemetry also gave indication that the solar array drive was rotating freely on the redundant hardware and gave no indication the mission was in immediate danger.
The spacecraft operations team (Lockheed Martin (LM) in Denver, Colorado) appropriately and immediately contacted the necessary engineering personnel to help troubleshoot the problem.
At the next scheduled contact, approximately 2 hours later, the normal spacecraft signal was not detected by the main DSN receivers. The operations team subsequently attempted to command the spacecraft multiple times, without success. On November 4, 2006, the operations team declared a spacecraft emergency to ensure long-term DSN antenna coverage. All attempts to command the spacecraft and reestablish communication were unsuccessful. During the week following the anomaly, it was discovered that radio science equipment at the DSN, operating on a pre-programmed observation schedule, had recorded signals from MGS just hours after the initial anomaly. However that signal was below the detection limits of the main DSN receivers. Beginning on November 14th, 2006, NASAâ„¢s Mars Reconnaissance Orbiter (MRO) and ESAâ„¢s Mars Express tried unsuccessfully to image and locate MGS. Formal recovery efforts were terminated on January 28th, 2007
6. 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.
6.1 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.
6.2 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.
7. 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 IDSN campus, 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)
Figure 7.1: 18-m Antenna simultaneously, whereas, the uplink is either RCP or LCP. The system will have a G/T of 30/39.5 dB/K (45Ã‚Âº 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 Figure 7.2: 32-m 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 (45Ã‚Â° 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.
The Indian Deep Space Network has been built to track and support India's first lunar mission Chandrayaan-1, an unmanned lunar exploration mission by the Indian Space Research Organisation (ISRO), India's national space agency. It was launched on 22 October 2008. The IDSN will be used for tracking, orbit control and housekeeping operations of India's lunar mission for its entire duration of two years. IDSN began to track Chandrayaan 17 minutes after its launch from the Satish Dhawan Space Launch Centre at Sriharikota, when the satellite separated from the launch vehicle.
8. INTRODUCTION TO SETI
Search for Extra-Terrestrial Intelligence (SETI) is the collective name for a number of activities to detect intelligent extraterrestrial life. The general approach of SETI projects is to survey the sky to detect the existence of transmissions from a civilization on a distant planet â€œ an approach widely viewed by the scientific community as hard science. The United States Government contributed to SETI early on, but recent work has been primarily funded by private sources.
There are great challenges in searching across the sky for a first transmission that could be characterized as intelligent, since its direction, spectrum and method of communication are all unknown beforehand. SETI projects necessarily make assumptions to narrow the search, and thus no exhaustive search has so far been conducted.
The first 360 feet (110 m) wide, 500 feet (150 m) long, and 70 feet (21 m) high Kraus-style radio telescope was powered up in 1963. In the March 1955 issue of Scientific American, Dr. John Kraus, Professor Emeritus and McDougal Professor of Electrical Engineering and Astronomy at the Ohio State University, described a concept to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within two years, his concept was approved for construction by the Ohio State University. With $71,000 total in grants from the National Science Foundation, construction began on a 20-acre plot in Delaware, Ohio. This Ohio State University radio telescope was called Big Ear. Later, it began the world's first continuous SETI program, called the Ohio State University SETI program.
9. FUTURE OF DEEP SPACE COMMUNICATION
There's a good chance that humans will travel to Mars before we see the beginning of a new century. How will we communicate with these distant travelers Scientists, engineers and programmers are already working to develop an interplanetary Internet that will connect us to probes and human space travelers, and allow more information to be sent back to Earth.
9.1 Interplanetary Internet
You can talk to almost anyone, in any corner of the world, almost instantly because of the Internet and other advances in electronic communication. Scientists and space explorers now are looking for a way to communicate almost instantly beyond Earth. The next phase of the Internet will take us to far reaches of our solar system, and lay the groundwork for a communications system for a manned mission to Mars and planets beyond. If we ever want to find out more about other planets, we will need a better communication system for future space missions. Today, communication in space moves at a snail's pace compared to communication on Earth. There are several reasons for this:
Â¢ Distance -- On Earth, we are only a fraction of a light second apart, making Earth communication nearly instantaneous over the Internet. As you move farther out into space, however, there is a delay of minutes or hours because light has to travel millions of miles, instead of thousands of miles, between transmitter and receiver.
Â¢ Line of sight obstruction -- Anything that blocks the space between the signal transmitter and receiver can interrupt communication.
Â¢ Weight -- High-powered antennas that would improve communication with deep space probes are often too heavy to send on a space mission, because the payload must be light and efficiently used.
Take a look at the 1997 Mars Pathfinder rover mission and you will understand space explorers need an interplanetary Internet for deep space communications. Data from the Pathfinder trickled back at an average rate of about 300 bits per second during its mission. Most likely, your computer can transfer data at least 200 times faster than that. An Internet between Mars and Earth would likely yield a data transfer rate of 11,000 bits per second. That is still much slower than your computer's transfer rate, but it would be enough to send back more detailed images of the Mars surface. Mars Network researchers think that the transfer rate could eventually go to about 1 Megabyte (8,288,608 bits) per second and allow anyone to take a virtual trip to Mars.
An interplanetary Internet is like the Earth's Internet on a grand scale and with some improvements. Here are the three basic components of the proposed interplanetary Internet:
Â¢ NASA's Deep Space Network (DSN).
Â¢ A six-satellite constellation around Mars.
Â¢ A new protocol for transferring data.
The DSN is the international network of antennas used by NASA to track data and control navigation of interplanetary spacecraft. It is designed to allow for continuous radio communication with the spacecraft. In an interplanetary Internet, the DSN will be the Earth's gateway or portal to that Internet. In a paper published by the MITRE Corp., a company that is financing the Interplanetary Internet Study, researchers suggest that the DSN's antennas could be pointed at Mars to connect Earth and Mars for at least 12 hours each day. Satellites orbiting Mars should provide a full-time connection between the two planets. A Martian rover, probe or human colony will provide a Mars portal to the interplanetary Internet. Under the Mars Network plan, the DSN will interact with a constellation of six microsatellites and one large Marsat satellite placed in low Mars orbit. These six microsats are relay satellites for spacecraft on or near the surface of the planet, and they will allow more data to come back from Mars missions. The Marsat will collect data from each of the smaller satellites and beam it to Earth. It will also keep Earth and distant spacecraft connected continuously and allow for high-bandwidth data and video of the planet, according to Mars Network officials. NASA could launch a microsat as early as 2003, with the six-microsat constellation orbiting Mars by 2009. In 2007, the Marsat is scheduled to be placed in a slightly higher orbit than the constellation. All of these dates are still very tentative.
Programmers are developing an Internet file transfer protocol to transmit the messages and overcome delays and interruptions. This protocol will act as the backbone of the entire system much as the Internet protocol (IP) and transmission control protocol (TCP) operate on Earth. IP and TCP, co-developed in the 1970s by Dr. Vinton Cerf, are the messenger service for our Earth-based Internet. These two protocols break up transmitted messages into packets of small data units and route them to a specified destination.
Cerf is part of the team of scientists who are developing a new protocol to enable reliable file transfer over the long distances between planets and spacecraft. This new space protocol must keep the Internet running even if some packets of data are lost during transmission. It must also block out noise picked up while crossing millions of miles. One idea for the space protocol is called the parcel transfer protocol (PTP), which will store and forward data at the gateway of each planet. The protocol would process an information request sent to a gateway and forward it to a final destination. The gateway would then check, process and forward information back down the path it came.
9.2 Astronomical Challenges
An interplanetary Internet will make data move drastically faster between Earth and the probes and other spacecraft that are millions of miles away. Engineers need to overcome several challenges before we plan our virtual journey to Mars through cyberspace. These challenges are:
Â¢ The speed-of-light delay.
Â¢ Satellite maintenance.
Â¢ The possibility of hacker break-ins.
On Earth, two computers connected to the Internet are only a few thousand miles away at the most. Because light travels at 186,000 miles per second, it takes only a few fractions of a second to send a packet of data from one computer to another. In contrast, distances between a station on Earth and one on Mars can be between 38 million miles (56 million km) and 248 million miles (400 million km). At these distances, it can take several minutes or hours for a radio signal to reach a receiving station. An interplanetary Internet will not be able to duplicate the real-time immediacy of the Internet that you use. The store-and-forward method will allow information to be sent in bundles and overcome the concern of data being lost due to delays.
The satellites of the Mars Network will be tens or hundreds of millions of miles from Earth and that means that it will be hard to get up there to fix things when they go wrong. The components of these satellites would have to be much more reliable than those circling Earth.
Hackers pose the biggest threat to an interplanetary Internet. Break-ins and corruption of navigation or communication systems could be disastrous for space missions, and even cause deaths in manned-spacecraft missions. Developers are taking every precaution to design a system that will be able to control access. The protocol selected will have to be impenetrable to hackers, something that has not been possible on Earth. Developers may look at the Secure Sockets Layer (SSL) protocol used for financial transactions as a model for securing the interplanetary Internet.
The interplanetary Internet will possibly wire us to Mars within the decade and to other planets in the decades to follow. It will no longer be necessary to go into space to experience space travel. Instead, space will be brought right to your desktop. With enhancements made to boost data rate transfers, you and I might soon be able to take a virtual space trip to the mountains of Mars, the rings of Saturn or the giant spot on Jupiter.
Since all these spacecraft are controlled by the DSN. There are a number of limitations to the current DSN, and a number of challenges going forward.
Â¢ There is only one DSN site in the Southern Hemisphere, Canberra. There are no DSN network dishes in South America or Southern Africa, so the DSN coverage of the Southern Hemisphere is limited.
Â¢ The need to support "legacy" missions that have remained operational beyond their original lifetimes but are still returning scientific data. Programs such as Voyager have been operating long past their original mission termination date. They also need some of the largest antennas.
Â¢ The DSN's deferred maintenance of its 70m antennas. This causes problems where they are out of service for months at a time. Furthermore, they are reaching the end of their lives. At some point they will need to be replaced. The leading candidate is an array of smaller dishes.
Â¢ By 2020, the DSN will be required to support twice the number of missions it was supporting in 2005.
Also the new development in Interplanetary Internet and optical communication networks it is possible for the DSN efficiency to increase.
By 2015 NASA is planning for a second Moon mission and by end of 2030 a manned mission to Mars. These all require a powerful Earth communication Network.
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