Satellite Communication With Mars

Satellite Communication With Mars

Satellite Communication

The use of Satellites Communication satellites for data and information transfer are now becoming common for both national as well, as international usage. As one pundit notes," Our world is becoming one of ever closer contact with our neighbors, and the most advanced system for relaying instantaneous messages is the communication satellite" (Cassata & Asante, 1979, p. 135). The International communications satellites that have been launched since 1965 have shown the effectiveness of the satellite system.

However, the use of satellites for communication and data transfer also extend beyond the parameters of the earth. This paper will examine various aspect of satellite communication with Mars.

The following discussion will therefore deal with the various architectures, protocols and methods of data transference that are necessary in order to facilitate satellite communication between Earth and Mars. This discussion will also deal worth the envisaged IPN or Interplanetary network and the data and relay requirements that are needed to deal with the demands and problematics of satellite communication and data sharing between planets.

Brief Overview and background

In essence a communication satellite is essentially a relay station that can be used to provide for the transfer of data and information. Put rather simplistically, "A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency" (Satellite Basics: Guide To Satellite-based Solutions).

One could also describe a satellite as a specialized wireless receiver or transmitter, as it receives radio waves from one location to another via a 'bent pipe" that is launched by a rocket and placed in orbit around the earth (SATELLITES). It is also important to note that they have many other uses besides communication. These include wide-area network communications, weather forecasting, television broadcasting, amateur radio communications, Internet access and the Global Positioning System (SATELLITES). Furthermore, scientific studies of our planet, the atmosphere and the universe all rely on satellites.

Most satellites are situated in a circular orbit about 35,800 kilometers above the surface of the earth (Cassata & Asante, 1979, p. 135). At its most fundamental, satellite communications have two main components. The first is the satellite itself, also known as the space segment. This is comprised of three distinct units; namely, the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the following elements: "… the receiving antenna to pick-up signals from the ground station, a broad band receiver, an input multiplexer, and a frequency converter which is used to reroute the received signals through a high powered amplifier for downlink" (Cassata & Asante, 1979, p. 135).

The second aspect of conventional satellite communication is the Ground Station or the earth segment. This Ground Station has a double function which is to act as an uplink to transmit data. This is usually in the form of baseband signals which are passed through a baseband processor, via an amplifier and through a parabolic dish antenna up to the orbiting satellite (Satellite Communication). However, as will be discussed in the following sections, this situation becomes much more complex and convoluted when communicating via satellite with Mars.

There are also other fundamental aspects so satellite communication that could be noted and that have reference to the topic of this paper. For instance, an important characteristic of satellite systems is that they can be implemented so as provide two basic types of circuits: permanently assigned and demand assigned (Cassata & Asante, 1979, p. 138). This is a factor that is relevant when it comes to the architecture of communication with satellites in deep space. Briefly, permanently assigned circuits provide "…fixed connections through a satellite from transit center to transit center via earth stations" (Cassata & Asante, 1979, p. 138). This is similar to the mode of transfer in cable and microwave circuits. Demand assigned circuits on the other hand, "…may also be established between two earth stations, with the component circuit sections being connected together automatically for the duration of the call" (Cassata & Asante, 1979, p. 138). These circuits are more flexible in terms of data routing.

In terms of the present study the advances in satellite and communications technology has meant that it has now become possible to launch and communicate with artificial satellites in orbits round the other planets. This refers especially to satellite communication with Mars. However, the situation with regard to space communication at this level of complexity presents a number of different criteria that have to be considered in terms of communication. This is due to factors such as distance and line of sight - which is the fact that obstacles to a signal transmission and its reception can prevent communication.

Another fact that needs to be considered in terms of space communication is weight - which refers to the fast that the high-powered sensors and antenna needed in space are often too heavy for practical purposes and transportation.

Protocols and architecture: overview

Conventional satellite architecture involves data transmission via a signal path known as a transponder. Usually satellites have between 24 and 72 transponders. A single transponder is capable of handling up to 155 million bits of information per second (Satellite Basics: Guide To Satellite-based Solutions). This transfer of information is facilitated within radio frequency bands. The frequency bands most used by satellite communications companies are called C-band and the higher Ku-band.

The architecture of satellite usage and transmission in terms of communicating in space and between Earth and other planets is based on the principle of providing reusable and sharable physical infrastructures which are intended to accommodate the communication needs of various space missions. The necessary architecture is furthermore intended to link network assets in terms three zones; namely earth, orbiting and deep space (Tahboub and Khan).

An important aspect of the theory of this architecture is that,

Space protocol architectures will transparently provide end-to-end communication services to space networks. These architectures also describe the design of the protocol suite applied by all assets such as satellites, rovers, and scientific equipments deployed in the space network. (Tahboub and Khan)

Furthermore, this architecture should be interoperable with standard terrestrial communication protocols, enabling both secured and real-time Internet-based access to on-going space missions.

In the light of this outline and considering satellite communication with Mars, there are a number of challenges that have to be considered with regard to the architecture. These include aspects such as long propagation delays, network mobility, link intermittency, limited resource allocation, extreme reliability and security (Tahboub and Khan). As a consequence, major space agencies and research labs have undergone design exploration for a new generation of space protocol architectures to address these design challenges.

The results of this exploration include the IPN or interplanetary networks that will be discussed in more detail in the following sections this paper. This will also be dealt with in the section on protocols and satellite communication with Mars.

Therefore, the issue of communication with Mars means that understanding the underlying and required architecture and protocols for information exchange and sharing should in the first instance distinguish between terrestrial and space communication architecture and protocols.

The environment that has be considered in terms of communication in space share a number of common characteristics. These are, "galactic geography, a set of standard features that consists of service constraints and environmental constraints" (Tahboub and Khan). As referred to, this galactic geography is divided into three interrelated zones. These are earth, orbital and deep space (Tahboub and Khan). As Tahboub and Khan state in a paper entitled Recent Developments in Space Communication Architectures, "…these network assets collectively provide a secured broadband network backbone that links scientist and investigators to mission operation centers (MOC)" (Tahboub and Khan). These mission operation centers are also connected with networks that are comprised of gateways to various space network assets.

Secondly, the orbiting zone is seen as an intermediately region that facilitates access and information transfer to the deep space mission. The orbiting zone network contain various satellites scales, such as LEO, MEO, GEO, micro, and nano satellites, space shuttles, ISS, and lunar orbiters (Tahboub and Khan). Galactic geography into a number of 'zonal-hops'; for example the space backbone refers to the regions between planets and "… consists of a set of relay satellites acting as an interplanetary communication backbone, which aims to interconnect different planetary networks into a global space network" (Tahboub and Khan).

Another important feature of this space communication architecture that will be explored in more detail is the issue of link intermittency. This is related to the dynamic nature of network and satellite communication in space. In many instances links are only active for a limited time period due to the mobility of the communication nodes (Tahboub and Khan). This present a number of problems sand envisaged solutions in terms of space architecture and protocols.

Current and Envisaged Space Communication Architectures

In the light of the above sketch of the architecture that would support Mars satellite communications, it is important to consider the most prevalent and accepted space communication architectures. These include Omni, CCSDS, Hi-DSN and SpaceVPN. OMNI and CCDS will be briefly considered, with the focus on CCSDS, as this architecture has been shown to be the most appropriate for satellite communication with Mars.

OMNI

Omni or the Operating Missions as Nodes on the Internet (OMNI) is an architecture proposed by NASA to provide the most simple and most effective infrastructure for space missions. Central to this architecture is the aim of integrating earth-based networks to space networks -- thereby being cost-effective and relatively easy to implement. This system also provides access to on-board spacecraft equipment by making use of standard remote access protocols (Tahboub and Khan). In other words, this architecture is particularly useful in that it makes use of standard Internet technologies.

OMNI is described as follows.

A space-based server network architecture which permits on demand transfer of mission and control data between client satellites in an orbit about earth and an earth station irrespective of the location of the client satellite relative to the earth station. (Space-based server network architecture)

Further identifying characteristics of this architecture are that it includes a plurality of server satellites which are in an earth orbit. Furthermore, "The server satellites provide substantially total world-wide communications coverage to and connectivity with designated and authorized earth stations and the plurality of client satellites" (Space-based server network architecture).

In this system each server satellite has a communications downlink, which provided communication with earth stations that are within its field of view; as well as "… communications crosslinks for providing intercommunications with other server satellites within its field of view" (Space-based server network architecture). The system works as follows. Client satellite data from Earth is passed to the server satellite. This is then transferred either to the intended client satellite or to another satellite having direct communications access to the intended client satellite. (Space-based server network architecture)

In term of the earth and orbiting galactic zones, this architecture forms the network backbone and combines different network assets. These assets are space missions and control centers, ground base-stations and users (Tahboub and Khan). More importantly, the orbiting zone consists of a satellite network in the vicinity of the earth.

In relation to actual data transfer, this architecture refers to the standard five protocol layers of the OSI-ISO reference model. The ISO or International Standards Organization developed a layered protocol model termed OSI or Open Systems Interconnect. The purpose of these layers is described as follows:"… to provide clearly defined functions to improve internetwork connectivity between "computer" manufacturing companies. Each layer has a standard defined input and a standard defined output" ( Introduction to the ISO - OSI Model). In effect each layer provides services to the layer above and requests service from the layer below (Introduction to the ISO - OSI Model).

Tahboub and Khan describe part of function of this process as follows:

The mechanisms for delivering bits across media like copper, fiber and RF are provided by the physical layer. This layer provides modulation, coding and forward error coding services along with the bit delivery mechanisms. The transmitted data (bits) are recovered by sampling the data line at each clock cycle… (Tahboub and Khan)

However, this robust architecture it is more suited to near -- earth space missions. As Tahboub and Khan state; "On the other hand, OMNI architecture is not expandable for deep space missions based on the fact that IP-based protocols are obsolete for deep space communications due to long propagation delay, mobility and link intermittency" (Tahboub and Khan). CCDS on the other hand is adaptable to deep space missions and for communication with planets like Mars.

CCSDS

Founded in 1982 the CCSDS or Consultative Committee for Space Data Systems is one of the major space agencies in the worlds. It is described as a " multi-national forum for the development of communications and data systems standards for spaceflight" (Welcome to CCSDS.org). In many regards CCDS is similar to the OMNI architecture, CCSDS also implements the standard ISO-OSI reference model but at a much wider scale supporting the deep space galactic zone. The CCSDS architecture is IP based in relation to the earth zone of satellite transmission, but "…IP-interoperable at the orbiting and deep space zones" (Tahboub and Khan). This means that while IP are employed for earth communications in terms of this architecture, IP-compatible protocols, which include SCPS-NP, -TP, and -- FP, are employed in the orbiting and deep space zones (Tahboub and Khan). Security, which is another important facet addressed by this architecture, is employed at the earth zone level by standard authentication and data encryption but by SCPS-NP in the other zones. (Tahboub and Khan) This refers to a protocol that is can deal with both static and dynamic connectivity, as well as multiple routing options (Durst, 1998). It has also been described as "…a new, bit-efficient protocol designed for use in space systems…A truly scalable network protocol for a broad range of spacecraft" (Durst, 1998).

The CCSDS architecture is also concerned with standard network protocols, which includes telemetry, tracking, and command, and information interchange processes, as well as radio-metric and orbit data. This in turn is linked to the concept of IPN and the need for interplanetary communication.

Briefly, the IPN is concept that stems from the model of the Internet as an interconnected system of networks. IPN extends this concept "… to higher level of abstraction, which envisions the entire Internet on a planet as single network, and the interconnection among these disconnected planetary Internets constitutes the IPN" (Tahboub and Khan).

In other words, the aim of IPN is to expand and use of standard Internet protocols to communicate in deep space and with deep space mission, such as Mars. In this regard the CCSDS has been suggesting that an integrated IPN protocol architecture for space communication be developed (Tahboub and Khan).

In this light the three zones that apply to OMNI - earth, orbiting and deep space- also apply to this architecture and its associated protocols; however, it differs in terms of deep space zone in that this area in CCSD architecture consist of "store-and-forward" relay satellites, communication, science spacecrafts (orbiters) and planetary colony networks. The issue of store -- and forward protocols will be addressed in more detail in the following section.

Communicating with Mars

In the 1990's, NASA developed its first deep space communication network, DSN, or NASA Deep Space Network to ensure control of space probes that they sent in exploration missions to Mars and other regions space. DSN is in effect "…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" (ABOUT THE DEEP SPACE NETWORK). NASA has also used this system to communicate with Mars rovers Spirit and Opportunity.

As was referred to briefly above, satellite communications with Mars or with outer space is envisaged as a form of interplanetary internet. In other words, the protocols, information architecture and transmission standards that work in the terrestrial environment are seen by many pundits as being the foundational constructs for the practical trajectory that should be followed for communication between earth and planet like Mars.

However, the literature also points out that the envisaged interplanetary Internet will in many respects be unlike the terrestrial communication backbone where there is connectivity and data transfer that is continuous with few delays and relatively clear channels for data transference. The issue of connectivity between planets presents a vast array of different issued and problems that have to be overcome in terms of data communication. As one important study on this aspect comments:

…the hallmarks of the interplanetary backbone are therefore intermittent connectivity, huge propagation delays and noisy data channels. While the Earth's backbone network is wired -- large numbers of fiber or copper circuits interconnecting fixed hubs -- the interplanetary backbone is dependent on fragile wireless links.

(Burleigh et al., 2003, p.2)

Another consideration in terms of communication with Mars via computer and satellite is that the interplanetary backbone, the relay spacecraft or gateways into remote local Internets, are in motion and moving in relation to one another (Burleigh et al., 2003. p.2). This means that, "Landed vehicles on remote planetary surfaces will move out of sight of Earth as the body rotates, and may have to communicate through local relay satellites that only provide data transmission contacts for a few minutes at a time" ( Burleigh et al., 2003, p.2)

Research into this area has a number of important ramifications for the present topic. In the first instance the discussion of an interplanetary Internet reveals the present state of the architecture, computer and information transmission systems and protocols that are being used for satellite communication with Mars. This research also highlights important areas such as packet transfer that need to be enhanced and developed in order to deal with the specific issues relate to a non-terrestrial communication environment. The following discussion will explore not only the present state of architecture and protocols required for communication between planets, but will also touch on the possibilities and challenges that exist.

As one pundit clearly states; "…successful program of Mars exploration will need a robust, dependable and high capacity space communications infrastructure" (Burleigh et al., 2003, p.3). The TCP/IP suite is mentioned as a requirement in this respect, but the authors of this study also note that "Programs of Mars exploration will need an analogous set of standard capabilities to support automated communications over the vast distances, heterogeneous and stressed environments that make up the Earth-Mars communications system" (Burleigh et al., 2003, p.3). This also refers to the above-mentioned IPN which envisages a six satellite constellation surrounding Mars, as well as new protocols for the transference of data.

The constellation of satellites around is discussed extensively in an article entitled, Constellation Design for a Mars-Orbiting Satellite Communication and Navigation Network by Kallemeyn et al. The article clearly outlines the aims of constellation satellites. "As part of NASA's effort to support Mars exploration, constellation design work has been done for a network of small computers that will provide communication relay and navigation support for a variety of future Mars missions…" (Kallemeyn et al.)., Furthermore, the central aims of this constellation is to "…provide increased data return, enable autonomous onboard navigation without relying on earth-based tracking data, and substantially lower the combined operation costs anticipated for Mars exploration" (Kallemeyn et al.)

Mars Communication: Architecture and Protocols

The following is a brief and selective overview of only of the aspects of Mars communication in terms of the archicture and protocol requirements discussed above The current situation with regard to the communication between Earth and space provides an overview of the present situation and the possibilities for the future. As an enlightening paper in this regard entitled THE INTERPLANETARY INTERNET: A COMMUNICATIONS INFRASTRUCTURE FOR MARS EXPLORATION, presented at the 53rd International Astronautical Congress The World Space Congress (2002) states, the current space / ground communications standards from the Consultative Committee for Space Data Systems takes the form of a set of protocol standards which provide insight into the problematics and possibilities of satellite communication with Mars.

A good example of the way that communication in a non-terrestrial environment and between, earth, satellites and Mars is facilitated is known as the bundling protocol. This protocol deals with a space environment in a number to ways. In the first instance it operates in terms of a "store and forward" method. This is similar or the way that ordinary email functions where bundles of data are held at routers until the establishment of a forward path.

The following is a more formal and detailed description of this protocol in terms of common networking demands.

Delay Tolerant Networking is an end-to-end architecture providing communications in and/or through highly stressed environments. Stressed networking environments include those with intermittent connectivity, large and/or variable delays, and high bit error rates. To provide its services, the DTN

protocols sit at the application layer of the constituent internets, forming a store-and-forward overlay network.

(Scott and Burleigh, 2004)

As Burleigh et al. state of this protocol:

It avoids the need for a sender to store data until an acknowledgement is received from the other end by operating in a "custodial" mode. In this mode, intermediate nodes in the network can assume responsibility for ensuring that bundles reach their destinations, allowing senders (and previous custodians) to reassign resources to new observations. ( Burleigh et al., 2003, p.2)

This also relates to a number of key factors and capabilities in terms of protocols for communication in deep space satellite communication. These include aspects such as custody-based reliability, the ability to cope with intermittent connectivity and the facility to take advantage of scheduled and opportunistic connectivity. (Scott and Burleigh, 2004)

The present communication structure for satellite communication with outer space takes the form of a layered protocol with the different elements stacked in modular fashion. Burleigh et al. (2003) also emphasize that in terms of the Mars Communications Protocol Stack, "The CCSDS File Delivery Protocol (CFDP) is emerging as the leading candidate for the ubiquitous "end-to-end" protocol for most near-term Mars operations" (Burleigh et al., 2003, p.7). This protocol needs to be explicated in detail in order to understand the fundamental aspects of information sharing and data communication between Earth and Mars. However, the parameters of this paper permit only a selective and discursive view of this important aspect

This protocol has a bidirectional architecture and "…allows users to exchange files between assets on and around Mars and facilities on the ground" (Burleigh et al., 2003, p.7 ). It is envisaged that, " Within the Earth's Internet, CFDP will be transported using standard Internet protocols…on the Deep Space backbone"(Burleigh et al.,2003, p.7).