Satellite Communication With Mars Satellite Term Paper

Excerpt from Term Paper :

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…

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