5.3 The Satellite Network Segment (SNS)

General

The SNS is the Wireless, Satellite based component of the HSTN. Possibly, in the final system, the HSTN will have several satellites. The number and types of satellites to be used in the final system depends on many factors that will be described in Requirements for SNS .

The SNS will be the most expensive sub-system of the HSTN. Hence, obviously, it would make sense to view it from the business perspective too. The most critical aspect of decision making for this sub-system is ``When ?'' . This problem can be seen as an interplay of :

1. Entering the market early on will lead to the capturing of a larger market share.
2. However, one has to note that especially in the area of satellites, new technology is constantly changing the capabilities and the economics. For example, launching costs are expected to fall drastically in the near future due to rapid advances in Materials technology and the emergence of new players in the satellite launching business. Similarly, advances in hardware technologies are leading the emergence of very small and light satellites with the comparable capabilities of present day satellites. In about 10 to 15 years the communication between Ground Stations and Satellites might be laser based leading to much higher bandwidths at much lower costs.

So, if a system is put up now, with an expected lifetime of say 10 years, it is possible that the systems that would be put up 5 years from now, would be much more cheaper and thus could potentially take over bigger market segments, the present system would end up making huge losses. Thus, the assessment of this risk is a very important business aspect.

Components and Relations Between Them

The SNS can be decomposed into the following subsystems:

• Satellites;
• Ground Stations.

Satellites

The satellites envisaged in this system is more than just a bent pipe. This system involves satellites with satellite-to-satellite communications capabilities. Currently, research efforts are being made to make this possible.

An experimental Japanese Store and Forward Satellite System

Further, decisions regarding the number of planes in the satellite system, the number of satellites in the satellite system, their altitude, inclination, eccentricity and technical capabilities of individual satellites and the satellite system architecture have to be made. These decisions can be directly mapped onto the requirements of the satellite system. We have done some analysis of the requirements for the satellite system, but, the resolution of the above issues require a much deeper understanding of the problem and its requirements and it is felt that it is beyond the scope of this project. However, references, [Ref8] and [Ref9] do deal with most of the issues involved in the design of a satellite system.

Ground Stations

The Ground Stations in this section is synonymous with the Output Subsystem in the NOS and its purpose is to transmit user data to the satellite for broadcast.

Requirements for SNS

1. [SNSR] The SNS will have a worldwide coverage.
2. [SNSR] The SNS will have a sufficient amount of redundancy inbuilt inorder to handle satellite failures.
3. [SNSR] The individual satellites in the SNS will be able to handle atleast 20MBps of traffic (this assumes that each satellite will handle only one Ground Station at a time).
4. [SNSR] Each individual satellite will be either a LEO (Low Earth Orbital) or a MEO(Middle Earth Orbital) satellite so as to give a minimum propagation delay that is consistent with the QoS requirements available from HSTN.
5. [SNSR] Each satellite in the system will be capable of store and forward of the data.
6. [SNSR] Based on the architecture of the system, the number of intersatellite links for each satellite will be decided. These are to be based on the level of fault tolerance of the system and the storage capabilities of individual satellites.

Standards Applicable at SNS level

• MIL-STD-1582 Satellite Data Link Standard

Design

After a literature survey, it was found that the ideal satellite system architecture is a seamless constellation network on account of the fact that these kinds of networks offer relatively fixed delays, where the delay encountered depends only upon the relative locations of the communicating stations (Ground Stations). The frequency ranges at which these satellites operate is expected to be in the 20-30GHz range. The biggest hurdle in this is expected to be the design and development of the antennae for use at ground stations and in the satellites themselves.

A major feature of satellite launching is that it does not exhibit economies of scale. The cost per satellite for any such system is expected to be between $1 million to $2 million.