Store-and-Forward and Data Relay Satellite Communications Services

Commercial telecommunications satellite systems that provide fixed (FSS), mobile (MSS), or broadcasting (BSS) satellite services provide essentially “real-time” communications to satisfy the market needs of their commercial customers or support military communications requirements. The service is not precisely simultaneous in that there is close to a quarter second delay in the case of a satellite relay that travels from Earth to a geosynchronous satellite and then back to Earth. For normal commercial satellite services to support voice, data links, radio or audio channels, videoconferencing, or television service, the satellite link is provided on as close to a real-time basis as is technically possible.

There are, however, a variety of communications satellite services that are variously known as store and forward, business-to-business (B2B) relay, machine to machine (M2M), or data relay satellites. These types of “data relay” satellite services are typically not as instantaneous as is the case with the big three services – namely, fixed, mobile, and broadcasting. This type of service is usually machine-to-machine data relays, and thus some delay in the transmission is usually not important.

Thus, what makes these types of satellite offering different is that there can be an acceptable time delay in the satellite data relay service. This delay, depending on the nature of the satellite service, can range from less than a second, to minutes, and to even hours. These various types of data relay satellite services will be addressed in this chapter. There is actually a wide variety of these satellite services that can also operate in different frequency bands. These diverse services and satellite types are designed to meet rather different types of communications and networking services. Some data relay satellites are very simple, small, and low-cost satellites that support amateur radio or volunteer efforts. Others are much more complex and actually support commercial customers. Yet others are designed for satellite-to-satellite interconnection and can be large, complex, and rather costly satellites.

Among these various types of data relay or machine-to-machine (M2M) services are the following: (i) amateur radio relay that is provided via so-called OSCAR satellites in the amateur radio band to allow global AMSAT connectivity and (ii) data networking using small satellites in various types of LEO or MEO orbits, or constellations, to provide non-real-time data relay services, often of a public service nature. Yet, this can also support commercial B2B or M2M services such as the Orbcomm satellite system. This commercial satellite system was designed to provide a minimum gap in connectivity and carry out such functions as near real-time tracking and communications with vehicles and ships and (iii) data relay services from GEO orbit to allow broadband communications with satellites or spacecraft in low or medium Earth orbit – or even UAV surveillance systems. These GEO-based data relay satellites are able to track and connect with lower orbit satellites and thus relay data from such satellites with minimum delay to ground communications centers half way round the world in close to real time. Such types of data relay satellites can be used to connect to spacecraft with passengers on board. These data relay satellites, such as NASA’s Tracking and Data Relay Satellites, were used to support flights during the US Apollo moon program and then with the Space Shuttle. These TDRS allowed NASA to maintain connection to ground control facilities on close to a real-time basis. The different technical aspects of the various types of store-and-forward or data relay satellite systems, the frequencies they utilize, and the various types of services they support are all addressed in this chapter.

The common denominator for these diverse types of satellite services is that they are not real time but rather involve some elements of time delay. In the case of the most sophisticated data relay satellites, the connection may involve a delay on the order of a second. In the case of the most basic and low-cost store-and-forward satellite systems, the delay may be a period of several hours from the initial uplink to the ultimate downlink of the data message. Today, new types of store-and-forward data relay satellites can be quite sophisticated and high-cost systems that can handle high data rates. These new type of data relay satellites have progressed a long way forward in terms of data throughput capabilities and can be more than a thousand times more capable than the first types of simple data relay satellites of the 1960s and 1970s. These much more broadband and sophisticated data relay satellites also operate in many higher frequency bands.