ACKNOWLEDGEMENTS

WMO would like to acknowledge the work of the lead author, Dr Mike Prior-Jones, the present Chair of Satcom Forum, and the contributions of the other members of the Satcom Executive Committee, especially Johan Stander, Sean Burns and Andy Sybrandy. Considerable thanks are also due to David Thomas, from the WMO Secretariat, for coordinating the work and producing the Web version.

1: Introduction and overview

1.1 Introduction

This is the first edition of the Satellite Data Telecommunication Handbook. This handbook is a guide to using satellite telecommunication systems and is provided as an attachment to the Guide to the WMO Information System (WMO-No.1061). It is intended for scientists and managers who are considering using satellite communications to collect data from remote instrumentation located either on land or at sea. The handbook aims to provide an overview of the state of the market at the time of writing (April 2018) so that users can quickly identify which satellite services are appropriate for their needs. Since the market is evolving rapidly, an online version will be made available via the WMO website https://wiswiki.wmo.int/Satcom-Guide and kept up to date as new systems are introduced to the market and older ones are retired.

This is the first publication from the joint WMO-Intergovernmental Oceanographic Commission (IOC) International Forum of Users of Satellite Data Communications, better known as the Satcom Forum. The Satcom Forum is a relatively new international body whose purpose is to provide clear information about satellite communications to scientific users and to liaise with the industry to advocate for appropriate new technical features and suitable pricing structures. For more details about the Satcom Forum, please see https://wiswiki.wmo.int/Satcom.

The editorial approach has been to work exclusively with publicly available information, including marketing brochures, manuals and other published documents. The handbook only considers systems that were in operation at the time of writing; several satellite networks had new services in development while this handbook was being drafted, but as these services were not yet functioning, their final specifications were unknown. The online version includes clearly marked sections contributed by the satellite operators which provide more details about their product offerings.

1.2 Why satellite communication?

Satellite communication is now more than sixty years old. The Soviet Union launched Sputnik I on 4 October 1957, and the first communications satellite was launched by the United States the following year. Since then, satellite communication has become firmly established as one of three basic techniques for long-distance telecommunications, alongside cables and terrestrial wireless networks.
The key benefit of satellite communication is its independence from ground-based infrastructure. This is particularly valuable in hazardous and isolated regions (such as oceans, deserts and the polar regions) and in places where conflict or natural disasters have destroyed the terrestrial infrastructure. For those in need of reliable emergency communications, satellite communications can literally save lives.

For scientific users, particularly meteorologists, hydrologists and marine scientists, satellite communication offers a convenient way to collect real-time data from instruments in the field. Marine science was an early adopter of remote instrumentation, with satellite services making it possible to immediately and reliably collect data from the open oceans. Satellite communication is also used to track marine creatures and larger birds.

1.3 Connectivity options for remote instruments

There are various connectivity options for equipment installed in remote locations:

(a) Physical retrieval. The instrument stores the data locally (for example, on a memory card, USB stick or local hard drive), and the operator visits the instrument periodically to retrieve the data. The operator may have to wait months or years for the data, but there is no ongoing cost apart from the site visits. This option is often used if a regular visit needs to be made to do maintenance work on the instrument.

(b) Cable/wireline/landline. A wired connection is typically a copper or fibre-optic cable, such as a phone line. These connections generally offer the highest data rates and can be reliable provided that the infrastructure is not disrupted. However, cables can be cut by construction work or damaged by high winds or storms. Power failures at the telephone exchange or router sites can also disrupt communications. In addition, in some countries, the landline network offers a poor quality of service and can be expensive. In truly remote regions, a wireline option is usually out of the question.

(c) Terrestrial wireless. Terrestrial networks include common services, such as cell phones and point-to-point microwave transmission, as well as more esoteric means of communication, such as packet radio, meteor burst, ionospheric scatter, tropospheric scatter and high-frequency (HF) (also known as short-wave) radio. Low-power wide-area networks (LPWANs), SigFox and LoRaWAN, for example, are being deployed commercially in some countries and are designed specifically to offer low data rates for telemetry and remote control applications at a low operating cost. Cellular networks are also starting to offer similar long-range, low-data-rate services (for example, LTE Cat NB1, also known as NB-IoT) to cater to this market. Terrestrial wireless systems are still very dependent on a fixed infrastructure, which can be disrupted by natural disasters, commercial failure or civil unrest.

(d) Satellite communication. Assuming that a terminal can obtain a clear view of the sky, satellite services allow connectivity from anywhere in the world, with no dependence on ground-based communication infrastructure. If the system includes its own independent power supply (batteries, solar, wind, and so forth), communications will not be disrupted unless the terminal itself is disabled or destroyed. The drawbacks to satellite communication are the relatively low data rates from small terminals when compared with terrestrial wireless and the possible higher cost per bit transmitted.

1.4 Types of satellite orbit

Communications satellites are commonly classified by their type of orbit. There is a huge variety of possible orbits, but in practice, only a small number of orbit types are used for communications systems:

(a) Geostationary orbit (GEO): Geostationary satellites sit directly above the Earth’s equator and have an orbital period of 24 hours. They are stationary relative to the Earth’s surface, which means that a single satellite can cover a large, fixed area of the Earth’s surface. Terminals do not require any active steering; they can be pointed at the satellite at the time of installation and then fixed. There are two key drawbacks to geostationary orbits: they cannot provide coverage to the polar regions beyond 80 degrees latitude, and they introduce an unavoidable latency in communications. Geostationary orbits are located at an altitude of 35 000 km, and at this distance, the speed at which light travels produces an end-to-end latency of about a quarter of a second. For a two-way link (such as a telephone call), this results in an apparent half-second delay on the line.

Note:The use of geostationary satellites at between 60 and 80 degrees latitude requires special care and attention and should be discussed with the supplier. When approaching 80 degrees latitude, multiple large antennas may be required in order to obtain a sufficiently strong signal.

(b) Low Earth Orbit (LEO): This is a broader classification of the term “orbit” and refers to satellites that are placed much closer to the Earth. An altitude of 300 km is the lowest practical orbit, and typically, LEO satellites orbit at altitudes of between 600 km and 2 000 km. The low altitude offers much lower latency compared with GEO. LEO satellites move rapidly with respect to the Earth’s surface; a typical LEO satellite will be in view of a ground station for 10 minutes on each pass. Providing real-time global coverage requires a large fleet of satellites (called a “constellation”) in slightly different orbits. Iridium and ORBCOMM both take this approach; Iridium had 66 active satellites in its original constellation. There are also networks that use smaller constellations (such as Argos and Gonets). In such cases, it may be necessary to wait for a satellite to come into view (called “waiting for a pass”) before a message can be sent. The message is often stored on the satellite and downlinked* when it passes over a ground station. This type of network design is called “store-and-forward” and can introduce minutes or even hours of delay between the transmission of the message to the satellite and its delivery. Many LEO satellites are in polar orbits, meaning that they pass over both poles and can thus “see” the whole surface of the Earth over the course of their orbital cycle. A key drawback to LEO satellites is that they are subject to small amounts of drag from the Earth’s upper atmosphere and thus tend to have a shorter service life than GEO satellites. The drag slows the satellites down, and eventually they will re-enter the atmosphere and burn up. Positioning thrusters are used to keep the satellites in the correct orbit, but once they are out of fuel, the satellite is either boosted into a higher “graveyard orbit” or put into a re-entry trajectory where it will burn up safely over the Pacific Ocean
Note:In the satellite industry, “uplink” refers to a signal travelling up to a satellite, and “downlink” refers to a signal travelling down to a terminal on the ground.

(c) Medium Earth Orbit (MEO): This term is used to describe orbits with altitudes between 2 000 km and 35 000 km. Global Positioning System (GPS), GLONASS and Galileo navigation satellites are MEO satellites, but this orbit is not commonly used for communications satellites because the terminals require a tracking antenna. However, O3B operates a constellation of 12 MEO satellites that provide high-speed, low-latency broadband to equatorial countries and to ships at sea.

1.5 Antenna issues

All radio systems experience path loss, which is caused by the spreading out of radio signals from the transmitter. The further the signal is from the transmitter, the weaker it becomes, just as light becomes fainter as it moves away from a lamp. This relationship is an inverse-square law; the signal power drops according to the square of the distance. A radio signal sent to a LEO satellite needs to travel at least 300 km, and to reach a GEO satellite, it must travel 35 000 km. Overcoming path loss is key for satellite communications.


There are three approaches to overcoming path loss:
(a) Increase the transmitter power;
(b) Increase the receiver sensitivity;
(c) Use directional antennas.

Increasing the transmitter power is practical up to a point. Satellites are solar powered, and their performance is usually constrained by the amount of electrical power available for the transmitters. On the ground, it is possible to use very powerful transmitters at the expense of electrical efficiency; provided that you have access to mains power or generators.

Receiver sensitivity has improved as electronics technology has advanced. First-generation satellite receivers in the 1950s were cooled with liquid nitrogen to improve their noise performance, whereas now we have sensitive solid-state electronics with superior performance.

The design of the antenna allows the radio signal to be directed into a narrow beam, rather than radiating equally in all directions. The beam can be thought of as a searchlight, as opposed to a bare light bulb. Concentrating the wave into a beam effectively increases the power of transmission in the direction of the beam. The classic parabolic dish antenna design produces this narrow beam; the larger the dish, the narrower and more powerful the beam. The effective amplification of a signal in the direction of the beam as compared to the signal produced by an omnidirectional antenna is known as “antenna gain”.


There is, however, always a trade-off: a directional antenna must be pointed accurately towards the satellite. This is what makes geostationary satellites so attractive; because the satellite appears “fixed” in the sky, a highly directional antenna can be lined up on the satellite and then fixed in position. For other types of orbit, either the antenna must track the satellite (with the consequent cost and complexity), or the user must forego the gain of a directional antenna and operate with a simpler wide-beam antenna with little or no gain. Note that an antenna used on a moving land vehicle, ship or aircraft must be of the tracking type.

Tracking antennas for ships are bulky and expensive because of the need to constantly correct for the ship’s motion. Traditionally they were gyro-stabilised with large rotating flywheels, but modern control electronics can sense the ship’s motion and automatically point the antenna. However, this requires powerful motors to move the heavy antenna rapidly as the ship pitches and rolls.