Small satellites mark their presence since the beginning of the Space Age. But the triumph of trunk communications via satellite, along with manned exploration of space has taken the space industry towards ever larger and more expensive missions. Smaller and cheap satellites used to be the exclusive domain of scientific and amateur groups. Now major advances in microelectronics, in particular microprocessors, have made smaller satellites a possible alternative. They provide cost-effective solutions to traditional problems at a time when space budgets are decreasing.
Interest in small satellites is growing fast world-wide. Businesses, Governments, Universities and other R&D Organisations around the world are starting their own small satellite programmes & are selling the satellite technology to various developing countries and helping their own Nations as well.
But what are the benefits to be gained from using small satellites? Traditionally satellites have become ever larger and more powerful. INTELSAT, a trunk communications satellite, has a design life of 10-14 years, weighs 4600kg at launch, and has deployed dimensions of 6.4 x 3.6 x 11.8m. It generates 2600W, and can support up to 120,000 two way telephone channels, and three TV channels. Consequently development times and satellite costs have been rising, and a single in-orbit failure can be costly. A typical modern micro-satellite weighs 50kg, has dimensions 0.6m x 0.4 x 0.3m, and generates 30W. Smaller satellites offer shorter development times, on smaller budgets and can fulfill many of the functions of their larger counterparts. As micro-satellites can benefit from leading edge technology, their design life time is often more limited by the rapid advances in technology rather than failure of the on-board systems.The significant reductions in costs make many new applications feasible. Recently it has been recognised that small satellites can complement the services provided by the existing larger satellites, by providing cost effective solutions to specialist communications, remote sensing, rapid response science and military missions, and technology demonstrators.
Recently constellations of satellites have been proposed to provide voice and data communications to mobile users world-wide. These systems are divided into "Little LEO's" and "Big LEO's and MEO's". The latter offer a real time mobile voice communication systems and require medium sized and powerful satellites, but the little LEO's will provide data services, and can be successfully implemented by small satellites. These systems no doubt will establish the small satellite in the marketplace. More...[Little LEO constellations]
The numbers of satellites launched in the three main small satellite categories are plotted below.
The mass distribution of small satellite (<500kg mass) is plotted below for the period 1980-1999. The trend line shows an upward trend, but this is deceptive. It can be seen that the number of mini satellites in the 100-500kg mass class has increased, and that their trend is towards lighter spacecraft. It could be argued that technology has permitted larger spacecraft to be built smaller making the mini satellite class spacecraft more popular.
For micro satellites the trend is also towards smaller satellites, and the first modern nano satellites have been launched towards the end of the 1990's. The general trend is also marginally downwards, although statistics are distorted by the early Soviet military constellations and communications satellite constellations of the late 1990's.
The clients type are plotted for the period 1980-1999, as well as the yearly distribution over this period. A clear trend can be seen from military to civil small satellite missions as the primary market. Government share of small satellite missions is also growing.
The applications are plotted for the period 1980-1999, as well as the yearly distribution over this period.
First of all, it is worth defining what we mean by a small satellite. The commercial space sector is typified by the Geostationary communications satellite, ranging in on-orbit mass from 1000kg to well over 4000kg. Current trends are for the in-orbit mass of these spacecraft to increase to 8000-12,000kg.
For many other applications, there have been trends towards smaller spacecraft, with particular emphasis on reducing cost and development time scales. For the purpose of these pages, a mass below 500kg is considered a small satellite, however the mission development philosophy is also relevant, to distinguish the new generation of small satellites from the more traditional small satellites that typified the early exploration of space.
The spirit of the small satellite world has been encompassed by the slogan "Faster, Better, Smaller Cheaper", although various high profile failures of some international spacecrafts have made this phrase less popular . Nevertheless, small satellite projects are characterised by rapid development scales for experimental missions when compared with the conventional space industry, with kick-off to launch schedules ranging from just six to thirty-six months. Leading-edge or terrestrial COTS technology is routinely employed in order to provide innovative solutions and cheaper alternatives to the established methods and systems. As such they permit lighter satellite systems to be designed inside smaller volumes. Frequently, traditional procedures, with roots in the military and manned space programmes, can no longer be justified, and low cost solutions are favoured to match the reducing space budgets. So in many ways "Faster, Better, Smaller, Cheaper" is a philosophy and an objective in small satellite programmes.
Many terms are used to describe this rediscovered class of satellites, including SmallSat, Cheapsat, MicroSat, MiniSat, NanoSat and even PicoSat and FemtoSat!
Group name
Wet Mass
Large satellite
>1000kg
Medium sized satellite
500-1000kg
Mini satellite
100-500kg
Small Satellites
Micro satellite
10-100kg
Nano satellite
1-10kg
Pico satellite
0.1-1kg
Femto satellite
<100g
Within this classification, the term "Small Satellite" is used to cover all spacecraft with in-orbit mass of less than 500kg. The small satellites we are concerned with throughout these pages are therefore satellites weighing approximately less than 500kg.
Another definition proposed by the IAA for inexpensive small satellites is
The mass distribution for small satellites as launched is plotted below, and illustrates that there are no clear mass boundaries. What can be said is that there are periods where spacecraft within certain mass classes are either rare or absent. For instance there is a general lack of spacecraft in the 100-200kg towards the end of the 1990's. These gaps appear as a result of available
aka Command and Data Handling System
The telecommand system provides for remote control of the spacecraft, and implements safety and security measures. This is typically achieved through radio control of on-board power switches. More complex tasks are sometimes catered for, often using an On-Board Computer, for instance loading new software tasks
In order to permit greater interoperability of spacecraft and ground segment, some International space agencies have proposed a common standard in 1982, recommended by the international Consultative Committee for Space Data Systems (CCSDS). As the protocol aims to include all possible applications, it is complex and has not yet established itself in smaller mission. Another factor in this is the cost of its implementation in both space and ground segment hardware and software. Nevertheless it is anticipated that this standard will soon take a foothold in the micro satellite world. One potential contender is becoming the use of IP "internet protocols", as the code is commonly used and often freely available. Many ground stations are already connected to the internet, and extending the link to the satellite has a number of attractions.
In modern TM/TM sub-systems, On-Board Computers play a vital role.
Attitude Control and Determination
ADCS equipment has traditionally been large and expensive, and only recently have small sub-systems become available. A common approach taken in particularly in early microsatellite, or in missions where requirements are not too demanding is to use no stabilisation, or rely on magnetic stabilisation. This is termed passive stabilisation, as no energy or control is required. A passive stabilisation scheme requires a design that can cope with the power and temperature variations, and has to rely on omnidirectional communication antennas. Examples of spacecraft employing these techniques are the AMSAT microsatellites (1990).
Where the power or thermal regimes must be guaranteed, inertial pointing is possible and sometimes utilised. The spacecraft is spun up about a suitable axis so that the spin axis remains fixed in inertial space. Another form of passive stabilisation is to use a gravity gradient boom, or aerodynamic boom for satellites in a very low Earth orbit. The gravity gradient effect tends to align the spacecraft in the gravitational field along the major axis. Active control systems typically employ actuators such as magnetorquers, reaction wheels, momentum wheels or thrusters to generate a torque. The simplest and most common active control system employs a gravity gradient boom and magnetorquers. Lately, wheel technology has become practical for micro satellites and is increasingly used. When a gravity gradient boom is carried, a pitch wheel can be used in momentum bias mode, or a yaw wheel in zero-momentum bias. As of yet, few micro satellites have attempted full three axis control using three orthogonal reaction wheels, and none have succeeded for sustained periods to my knowledge.
Orbit Control and Determination
Orbit control is relatively common in mini satellites, but much less common in the smaller classes of satellites.
Few modern Nano and Pico-Satellites [classification] weighing less than 10kg have been launched, although there is increasing interest in this area as advanced Microsat technology is applied to miniaturise satellite systems even further. Some microsats that have been launched in the early 1990's almost fall into this category with weights of 11-14kg, most notably the AMSAT microsat-series. These satellites are cubical in shape and measure less than 150mm on each side. A plot below showing satellite launched in the last decade under 20kg shows that the number nano- and picosats launched has recently been increasing. Those launched in 2000 could well be classed as the first "modern" nanosatellites. 2002 will see the first launch of swarms of "Cubesats" measuring in on the 1kg nanosat/picosat class boundary, and it looks like due to their cost-effectiveness the picosat and nanosat class spacecraft will be employed within the space research, technology demonstration and education sectors.
Nanosatellites are attractive to many educational institutions to get involved in space, as commonly available technology now makes this type of satellite feasible and most importantly affordable. If you think about it, the electronics in a mobile phone or PDA include most of the electronics you would need for a passively stabilized satellite. If you now couple this with the fact that you can launch such a spacecraft for less than US$50k or Rs.25Lakhs(INR) you can appreciate you can afford to take risks to learn how to use such Commercial Off-The Shelf (COTS) technology in the space environment. Pico satellites weighing less than 1kg are still some time off for commercial applications, but will become commonplace in niche sectors before 2005.
For Nano satellites, autonomous operation using a single on-board computer is feasible, making use of technology developed for laptop and palmtop computers. To minimise mass, active attitude and orbit control are often ignored, and omni-directional antennas are employed. The main limits are set by the downlink and power generation systems. The downlink data rate is limited by the orbit average power generation, and has to be operated at low data rates, or in burst mode.
Increasingly, micro and nanotechnology makes it possible to fabricate entire satellite sub-systems, and possibly entire satellites on a chip! Considerable effort is being spent on these Femto satellites weighing less than 0.1kg, with applications in remote inspection, distributed measurement and disposable sensors.
