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Small satellite technology
This page provides a brief overview of the various sub-systems and parts of a
satellite
Under construction - link with here
Systems
- Mechanical structure
- Power system
- Solar panels
- Batteries
- Power regulation and distribution
- Telemetry and Telecommand
- Communications
- Transmitter
- Receiver
- Antennas
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- Telemetry and Telecommand (TM/TC)
- 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 cometimes
catered for, often using an On-Board Computer, for instance loading new
software tasks
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- A widely used standard amongst the microsatellite and amateur community is the PACSAT
protocol developed for the missions on the first ARIANE ASAP in 1990,
including UoSAT-3, 4, AMSAT-OSCAR-16, LuSAT, DOVE and WeberSAT. It employs packet
protocols based on the AX.25 protocols.
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- In order to permit greater interoperability of spacecraft and groundsegment, NASA and
ESA 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
microsatellite world. One potential contender is becoming the use of IP
"internet protocols", as the code is commonly used and often
freely available. Many groundstations are already connected to the internet,
and extending the link to the satellite has a number of attractions.
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- In modern TM/TM sub-systems, On-Board Computers play a vital role.
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- 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).
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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 microsatellites 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 microsatellites 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 minisatellites, but much less common in
the smaller classes of satellites. Simple systems employ cold-gas
propulsion, using Nitr
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More...
[links to spacecraft systems and technology]
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