Appendix 4 – Survey of nanosatellite sub-systems Based on the study presented in paper [1], we identified the following general characteristics of nanosatellites: Shape: box, cubic, cylinder, hexagonal, octagonal Dimension of sides: between 10 and 40 cm Weight: 1 to 10 kg. Design life: 3 months to 2 years Payload As seen in Figure 6.1, the most common payloads for nanosatellites are: cameras, spectrometers, transceiver, GPS receivers, propulsion system, inter-satellite separation system.
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Figure 6.1 – Nanosatellite payload examples: a) SwissCube camera (40 g); b) NanoCam 1U Cubesat - 3MP CMOS sensor, 10-bit color (credit: GomSpace); c) CMOS sensor (resolution: 4 MP, 700mW@250 images/second); d) CanX-2 spectrometer (credit: York University); e) GPS receiver for RAX (Radio Aurora eXplorer) nanosatellite; f) Inter-satellite separation system
Structural Subsystem Most nanosatellites have the mechanical structure made of Aluminum alloy (Al 6061, Al 7075, Al 5052), Magnesium alloy or Titanium rather than composite materials. Aluminum is less expensive than composite materials (silicone) but, it is not a good radiation tolerant material. If mission design imposes a severe radiation shield requirement, silicone is an ideal candidate for nanosatellites for 3 reasons: its hardness is one order of magnitude higher than titan and stainless steel; its thermal, optical and electrical properties; a structure using silicon substrate can act as: - radiation shield; - optical material; - semiconductor substrate;
- thermal regulation system. Figure 6.2 illustrates the main configuration for CubeSat standard.
Figure 6.2 – Possible structural configurations for CubeSat standard Some types of structures proposed by DSRI (Danish Space Research Institute) researchers are presented in Figure 6.3.
Figure 6.3 – Mechanical structures proposed by Danish Small Satellite Programme [14] Attitude Determination and Control (ADAC) Subsystem This system consists of equipment to measure, report and change the orientation of the satellite. Controlling satellite attitude requires sensors to measure attitude, actuators to apply the torques needed to re-orient the satellite to a desired attitude, and algorithms to command the actuators based on: - sensor measurements of the current attitude and, - specification of a desired attitude. The integrated field that studies the combination of sensors, actuators and algorithms is called Guidance, Navigation and Control (GNC). Figures 6.4 and Figure 6.4 illustrate a comparison of various types of attitude determination and positioning and various techniques of attitude control respectively.
Figure 6.4 – Comparison of various methods of attitude determination and positioning [16]
Figure 6.5 – Comparison between various techniques of attitude control [16] Most of nanosatellites are three-axis stabilized and are using COTS (commercial-offthe-shelf) magnetorquers onboard for handling the orbit control. For position determination, almost all nanosatellites use GPS receivers. Figure 6.6 illustrates some sensors and actuators used in nanosatellite ADAC subsystem.
a b c Figure 6.6 – Sensors and actuators examples: a) Sun sensor (40g, 16 000$); b) Reaction wheel (credit: UTIAS/SFL, Sinclair Interplanetary); c) Magnetometer and sun sensors (credit: UTIAS/SFL, Sinclair Interplanetary) Communication Subsystem It assures the communication with the ground station for transmitting measurements data and telemetry data. For some nanosatellites (i.e., CanX-2, Can-X-3, CanX-4, CanX-5), the communication subsystem consists of 3 components: an UHF receiver used for data uplink from the ground station to the nanosatellite, operating at 4000bps; it uses a pre-deployed quad-canted monopole antenna array with near omni-directional coverage; an S-band transmitter which provides the primary downlink (e.g., data rates from 32Kbps to 256Kbps for CanX-4); it uses two patch antennas mounted on opposite
sides of the satellite; a VHF beacon will transmit satellite identification and basic telemetry in Morse code during the early stages of the mission to assist in commissioning the satellites.
In terms of communication spectrum, nanosatellites use the radio amateur frequency bands of 144 – 148 MHz, 420 – 450 MHz and 2.4 GHz. Table 6.4 [13] presents the modulation schemes that are commonly used for small satellite transmissions.
Table 6.4 – Modulation schemes for various nanosatellites Propulsion Subsystem Propulsion on nanosatellites is needed for: Compensation for disturbance torques: atmospheric drag, gravity gradient, solar pressure magnetic forces, etc; Spin-up and spin down; Attitude control: - Communication improvement (antenna pointing); - Scientific payload requires control/fine-pointing (spectrometry, camera); - Improved power generation (solar panels). Orbit control: - Improved mission autonomy (compensation for errors during orbit insertion, change of orbit, etc.); - Broader mission range (e.g., formation flying); - Higher mission pay-off /success. De-orbiting after EOL (End Of Life) De-orbiting ability might become compulsory in the near future for all spacecrafts. In order to perform thrusting maneuvers required for formation flying, nanosatellites use often cold gas or plasma thrusters. There are also nanosatellites without any propulsion system, thus relying solely on the torquers. This is the case of those nanosatellites which are used as test beds for different subsystems. Table 6.5 summarizes propulsion systems dedicated for CubeSat Programme 2009.
Table 6.5 – CubeSat propulsion systems summary
There are three types of propulsion commonly used for nanosatellites: electrical propulsion; chemical propulsion; solar propulsion (i.e., solar sails). Figure 6.7 illustrates some examples of propulsion systems used for nanosatellites.
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c d Figure 6.7 – Examples of propulsion systems for nanosatellites: a) Nanosail – D; b) MEMS thruster; c) Micro pulsed plasma thruster; d) CNAPS – Canadian Nanosatellite Advanced Propulsion System
Command & Data Handling Subsystem The C&DH subsystem uses a high speed data bus to facilitate data transfer between subsystems, prepares telemetry for downlink, distributes telemetry from uplink, and performs system maintenance and checks.
The CPU is the center for all of the spacecraft’s operations, as it can receives and processes data as necessary, stores data in bulk memory, or passes data on to other subsystems, such as communications. Some examples of processors available on nanosatellite systems are: Hitachi SH7709, Intel 80C188EC, Texas Instrument TMS320C50, ARM7, Motorola 68332, Power PC 750. Some commercial C&DH systems are presented in Figure 6.8.
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Figure 6.8 – OBC examples: a) Flight Computer for CubeSat Kit FM430 FCPU; b) NanoMind A702 (credit: GomSpace); c) CanX-2 On Board Computer
Power Supply Subsystem The main purpose of the power supply subsystem is to take power from the solar cells placed on the satellite sides and store it in the batteries as well as deliver it to other subsystems, by using a power bus. Satellite life time strongly depends on power system as well as the type of instruments carried by the nanosatellite. Energy consumption is depending on orbit type and on the mission’s functional scenarios. For a good sizing of energy budget, it is mandatory to consider also the worst case scenario. On average, only 2 W are available for subsystems at a given time. Therefore, it is impossible to conduct scientific measurements and data transmission simultaneously. For example, instrumental experiments can be conducted during at least 20 minutes for an orbit period of 90 minutes. A nanosatellite power system consists of 3 elements: energy generation system is represented by plug-n-play solar panels (e.g., GaAs, Ge, Li-Ion, silicon, etc) Observation: The best efficiency (40.7%) was recorded for multi-junction GaInP/GaAs/Ge cells manufactured by Spectrolab. energy storage system consists of battery packs (e.g., Li-Ion, Li-polymer, NiCd) energy distribution system is represented by the cables that connects the other nanosatellite subsystem with the power system. Figure 6.9 illustrates various power system components used for nanosatellites.
a b c Figure 6.9 – Nanosatellite power system examples: a) Electrical Power System (credit: Clyde Space); b) Solar panels (4 grams, efficiency=27 %); c) Battery pack (
[email protected] V) for 3U CubeSat (credit: Clyde Space) Thermal Control Subsystem The purpose of thermal subsystem is to control and maintain spacecraft component temperatures within their specified limits throughout all mission phases. Nanosatellites require a passive approach due to the severe restrictions on both mass and power. Two types of hardware used for thermal control have been identified: sensors and radiators. Thin film resistance temperature detectors are selected as sensors because they provide the most versatility and stability when compared to other temperature sensors. For example, SwissCube nanosatellite has 17 temperature sensors for assuring an optimal thermal control.
