The Guidance, Navigation & Control (GNC) subsystem includes both the components used for position determination and the components used by the Attitude Determination and Control System (ADCS).
In Earth orbit, an onboard position determination capability can be provided by a GPS receiver. Alternatively, ground based radar tracking systems can be used. If onboard knowledge is required then these radar observations can be uploaded and paired with a suitable propagator. Commonly, the USAF published two line element (TLE) sets  are paired with the SGP4 propagator . In deep space, position determination is performed using the Deep Space Network (DSN) and an onboard radio transponder .
ADCS includes sensors used to determine attitude and attitude rate, such as star trackers and gyros, and actuators designed to change a spacecraft’s attitude, such as reaction wheels and thrusters. There are many attitude control and determination architectures and algorithms suitable for use in small spacecraft .
The continuing trend in small spacecraft GNC is the miniaturization of existing technologies. While 3-axis stabilized, GPS equipped 100 kg class spacecraft have been flown for over a decade, it has only been in the past few years that such technologies have become available for 10 kg class spacecraft. Table 5.1 summarizes the current state of the art performance for GNC subsystems in small spacecraft.
|Reaction Wheels||0.1 Nm peak torque, 1.5 Nms storage||TRL 9|
|Magnetorquers||0.1 Nm peak torque, 1.5 Nms storage||TRL 9|
|Star Trackers||25 arcsec pointing knowledge||TRL 9|
|Sun Sensors||0.1° accuracy||TRL 9|
|Earth Sensors||0.25° accuracy||TRL 9|
|Gyroscopes||1°h-1 bias stability, 0.1°h-1/2 random walk||TRL 9|
|GPS Receivers||1.5 m position accuracy||TRL 9|
|Integrated Units||0.007° pointing capability||TRL 6|
State of the Art
Integrated units combine multiple different attitude and navigation components into a single part with the aim to provide a simple, single component solution to a spacecraft’s GNC requirements. Typical components included are reaction wheels, magnetometers, magnetorquers and star trackers. The units often include built-in attitude determination and momentum management algorithms. Table 5.2 describes some of the integrated units currently available, though none have flown yet. Both units described in the table are ½U units, and the unit from Blue Canyon Technologies is illustrated in Figure 5.1.
|Product||Manufacturer||Mass (kg)||Components||Pointing Capability||Status|
|MAI-400||Maryland Aerospace||0.694||3 reaction wheels, 3-axis magnetometer, 2 HCIs, 3 torque rods||Unkn.||TRL 6|
|BCT XACT||Blue Canyon Technologies||0.85||3 reaction wheels, 3-axis magnetometer, star tracker, 3 torque rods||0.007°||TRL 6|
Miniaturized reaction wheels provide small spacecraft with aprecision pointing capability. Reaction wheels can provide arbitrary torques limited only by the wheel’s peak torque and momentum capacity. Table 5.3 lists a selection of high heritage miniature reaction wheels, and Figure 5.3 illustrates one of the wheels offered by Sinclair Interplanetary. For full three-axis control, a spacecraft requires three wheels. However, a four wheel configuration is often used to provide fault tolerance . Due to parasitic external torques, reaction wheels need to be periodically desaturated using an actuator that provides an external torque, such as thrusters or magnetorquers .
|Product||Manufacturer||Mass (kg)||Peak Torque (Nm)||Momentum Capacity (Nms)||Radiation Tolerance (krad)||Status|
|10SP-M||Surrey Satellite Technology||0.96||0.011||0.42||5||TRL 9|
|100SP-O||Surrey Satellite Technology||2.6||0.11||1.5||5||TRL 9|
|RW-0.03||Sinclair Interplanetary||0.185||0.002||0.04||20||TRL 9|
Magnetorquers are an established technology used in small spacecraft and can provide control torques perpendicular to the local external magnetic field. Table 5.4 lists a selection ofhigh heritage magnetorquers and Figure 5.3 illustrates some of ZARM Technik’s product offerings. Magnetorquers are often used in combination with wheels to remove excess momentum. As control torques can only be provided in the plane perpendicular to the local magnetic field, full 3-axis stabilization is not possible at any given time. However, orbit periodic control is possible using only magnetorquers as the spacecraft moves through the magnetic field .
|Product||Manufacturer||Mass (kg)||Peak Dipole (A m2)||Radiation Tolerance (krad)||Status|
|MTR-5||Surrey Satellite Technology||0.5||5||5||TRL 9|
|0-1-1||Spaceflight Industries||0.727||15||Unkn.||TRL 9|
Thrusters used for attitude control are described in Propulsion Chapter. Pointing accuracy is determined by minimum impulse bit, and control authority by thruster force.
A star tracker can provide an accurate, standalone estimate of the spacecraft’s attitude by comparing a digital image captured with a CCD or CMOS sensor to an onboard star catalog . Table 5.5 lists some models suitable for use on a small spacecraft, one of which is illustrated in Figure 5.4.
|Product||Manufacturer||Mass incl. baffle (kg)||Accuracy (arcsec)||Radtion Tolerance (krad)||Status|
|Rigel-L||Surrey Satellite Technology||2.2||25||5||TRL 9|
|Procyon||Surrey Satellite Technology||1.7||50||Unkn.||TRL 9|
|ST-16||Sinclain Interplanetary||0.12||74||9||TRL 9|
Magnetometers provide a measurement of the local magnetic field, and this measurement can be used to provide both estimates of attitude  and also orbital position . Table 5.6 provides a summary of some 3-axis magnetometers available for small spacecraft, one of which is illustrated in Figure 5.5.
|Product||Manufacturer||Mass (kg)||Resolution (nT)||Orthogonality (°)||Radiation Tolerance (krad)||Status|
|Magnetometer||New Space Systems||0.2||6.5||<1||10||TRL 9|
|MicroMag3||PNI Corp||0.2||15||<1||Unkn.||TRL 9|
|Magnetometer||Surrey Satellite Technology||0.19||10||<1||5 (Si)||TRL 9|
Sun sensors are used to provide an estimate of the location of the Sun in the spacecraft body frame, which in turn can be used as an input in attitude estimation. Coarse sensors only provide a non-directional cosine reading , and a spacecraft would require a minimum of six. Fine sun sensors provide a full 2-axis estimate of Sun location  and a minimum of four are required. A selection of sun sensors are described in Table 5.7 one of which is illustrated in Figure 5.6.
|Product||Manufacture||Mass (kg)||Accuracy (°)||Radiation Tolerance (krad)||Status|
|Fine (digital) Sun Sensor||New Space Systems||0.035||0.1||10||TRL 9|
|Analog Sun Detector||Adcole||0.068||0.75||Unkn.||TRL 9|
|CSS-01||Space Micro||0.0141||5||Unkn.||TRL 9|
Earth sensors can be simple infrared horizon crossing indicators (HCI) or can utilize more advanced thermopile sensors to detect the temperature differences between the poles and the equator. Examples of such technologies are described in Table 5.8 and illustrated in Figure 5.7.
|Product||Manufacturer||Mass (kg)||Accuracy (°)||Status|
|Static Earth Sun Sensor||Maryland Aerospace||0.033||0.25||TRL 9|
|Mini Digital HCI||Servo||0.050||0.75||TRl 9|
Gyroscopes provide a measurement of angular velocity. The main gyro types used in small spacecraft are fiber optic gyros (FOGs) and MEMS gyros, with FOGs offering better performance at a mass and cost penalty . Table 5.9 lists a sample of gyros available for small spacecraft, one of which is illustrated in Figure 5.8.
|Product||Manufacturer||Type||Mass (kg)||Bias Stability (°h-1)||Random Walk (°h-1/2)||Radiation Tolerance (krad)||Status|
|MIRAS-01||Surrey Satellite Technology||3-axis MEMS||2.8||10||0.6||5||TRL 9|
|LN-200S||Northrop Grumman||3- axis FOG||0.75||1||0.1||10||TRL 9|
|ADIS16405||Analog Devices||3-axis MEMS||0.016||25||2.0||Unkn.||TRL 9|
For Low Earth Orbiting spacecraft GPS receivers are now the primary method for performing orbit determination, replacing ground based tracking methods. Onboard GPS receivers are now considered a mature technology for small spacecraft, and some examples are described in Table 5.10. The NovaTel OEM615 board, replacing the ubiquitous OEMV1, is illustrated in Figure 5.9. GPS accuracy is limited by propagation variance through the exosphere and the underlying precision of the civilian use C/A code . GPS units are controlled under the Export Administration Regulations (EAR) and must be licensed to remove COCOM limits .
|Product||Manufacturer||Mass (kg)||Accuracy (m)||Radiation Tolerance (krad)||Status|
|SGR-05U||Surrey Satellite Technology||0.040||10||5||TRL 9|
|SGR-10||Surrey Satellite Technology||0.95||10||10||TRL 9|
Deep Space Navigation
In deep space, navigation is performed using radio transponders in conjunction with the Deep Space Network (DSN). At the time of writing the only small spacecraft suitable deep space transponder to have flown previously is the JPL designed and General Dynamics manufactured Small Deep Space Transponder (SDST). However, JPL has designed a deep space transponder suitable for use in a cubesat, IRIS. Table 5.11 details these two radios, and the SDST is illustrated in Figure 5.10. IRIS is derived from the Low Mass Radio Science Transponder (LMRST) and is scheduled to fly on INSPIRE .
|SDST||General Dynamics||3.2||X, Ka||TRL 9|
|IRIS V2||JPL||0.5||X, Ka, S, UHF||TRL 6|
On the Horizon
Given the high maturity of existing GNC components, future developments in GNC are mostly focused on incremental or evolutionary improvements, such as decreases in mass and power, increases in longevity, or higher accuracy. This is especially true for GNC components designed for deep space missions, where small spacecraft focused missions have only very recently been proposed.
Small spacecraft GNC is a mature area, with many previously flown and high TRL components offered by several different vendors. Soon-to-be-flown integrated units will offer a simple, single vendor single component solution for ADCS which will simplify GNC subsystem design.
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