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 also be used. If onboard knowledge is required, then these radar observations can be uploaded and paired with a suitable propagator. Commonly, the USAF publishes two line element (TLE) sets 1 are paired with the SGP4 propagator 2. In deep space, position determination is performed using the Deep Space Network (DSN) and an onboard radio transponder 3.
ADCS includes sensors used to determine attitude and attitude rate, such as star trackers, sun sensors, horizon sensors, magnetometers, and gyros. Actuators are designed to change a spacecraft’s attitude. Common spacecraft actuators include magnetorquers, reaction wheels, and thrusters. There are many attitude determination and control architectures and algorithms suitable for use in small spacecraft 4.
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 decades, it has only been in the past few years that such technologies have become available for micro and nano class spacecraft. Table 5.1 summarizes the current state of the art of performance for GNC subsystems in small spacecraft. Performance greatly depends on the size of the spacecraft and values will range for nano- to micro-class spacecraft.
|Table 5-1: The state of the art for GNC subsystems|
|Reaction Wheels||0.001 – 0.3 Nm peak torque, 0.015 – 8 Nms storage||9|
|Magnetorquers||0.1 Nm peak torque, 1.5 Nms storage||9|
|Star Trackers||25 arcsec pointing knowledge||9|
|Sun Sensors||0.1° accuracy||9|
|Earth Sensors||0.25° accuracy||9|
|Gyroscopes||1°h-1 bias stability, 0.1°h-1/2 random walk||9|
|GPS Receivers||1.5 m position accuracy||9|
|Integrated Units||0.002° pointing capability||9|
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. Blue Canyon Technologies’ XACT is currently flying on the NASA-led missions MarCO and ASTERIA, both of which are 6U platforms, and have also flown on 3U missions (MinXSS deployed from NanoRacks February 2016). The XACT is illustrated in Figure 5.1.
|Table 5-2: Integrated GNC Units|
|Product||Manufacturer||Mass (kg)||Components||Pointing Accuracy||Status|
|High-Precision Attitude Determination and Control System||AAC-Clyde||0.086||ADCS processor, sensors and actuators, magnetorquers, GPS receiver chip, single and 3-axis reaction wheels, µPPT||0.5°||9|
|Inertial Reference Module (IRM)||Tyvak||0.61||2 Orthogonal Star Trackers, 3-Axis MEMS Gyro, Reaction Wheels (x3), Torque Coils (x3), C&DH processor, ADCS processor||0.057° 1-σ||9|
|MAI-400||Adcole Maryland Aerospace||0.694||3 reaction wheels, 3-axis magnetometer, 2 IREHSs, 3 torque rods||1°||9|
|MAI-401||Adcole Maryland Aerospace||0.56||3 reaction wheels, 3-axis magnetometer, star tracker, 3 torque rods||<0.1°||7|
|MAI-500||Adcole Maryland Aerospace||0.694||3 reaction wheels, 3-axis magnetometer, 2 star trackers, 3 torque rods||<0.1°||7|
|XACT||Blue Canyon Technologies||0.91||3 reaction wheels, 3-axis magnetometer, star tracker, 3 torque rods||0.007°||9|
|XACT-50||Blue Canyon Technologies||1.23||3 reaction wheels, 3-axis magnetometer, star tracker, 3 torque rods||0.007°||9|
|iADCS-100||Berlin Space Technologies||0.345||Star tracker, 3 gyro modules, 3 reaction wheels, 3 magnetorquers, optional sensors||<<1°||9|
Miniaturized reaction wheels provide small spacecraft with precision pointing capability. Reaction wheels can provide arbitrary torques limited by the wheel’s peak torque, momentum capacity, and wheel dead-band. Table 5.3 lists a selection of high-heritage miniature reaction wheels, and Figure 5‑2 depicts one of the wheels offered by Sinclair Interplanetary. With the exception of three units, all of the reaction wheels listed in Table 5.3 have spaceflight heritage. For example, since 2015, Blue Canyon’s RWp500 is flying on NASA’s CYGNSS mission, and Millennium Space Systems has 20 RWA1000s on orbit. For full three-axis control, a spacecraft requires three wheels. However, a four wheel configuration is often used to provide fault tolerance 5. 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 6.
|Table 5-3: Reaction Wheels|
|Product||Manufacturer||Mass (kg)||Peak Torque (mNm)||Momentum Capacity (Nms)||Radiation Tolerance (krad)||TRL Status|
|10SP-M||Surrey Satellite Technology||0.96||11||0.42||5||9|
|100SP-O||Surrey Satellite Technology||2.6||110||1.5||5||9|
|MAI-400 Reaction Wheel||Adcole Maryland Aerospace||0.11||0.635||.0111||Unkn.||9|
|MicroWheel||Blue Canyon Technologies||0.13||4||0.015||Unkn.||9|
|RWp500||Blue Canyon Technologies||0.75||25||0.5||Unkn.||9|
|RWp050||Blue Canyon Technologies||0.24||7||0.05||Unkn.||6|
|RWp100||Blue Canyon Technologies||0.35||7||0.1||Unkn.||6|
|SmallSat Reaction Wheel||ClydeSpace||1.5||40||Unkn.||10||9|
|RWA1000||Millenium Space Systems||Unkn.||1000||0.1||Unkn.||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 of high 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 7.
|Table 5-4: Magnetorquers|
|Product||Manufacturer||Mass (kg)||Peak Dipole (A m2)||Radiation Tolerance (krad)||TRL Status|
|MTR-5||Surrey Satellite Technology||0.5||5||5||9|
|Electromagnet (Type A)||Adcole Maryland Aerospace||0.018||0.15||Unkn.||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 8. Table 5.5 lists some models suitable for use on a small spacecraft, one of which is depicted in Figure 5‑4. Sinclair Interplanetary has flown about 38 ST-16RT2 units, Blue Canyon Technologies has flown the Extended NST onboard the DARPA High Frequency Receiver Experiment, and many other star trackers also have notable flight heritage.
|Table 5-5 Star Trackers|
|Product||Manufacturer||Mass incl. baffle (kg)||Accuracy (arcsec)||Radiation Tolerance (krad)||TRL Status|
|Rigel-L||Surrey Satellite Technology||2.2||25||5||9|
|Procyon||Surrey Satellite Technology||1.7||30||5||9|
|MAI-SS Space Sextant||Adcole Maryland Aerospace||0.282||27||75||9|
|Standard NST||Blue Canyon Technologies||0.35||40||Unkn.||9|
|Extended NST||Blue Canyon Technologies||1.3||40||Unkn.||9|
|ST200||Berlin Space Technologies||0.04||10||11||9|
Magnetometers provide a measurement of the local magnetic field, and this measurement can be used to provide both estimates of attitude 9 and also orbital position 10. Table 5.6 provides a summary of some 3-axis magnetometers available for small spacecraft, one of which is illustrated in Figure 5‑5.
|Table 5-6: Magnetometers|
|Product||Manufacturer||Mass (kg)||Resolution (nT)||Orthogonality (°)||Radiation Tolerance (krad)||TRL Status|
|Magnetometer||New Space Systems||0.085||10||<1||10||9|
|Magnetometer||Surrey Satellite Technology||0.14||10||1||10 (Si)||9|
|3-axis Magnetometer||Adcole Maryland Aerospace||Unkn.||Unkn.||Unkn.||Unkn.||9|
|MAG-3 Three-Axis Magnetometer||SpaceQuest||0.1||Unkn.||<1||10||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. A digital two-axis sun sensor can provide perfectly fine sun vector solutions, but we manifest multiple sensors in case we are “lost in space”. Fine sun sensors provide a full 2-axis estimate of Sun location 11, and a spacecraft would require a minimum of six .and a minimum of four are required. A selection of sun Figure 5‑6.
|Table5-7: Sun Sensors|
|Product||Manufacturer||Mass (kg)||Accuracy (°)||Radiation Tolerance (krad)||TRL Status|
|Fine (digital) Sun Sensor||New Space Systems||0.035||0.1||10||9|
|Analog Sun Detector||Adcole Maryland Aerospace||0.068||0.75||Unkn.||9|
|BiSon64||Lens Research & Development||0.0217||0.5||1100||8|
|BiSon64-B||Lens Research & Development||0.0217||0.5||1100||8|
|BiSon74-ET-RH||Lens Research & Development||0.0245||0.7||1100||~6|
Horizon 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. For terrestrial applications, these sensors are referred to as Earth Sensors, but can be used for other planets. Examples of such technologies are described in Table 5.8 and illustrated in Figure 5‑7.
|Table 5-8: Horizon Sensors|
|Product||Manufacturer||Mass (kg)||Accuracy (°)||TRL Status|
|MAI-SES Static Earth Sensor||Adcole Maryland Aerospace||0.033||0.25||9|
|Mini Digital HCI||Servo||0.050||0.75||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 12. Table 5.9 lists a sample of gyros available for small spacecraft, one of which is illustrated in Figure 5.8.
|Table 5-9: Gyros|
|Product||Manufacturer||Type||Mass (kg)||Bias Stability (°h-1)||Random Walk (°h-1/2)||Radiation Tolerance (krad)||TRL Status|
|MIRAS-01||Surrey Satellite Technology||3-axis MEMS||2.8||10||0.6||5||9|
|LN-200S||Northrop Grumman||3- axis FOG||0.75||1||0.07||10||9|
|ADIS16405||Analog Devices||3-axis MEMS||0.016||25||2.0||Unkn.||9|
|MASIMU04||Micro Aerospace Solutions||3-axis MEMS||0.03||0.6||Unkn.||Unkn.||Unkn.|
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 board NovaTel OEM615, 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 13. GPS units are controlled under the Export Administration Regulations (EAR) and must be licensed to remove COCOM limits 14.
|Table 5-10: GPS Receivers|
|Product||Manufacturer||Mass (kg)||Accuracy (m)||Radiation Tolerance (krad)||TRL Status|
|SGR-05U||Surrey Satellite Technology||0.04||10||5||9|
|SGR-10||Surrey Satellite Technology||0.95||10||10||9|
Deep Space Navigation
In deep space, navigation is performed using radio transponders in conjunction with the Deep Space Network (DSN). As of 2018, the only deep space transponder that is suitable for small spacecraft and that has flown previously is the JPL-designed and General Dynamics-manufactured Small Deep Space Transponder (SDST). JPL has also designed IRIS V2, which is a deep space transponder that is more suitable for the CubeSat form factor. Table 5.11 details these two radios, and the SDST is illustrated in Figure 5‑10. IRIS V2 is derived from the Low Mass Radio Science Transponder (LMRST), is currently flying on the MarCO CubeSats, and is scheduled to fly on INSPIRE 15.
|Table 5-11: Deep Space Transponders|
|Product||Manufacturer||Mass (kg)||Bands||TRL Status|
|SDST||General Dynamics||3.2||X, Ka||9|
|IRIS V2||JPL||0.4||X, Ka, S, UHF||9|
Atomic clocks have been used on larger spacecraft in Low Earth Orbit for several years now, however integrating them on small spacecraft is relatively new. The conventional method for spacecraft navigation is a two-way tracking system of the ground-based antennas and atomic clocks. The time difference from a ground station sending a signal and the receiving the response the spacecraft can be used to determine the spacecraft’s location, velocity and path. This is not a very efficient process as the spacecraft must wait for navigation commands from the ground station instead of making real time decisions, and the ground station can only track one spacecraft at a time as it must wait for the spacecraft to return a signal 16.
In deep space navigation, the distances are much greater from the ground station to spacecraft, and the accuracy of the radio signals need to be measured within a few nanoseconds. JPL’s Deep Space Atomic Clock (DSAC) project plans to launch a prototype of a small, low mass (16 kg) atomic clock based on mercury-ion trap technology, which underwent demonstration testing Autumn 2017. The project aims to produce a <10 kg configuration in the second generation. DSAC will launch in 2018 as a hosted payload on General Atomic’s Orbital Test Bed spacecraft aboard the U.S. Air Force Space Technology Program (STP-2) mission 17.
More designers of small spacecraft technology are developing their own version of atomic clocks and oscillators to be used in space and need to ensure they are properly synchronized. They are designed to fit small spacecraft and for missions that are power and volume limited, requiring multiple radios. Table 5-12 lists the atomic clocks and oscillators available for small spacecraft missions.
Bliley Technologies has developed Miniature Half-DIP package Low Power Oven-Controlled Crystal Oscillation (OCXO) and Iris Series 1″x1″ OCXO for LEO that is desired for power constrained missions. The Half-DIP package has 135 mW power consumption, and has superior phase noise with close in phase noise of -125 dBc/Hz at 10 Hz 18. The Iris series can range from 10-100 MHz in frequency and has a stability vs temperature performance of +/-25ppb with a sine output and has a radiation tolerance of 38kRAD TID. As no radiation testing against this components of this part have been characterized, the TRL for small spacecraft technology is 6.
|Table 5‑12: Atomic Clocks|
|Product||Manufacturer||Dimensions (mm)||Power Consumption||Frequency Range (MHz)||TRL Status|
|Miniature Half-DIP package Low Power OCXO||Bliley Technologies, Inc.||Up to 12 x 12 x 10||135 – 180 mW at steady state||10 – 60||6|
|Iris Series 1″x1″ OCXO for LEO||Bliley Technologies, Inc.||19 x 11 x 19||1.5 W at steady state||10 – 100||6|
|Ultra Stable Oscillator||AccuBeat, Ltd.||120x120x120||3.8 W||Unkn.||6|
|9635QT||Microsemi||33 x 33 x 33||Unkn.||Unkn.||6|
|Miniature Atomic Clock (MAC) SA.3Xm||Microsemi||50.8 x 50.8 x 18||5 – 8 W||10||Unkn.|
|Space Chip Scale Atomic Clock (CSAC)||Microsemi||40 x 35 x 11||<120 mW||10||9|
On the Horizon
Technological progress in the area of guidance, navigation, and control is slow. 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 and/or accuracy. This is especially true for GNC components designed for deep space missions, where small spacecraft-focused missions have only very recently been proposed. However, in a collaborative effort between the Swiss Federal Institute of Technology and Celeroton, there is progress being made on a high-speed magnetically levitated reaction wheel for small satellites see Figure 5‑11. The idea is to eliminate mechanical wear and stiction by using magnetic bearings rather than ball bearings. The reaction wheel implements a dual hetero/homopolar, slotless, self-bearing and a permanent-magnet synchronous motor (PMSM). The fully active, Lorentz-type magnetic bearing consists of a heteropolar self-bearing motor that applies motor torque and radial forces on one side of the rotor’s axis, and a homopolar machine that exerts axial and radial forces, which allows active control of all six degrees of freedom. It is capable of storing 0.01 Nm of momentum at a maximum 30,000 rpm, and applying a maximum torque of 0.01 Nm 19. Another team from Johns Hopkins University is doing work that aims at designing a fully robotic CubeSat by conducting ground simulations of docking, charging, relative navigation, and deorbiting 20. The RANGE mission from Georgia Institute of Technology is a pair of 1.5U CubeSats with the goal of improving the relative and absolute positioning capabilities of nanosatellites 21.
The rising popularity of smallsats in general and CubeSats in particular means there is a high demand for components, and engineers are often faced with prohibitive prices. The Space Systems Design Studio at Cornell University is tackling this issue in the area of GNC with their PAN nanosatellites. A paper by Choueiri, et al. outlines an inexpensive and easy-to-assemble solution for keeping the ADC system below $2,500 lowering the cost of components means we will likely witness a burgeoning of the smallsat industry, and this holds exciting implications for the future 22.
Small spacecraft GNC is a mature area, with many previously flown and high TRL components offered by several different vendors. The progress that is being made in the area of integrated units will offer simple, single vendor, and modular devices for ADCS which will simplify GNC subsystem design. Other areas of GNC have potential for improvements as more research is being conducted. For example, a team at the University of Michigan is studying the use of a multi-algorithmic hybrid ADCS system for CubeSats that aims to manage issues that occur when implementing multiple estimation and control algorithms 23