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Wind speeds at the NREL National Wind Technology Center (NWTC) vary dramatically from summer to winter. Wind speeds are low over the summer and high over the winter. The data collected from this site will allow validation of the survivability of the Sunfolding tracker under high wind events, increased understanding of the dynamic behavior of the Sunfolding tracker at different wind speeds.
A method is presented by which the attitude of a low-Earth orbiting spacecraft may be determined using a vector magnetometer, a digital Sun sensor, and a mathematical model of the Earth's magnetic field. The method is currently being implemented for the Solar Maximum Mission spacecraft (as a backup for the failing star trackers) as a way to determine roll gyro drift.
A variety of sensors are studied on a one-axis tracking surface and a horizontal surface in Golden, Colorado, and Eugene, Oregon. This is the first year of a long-term study that will look at not only a comparison between the instruments but also the longer-term degradation in calibration and/or performance. Initially, results from each location will be analyzed, and then results will be compared and contrasted between the two locations. A quick comparison at Eugene indicates that reference solar cells seem to compare better against a secondary standard pyranometer on a one-axis tracker than photodiode-based pyranometers. More study is needed to characterize and confirm this finding.
The capability to build complex electronic functions into compact packages is opening the path to miniature satellites on the order of 1 kg mass, 10 cm across, packed with the computing processors, motion controllers, measurement sensors, and communications hardware necessary for operation. Power generation will be from short strings of silicon or gallium arsenide-based solar photovoltaic cells with the array power maximized by a peak power tracker (PPT). Energy storage will utilize a low voltage battery with nickel cadmium or lithium ion cells as the most likely selections for rechargeables and lithium (MnO2-Li) primary batteries for one shot short missions.
On this eleventh day of the STS-88 mission, the flight crew, Commander Robert D. Cabana, Pilot Frederick W. Sturckow, and Mission Specialists Nancy J. Currie, James H. Newman, Jerry L. Ross, and Sergei Krikalev are awakened with the song "Goodnight, Sweetheart, Goodnight". Pilot Rick Sturckow undocks Endeavour from the station and backs the shuttle away to a distance of 450 feet above the station before beginning a nose-forward fly-around. Later Cabana, Sturckow and Ross deploy the SAC-A satellite from Endeavour's payload bay. SAC-A is a small, self-contained, non-recoverable satellite built by the Argentinean National Commission of Space Activities. The cube-shaped, 590-pound satellite will test and characterize the performance of new equipment and technologies that may be used in future scientific or operational missions. The payload includes a differential global positioning system, a magnetometer, silicon solar cells, a charge-coupled device Earth camera and a whale tracker experiment.
The animation depicts NASA's concept for a next-generation Advanced X-ray Timing Mission. The models and their textures doe not necessarily represent the final iteration. Delivery specifications include launch with Taurus II or Falcon 9, mass of 2650 kg, with a circular low earth orbit at approximately 600 km. The inclination depends on the launch vehicle and spacecraft mass. AXTAR's prime instrument will probe the physics of neutron stars and black holes through X-ray timing and spectral measurements. The primary instrument will be the Large Area Timing Array (LATA). The Sky Monitor Clusters configuration consists of 27 Sky Monitor cameras th at are grouped in five clusters. This configuration will achieve approximately 85 percent all sky coverage. Spacecraft components include a science bus to house the LATA of supermodules; a spacecraft bus to house components such as propulsion tanks, avionics, and reaction wheels; solar arrays configured from space-qualified GaAs 3-junction cells; star trackers for attitude knowledge; a propulsion system of four pods, each containing one 100 lbf and two 5 lbf engines; a launch vehicle adaptor; and a radiation shield.
Solar Cruiser is a small satellite Technology Demonstration Mission (TDM) to mature solar sail propulsion technology using a solar sail larger than 1600 square meters, demonstrating performance as a propulsion system and a stable pointing platform for science observations in an artificial halo orbit sunward of the Sun-Earth Lagrange Point 1 (sub-L1). For the “sailcraft” to meet mission ob- jectives, there are several unique attitude control challenges that the Attitude De- termination and Control System (ADCS) must overcome. Large disturbance tor- ques, primarily due to sail deformations coupled with off-sun pointing angles, make it more difficult to maintain adequate controls performance and manage accumulated momentum on the control actuators. The large-amplitude, low- frequency flexible body modes of the sail, in concert with noisy sensors and ac- tuators, make it challenging to mitigate control-structure interactions and main- tain fine pointing capabilities. Stability and control performance during sail de- ployment is complicated by rapidly and widely varying inertias. The Solar Cruiser ADCS effectively addresses these challenges using a simple, traditional control system – including momentum management actuators that use bang- bang control to constrain internal accumulated momentum, a low-pass controls filter and an attitude determination Kalman Filter (KF) that blends multiple star tracker solutions, and a reaction wheel assembly (RWA) with controller gains tuned to specific configurations or inertias. Solar Cruiser’s attitude control ap- proach, and lessons learned from its development, establishes a state of the art of high value to future solar sail missions and other small spacecraft with large de- ployable structures operating in deep space.
Solar Cruiser is a small satellite Technology Demonstration Mission (TDM) to mature solar sail propulsion technology using a solar sail larger than 1600 square meters, demonstrating performance as a propulsion system and a stable pointing platform for science observations in an artificial halo orbit sunward of the Sun-Earth Lagrange Point 1 (sub-L1). For the “sailcraft” to meet mission objectives, there are several unique attitude control challenges that the Attitude Determination and Control System (ADCS) must overcome. Large disturbance torques, primarily due to sail deformations coupled with off-sun pointing angles, make it more difficult to maintain adequate controls performance and manage accumulated momentum on the control actuators. The large-amplitude, low-frequency flexible body modes of the sail, in concert with noisy sensors and actuators, make it challenging to mitigate control-structure interactions and maintain fine pointing capabilities. Stability and control performance during sail deployment is complicated by rapidly and widely varying inertias. The Solar Cruiser ADCS effectively addresses these challenges using a simple, traditional control system – including momentum management actuators that use bang-bang control to constrain internal accumulated momentum, a low-pass controls filter and an attitude determination Kalman Filter (KF) that blends multiple star tracker solutions, and a reaction wheel assembly (RWA) with controller gains tuned to specific configurations or inertias. Solar Cruiser’s attitude control approach, and lessons learned from its development, establishes a state of the art of high value to future solar sail missions and other small spacecraft with large deployable structures operating in deep space.
Direct current motors are used in photovoltaic systems. Important characteristics of electric motors are the starting to rated current and torque ratios. These ratios are dictated by the size of the solar cell array and are different for the various dc motor types. Discussed here is the calculation of the starting to rated current ratio and starting to rated torque ratio of the permanent magnet, and series and shunt excited motors when powered by solar cells for two cases: with and without a maximum-power-point-tracker (MPPT) included in the system. Comparing these two cases, one gets a torque magnification of about 3 for the permanent magnet motor and about 7 for other motor types. The calculation of the torques may assist the PV system designer to determine whether or not to include an MPPT in the system.
Direct current (dc) motors are used in terrestrial photovoltaic (PV) systems such as in water-pumping systems for irrigation and water supply. Direct current motors may also be used for space applications. Simple and low weight systems including dc motors may be of special interest in space where the motors are directly coupled to the solar cell array (with no storage). The system will operate only during times when sufficient insolation is available. An important performance characteristic of electric motors is the starting to rated torque ratio. Different types of dc motors have different starting torque ratios. These ratios are dictated by the size of solar cell array, and the developed motor torque may not be sufficient to overcome the load starting torque. By including a maximum power point tracker (MPPT) in the PV system, the starting to rated torque ratio will increase, the amount of which depends on the motor type. The starting torque ratio is calculated for the permanent magnet, series and shunt excited dc motors when powered by solar cell arrays for two cases: with and without MPPT's. Defining a motor torque magnification by the ratio of the motor torque with an MPPT to the motor torque without an MPPT, a magnification of 3 was obtained for the permanent magnet motor and a magnification of 7 for both the series and shunt motors. The effect of the variation of solar insolation on the motor starting torque was covered. All motor types are less sensitive to insolation variation in systems including MPPT's as compared to systems with MPPT's. The analysis of this paper will assist the PV system designed to determine whether or not to include an MPPT in the system for a specific motor type.
The objective of this experiment was to demonstrate electrical power generation using a small free-piston Stirling engine and linear alternator in conjunction with a parabolic solar collector. A test bed collector, formerly used at the JPL Table Mountain Observatory, was renovated and used to obtain practical experience and to determine test receiver performance. The collector was mounted on a two-axis tracker, with a cold water calorimeter mounted on the collector to measure its efficiency, while a separate, independently tracking radiometer was used to measure solar insolation. The solar receiver was designed to absorb energy from the collector, then transfer the resulting thermal energy to the Stirling engine. Successful testing of receiver/collector assembly yielded valuable inputs for design of the Stirling engine heater head.
While wind tunnel testing and proprietary industry modeling tools have been used for years to design and develop PV tracker systems, wind induced dynamic failures are becoming more prevalent and high visibility. Designers and manufactures have reacted by developing new systems or add-on products to improve system dynamics adding to overall system costs. To better understand the physics and aeroelastic behavior that leads to dynamic instabilities NREL researches have taken up a first of a kind study to both develop open source aeroelastic modeling tools and measure tracker loads on a single axis full-scale tracker system at a high wind site. These modeling tools can simulate the fluid-structure interaction driving torsional instabilities under a wide range of turbulent inflow conditions and stow angles. The results of these simulations reveal the flow features associated with panel rotation and the induced loads in the structure. The experimental load measurements can be used to validate models and provide a quantifiable understanding of tracker dynamics, critical loads paths, identify instability markers, inform resilient design, and suggest the most favorable stow approach.
A metric is developed to analyze the daily performance of single-axis photovoltaic (PV) trackers. The metric relies on comparing correlations between the daily time series of the PV power output and an array of simulated plane-of-array irradiances for the given day. Mathematical thresholds and a logic sequence are presented, so the daily tracking metric can be applied in an automated fashion on large-scale PV systems. The results of applying the metric are visually examined against the time series of the power output data for a large number of days and for various systems. The visual inspection results suggest that overall, the algorithm is accurate in identifying stuck or functioning trackers on clear-sky days. Visual inspection also shows that there are days that are not classified by the metric where the power output data may be sufficient to identify a stuck tracker. Based on the daily tracking metric, uptime results are calculated for 83 different inverters at 34 PV sites. The mean tracker uptime is calculated at 99% based on 2 different calculation methods. The daily tracking metric clearly has limitations, but as there is no existing metrics in the literature, it provides a valuable tool for flagging stuck trackers.
An electrooptical position-measuring instrument, the Retroreflector Field Tracker or RFT, is described. It is part of the Dynamic Augmentation Experiment - a part of the payload of Space Shuttle flight 41-D in Summer 1984. The tracker measures and outputs the position of 23 reflective targets placed on a 32-m solar array to provide data for determination of the dynamics of the lightweight structure. The sensor uses a 256 x 256 pixel CID detector; the processor electronics include three Z-80 microprocessors. A pulsed laser diode illuminator is used.
Attitude determination algorithms for a multimission spacecraft are derived and their performance analyzed. The attitude determination system is composed of a strapdown Inertial Reference Unit (IRU), two fixed head star trackers, and an onboard computer. IRU data is processed to maintain real-time knowledge of spacecraft attitude relative to an inertial reference frame. Star tracker data is processed using Kalman filtering techniques to estimate and correct the attitude determination errors and the gyro drift compensation errors. The results of a star availability analysis for stellar, solar and earth pointing missions are presented. Linear covariance analysis techniques are used to evaluate nominal attitude determination performance, the effects of sensor measurement accuracy variations, the effects of errors in knowledge of sensor measurement accuracy, and the effects of star tracker misalignment errors. Results of a nonlinear simulation analysis of attitude determination performance are also presented. These analyses show that precision attitude determination for stellar, solar and earth pointing missions is achieved.
The purpose of the Solar Array Flight Dynamic Experiment (SAFDE) is to demonstrate the feasibility of on-orbit measurement and ground processing of large space structures dynamic characteristics. Test definition or verification provides the dynamic characteristic accuracy required for control systems use. An illumination/measurement system was developed to fly on space shuttle flight STS-31D. The system was designed to dynamically evaluate a large solar array called the Solar Array Flight Experiment (SAFE) that had been scheduled for this flight. The SAFDE system consisted of a set of laser diode illuminators, retroreflective targets, an intelligent star tracker receiver and the associated equipment to power, condition, and record the results. In six tests on STS-41D, data was successfully acquired from 18 retroreflector targets and ground processed, post flight, to define the solar array's dynamic characteristic. The flight experiment proved the viability of on-orbit test definition of large space structures dynamic characteristics. Future large space structures controllability should be greatly enhanced by this capability.
The purpose of the Solar Array Flight Dynamic Experiment (SAFDE) is to demonstrate the feasibility of on-orbit measurement and ground processing of large space structures' dynamic characteristics. Test definition or verification provides the dynamic characteristic accuracy required for control systems use. An illumination/measurement system was developed to fly on space shuttle flight STS-41D. The system was designed to dynamically evaluate a large solar array called the Solar Array Flight Experiment (SAFE) that had been scheduled for this flight. The SAFDE system consisted of a set of laser diode illuminators, retroreflective targets, an intelligent star tracker receiver and the associated equipment to power, condition, and record the results. In six tests on STS-41D, data was successfully acquired from 18 retroreflector targets and ground processed, post flight, to define the solar array's dynamic characteristic. The flight experiment proved the viability of on-orbit test definition of large space structures dynamic characteristics. Future large space structures controllability should be greatly enhanced by this capability.
The purpose of the Solar Array Flight Dynamic Experiment (SAFDE) is to demonstrate the feasibility of on-orbit measurement and ground processing of large space structures dynamic characteristics. Test definition or verification provides the dynamic characteristic accuracy required for control systems use. An illumination/measurement system was developed to fly on Space Shuttle flight STS-31D. The system was designed to dynamically evaluate a large solar array called the Solar Array Flight Experiment (SAFE) that had been scheduled for this flight. The SAFDE system consisted of a set of laser diode illuminators, retroreflective targets, an intelligent star tracker receiver and the associated equipment to power, condition, and record the results. In six tests on STS-41D, data was successfully acquired from 18 retroreflector targets and ground processed, post flight, to define the solar array's dynamic characteristic. The flight experiment proved the viability of on-orbit test definition of large space structures dynamic characteristics. Future large space structures controllability should be greatly enhanced by this capability.