ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
373 BESST:  A Miniature,  Modular Radiometer 

Robert Warden*, William Good* and Erik Baldwin-Stevens* 

Abstract 

A new radiometer assembly has been developed that in corporates modular design principles in order to  provide flexibility and versatility. The assembly, shown in Figure 1, is made up of six modules plus a  central cubical frame. A small thermal imaging detecto r is used to determine the temperature of remote  objects. To improve the accuracy of the temperat ure reading, frequent calibration is required. The  detector must view known temperature targets before viewing the remote object. Calibration is achieved  by using a motorized fold mirror to select the desire d scene the detector views. The motor steps the fold  mirror through several positions, which allows the dete ctor to view the calibration targets or the remote  object. The details, features, and benefits of t he radiometer are described in this paper. 

Introduction 

The availability of small, low-cost infrared detecto rs has enabled the development of small, low-cost  thermal imaging instruments. The radiometer, which is the subject of this paper, uses this technology to  remotely detect the surface temper ature of large bodies of water. Knowledge of the surface temperature  of the ocean is an important factor in weather predi ction. Currently, the temper ature is recorded by large  instruments onboard ocean going ships. These instrum ents are very accurate but are also large and  heavy. The temperature readings are limited to the pat h of the ship so relatively few data points are  available for weather forecasting models. Achievin g the high accuracy desired requires frequent and  regular calibration. This is challenging for the in strument design and usually requires increased volume,  mass, and complexity.  

The Ball Experimental Sea Surface  Temperature (BESST) radiometer was designed and built by Ball Aerospace as a small, modular, light-weight and low-power sensor for sea surface temperature (SST) studies from an Unmanned Aerial Vehicle (UAV). BESST has been demonstrated on a conventional Twin Otter airplane and is part of several NASA proposals. It incorporates a commercial detector to sense the infrared radiation from the surface of the water and features two on-board calibration targets in addition to the ability to view the sky for background readings. BESST uses a combination of windows and filters to improve accuracy by minimizing the effect of water vapor and aerosols in the atmosphere. Special absorptive or reflective paints and treatments are used on the various surfaces, as needed. The resulting measurements of the absolute sea surface  temperature are accurate to within 0.3 °C.  

                                                  * Ball Aerospace & Technologies, Boulder, CO 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  RKW18cm (7.1”) Figure 1.  BESST Radiometer   NASA/CP-2010-216272
375 temporal/spatial variability greatly  increases its importance in being able to augment the ship-based  surface temperature measurements. 

Design Development 

The accuracy of the temperature reading relies on fre quent calibration, which involves having the detector  look at known temperature sources before looking at the remote object. The initial design requirement  was very simple:  place a black-body target in front of the optical path in order to calibrate the detector.  The initial configuration was to have some sort of flip mechanism to move a black-body target in and out of the detector field of view. It was then determined that two calibration targets should be used:  one for  the warm black-body and another for ambient temperat ure. To accommodate two calibration sources, a  three position flip mechanism was proposed.  The final design requirement was that a second scene select was needed to look at the sky. It was apparent that in order to get a second sky view, eit her the detector had to move or a fold mirror was  needed to reflect the image through a second baffle tube. The development of the design is shown  graphically in Figure 3. Note that each doubl e-ended arrow represents a separate mechanism. 

Detector  Filter(s)  Lens  Cal Target 1  Baffle Tube Cal Target 2  2nd Configuration  Detector  Filter(s)  Lens  Cal Target 1  Baffle Tube 1 Cal Target 2  Baffle Tube 2  3rd Configuration  Detector  Filter(s)  Lens  Calibration  Target  Baffle Tube  1st Configuration  Fold Mirror  Figure 3. Design Iteration Showing Increasing Complexity 

The key design breakthrough was realizing that if a fold mirror was acceptable for one scene, it could be  acceptable for all scenes. By rotating the fold mirror  as shown in Figure 4, the detector could easily be  exposed to different scenes. It is also a benefit to hav e the fold mirror always in place so that the only  variable is the scene.  The design iteration went as shown in Figure 3 with  the development progressing at a sketch level before  detailed design work was started. A set of well de fined concepts and parameters were evaluated before  the design work started. The final configuration is th e best compromise between optics, thermal, cost and  schedule.  NASA/CP-2010-216272
376  

Operation 

The basic principal of the design is to have the motor/fold-mirror opposite the detector assembly as shown in Figures 5 and 6. The motor rotation axis is dire ctly in-line with the center of the detector. In this  way, the optical path reflects off the fold mirror and can be directed by rotating the fold mirror. The optical path always reflects off the mirror so that the only difference to the detector is the actual scene. In addition, the optical stop is built into the tu rret, so the stop rotates with the mirror.     On-board calibration is done by alternating the observed scene from the ocean and two temperature-monitored black-body references, the common "two-point" method in radiometers with one heated and the other at ambient instrument temperature. An absolute calibration of these reference blackbodies is performed in the laboratory prior to deployment. A third "sky" port is also available to measure sky radiance to extract its reflection from the ocean temperature measurement. The use of microbolometers eliminates the need for cryogenic fluids, greatly simplifying flight requirements.   For calibration, the mechanical design has three desirable features. First, the tight fit between the mirror housing and the lens housing keeps stray light off of the calibration sources. Second, the angle of  incidence of the mirror is equal for the calibration s ources and the scene. Third, the area of the mirror  used is the same for both the calibration sources and the scene.   Description   The six modules that go into the assembly are shown in Figure 7. The central cubical frame is universally symmetric so that any module can mount to any fa ce. The detector module is mounted opposite the  motor module previously shown in Figure 5. The motor rotates to position the turret and fold mirror to one  of four quadrants. These faces can accommodate eit her a calibration target or a baffle tube. Light (or  heat) from any one of the four faces can then be directed to the detector. The calibration and baffle modules typically mounted on these face s are discussed later in the paper. 1 2  3  4 Rotating Mirror  4 scene select  Figure 4. Final Single-Mechanism  Configuration  

Rotating turret assemblyDetector Optical  Path Optical sto pMo torFo ld m irr or Filter(s) LensLens & fi lter  housi ng Rotation Axis Figure 5.  Detector, Mirror & Motor NASA/CP-2010-216272
378 Mounting   The BESST instrument can be mounted on any of the si x faces as shown in Figure 8 by using a custom  bracket to interface with the airborne or terrestrial vehicle. The bracket can also provide additional thermal  isolation from the vehicle. The lightest-weight opti on is to have the support vehicle provide a mounting  interface. 

Cal Side Mount Motor Side Mount Axial Mount   Figure 8.  Mounting Options 

Cubical Frame   The central structure or chassis for  the assembly is a simple hollow cube as shown in Figure 9. Each side is 70-mm long and each face has four tapped holes with which to attach the modules. The six faces are machined out of a single piece of aluminum so that the cube is lightweight but structurally sound. The mass of the aluminum cube is only 180 grams. The cube could be scaled up to accommodate a larger optical path.  Each face also has a diametral centration feature in the form of a close tolerance bore. Each module has a corresponding lip that fits into the bore. As the modules are attached to a given face, they are automatically aligned to the center  of the cube. Modules can be replaced or rearranged while maintaining alignment. The focal length is unaffected by rearranging the modules because the distance from the face to the center of the cube is the  same for each location.  Detector Module

The detector module consists of the detector as well as any lenses or filters required for the desired  operation (Figure 10). A single housing is used to posi tion the optical components so that this module can  be aligned and verified as a stand-alone unit. The detector module has a total mass of 225 grams of which the detector assembly is 125 grams. 

Centration Feature 50mm  70mm   Figure 9.  Cube Frame   NASA/CP-2010-216272
379 Two different detectors have been mounted to the radiometer. The original detector was made by  Raytheon and the second detector was made by FLIR. Both are un-cooled microbolometers with a 324 X  256 array with a pixel size of 38 microns. 

Lens Detector Assembly Filter(s)  Housing Centering Lip    

Figure 10.  Detector Module 

Motor Module

The key to the performance of the r adiometer is the ability to easily calib rate the detector. This involves  having the detector look at objects that have a known temperature in order to calibrate the output. 

The motor module (Figure 11) supports a 45-degree fold mirror that directs the source of light to the  detector. The fold mirror is mounted in a rotating turr et and the entrance to the turret is the Lyot stop for  the optical path.  The only load on the motor is from the turret and fold mirror, which are cantilevered off of the output shaft of the motor. The total offset load is only 33 grams. The total mass for the entire motor module is 380 grams; however, this includes a gearhead and control electronics.   

Stepper Motor  Figure 11.  Drive Module Mounting FlangeFold Mirror Mirror Turret  To DetectorFrom Source Lyot Stop  Lip   NASA/CP-2010-216272
380 Motor Details   The MDrive 17 motor/controller was selected for this program because  it is a readily available commercial unit that comes with an integrated  controller. At 300 grams, it is reasonably light-weight and the long  shaft was convenient for attaching the turret and fold mirror. The unit also comes with a canned software package that loads easily onto a computer. Simple commands can be issued to rotate the mirror to the required quadrant. If step counts are lost, the unit can be commanded to find its home position in order to reset the zero degree location.  An even lighter-weight motor was desired to further reduce the weight  of the system. When Sanyo Denki announced a new li ne of pancake stepper motors that are only 11-mm  thick, the program purchased both the 42-mm versi on and the 50-mm version, as shown in Table 1.  Although the new motors were very small and light, the output shaft was found to be too short. A longer version of the 42-mm motor is available with a longer s haft but it is the same weight as the original unit. 

Table 2.  Alternate motors considered 

42mm pancake motor  (70 grams) 42mm cube motor (240 grams)50 mm pancake motor (90 grams)MForce Controller (74 grams)

Overall mass and complexity could be reduced by  using a simple 4-position motor; however, a  commercial unit does not currently exist. The ideal motor would be a 90 ° stepper motor that could be  rotated 90 ° with a single switch command. A detent at each quadrant position might be needed to assure  repeatability if the natural cogging of the motor is not accurate enough.  

Baffle Module     The baffle tube is constructed from a single piece as shown in Figure 13. The purpose of the baffle is to  reduce the amount of stray light getting to the detec tor for the Earth and sky view scenes. The inside of  the baffle is painted with a black absorptive paint. Baffle fins are placed so that undesired light is directed to the black surface after one bounce. T he total mass for each baffle is 110 grams. 

Mounting Flange Baffle Fins  Centering Lip 

Figure 13.  One Piece Baffle Tube 

Figure 12 .  MDrive 17 Motor  and controller (300 grams)   NASA/CP-2010-216272
381 Calibration Module   The calibration target consists of an aluminum tube with a conical end that forms a black-body calibration  source. The inside is painted with a black absorptive paint so that when the detector views the target, it  sees an accurately defined and uniform temperature.  The Calibration Module shown in Figure 14 can be  built up in one of two configurations depending on the calibration temperature required. For warm  calibration, a fiberglass mounting flange is used so that  the black-body target is thermally isolated from  the rest of the system. A strip heater, attached to t he outside of the aluminum tube, heats the tube to a  predefined temperature and a temperature sensor reports the temperature.     Using an aluminum mounting flange and no external heater, the temperature of the target is the same as  the temperature of the cube, detector and baffles. No heater is used but a temperature sensor is mounted to the black-body to report the temperature of the assembly. The black-body a nd temperature sensor are  covered by a "hat" made up of Multi-Layer Insulation  (MLI). The wires from the temperature sensor and  strip heater exit through a small hole in the MLI cover. The mass of the calibration module with the  fiberglass mounting flange is 95 grams and the module with the aluminum plate is 110 grams. 

Mounting Flange  -Aluminum for ambient -Fiberglass for warm MLI Cover  Black Body Strip Heater  Temp. Sensor Centering Lip 

Figure 14.  Calibration Modules 

Mass   The total mass of the BESST instrument shown in Table 2 is for the radiometer and mechanism only and does not include the electrical cables, computer or battery required for the entire system. Reasonable dimensions and tolerances were used for this development unit. Additional mass savings could be achieved using more complicated machining.    The heaviest component on the list is the motor. For this application, the motor also includes a gearhead and control electronics. A much simpler and lighter motor could be procured but the selected unit was inexpensive, easy to interface with and easy to control.  Alternate configurations    Several different configurations are possible as show n in Figure 15. Using the existing central cube, the  baffle tubes and calibration targets can be moved around to accommodate the desired mission. The central cube can be replaced with a hexagonal housi ng to provide additional target viewing options. Table 2.  Mass Summary  Module Mass (grams)  Cube Frame 180  Motor 380  Detector 225  Baffle 110  Baffle 110  Cal Source 95  Cal Source 110  Mounting Bracket 150  Total 1.36 kg (3 lb)  NASA/CP-2010-216272
382  

Baffle Tubes Cal Targets  Baffle Tubes Cal Targets 

Figure 15.  Alternate Configurations 

The resulting assembly is a compact sensor approximately the size of a 20-cm cube as shown in Figure  16. Not included in this volume are the mounting brackets, electrical cables, computer and batteries. Also, extended baffle tubes may be needed for specific scene viewing. 

   Flight Performance 

The first “flight” of the BESST instrument was actually a field test at a local Colorado lake. The instrument  was cantilevered off the side of a small dock as shown in Figure 17. This test simulated the important  parts of the system by having a view of the water as well as a view of the sky. 

Figure 16 .  Overall assembly  NASA/CP-2010-216272
383   Figure 17.  Field Testing  

Electronics    The block diagram of the electronics for BESST is  shown in Figure 18. The Airborne Computer  commands the motor position and records the data from the detector. The motor is commanded to rotate  the central fold mirror to view the four scenes:  water, sky, warm black-body, and ambient black-body.  The detector captures images of each scene through the sequence and the black- body readings are used  to provide a highly accurate calibration of the sc ene measurement. The sequence is automated through  the software to collect data over a region of interest. 

Airborne  Computer Temperature  Monitor  Ch.  1    2    3  Moto r Power  Supply Heater  Power  Supply BESST  Radiometer        

Figure 18:  BESST electronics block diagram   Test  Engineer  Lake  Temperature Probe BESST Computer  NASA/CP-2010-216272
384 Test Results 

The BESST radiometer demonstrated exce llent performance in a test flight over Salt Lake, Utah. Flying at  low altitude, it provided some of  the highest resolution water surface temperature measurements yet  achieved, as shown in Figures 19 and 20. Each pi xel represents roughly 10 meters. The resulting detail  uncovers significant  spatial variability, with 1 to 2oC warm and cold pools on the order of 50 to 100 meters  across.    Predicted noise levels on the order of 0.1° C have been  achieved, and the instrument  is sensitive to water  temperature changes on this level. While absolute  accuracy of the measurements cannot be verified for the  test flight over Salt Lake, lab testing with controlled,  uniform blackbodies has shown the instrument to be  accurate to better than 0.3 °C. Repeated flight passes  have an RMS variability of 0.1° C.  

Additionally, the orbiting Moderate Resolution Imaging Spectroradiometer (MODIS) passed over Salt Lake on the day of the test flight, collecting 1-km-resolution surface temperature data. The newly developed radiometer compares well to MODIS, with an average resolved surface temperature difference of roughly  0.9°C.  

Much of this difference may be attributed to the disparity in resolution between the radiometer and MODIS. Considering the small-scale surface temperature variation seen during the test flight and the small fraction of each MODIS pixel that was sampled (roughly 7%), this data is not expected to directly represent the average temperature over a 1-km 2 area measured by MODIS. The m easurements match MODIS to within  the level of spatial variation observed, c onfirming the accuracy of the instrument. 

Figure 19.  Google Earth overlay showing BESST r adiometer data (narrow strips)   and MODIS data (large pixels) over Salt Lake, Utah.  Figure 20 . One data transect across a  railroad with water on either side  BESST MODIS  NASA/CP-2010-216272
385 Applications 

The Modular Radiometer can be  used in several applications and environments. It was originally designed to fly on a UAV such as the Aerosonde shown in Figure 22.  In this application, sea surface temperatures could be measured over long strips of ocean. The assembly is small and light enough to be compatible with this type of aircraft.   

To demonstrate capability,  however, it was re-configured with a different mounting bracket to fly on a Twin Otter, which is a plane commonly used to test prototype instruments (Figure 21). For this application, the unit was mounted to a central structure and controlled using a laptop computer. Aircraft inverters were available for power to the unit and computer. 

With a space-qualified motor, detector and electronics, the Modular Radiometer could be mounted to a  satellite to provide remote radiometric sensing capab ilities (Figure 23). 

Modular Radiometer on  board small satellite for  remote temperature sensing  Figure 23.  Mounted on Satellite 

Figure 21.  Twin Otter 

Figure 22 .  Modular Radiometer on board UAV  NASA/CP-2010-216272
386 Lessons Learned 

Much of the development of  the BESST instrument went very well but  there were some specific lessons  that were learned that may be of use to the reader:  Communication

Always communicate the status and requirements of  a program to all relevant disciplines. If the  requirements are not known, make assumptions and communicate those assumptions to the team. For this program, mass was not an actual requirement ot her than to enable the instrument to be flown on a  UAV or satellite at some later date. Consequently, a reasonable effort was made to make the parts lightweight. The mass of each component as well as the entire assembly was ca lculated regularly and  distributed to the team members. Likewise, the voltage and current needed for the motor and heaters were also researched and reported. The systems engineers could then work with these values to  anticipate any problems.    Documentation

Maintaining a centralized location for documenting c hanges to the design and layout of the system was  crucial to the success of the instrument. In a development environment, the c onceptual layout changes  discussed herein impacted all area s of the system and needed to be well understood by each engineer  on the project. Turnover of personnel during the lifetime of a project makes documentation even more important. 

Detector

In the interest of making the cost  per unit of the BESST instrument as low as possible while maintaining calibration accuracy, the original Raytheon Vision Systems bolometer was replaced with a FLIR bolometer which has a larger detector area. The initial unit was designed for the smaller Raytheon Vision Systems detector so that the initial baffle tubes were clipping the edges of the new light path. It was fairly easy to replace the original baffles with two new baffles tubes because of the modularity of the design as shown in Figure 24. The lesson learned is that it is advantageous to provide accommodations  for potential detector upgrades or modifications.  Motor

At the beginning of the program, it  was envisioned that a small, simple motor could be procured that  would rotate the fold mirror to the four positions. It was thought that a rotary solenoid or a four-pole motor  could be used. A detent of some sort might be necessar y to hold the mirror in position at the quadrant,  but a command to move the motor 90 degrees should be very simple.    In practice, such a device could no t be found. Motor vendors were willing to develop a custom motor for  this program but the cost was too great for a develop ment effort. The motor that was eventually used is  compact and heavy although easy to command. The lesson learned from this is that a motor as a single design component can still be very co mplicated and difficult to obtain.    Initial Baffle Tube Final Baffle Tube  Figure 24.  Baffle Tubes   NASA/CP-2010-216272