Electromagnetic Formation Flight NRO-000-02-C0387-CLIN0001 MIT Space Systems Laboratory 1 Electromagnetic Formation Flight Progress Report Submitted to: Lt. Col. John Comtois Technical Scientific Officer National Reconnaissance Office Contract Number: NRO-000-02-C0387-CLIN0001 MIT WBS Element: 6893087 Submitted by: Prof. David W. Miller Space Systems Laboratory Massachusetts Institute of TechnologyElectromagnetic Formation Flight NRO-000-02-C0387-CLIN0001 MIT Space Systems Laboratory 2 DESCRIPTION OF THE EFFORT The Massachusetts Institute of Technology Space Systems Lab (SSL) and the Lockheed Martin Advanced Technology Center (ATC) are collaborating to explore the potential for a Electro-Magnetic Formation Flight (EMFF) system applicable to Earth-orbiting satellites flying in close formation. PROGRESS OVERVIEW Work at the MIT SSL is continuing on two fronts: the CDIO class, and the MIT SSL graduate research group. The CDIO class is currently in the process of designing an electromagnetic test bed called EMFFORCE. One requirement of this testbed is the ability to determine each satellites relative position to one another. This month’s report will discuss the analysis and trades used to determine the preliminary design of the EMFFORCE test bed metrology subsystem. Metrology 1. Subsystem Overview Fig. 1: Metrology Block Diagram Extracted from the requirements of the overall project, the goal of the metrology system is to accurately calculate relative distance and attitude. Per the requirements document, accurately is defined as 1/10 of the control tolerance for both distance and angular readings. In addition, the metrology system needs to have a field of view of 360º in a 2-D plane. Finally, the system needs a detection range compatible with test facilities. These test facilities include the test facility at MIT and the Lockheed flat floor facility in Denver, CO. Interface Board Rate Gyro 2-axis accelerometer TT 8 Processor A/D Converter IR Receiver x3 Ultrasonic Receiver x3 Ultrasonic TransmitterIR Transmitter x2Electromagnetic Formation Flight NRO-000-02-C0387-CLIN0001 MIT Space Systems Laboratory 3 2. System Trade Analysis The initial trade analysis for the metrology subsystem was to compare sonic ranging systems, indoor GPS, and inertial navigation. Sonic ranging may be implemented in many ways. In the current incarnation, the sonic system uses time differences between transmitted sonic signals to triangulate the position of a vehicle (this is explained in much greater detail in the design section below). Inertial navigation uses velocity and acceleration information from rate gyros and accelerometers to calculate the position of a vehicle. The second derivative of acceleration gives linear position and the first derivative of angular rate gives angular orientation. The indoor GPS system is very similar to the current design for the sonic system except that it uses radio frequency. Indoor GPS relies on several radio frequency antennas. The system measures the time difference between incoming RF signals to triangulate position. The metrology team could not find substantial technical information on the indoor GPS system. It was also assumed that the system would require more computational cycles than the other systems since it was assumed that the RF interface would be more complicated. Additionally, the RF signals are prone to interference from the electromagnet. For these reasons the indoor GPS system was deemed infeasible. The inertial navigation system seemed well suited to our purposes; however, it also seemed to require excessive computational power. Also, inertial navigation systems may experience unacceptable drift. Each experiment is expected to run for approximately 5 minutes. Accelerometers with sufficiently low drift rate to meet this time requirement are prohibitively expensive. A sonic system is very desirable because there are many people within the department who have experience with sonic ranging. SPHERES has demonstrated and documented one functional system for sonic ranging. Also the Virtual Ink Corporation has developed an electronic whiteboard system (Mimio) based on sonic ranging which seems to demonstrate performance that meets our requirements. These design considerations lead us to choose the third option of sonic ranging. The SPHERES metrology system has several mounted ground units that emit ultrasonic pulses, which are received by the vehicles. The Mimio whiteboard system also uses a fixed unit to track a moving unit. The original designs for the EMFFORCE metrology system utilized similar ground units to give position of the vehicles relative to the fixed ground coordinates. This design is desirable since it has already been tested and proven to be effective. However, the system architecture dictates that the actuation system will have only relative control authority. The electromagnets will allow only for control of the vehicles relative to each other and not relative to the ground, therefore ground referenced positioning is unnecessary. The best option is to eliminate the ground units and put both the emitters and receivers on the vehicles. The sensors will determine the position of the other vehicles just as it would have determined the position of the ground units. This is the current design of the metrology system.Electromagnetic Formation Flight NRO-000-02-C0387-CLIN0001 MIT Space Systems Laboratory 4 3. Design Overview The current design of the system relies only on distance readings. Previous designs used distance readings and the time difference between each ultrasonic receiver on board (in addition to the time difference between the IR and ultrasonic signals) to determine the angle. It was decided that this data might not be precise enough. SPHERES uses a similar system, but cannot get good accuracy on distance using the time difference between the signals. However, the Virtual Ink Corporation has achieved millimeter accuracy with the Mimio system, and has shown interest in helping the EMFFORCE Team. The current design uses data from all three sensors, while a few previous designs used only data from two sensors. The third sensor served only to determine a positive of negative reading. This design was flawed since there was a range where the sensor wasn’t able to determine if the signal had a