Abstract:
A charging sensor that uses photoemission to directly measure the electrical potential of a spacecraft in orbit relative to the space environment. The photoemission based charging sensor uses photoemission from a photoemitting conductive plate of a charge probe as a reference point for voltage readings and provides direct measurements of spacecraft charging.

Description:
BACKGROUND 
     The present invention relates generally to spacecraft, and more particularly, to a photoemission based spacecraft charging sensor for use on spacecraft. 
     Previously developed charge plates measured the buildup of charge on an electrically isolated dielectric surface. Although this produces voltage data, there is no available voltage reference. As a result, the data produced cannot be easily convened to a spacecraft charging measurement. 
     A surface charging detector (or charge plate) has heretofore been developed and flown by Lockheed Martin (formerly Martin Marietta). This surface charging detector is described in an article entitled “Integrated Environmental Monitoring System for Spacecraft.” 
     This paper describes an integrated space environmental monitoring system for geosynchronous satellites. The system provides measurements of surface charging, internal charging, and total dose radiation from the space environment. There are two components of the system, including an internal charge monitor and a surface charge detector. The surface charging monitor measures charging by magnetospheric plasma electrons from 3 keV to 20 keV and with worst-case current density from 0.1 to 1 nA/cm 2 . The surface charge detector has two components, including a charging plate and control electronics. The charging plate is mounted to the spacecraft structure outside the spacecraft and is exposed directly to the space environment. 
     However, this prior art monitoring system does not directly measure the electrical potential of a spacecraft in orbit relative to the space environment. Furthermore, this prior art monitoring system does not use photoemission from a photoemitting metal plate as a reference point for voltage readings or provide direct measurements of spacecraft charging. 
     It would be therefore be advantageous to have an improved photoemission based spacecraft charging sensor for use on spacecraft. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a charging sensor that uses photoemission to directly measure the electrical potential of a spacecraft in orbit relative to the space environment. The photoemission based charging sensor uses photoemission from a photoemitting conductive (metal) plate to create a reference point for voltage readings and provides direct measurements of spacecraft charging. Conventional devices have lacked such a reference point. 
     The charging sensor includes a charge probe and probe electronics coupled between the charge probe and a ground plane of the spacecraft. The charge probe comprises the photoemitting conductive plate which is isolated from the body of the spacecraft. The charge probe measures charge accumulation on the spacecraft by measuring the potential of the spacecraft relative to ambient plasma surrounding the spacecraft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 illustrates an exemplary charge probe used in a charge sensor in accordance with the principles of the present invention; 
     FIG. 2 illustrates an I-V curve for photoemission of aluminum; and 
     FIG. 3 illustrates an exemplary charge sensor in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, FIG. 1 illustrates an exemplary charge probe  10  used in a charge sensor  20  in accordance with the principles of the present invention. The charge probe  10  and charge sensor  20  are designed for use on a spacecraft  11 , such as an FS 1300 spacecraft  11 , developed by the assignee of the present invention. 
     The charge probe  10  comprises a conductive metal plate  13 , such as an aluminum or gold plate  13  which is isolated (insulated) from the body of the spacecraft  11  by means of an insulative adhesive  14 , for example. The metal plate  13  is coupled to probe electronics  15  which are coupled to the ground plane  12  of the spacecraft  11 . The charge probe  10  and charge sensor  20  measures the potential of the spacecraft  11  relative to the ambient plasma surrounding the spacecraft  11 , and thus determines charge accumulation on the spacecraft  11 . 
     During solar substorms, the spacecraft  11  tends to collect negative charge from the surrounding plasma. The spacecraft  11  can charge several hundred volts negative to its surroundings in a period of minutes. A typical spacecraft could use two charge probes  10  for redundancy in combination with additional charging probes of differing design. The charge probe  10  measures the potential difference between the ground plane of the spacecraft  11  and the isolated metal (aluminum) plate  13  mounted on a sun facing surface of a solar array  18 , for example, on the spacecraft  11 . 
     Aluminum is a strong photo-emitter. Alternative conductive photoemitting materials may also be used, including metals such as gold, for example. When exposed to sunlight  16 , the isolated metal plate  13  emits  17  electrons (e − ) and its potential starts to rise with respect to its surroundings. Once the potential of the metal plate  13  rises above the potential of the surrounding plasma, any electrons (e − ) that are emitted are attracted back to the surface of the metal plate  13 . The result is illustrated in the generic I-V curve shown in FIG.  2 . 
     With reference to FIG. 2, since the current drops rapidly at positive voltages, the metal plate  13  sits a few volts positive of its surroundings. The charge sensor  20  measures the potential of the spacecraft  11  by measuring the voltage difference between the metal plate  13  and the ground plane  12  of the spacecraft  11 . The measured value is proportional to the actual potential of the spacecraft  11 . The charge sensor  20  measures relatively large negative voltages (up to 1000 V) at very low currents. 
     The unique aspect of the design of the charge sensor  20  relative to previous designs is that the photoemitting metal plate  13  is used as a reference for the voltage measurement. The photoemission effectively “clamps” the potential of the metal plate  13  to the potential of the surrounding environment. As a result, measurements of the potential between the metal plate  13  and the body of the spacecraft  11  are a direct measurement of the potential between the spacecraft  11  and the environment around the metal plate  13 . If the metal plate  13  is located in the middle of a panel of solar cells of a solar array  18 , for example, the metal plate  13  measures the voltage between the body of the spacecraft  11  and solar cell cover glass surrounding the metal plate  13 . 
     FIG. 3 illustrates an exemplary charge sensor  20  in accordance with the principles of the present invention. The charge sensor  20  comprises the charge probe  10 , which includes a preamplifier  21  that is coupled between the metal plate  13  and the ground plane  12  of the spacecraft  11 . The metal plate  13  is disposed adjacent to and insulated from the solar array  18 . The preamplifier  21  is coupled to an electrostatic discharge (ESD) suppression circuit  24 . The ESD protection unit  24  outputs an analog telemetry signal  25  to a data collecting unit (DCU)  26  (or computer  26 ) on the spacecraft  11 . The preamplifier  21  is disposed relatively close to the metal plate  13 . The electrostatic discharge suppression circuit  24  and data collecting unit  26  are disposed within the body of the spacecraft  11 . The electrostatic discharge suppression circuit  24  and data collecting unit  26  are connected via a wire running through a solar array drive assembly (SADA) on the spacecraft  11 . 
     An exemplary voltage probe  10  and charge sensor  20  for use on an FS1300 spacecraft  11  that was reduced to practice has the following characteristics. The location of the metal plate  13  is preferably adjacent to the center panel of the solar array  18 , next to a center holddown of the solar array  18 . This minimizes interaction with the body of the spacecraft  11  and maximizes illumination. One voltage probe  10  is located on each solar array  18  (two per spacecraft  11 ). The lifetime of the exemplary voltage probe  10  and charge sensor  20  is estimated to be from 2 to 15 years. 
     Photoemission current density (j P ) is 42 mA/m 2  measured for an aluminum plate  13  with naturally occurring oxide. The size of the metal plate  13  is 1.2 inches by 1.2 inches. The total measured current (I s ) is between 0-39 mA. The duty cycle of the exemplary voltage probe  10  and charge sensor  20  is continuous when it is sunlit, and does not operation during eclipse periods. The output of the exemplary charge sensor  20  is a standard analog telemetry signal. The measurable voltage range for the exemplary charge sensor 20 is −1000 Volts to +50 Volts. 
     The use of the charge probe  10  and charge sensor  20  of the present invention may be used to verify and correlate simulations of the spacecraft  11 . The charge probe  10  and charge sensor  20  may also be used to provide data for anomaly investigations and aid in the design and verification of charge control measures taken on present or future spacecraft  11 . 
     Thus, a photoemission based spacecraft charging probe and sensor for use on spacecraft has been disclosed. It is to be understood that the above-described embodiment is merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.