Abstract:
The present invention provides a charge monitor for satellites for monitoring build up of electric charge due to charge particle fluxes impinging on the satellite. In one aspect of the invention the charge monitor includes an electrically conducting housing including at least two compartments with each compartment including a dielectric slab contained therein. An electrically conducting electrode is embedded in each dielectric slab a pre-selected distance below a top surface of the dielectric slab, the housing being mountable on a satellite with the top surface of the dielectric slab facing outwardly into space away from the satellite whereby charged particles emitted from sources external to the satellite penetrate the dielectric slab resulting in charge accumulation in each electrically conducting electrode, each electrically conducting electrode being electrically isolated from all other electrically conducting electrodes in the electrically conducting housing. The monitor includes an electrical voltage detector for sensing the floating DC voltage developed on the electrically conducting electrode in each dielectric slab, due to the charge accumulation, and converting the DC voltage into an AC voltage representative of a charge buildup on each electrically conducting electrode.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a satellite charge monitor for monitoring electric charge buildup in various components making up a satellite.  
         BACKGROUND OF THE INVENTION  
         [0002]    Earth-orbiting satellites are now very numerous and provide a wide range of services, mostly in the areas of communications and remote sensing. These satellites find themselves immersed in a hostile environment of high-energy charged particles (electrons and ions) that originate in the Sun, are emitted as the “solar wind”, and eventually swirl around the Earth following complex paths that are strongly influenced by the Earth&#39;s magnetic field. Some of these charged particles enter the Earth&#39;s atmosphere to generate the visible aurora and others swoop past the Earth only to become involved in unstable oscillations that drive them backwards, once again towards the Earth. A fraction of all these particles eventually become trapped in relatively stable patterns known as the Van Allen belts (sometimes called “radiation belts”).  
           [0003]    Unfortunately, the regions most heavily populated with high-energy charged particles are also the regions most frequently used by satellites, so that encounters between them are inevitable. In general, the impacts of most types of energetic charged particles are capable of causing either temporary or permanent damage to satellite electronic circuits. In particular, the electrons usually are not sufficiently energetic to pass through an entire satellite but they are energetic enough to become deeply embedded in the satellite dielectric materials that are used for both thermal and electrical insulation. There, the electric charge can build up to the point of electrical breakdown and arc discharge, producing both electromagnetic interference and physical damage. Wires or other conductors that are electrically “floating” (i.e. not grounded to the satellite frame) represent a special danger because they can store a large quantity of electrostatic energy which can be severely damaging when it discharges.  
           [0004]    It has been determined that over 90% of the satellite anomalies that have occurred over the last 25 years are likely owing to environmental effects with at least 50% of the total owing to charging and discharging within the satellite (H. C. Koons et al., “The Impact of the Space Environment on space Systems”, The Aerospace Corporation, paper at the Spacecraft Charging Technology Conference, 2-6 Nov., 1997, Hanscom AFB).  
           [0005]    Therefore, it would be very advantageous to provide satellite operators with sufficient warning of an impending discharge or breakdown, so that they can take mitigating measures to avoid problems. This necessitates providing a satellite charge monitor which is compact and can sense the voltage buildup in the dielectric materials representative of the dielectric materials from which the satellite is produced.  
         SUMMARY OF THE INVENTION  
         [0006]    This present invention provides a compact charge monitor for monitoring buildup of charge in a satellite and includes probes embedded in various samples of dielectric materials and different embodiments of devices that measures the floating voltage potentials developed on the probes due to charge particle fluxes impinging on the monitor.  
           [0007]    In one embodiment there is provided a charge monitor for satellites, comprising:  
           [0008]    a) an electrically conducting housing including at least two compartments with said at least two compartments each including a dielectric slab contained therein, each dielectric slab having a selected thickness;  
           [0009]    b) an electrically conducting electrode embedded in each dielectric slab a pre-selected distance below a top surface of the dielectric slab, said housing being mountable on a satellite with said top surface of said dielectric slab facing outwardly into space away from said satellite whereby charged particles emitted from sources external to the satellite penetrate said dielectric slab resulting in charge accumulation in each electrically conducting electrode producing a floating DC voltage on said electrically conducting electrode, each electrically conducting electrode being electrically isolated from all other electrically conducting electrodes in said electrically conducting housing; and  
           [0010]    c) detection means for sensing of the floating DC voltage developed on the electrically conducting electrode embedded in each dielectric slab, due to the charge accumulation, and converting the floating DC voltage into an AC voltage representative of a charge buildup on each electrically conducting electrode.  
           [0011]    In this aspect of the invention the voltage detection means may include either moving parts or non-moving parts. In the embodiment having non-moving parts, the detection means comprises a pre-selected number of transducers equal to the number of said electrically conducting electrodes,  
           [0012]    each transducer including a first metallic diaphragm electrically connected to an associated electrically conducting electrode, a second metallic diaphragm spaced from said first metallic diaphragm and substantially parallel thereto on one side of said first metallic diaphragm, a means for vibrating said second metallic diaphragm at a pre-selected frequency, and a third metallic diaphragm located on the other side of said first metallic diaphragm and substantially parallel thereto, said first metallic diaphragm being electrically insulated so that it acquires the floating voltage potential developed on the electrically conducting electrode to which it is attached,  
           [0013]    said detection means including an oscillator circuit connected to said vibration means and a synchronous detection circuit electrically connected to said third metallic diaphragm, wherein vibrating said second metallic diaphragm at said pre-selected frequency produces an oscillating capacitance to the first metallic diaphragm which then induces an oscillating charge on said third metallic diaphragm, resulting in an oscillating current at said pre-selected frequency being injected into the synchronous detection circuit which converts said oscillating current into a DC voltage proportional to the floating DC voltage on the electrically conducting electrode.  
           [0014]    In another aspect of the present invention there is provided a charge monitor for satellites, comprising:  
           [0015]    a sensor array including at least one sensor unit with the sensor array being attachable to a satellite;  
           [0016]    said at least one sensor unit including  
           [0017]    a) an electrically conducting housing defining a compartment containing a dielectric slab therein, said dielectric slab having a selected thickness, the electrically conductive housing being attachable to a satellite;  
           [0018]    b) an electrically conducting electrode embedded in said dielectric slab a pre-selected distance below a top surface of the dielectric slab, said housing being mountable on a satellite with said top surface of said dielectric slab facing outwardly into space away from said satellite whereby charged particles emitted from sources external to the satellite penetrate said dielectric slab resulting in charge accumulation in the electrically conducting electrode to produce a floating DC voltage, and  
           [0019]    c) detection means for sensing the floating DC voltage developed on the electrically conducting electrode in said dielectric slab, due to the charge accumulation, and converting the floating DC voltage into an AC voltage representative of a charge buildup on said electrically conducting electrode.  
           [0020]    In this aspect of the invention the at least one sensor unit may be two or more sensor units pre-positioned with respect to each other.  
           [0021]    In another aspect of the invention there is provided a satellite, comprising;  
           [0022]    a) a satellite housing containing a power supply and a satellite payload including communication means for communicating with a satellite control center;  
           [0023]    b) a charge monitor including  
           [0024]    an electrically conducting housing including at least two compartments with said at least two compartments each including a dielectric slab contained therein, each dielectric slab having a selected thickness, an electrically conducting electrode embedded in each dielectric slab a pre-selected distance below a top surface of the dielectric slab, said housing being mounted on said satellite housing with said top surface of said dielectric slab facing outwardly into space away from said satellite whereby charged particles emitted from sources external to the satellite penetrate said dielectric slab resulting in charge accumulation in each electrically conducting electrode producing a floating DC voltage on each electrically conducting electrode, each electrically conducting electrode being electrically isolated from all other electrically conducting electrodes in said electrically conducting housing; and  
           [0025]    detection means for sensing of the floating DC voltage developed on the electrically conducting electrode embedded in each dielectric slab, due to the charge accumulation, and converting the floating DC voltage into an AC voltage representative of a charge buildup on each electrically conducting electrode,  
           [0026]    said charge monitor being connected to said communication means for communicating charge build-up data to the satellite control center.  
           [0027]    The present invention also provides a satellite, comprising;  
           [0028]    a satellite housing containing a power supply, satellite payload including communication means for communicating with a satellite control center; and  
           [0029]    a charge monitor including  
           [0030]    a sensor array including at least one sensor unit with the sensor array being attached to the satellite housing,  
           [0031]    said at least one sensor unit including  
           [0032]    a) an electrically conducting housing defining a compartment containing a dielectric slab therein, said dielectric slab having a selected thickness, the electrically conductive housing being attachable to a satellite;  
           [0033]    b) an electrically conducting electrode embedded in said dielectric slab a pre-selected distance below a top surface of the dielectric slab, said housing being mountable on a satellite with said top surface of said dielectric slab facing outwardly into space away from said satellite whereby charged particles emitted from sources external to the satellite penetrate said dielectric slab resulting in charge accumulation in the electrically conducting electrode to produce a floating DC voltage, and  
           [0034]    c) detection means for sensing the floating DC voltage developed on the electrically conducting electrode in said dielectric slab, due to the charge accumulation, and converting the floating DC voltage into an AC voltage representative of a charge buildup on said electrically conducting electrode.  
           [0035]    In this aspect of the invention the at least one sensor unit may be two or more sensor units pre-positioned with respect to each other. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    The present invention will now be described by way of example only, reference being had to the accompanying drawings in which:  
         [0037]    [0037]FIG. 1 is a perspective view of a satellite charge monitor constructed in accordance with the present invention;  
         [0038]    [0038]FIG. 2 shows a perspective view of the charge monitor disassembled to reveal the underside of the top layer that is visible in FIG. 1;  
         [0039]    [0039]FIG. 3 is an exploded view of part of the charge monitor of FIG. 1, showing the interrupter drum located between the partitioned sample container (above the drum) and the probe array and motor (below the drum);  
         [0040]    [0040]FIG. 4 is a view of the charge monitor similar to FIG. 3 but with the interrupter drum in position;  
         [0041]    [0041]FIG. 5 is a block diagram showing the part of one embodiment that constitutes a device for generating AC voltages from the DC voltages on the sensor wires embedded in the dielectric slices;  
         [0042]    [0042]FIG. 6 is a circuit diagram for a 12 channel satellite charge monitor;  
         [0043]    [0043]FIG. 7 shows a perspective view, cut-away, of an alternative embodiment of a satellite charge sensor that has no moving parts;  
         [0044]    [0044]FIG. 8 is a schematic circuit diagram of the electronic driver/processor detector circuit that drives the piezoelectric ceramic vibrator in the satellite charge monitor of FIG. 7 and measures the floating potential induced in the electrode contained in the dielectric slab;  
         [0045]    [0045]FIG. 9 is a schematic diagram showing the various system capacitances in the satellite charge monitor of FIG. 7;  
         [0046]    [0046]FIG. 10 is a cross sectional view of an alternative embodiment of an integrated charge monitor that incorporates the functions of the charge monitor embodiments of FIGS. 7, 8, and  9 , along with a single test cell and embedded probe generally of the embodiment of a charge monitor shown in FIG. 1; and  
         [0047]    [0047]FIG. 11 is a schematic diagram showing the various system capacitances in the satellite charge monitor of FIG. 10. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0048]    Referring to FIG. 1, a satellite charge monitor shown generally at  10  includes a metal or otherwise electrically conducting housing  12  made for example of lightweight aluminum or alloy of aluminum. Housing  12  is divided into twelve pie-shaped compartments  13  separated by electrically conducting partitions  14  (which form part of the electrically conducting housing  12 ) with a pie-shaped slab of dielectric material  16  located in each of the twelve compartments  13 . Each dielectric slab  16  has an electrical conductor  18  (e.g. wires) embedded in it. These wires  18  are embedded at different depths in the dielectric slabs  16  so for example three of the wires  18  may be flush with the top surface exposed to space being located in a groove formed in the top surface of the dielectric slab  16 . It will be understood that wires per se are not necessary but any electrically conducting material may be used such as conducting foils, ribbons to mention just a few. Another set of three wires may be embedded at a depth of 2 mm in the interior of the dielectric slice and the other three slices may have the wires embedded to a depth of 4 mm from the top surface with the dielectric slices  16  being 6 mm thick. It will be appreciated however that the dielectric slices need not be restricted to being 6 mm thick and the depths of the wires  18  embedded in the slabs  16  may vary as well. Each compartment  13 , surrounded by a partition wall  14 , includes a metal or electrically conductive electrical conductors (e.g. wires)  18  lying horizontally in the dielectric slabs  16  that are connected to a metal wire  20  located in that particular compartment which in turn is connected to a electrically conductive charging post  22  (FIG. 2).  
         [0049]    The dielectric slices or slabs  16  are preferably good insulators with low inherent conductivity and are selected to be representative of the materials used in the satellite itself. These could include typical insulators such as Mylar, Kapton, Rexolite, Kevlar, Teflon, acrylic, ceramic, or composites such as fiberglass, or multi-layer printed circuit boards, coaxial cables, etc. The electrical conductors or probes  18  may be embedded in the dielectric slabs  16  at various depths to allow monitoring at those depths of the potentials due to charge accumulation. By digital means, this device enables the measurement of many probes at frequent intervals, allowing the operators of the satellite system to monitor electric fields (potential gradients) building up within the material as described hereinafter.  
         [0050]    The charge monitor of FIG. 1 is mounted on the satellite so that the upper surface of the monitor as shown in FIG. 1 is pointing out into space away from the satellite while the bottom portion of the monitor seen in FIG. 2 is facing the satellite.  
         [0051]    [0051]FIG. 2 shows the bottom of housing  12  disassembled, and metal charging posts  22  protruding through from the partitioned compartments  13  on the top side of the housing seen in FIG. 1. The metal wires  20  embedded in the dielectric slice  16  that are electrically connected to the probe wires  18  are extended by a short wire that passes through a hole in the base of its metal compartment and connects to an associated metal charging post  22  beneath the housing  12  thereby ensuring each electrical conductor  18  on the top of the housing  12  is electrically connected to an associated charging post  22  on the bottom of the housing  12 .  
         [0052]    Referring now to FIGS. 3 and 4, housing  12  includes a base  26  (shown separated from the rest of the housing  12 ), which includes twelve metal sensing posts  30  arranged in a circle. A motor  32  is mounted for rotation inside the circle defined by the sensing posts  30 . Motor  32  rotationally drives a metal drum  34  with the drum having a periodically perforated wall, the open perforations or slots  37  acting as windows so that between the sensing posts  30  and the electrically conductive charging posts  22  is the thin, periodically perforated wall of the rotating metal drum  34 . The rim of drum  34  is notched to form tabs  35  which extend outwardly from the drum wall. A hole  33  is located in base  26  and is positioned to be below tabs  35  which periodically cover hole  33  so that a light beam emerging from a light source below base  26  is reflected back to an optical reader  51  (FIG. 6) discussed hereinafter.  
         [0053]    Charge monitor  10  is designed to test a number of samples nearly simultaneously, the number being twelve in the monitor  10  shown in the FIG. 1 but it will be understood that there may be more or fewer than twelve dielectric slabs. Each dielectric slab  16  under test is fitted tightly into the pie-segment-shaped metallic-partitioned compartment  13  which is electrically connected to the satellite frame thereby grounding the housing to the satellite. The electrically conductive partitions  14  serve to provide electrical shielding between the compartments so that a charge buildup on one conductor  18  does not affect any charge buildup in any of the other compartments thereby preventing unwanted coupling between wires  18  and its associated charging post  22  with the wires/posts in neighboring compartments  13 . Each metallic-partitioned compartment  13  has one side open to the energetic-electron environment of the satellite&#39;s orbit in space. Alternatively the open side may be covered with a thin metallic foil that is penetrable only by the higher-energy electrons in the environment. The metallic probe  18  has no low-resistance, direct-current connection either to ground or to the external measurement circuit, so the probe may properly be regarded as electrically isolated or “floating”. Thus, due to the energetic-electron environment, the probe acquires an electric potential (a voltage) which may be termed its “floating potential”.  
         [0054]    Referring to FIG. 5, each electrically conductive sensing post  30  is connected to an operational amplifier  50  which, in turn, provides an amplified signal proportional to the voltage on the sensing post  30 . The rotation of drum  34 , in effect, opens and closes the windows, thus periodically turning on and off the weak electric field that passes between each charging post and its corresponding sensing post. Thus the steady direct current (DC) voltage on the embedded probe  18  is converted into an alternating voltage (AC) on the sensing post  30 .  
         [0055]    The system is synchronized using a light source  52  and light detector  54  shown in FIGS. 5 and 6. FIG. 6 shows an exemplary circuit diagram for a twelve (12) channel charge monitor which would include housing  12  having twelve different compartments with twelve dielectric slabs  16  (FIG. 1) contained therein. The alternating voltage developed on the electrically conductive sensing post  30  is then passed through AC amplifiers  50  and  56  which are inherently much more stable and interference-free than would be the case if only DC voltages were amplified. The alternating signal from the sensing post  30  is passed through the two operational amplifier stages, the first amplifier  50  serving only as an amplifier and the second amplifier  56  functioning as part of a synchronous detector. The switching signal for this synchronous detection process is derived from periodic optical reflections from the rotating drum  34  wherein the periodic optical reflections come from tabs  35  formed by notching a rim around the edge of drum  34 . The circuit that achieves this is built around an optical reader package  51  that contains a light-emitting diode  52  which shines light up through hole  33  from below base plate  26  (FIG. 4) with hole  33  positioned to be below tabs  35  which periodically cover hole  33  to permit reflection of light back to the optical reader  51  shown in the broken box in the lower right of FIG. 6.  
         [0056]    A light-sensitive photo-transistor  54  which is part of the optical reader package  51  serves as a light detector. The detector signal is then passed through AC amplifier  57 . The alternating signal from the optical reader  51  is passed through a 2N4557 FET  55  functioning as a switch whose output turns on and off its associated operational amplifier  56  in synchronism with the opening and closing of the window formed by the slot  37  in the wall of drum  34  between the charging post  22  and the sensing post  30 . The coupling between the charging posts  22  and the sensing posts  30  cannot pass DC so differentiation is involved and the unipolar field around the charging post  22  is transformed into a bipolar-pulse signal emerging from the sensing post  30 . The electro-mechanical design is such that the associated operational amplifier  56  is “on” only during a unipolar part of the bipolar pulse. Thus the rectified and filtered output of the synchronous detector can be designed to have the same sign as the charging potential that is being measured, so that magnitude calibration is all that is needed in order to complete the measurement. The end result is a recorded steady voltage proportional to the original steady probe floating potential and possessing the needed long-term stability and low noise level. Once functioning, the system can be calibrated by temporarily connecting a known voltage to each charging post through a large-value resistor. The calibration process can also be extended to ensure that no significant voltage is registered on a particular sensing post by a neighboring charging post, a deleterious effect known as “crosstalk”. The minimization of crosstalk is of critical importance and is achieved by careful design of three components, 1) the electrically conducting partitions  14  that separate the dielectric slabs above the container base shown in FIG. 1, 2) the same electrically conducting partitions  14  that separate the posts beneath the base shown in FIG. 2 (both points having been discussed above), and 3) the spacing between the windows in the wall of the rotating drum.  
         [0057]    The motor control circuit for control of motor  32  is shown in the insert in FIG. 6. The circuit uses an RC network as a frequency-to-DC-voltage converter. The resulting voltage is compared with a reference voltage and the error signal controls the speed of the motor. The motor (M)  32  is a brushless DC type motor with a built-in field generator whose output is an AC signal at the rotational frequency of the motor. This AC voltage is passed through a feedback loop and then through AC amplifier A 1 . A capacitor C and a resistor R together form a frequency-sensitive network whose output feeds the voltage-follower A 2  functioning as a rectifier. Stage A 3  is a DC amplifier serving as the motor driver driving the motor at a speed determined by the reference voltage marked “REF” in FIG. 6. Stage A 3  is configured as an integrator whose function is to enable rapid system response to any change in motor speed.  
         [0058]    Referring again to FIG. 1, a multi-pronged electrical port  24  is used to interface the electronics in monitor  10  to the electronics of the satellite to which the monitor is affixed. More specifically, port  24  has the following lines: two power lines for ±12 volts, ground, analog output, and four digital address lines A 0  to A 3  seen in FIG. 5.  
         [0059]    While the system has been described using a mechanical shutter (perforated drum  34 ) to generate the AC signal from the DC voltage on the charging posts  20 , it will be understood that this AC signal could be achieved with no moving parts using an electronic approach.  
         [0060]    [0060]FIG. 7 shows the physical core sensor unit of an alternative embodiment of a charge monitor with no moving parts shown generally at  70 . Core unit  70  includes an aluminum outer cylindrical housing  72  which is internally threaded and an inner aluminum cylindrical housing  74  which is threaded on its exterior in order to engage the internal thread on housing  72 . A grounded metal disc  76  is mounted in housing  74  and a piezoelectric ceramic driver  78  is mounted on the metal disc or diaphragm  76  in order to vibrate it at a selected frequency. Spaced from metal disc  76  is another metal disc  80  mounted in housing  74  which is electrically floating and is connected to an external test voltage which is generally the floating potential of a probe  18  embedded in a dielectric slab  16  (FIG. 1). A third metal disc  82  spaced from disc  80  mounted in housing  74  provides a signal to an external processor. Metal disc  80  is mounted in a Teflon ring  84  to insulate it from housing  74  and is spaced from metal disc  76  by a Teflon ring  86 . Metal disc  82  is mounted in a Teflon ring  88  to insulate it from housing  74  while at the same time being spaced from disc  80  by Teflon ring  84 .  
         [0061]    [0061]FIG. 8 is a circuit diagram of the electronic driver/processor that drives the piezoelectric ceramic driver  78  in housing  74  and processes the output signal from the core unit by means of a synchronous detection function leading to an analog DC output proportional to the floating test voltage input. FIG. 9 is an electrical schematic diagram showing the various system components. C 1  is the steady part of the oscillating capacitance between the vibrating diaphragm  76  and the metal disc  80  connected to the test voltage of the floating probe, and C 2  is the coupling capacitance leading to the driver/processor unit. V is the steady floating potential of the wire probe embedded in the dielectric slab under test, R is the input resistance of the first-stage operational amplifier in the driver/processor, and v is the oscillating voltage at the driver/processor input.  
         [0062]    Charge monitor  70  of FIG. 7 measures the electrical floating potential of a metallic probe  18  embedded in a sample of dielectric material  16  shown in FIG. 1. This represents configurations such as the inner conductor of a coaxial cable or a metallic trace on a printed-circuit board, where the metal has become charged as a consequence of environmental conditions such as the presence of a beam of energetic electrons in space (the Van Allen belt environment of a communication satellite) or friction between dissimilar materials (triboelectric charging).  
         [0063]    The metallic probe  18  (FIG. 1) is connected to the “Test Voltage” point in FIG. 7 which in turn connects to the electrically floating metallic disc  80  sandwiched between the 500 Hz vibrating piezoelectric-driven metal disc  76  above it and the capacitively-coupled signal-pickup disc  82  located below it in FIG. 7. The vibrating disc  76  (driven by the piezoelectric  78  connected to an oscillator circuit at Point B in FIG. 8) produces an oscillating capacitance to the middle, charged disc  80 , which then induces an oscillating charge on the lower signal-pickup disc  82 , resulting in a 500 Hz oscillating current injected into the signal-processing unit at Point A in the circuit of FIG. 8.  
         [0064]    Referring again to the circuit diagram in FIG. 8, the oscillating signal at Point A passes through two stages (Stage 1 and Stage 2) of AC amplification to Stage 3 which provides synchronous detection. The amplifier function of Stage 3 is turned on and off at half-cycle intervals as a consequence of the half-wave rectifier function of the 2N4557 FET operating on a fraction of the 500 Hz oscillator/driver signal, the other fraction driving the piezoelectric vibrator. The half-wave-rectified signal is filtered through two R-C circuits to remove ripple and is then passed via the DC amplification of Stage 4 to the DC output connector where the DC signal is proportional to the original DC voltage on the probe embedded in the dielectric slab. At this point, the DC output is then ready for conversion to a digital signal for transmission to other units for data storage, retrieval and display.  
         [0065]    With reference to FIG. 9, circuit analysis shows that the oscillating voltage v provided to the first operational amplifier stage of the driver/processor unit is given approximately by:  
           v=−jωRC   2   Vc   1   [C   1 (1 +jωRC   2 )+C 2 ] −1    
         [0066]    where v is the oscillating voltage input to the processor, using complex/phasor notation, ω is the radian frequency of the piezoelectric driver, R is the input resistance of the processor first stage, C 2  is the capacitance of the coupling capacitor, V is the steady voltage induced on the dielectric-embedded wire probe, c 1  is the oscillating part of the capacitance C 1 , driven at frequency ω, C 1  is the steady part of the driven capacitance. Note that the normally small capacitance of the embedded thin-wire probe is assumed to be negligible in the above, but it could be included in the analysis if necessary.  
         [0067]    For circuit values as follows: C 1 =1 pF, C 2 =1 pF, c 1 =0.01 pF peak, R=20 MΩ (shunted by approximately 3 pF), operating frequency f=ω/2π in the range 300-600 Hz, the resonant frequency of the piezoelectric disc is approximately 4 kHz (note that operation well below this frequency reduces undesirable system sensitivity to interference and temperature variations).  
         [0068]    A test voltage V of 100 volts produced v=1.5 millivolts peak. This not only demonstrates system feasibility by providing sufficient signal for subsequent processing, but also it demonstrates system calibration.  
         [0069]    [0069]FIG. 10 shows an integrated charge monitor unit  100  that may be operated in a laboratory vacuum chamber or mounted on a spacecraft. It combines the functions of the core unit in FIG. 7, the electronic driver/processor in FIG. 8, and, as seen in FIG. 1, one of the dielectric samples  16  with its embedded metallic probe  18 . Charge monitor  100  provides a compact monitor which measures the voltage on a single electrode  18 . With reference specifically to FIG. 10, the entire charge monitor device  100  is contained within a cylindrical aluminum housing  102 . A cylindrical aluminum screw  104  holds in place metallic probe  18  embedded in a dielectric slab  16 . To ensure against residual-gas breakdown, an elastomer O-ring  110  seals a potential breakdown path defined through the channel that carries an electrically conducting rod  114  which guides a metal spring  112  from probe  18  through dielectric slab  16 .  
         [0070]    In order to make electrical contact with the metallic probe  18 , a pressure contact is maintained via the metal spring  112  that presses directly against the probe  18 . Metal spring  112  is mounted over and concentric with metal conductor rod  114  which is shown threaded into an insulating Teflon web  118 . Because the probe  18  can charge to a floating potential of around 15 kV, everything connected to the probe  18  must be contained in a Teflon insulating chamber defined by  116 ,  118  and  120 . An electrically floating metal disc  122  is connected to the probe  18  through the pressure contact of spring  112  bearing against probe  18  which is electrically contacting metal rod  114  which is connected to metal disc  122 , and this disc  122  is also connected to three identical 80 pF high-voltage (10 kV each) capacitors C 3 , C 4  (partially hidden from view) and C 5 , which together perform the function of C 2  in FIG. 9 as illustrated in FIG. 11, while ensuring a sufficiently high end-to-end voltage rating of 30 kV to protect against the expected maximum floating potential of 15 kV with a considerable safety margin. A circuit board  126  is mounted in housing  102  which contains the amplification and detection circuit shown in FIG. 8. Capacitor C 5  is then electrically connected to circuit board  126  at the point “A” in the circuit of FIG. 8. A grounded metal disc  130  supports a piezoelectric vibrator  128 , so that the capacitance between disc  130  and disc  122  oscillates, thus functioning as the oscillatory capacitance C 1  in FIG. 11 (same as C 1  in FIG. 9). In FIG. 10, a cylindrical aluminum screw  132  holds in place the aluminum spacer  134 , the circuit board  126 , the aluminum spacer  138 , and the connector board  140  to which electrical connection is made to provide power to circuit board  126  and which also includes a data bus  144  for transmitting voltage data from the charge monitor representative of the voltage buildup on the electrical conductor  18  to the satellite communication system. A connection between the piezoelectric vibrator  128  and the circuit board  126  is shown, the circuit connection point being the one labeled “B” in FIG. 8. FIG. 11 is a schematic diagram showing the various system capacitances in the satellite charge monitor of FIG. 10.  
         [0071]    Several charge monitor sensor units  100  may be placed in close proximity to each other with each individual unit  100  having its dielectric slabs aligned with the dielectric slabs in the other sensor units but with each having its electrical probe  18  located at a different depth in its associated dielectric slab relative to the other units  100  to provide the depth profile of charge buildup local to that part of the satellite similar to the depth profile obtained with the charge monitor  10  of FIG. 1. A pre-selected number of units  100  may be pre-assembled together as a sensor array and affixed as a unit to the satellite.  
         [0072]    Typical commercial instruments are designed for the measurement of potentials at various points a short distance above a charged surface, generally for the purpose of creating a map of the surface potential distribution, by scanning the surface potential probe back and forth over the surface to acquire the needed data. Such a probe scanning procedure must not disturb the overall charge distribution (hence potential distribution) as the probe is moved, so at each measurement point the potential of the entire probe system has to be automatically adjusted to be equal to the potential being measured. This requires a complex and costly control system, and it also creates situations of significant danger due to the high voltages involved. For example, the probe can be measuring a potential of many kilovolts as it approaches grounded metal. The result is a probe-to-ground arc discharge that destroys the probe, its control circuits and the data in memory within nearby computers. While widely used for relevant laboratory research, such a probing technique is clearly not suitable for assessing the charging threat on a satellite, where reliability, cost, weight, size and power consumption are crucial. The problematic nature of measuring potentially very high voltages in space applications also means that it is not practical to have more than one electrical probe embedded in each dielectric slab. The inventors have discovered that having more than one electrical probe in the same dielectric slab which must be relatively small in surface area in a compact design greatly deceases the chance of getting a true reading at the particular depth of each wire due to interference and cross talk from the other wires in the same dielectric slab albeit they may be spaced from each other. Thus, in a compact design of a charge monitor as disclosed herein it is preferable to have only one probe per slab.  
         [0073]    Existing commercial probes typically employ a metal box with a hole in one of its walls, with the hole positioned just over and close to the surface whose potential is to be measured. A metallic vane within the box is by some means mechanically driven to wag back and forth over the hole in the presence of such electric field that manages to leak through the hole, thus generating an AC signal in the wire connected to the vane. The control system then adjusts the potential of the box and everything in it to minimize the AC signal, thus making the box potential identical to the surface potential being measured.  
         [0074]    In the present application of assessing the charging threat by measuring individually the floating potentials on an array of metallic probes embedded in samples of dielectric materials, once this guiding principle has been established, everything depends on the details of the technique to be implemented. Since the probes are small and electrically isolated, the charge on each probe is largely immobile and cannot be moved around by the process of making a measurement of its floating potential or that of its neighbors. Thus the wagging-vane process of generating an AC signal together with minimizing it using a complex control system seems unnecessary. Instead, the present inventors have developed different techniques to generate the AC signal, and then used its proportionality to the probe floating potential together with calibration in order to complete the measurement processes.  
         [0075]    As used herein, the terms “comprises”, “comprising”, “including” and “includes” are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in this specification including claims, the terms “comprises” and “comprising” and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.  
         [0076]    The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.