Patent Publication Number: US-6909235-B2

Title: Power regulator for intermittent use of traveling wave tube amplifiers in communications satellites

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
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   STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   BACKGROUND OF THE INVENTION 
   This invention relates to improvements in power consumption of intermittently-operated traveling wave tube based amplifiers, particularly as used in satellite systems. 
   Amplifiers based on traveling wave tubes have numerous applications in wireless telecommunications and radar systems. Satellite communication is one class of applications, which, because of high efficiency and high reliability requirements, presents unique environmental constraints. On many satellites, traveling wave tube amplifiers, or TWTAs, are used to amplify low-level communication signals such that they can be transmitted to distant ground and/or space stations. For many satellite communication applications, TWTAs are the amplifier technology of choice because of their relatively high power and high conversion efficiency. 
   A satellite&#39;s cost is directly related to the amount of power and the thermal rejection capacity it must have. The amount of power required dictates the size and cost of the solar arrays, batteries and power conversion electronics that make up a satellite&#39;s power subsystem. The amount of thermal rejection capacity required dictates the size of the satellite&#39;s thermal radiators and the number of heat pipes, heaters and thermal blankets required in its thermal subsystem. In addition to the basic cost of these components, their mass directly affects cost of a satellite system since the cost of the required launch vehicle is directly related to the mass of the payload, such as a satellite. 
     FIG. 1  illustrates characteristics of the trajectory of an idealized non-geosynchronous orbiting (NGSO) satellite, which is a primary application of the present invention, showing a satellite  12  in positions A and B in line of sight with a tracking ground station  14  on earth  16 . It will be noted that the satellite is in view of its target/source ground station(s)  14  for only a fraction F of the orbit T. Thus, power consumption reduction during non active times is significant in that it means that if communications equipment power consumption can be terminated during these periods, the satellite&#39;s required power and thermal system capacities can be reduced. 
   Although many ways of terminating communications equipment power consumption during these no-service periods of the orbit have been considered previously, none has been able to achieve desired substantial efficiencies. For example, in the past, several methods that have been used to reduce power consumption, primarily by disabling the TWTAs. The most obvious and straightforward is to turn it off. This achieves the intended result, but the electronic power conditioner (EPC) units used to power the typical TWTA (see  FIG. 3  for a simplified TWTA block diagram comprising an EPC and TWT) are complex and sensitive devices, so that subjecting them to the tens of thousands of on/off cycles required during the life of a satellite mission is a significant reliability risk considered unacceptable by most satellite customers. 
   Disabling the pre-amplifier used to drive the TWTA is another technique used to disable these devices. Unfortunately this does not prevent the TWTA from amplifying spurious noise power. Amplification of noise is undesirable because it creates a source of interference that degrades the signal of other active satellites. Additionally, disabling the driver amplifier only marginally reduces the TWTA power consumption and thermal dissipation. Therefore a satellite using it would require larger than desired power and thermal system capacity and is therefore for reasons mentioned earlier a more expensive system. 
   Biasing anode voltage is yet another method of disabling TWTA operation and reducing satellite power and thermal resource requirements. A substantial bias potential for the anode (˜5700V for example) is required in order to be effective. Although possible to do, creating this high voltage signal is complex and expensive. 
   Since TWTAs contribute disproportionately to a satellite&#39;s power and thermal requirements—TWTAs typically consume around 90% of the power generated and distributed on a satellite and typically dissipate directly or indirectly about 70% of the heat that must be eliminated by the satellite&#39;s thermal control system—what is needed is a mechanism for providing more efficient implementations and designs for TWTA based amplifiers used in satellites, particularly NGSO satellite designs. 
   SUMMARY OF THE INVENTION 
   According to the invention, a traveling wave tube (TWT) amplifier in a spacecraft such as an orbiting satellite includes a beam forming electrode (BFE) and a BFE modulator having a bias-based keyer so that the TWT amplifier signals are keyed off during periods when RF power is not to be amplified. Biasing the beam forming electrode voltage off relative to cathode voltage effectively modulates or shuts down the electron beam so that low-level RF signal input is not amplified, and no significant RF power is output. It also does not require shutting down the power supply and thus reduces undesired power supply stress and improves reliability. The voltage required to bias off the BFE to eliminate RF output is for example a cutoff voltage of approximately less than 10% of that required for an anode-based biasing scheme or only about 500V below cathode potential. Likewise, given that no amplification has taken place, no power associated with the TWT amplification function is consumed. Thus, the only power consumed by the TWT amplifier is that associated with the cathode heater and the electronic power conditioner (EPC) used as the electron beam source. 
   A TWT amplifier according to the invention is readily applicable for deployment in a non-geosynchronous (NGSO) satellite, or a satellite whose orbital period is not equal to terrestrial orbit period. Teledesic, Global Radio, Landsat and Spot are examples of systems that are known to use NGSO satellites. Unlike geosynchronous satellites, these satellites move relative to fixed locations on the earth. For this reason, line-of-sight based communications are not possible for large portions of the satellite&#39;s orbit, so it is prudent and beneficial to shut off amplification functions during periods when the satellites are out of communication range with communication locations. 
   The invention will be better understood by reference to the following detailed description of the invention and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates characteristics of a non-geosynchronous (NGSO) satellite trajectory. 
       FIG. 2A  is a schematic diagram of a traveling wave tube (TWT) according to the invention. 
       FIG. 2B  is a timing diagram illustrating operation of a beam forming electrode modulator according to the invention. 
       FIG. 3  is a simplified block diagram of a traveling wave tube amplifier according to the prior art. 
       FIG. 4  is a diagram illustrating typical power demand of a satellite absent the present invention. 
       FIG. 5  is a diagram illustrating power demand of a satellite employing the present invention. 
       FIG. 6  is a table illustrating comparisons between power consumption with and without a TWTA operated in accordance with the invention in comparison with a prior art technique. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2A  provides schematic diagram of a traveling wave tube (TWT)  10  according to the invention.  FIG. 2B  is a graphical representation of the principles underlying the invention. In  FIG. 2A , the TWT  10 , which is enclosed in a vacuum chamber (not shown) as part of a TWT amplifier  18  includes for example a circular or parabolic cathode surface  20  coupled to a first voltage source  22  that provides a source of electrons in an electron beam  21 , a properly shaped anode  24  through which the electron beam passes and which is coupled to a second voltage source  26 , a helix  28  through which the electron beam passes along a significant length and which is coupled to receive an RF modulation input signal from an RE input  30 , typically via a coupling capacitor  32  which isolates the RE signal from the DC voltages, and produces an amplified RE output signal at an RE output terminal  34  which in turn is coupled to an antenna system (not shown). A series of collector electrodes  36 ,  38 ,  40  and  42  capture the electron beam  21  as it passes through them, each of which is coupled to a voltage source  44 ,  46 ,  48 ,  50 . According to the invention, an appropriately shaped beam forming electrode (BFE)  52  is provided adjacent the cathode  20  which is in turn coupled directly to a BEE module input source  54  and via a choke  56  to a BFE cut-off bias source  58 . The BFE modulation input source  54  is coupled to and controlled by a bias control circuit  60 . The bias control circuit  60  controls the BFE modulation input source  54  in such a way that an adjustable bias voltage based on the BFE cut-off bias source  58  is provided to the BFE  52 . The bias control circuit  60  may include a keyer  62  that is used to key off the TWT amplifier signals during periods when RF power is not to be amplified. By controlling the adjustable bias voltage, the electron beam from the BFE  52  can be turned on or off as desired. Based on the disclosure and teachings provided herein, a person of ordinary skill in the art should appreciate how to implement the bias control circuit according to the present invention. During normal operation, the beam-forming electrode (BFE)  52  serves to focus the electron beam into the traveling wave section of the TWT  10  and ultimately into the collectors  36 ,  38 ,  40 ,  42 . 
   Referring to  FIG. 2B , during periods of desired RF output (trace c, periods  2  and  4 ), the BFE  52  is held at a BFE bias voltage (trace b, periods  2  and  4 ) approximately equal to cathode voltage (Vk, ˜6000V for example, for a 120-150W TWT). By biasing the beam forming electrode voltage off relative to cathode voltage, requiring for example 500V below cathode potential, the electron beam representing the bulk of DC power consumption is effectively shut down (trace d, periods  1 ,  3 ,  5 ). The low-level RF signal input is therefore not amplified, and no significant RF power is output (trace c, periods  1 ,  3 ,  5 ). Likewise, given that no amplification has taken place, no power is consumed by the TWT associated with the amplification function. Given this condition, the only power consumed by the TWTA is that associated with the cathode heater and EPC used as the electron beam source. 
   As an example of advantages, an exemplary NGSO system uses a constellation consisting of two or three planes of seven satellites providing coverage to twenty spot beam locations scattered around the globe. Because of the satellite&#39;s orbits and the diurnal variations telecommunications service demand, the average traffic through each satellite varies, but it is about a 23% activity level. In order to make a minimum cost satellite network, each satellite must be able to minimize power consumption/thermal dissipation during periods of reduced or non-existent communications systems demands. 
   In order to quantify the potential savings of this invention, three test cases assuming a representative satellite in the exemplary system have been simulated.
         Case 1 is a configuration that envisions that all amplifiers on the satellite are left on regardless of the amount of traffic demand. This is the simplest design/operational mode.   Case 2 envisions that the TWTA pre-amplifiers be turned off as a function of traffic demand and (lack of) line of sight visibility.  FIG. 4  is a graphical illustration showing the satellite dc power demand profile for a satellite operating with this assumption.   Case 3 envisions use of the subject invention.  FIG. 5  is a graphical illustration showing the dc power demand profile for the same satellite with the invention in use.       

   As can be seen by comparing  FIGS. 4 and 5 , the minimum and average power demands required drop significantly when the invention is used (FIG.  5 ). 
     FIG. 6  provides a table that summarizes key power demand parameters for the various cases and summarizes the savings associated with the invention (Case  3 ). 
   Standard metrics can be used to estimate the cost savings associated with the invention relative to cases 1 and 2 as follows:
         Cost savings due to reduced power required and therefore reduced spacecraft power subsystem equipment (batteries, solar arrays, electronics) can be estimated as assuming $1650 saved per Watt of average power saved. Based on this metric, the invention would save ˜$1.5 million dollars per spacecraft relative to Case 1 and ˜$0.65 million dollars per spacecraft relative to Case 2.   The mass savings associated with the smaller amount of power equipment required can be estimated using the metric 1 kg is saved for every 15 Watts of power saved. Based on this, the invention saves ˜61 kg relative to Case 1 and 26 kg relative to Case 2.   As a consequence of reduced power consumption requirements, the amount of thermal waste heat the satellite thermal radiators need to be sized to eliminate is reduced. This results in some cost and mass savings. For the purposes of this analysis, this factor has been ignored due to lack of a good metric.   Spacecraft mass directly affects the costs to put a satellite in operation in that the more massive a satellite, the more it costs to launch. The value of mass saved for a satellite launched into orbits can be estimated as ˜$30,000/kg. Therefore, the invention saves $1.8 million per satellite in launch costs relative to Case 1 and ˜$0.78 million per satellite in launch costs relative to Case 2.   Summing the savings per satellite associated with power system equipment and launch costs and multiplying it by the number of satellites required to operate a satellite network system as simulated ( 14 ), the total estimated savings amount associated with invention is $46 million relative to Case 1 and $20 million relative to Case 2.       

   The invention has now been explained with reference to specific embodiments. Other embodiments will be evident to those of ordinary skill in this art. Therefore the invention should not be limited, except as indicated by the appended claims.