Abstract:
Methods that test multibeam satellite communication systems, including its antennas and transponders. The methods use input power telemetry and output noise power to test satellite transponders and antennas while the satellite is in orbit. One method that tests a satellite receive antenna employs at least two earth stations, one for RF testing and one for telemetry and commanding, with each providing a backup for the other. Other methods may use one or more earth stations to perform testing. Methods are disclosed that generate receive antenna pattern measurements, transmit pattern measurements, input chain frequency response curves, input chain transfer curves, and output chain frequency response curves.

Description:
BACKGROUND 
     The present invention relates generally to satellite communication systems, and more specifically, to methods for testing multibeam satellite communication systems in orbit using input power telemetry and output noise power. 
     The assignee of the present invention manufactures and deploys satellites carrying communication systems into geosynchronous and low earth orbits. Certain coverage patterns provided by communication systems produce many small spot beams. In many cases, the receive pattern and the transmit pattern are not over the same geographical location on the earth. Therefore, many payload test earth stations are required for payload testing while the satellite is in orbit. 
     In general, in-orbit testing of a satellite includes verifying the health of transponders and verifying the pointing and shape of the antenna patterns. Typically, these tests are end-to-end and require an earth station to provide an uplink and an earth station to receive and analyze the downlink signal. If the satellite&#39;s transmit and receive antenna patterns do not cover the same geographical location, two or more earth stations are required. 
     For satellites whose receive and transmit footprints cover essentially the same geographical area, (or if there are two earth stations, one in the receive and one in the transmit pattern), the antenna pattern verification is conducted by performing RF measurements while maneuvering the satellite through a set of attitude maneuvers. Saturated flux density (SFD) and Effective Isotropic Radiated Power (EIRP) at saturation are recorded along with satellite attitude telemetry, then analyzed, to determine the receive pattern and the transmit pattern. This method requires an uplink from an earth station and downlink reception at an earth station. This method is used throughout the industry. 
     For satellites whose receive and transmit footprints cover essentially the same geographical area (or if there are two earth stations, one in the receive and one in the transmit pattern), the health of the transponders is verified by executing transponder tests such as SFD/EIRP, power input versus power output, frequency response, etc. These test also require an uplink from an earth station and downlink reception at an earth station. This method is used throughout the industry. 
     For satellites that do not have an earth station in both the receive and transmit patterns, there are two known methods that achieve receive antenna pattern measurements in-orbit. One of these methods is used to measure receive antenna patterns of NSTARa and NSTARb satellites deployed by the assignee of the present invention, and the other is a method disclosed in U.S. Pat. No. 6,157,817. There is currently no method to achieve transmit antenna patterns for satellites unless both receive and transmit footprints are covered by earth stations. There is also no currently-available method to evaluate transponder health unless both receive and transmit footprints are covered by earth stations. 
     The NSTAR antenna pattern measurement method verifies a receive antenna pattern by recording signal strength telemetry resulting from an RF uplink at discrete attitude positions. The satellite attitude is commanded to a specified attitude position, an RF uplink test carrier is applied, the signal strength telemetry from an on board power sensing device is recorded, the uplink is removed, then the satellite attitude is commanded to the next attitude position. These steps are repeated until sufficient data is taken to analyze the RF pattern. The earth station that provides the RF test uplink is a geographically separate earth station from the earth station that provides command and telemetry. 
     In method disclosed in U.S. Pat. No. 6,157,817, the same earth station provides the RF test uplink and receives the telemetry. This presents a risk to the mission in the event the earth station becomes inoperable. 
     It would be desirable to have the capability of testing both the transponders and antenna patterns of a satellite-based communication systems without the requirement of having an earth station in both the receive and transmit footprints. This would allow for testing of the antenna patterns and transponders of multibeam satellites with a minimum number of earth stations. Accordingly, it is an objective of the present invention to provide for improved methods of testing multibeam satellite communication systems with a minimum of earth stations using input power telemetry and output noise power. 
     SUMMARY OF THE INVENTION 
     To accomplish the above and other objectives, the present invention provides for methods for testing multibeam satellite communication systems, including antennas and transponders. The methods use input power telemetry and output noise power to test satellite transponders and antennas while the satellite is in orbit. One of the methods employs at least two earth stations, one for RF testing and one for telemetry and commanding, with the RF test earth station providing a backup for the telemetry and commanding earth station. The other methods may use one or more earth stations to perform testing. 
     A first exemplary method that generates receive antenna pattern measurements comprises the following steps. The satellite attitude is positioned so the starting orientation angle of the slew for the uplink beam under test is over the payload test earth station providing the RF test uplink. Typically, the edge of the uplink beam pattern is chosen as the start point for the subsequent slew. A test signal is uplinked from the earth station to a receive antenna on the satellite. Commands are uplinked from a second earth station that cause the satellite to perform a slow constant attitude translation (slewing) over predetermined orientation angles. The power level of the uplink test signal is sensed while the satellite is slewed. Downlink telemetry corresponding to the sensed power level and orientation angles are generated and transmitted to a second ground station that is located at a geographically distinct location from the first earth station. The sensed power level and orientation angles contained in the downlinked telemetry are processed and analyzed to verify the operation of the receive antenna. 
     The first exemplary method uses more than one earth station to perform receive antenna pattern measurements, compared to the use of a single earth station disclosed in U.S. Pat. No. 6,157,817. The improvement provided by the present invention over the NSTAR method is that, instead of commanding discrete attitude steps to perform receive antenna pattern measurements, the present method commands a slow, continuous, constant attitude sweep. This allows for more data points to be taken, and reduces the time for the sweep. Slewing of the satellite is faster than the stop and measure technique used to test NSTAR satellites. 
     A second exemplary method uses a single earth station to generate transmit antenna pattern measurements without using an uplink carrier. The second exemplary method comprises the following steps. The satellite attitude is positioned so the start orientation angle of the slew for the downlink beam under test is over the payload test earth station receiving the downlink noise. Typically, the edge of the downlink beam pattern is chosen as the start point for the subsequent slew. Commands are uplinked from an earth station that causes the satellite to perform a slow constant attitude translation over predetermined orientation angles. Downlink noise power of a transponder is received at the earth station and measured in a specified bandwidth while the satellite is slewed. Downlink telemetry corresponding to the orientation angles are generated and transmitted to the earth station. The measured noise power levels and orientation angles contained in the downlink telemetry are processed and analyzed to verify the operation of the transmit antenna. 
     A third exemplary method generates an input chain frequency response curve that serves to verify the frequency characteristics of the transponder equipment up to the signal strength telemetry monitoring circuit. This method comprises the following steps. The uplink beam corresponding to the transponder equipment under test is positioned over a payload test earth station. RF signals at selected frequencies having the same power level are uplinked from the earth station to the satellite. Downlink telemetry corresponding to the signal strength is generated and transmitted to the earth station. The signal strength telemetry, earth station uplink power and frequency are recorded and processed produce the input chain frequency response curve. 
     A fourth exemplary method generates an input chain transfer curve that serves to verify the power characteristics of the transponder equipment up to the signal strength telemetry monitoring circuit. This method comprises the following steps. The uplink beam corresponding to the transponder equipment under test is positioned over a payload test earth station. RF signals at selected power levels having the same frequency are uplinked from the earth station to the satellite. Downlink telemetry corresponding to the signal strength is generated and transmitted to the payload test earth station. The signal strength telemetry and earth station uplink power is recorded and processed to produce the input chain transfer curve. 
     A fifth exemplary method that generates an output chain frequency response curve that serves to verify the frequency characteristics of the entire transponder if the transponder is in a linear gain mode or from the output of an amplifier (TWTA) to the downlink antenna if the transponder is in automatic level control mode. This method comprises the following steps. A downlink beam is positioned over an earth station. The noise power within a small bandwidth centered around a selected one of a plurality of frequencies of interest is measured at an earth station. The noise power measurements are continued until the noise power at all frequencies of interest are measured. The recorded noise power measurements are processed to generate an output chain frequency response curve. 
     A sixth exemplary method that verifies the gain of the transponder comprises the following steps. A downlink beam is positioned over an earth station. The noise power over a small bandwidth at center frequency or other frequency of interest is measured at an earth station. The noise power measurements are made in both linear mode and automatic level control mode at a variety of gain/level steps, if the satellite is equipped with commandable gain/level steps. The recorded noise power measurements are processed to generate gain characteristics of the transponder. 
    
    
     
       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 system in which methods in accordance with the principles of the present invention are employed; and 
         FIGS. 2-7  are flow diagrams that illustrate exemplary methods in accordance with the principles of the present invention for use with the system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures,  FIG. 1  illustrates an exemplary system  9  in which the present methods  20 ,  30 ,  40 ,  50 ,  60 ,  70  are employed. The system  9  comprises a satellite  10 , payload test earth stations  18 , and telemetry and command earth station  19 . The satellite  10  comprises transponders  11 , receive antennas  12 , and transmit antennas  13  that are to be tested. The payload test earth stations  18  and telemetry and command earth station  19  are at geographically separate and distinct locations. The exemplary system  9  is used in conjunction with various methods  20 ,  30 ,  40 ,  50 ,  60 ,  70  in accordance with the principles of the present invention to provide for testing of the transponders  11  and antennas  12 ,  13 . 
     The present systems  9  and methods  20 ,  30 ,  40 ,  50 ,  60 ,  70  provide for an improvements over the technique used with regard to NSTARa and NSTARb satellites deployed by the assignee of the present invention. The present systems  9  and methods  20 ,  30 ,  40 ,  50 ,  60 ,  70  also provide improvements over the teachings of U.S. Pat. No. 6,157,817. The contents of U.S. Pat. No. 6,157,817 are incorporated herein by reference in its entirety. 
       FIG. 2  is a flow diagram that illustrates a first exemplary procedure or method  20  in accordance with the present invention that is employed in conjunction with the system  9  shown in  FIG. 1 . The method  20  generates receive antenna pattern measurements that verify operation of the receive antenna  12  on the satellite  10 . The steps of the exemplary receive antenna pattern measurement method  20  are as follows. 
     A receive antenna pattern is positioned  21  over a payload test earth station  18 . This position is the start point for the subsequent slew and typically is the edge of the pattern. An RF test signal is uplinked  22  from a payload test earth station  18 , to a receive antenna  12  on the satellite  10 . 
     Commands  16  are uplinked  23  to the satellite  10  from a telemetry and command earth station  19  that cause a slow constant attitude translation (slewing) over predetermined orientation angles. The power level of the uplink test signal is sensed  24  in a transponder  11  on-board the satellite  10  while the satellite is slewed. Downlink telemetry  17  corresponding to the sensed power level and orientation angles is generated  25  on-board the satellite  10  and downlinked. 
     The downlink telemetry  17  is received  26  at a telemetry and command earth station  19  that is located at a geographically separate location from the payload test earth station  18 . The sensed power level and orientation angles contained in the downlinked telemetry  17  are recorded and processed  27  to verify the operation of the receive antenna  12  on the satellite  10 . 
       FIG. 3  is a flow diagram that illustrates a second exemplary method  30  that generates transmit antenna pattern measurements to verify operation of the transmit antenna  13 . The purpose of the transmit antenna pattern measurement method  30  is to allow one earth station  18  to accurately map any of the transmit spot beams generated by the satellite  10  without using multiple payload test earth stations. 
     The second method  30  requires no uplink carrier, but the satellite transponder  11  must be able to generate enough noise to be received at the payload test earth station  18 . This may be achieved by placing the transponder in automatic level control mode. An exemplary transmit antenna pattern measurement method  30  is as follows. 
     The gain of the transponder  11  is configured  31  to establish a suitable noise pedestal at the payload test earth station  18 . The transmit antenna  13  pattern is positioned  32  over the payload test earth station  18 . This position is the start point for the subsequent slew and typically is the edge of the pattern. 
     Commands  16  are uplinked  33  to the satellite  10  from a telemetry and command earth station  19  that cause a slow constant attitude translation (slewing) or discrete steps in attitude over predetermined orientation angles. Downlink noise  15  of the transponder  11  is transmitted  34  to the payload test earth station  18 , where it is measured and recorded. 
     Downlink telemetry  17  corresponding to the orientation angles is generated  35  on-board the satellite  10  and transmitted  36  to a command and telemetry earth station  19 . The noise power measurements and satellite attitude are recorded and processed  37  to verify the operation of the transmit antenna  13 . 
       FIG. 4  is a flow diagram that illustrates a third exemplary method  40  that generates an input chain frequency response curve. The objective of this method is to verify the health of the input chain using signal strength telemetry. The input chain typically includes receive antenna feeds, input multiplexer, receiver, and channel amplifier. This method will verify all equipment up through the signal strength telemetry monitoring point, which is typically, but not necessarily, in the channel amplifier. The method  40  comprises the following steps. 
     An uplink beam  12  is positioned  41  over a payload test earth station  18  and the attitude of the satellite  10  is held stationary. An RF test signal at one of several selected frequencies of interest at a specified power level is uplinked  42  from the payload test earth station  18  to the satellite  10 . 
     The signal strength of the uplink test signal is sensed  43  in the transponder  11  on-board the satellite  10 . Downlink telemetry  17  corresponding to the signal strength is generated  44  on-board the satellite  10  and downlinked. 
     The downlinked signal strength telemetry  17  is received  45  at a telemetry and command earth station  19 . The signal strength telemetry  17  and RF test carrier frequency is recorded  46 . 
     The above steps ( 42 - 43 ) are repeated  47  until all frequencies of interest are uplinked and signal strength telemetry is recorded  46 . The recorded signal strength telemetry and RF test carrier frequency are processed  48  to produce the input chain frequency response curve. 
       FIG. 5  is a flow diagram that illustrates a fourth exemplary method  50  that generates an input chain transfer curve. The objective of this method  50  is to verify the health of the input chain using signal strength telemetry. The input chain typically includes receive antenna feeds, input multiplexer, receiver, and channel amplifier. This method  50  verifies all equipment up through the signal strength telemetry monitoring point, which is typically, but not necessarily, in the channel amplifier. The method  50  comprises the following steps. 
     An uplink beam  12  is positioned  51  over an earth station  18  and the attitude of the satellite  10  is held stationary. An RF test signal at one of several power levels of interest at a specified frequency is uplinked  52  from the payload test earth station  18  to the satellite  10 . 
     The signal strength of the uplink test signal is sensed  53  in the transponder  11  on-board the satellite  10 . Downlink telemetry  17  corresponding to the signal strength is generated  54  on-board the satellite  10  and downlinked. 
     The downlinked signal strength telemetry  17  is received  55  at a telemetry and command earth station  19 . The signal strength telemetry  17  and RF test carrier power level are recorded  56 . This is repeated until all power levels of interest are uplinked and signal strength is recorded. The recorded signal strength telemetry and RF test carrier power level are processed  57  to produce the input power transfer curve. 
       FIG. 6  is a flow diagram that illustrates a fifth exemplary method  60  that generates an output chain frequency response curve. The objective of this method is to verify the health of the output chain using downlink noise power. No uplink is required. The output chain includes typically includes high power amplifier (traveling wave tube (TWT) or solid state power amplifier (SSPA), filter, output multiplexer, transmit antenna feeds. The method  60  comprises the following steps. 
     A downlink beam  13  is positioned  61  over an earth station  18  and the attitude of the satellite  10  is held stationary. The downlink noise power  15  within a small bandwidth centered around a selected frequency of interest is measured  62  at the earth station  18 . 
     The noise power measurements are continued  63  until the noise power at all frequencies of interest are measured. The recorded noise power measurements are processed  64  at the earth station  18  to generate an output chain frequency response curve. 
       FIG. 7  is a flow diagram that illustrates a sixth exemplary method  70  that generates a power level measurement of the transponder. The objective of this method is to verify the health of the output chain using downlink noise power. No uplink is required. The output chain includes typically includes high power amplifier (traveling wave tube (TWT) or solid state power amplifier (SSPA), filter, output multiplexer, transmit antenna feeds. The method  70  comprises the following steps. 
     A downlink beam  13  is positioned  71  over an earth station  18  and the attitude of the satellite  10  is held stationary. The downlink noise power  15  within a small bandwidth at the center frequency is measured  72  at the earth station  18 . These steps are repeated  73  for a variety of gain steps, if the satellite is equipped with commandable gain steps. 
     Thus, various methods for testing in-orbit multibeam satellite communication systems using input power telemetry and output noise power been disclosed. It is to be understood that the above-described embodiments are 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.