Patent Publication Number: US-2007099565-A1

Title: Method of remotely estimating a rest or best lock frequency of a local station receiver using telemetry

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit U.S. Provisional Application No. 60/434,259, filed on Dec. 18, 2002, the contents of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to systems including a control station and a remote receiver, and more specifically, to such a system wherein the control station remotely monitors an operating frequency of the receiver using telemetry.  
      2. Discussion of the Related Art  
      In a typical satellite system, a ground station transmits an uplink signal to a satellite and the satellite transmits a downlink signal to the ground station. The satellite includes a receiver configured to phase and/or frequency track the uplink signal. That is, in the presence of the uplink signal, the receiver locks onto and tracks the uplink signal frequency (referred to as the uplink frequency) and/or phase. However, in the absence of the uplink signal, the receiver is unlocked, i.e., free-running, and thus settles to a rest frequency, also referred to as best lock frequency (BLF). The BLF corresponds to the uplink frequency that, if present, would cause the receiver to transition from the unlocked state to the locked state in a minimum amount of time and with a minimum amount of frequency pull-in. The BLF can be considered a natural frequency of the receiver.  
      The receiver may have a relatively narrow uplink frequency pull-in range typically encompassing the BLF. However, the BLF is typically uncertain, i.e., not precisely known. Thus, in a known technique for uplink frequency acquisition, the ground station sweeps the uplink frequency over a relatively wide frequency range anticipated to include the uncertain BLF. When the swept uplink frequency moves near to the BLF and is within the frequency pull-in range, the receiver captures or locks onto and tracks the uplink frequency, and is said to have acquired the uplink signal.  
      Causes of BLF uncertainty include, for example, receiver temperature variations and component (e.g., oscillator) aging, or other physical effects. The larger the BLF uncertainty, the larger the uplink frequency sweep range, and disadvantageously, the larger the uplink signal acquisition time. It is desirable to minimize or eliminate the BLF uncertainty, and correspondingly narrow the uplink signal sweep range, so as to minimize the uplink signal acquisition time. Therefore, it would be advantageous to be able to accurately determine the BLF at the ground station. In other words, it would be advantageous to be able to monitor at the ground station the BLF of the spacecraft receiver.  
      After the uplink frequency is acquired, the ground station performs two-way Doppler tracking of the uplink and downlink signals (i.e., frequencies). The accuracy of the Doppler tracking depends on an accuracy with which the ground station can determine the uplink frequency as received at the satellite. Thus, there is a need to be able to accurately determine at the ground station the uplink frequency as received at the satellite. In other words, there is a need to monitor at the ground station the uplink frequency at the satellite.  
      These and other embodiments of the present invention will become apparent from the ensuing description.  
     SUMMARY OF THE INVENTION  
      An embodiment of the present invention includes a method of remotely monitoring a BLF of a local receiver.  
      Another embodiment of the present invention includes a method of remotely monitoring an uplink frequency received at a local station.  
      A system in which the embodiments may operate includes a remote station, such as a ground station, and a local station, such as a spacecraft. The local station includes a receiver having first and second digital counters. The first counter is clocked at a relatively low rate related to a down-converted and scaled frequency that is input to the receiver, which may represent either (i) the BLF of the receiver in the absence of an uplink frequency, or (ii) an uplink frequency input to the receiver when the uplink signal is present. The second counter is clocked at a high rate related to a frequency of a local reference frequency of the receiver.  
      The receiver includes latch logic configured to simultaneously latch first and second counter output values of the first and second counters, respectively, responsive to a trigger signal in the receiver. The latch logic operates repeatedly to generate successive pairs of first and second counter values. The successive pairs of counter values are indicative of a ratio of either (i) the uplink frequency, or (ii) the BLF, to the reference frequency.  
      The local station telemeters the successive pairs of counter values to the remote station. The remote station determines either (i) the uplink frequency, or (ii) the BLF, based on the successive pairs of telemetered counter values and a predetermined estimate of the reference frequency available at the remote station.  
      In the absence of the uplink signal, the present invention enables a user at the remote station (e.g., ground station) to remotely determine the BLF of the receiver. The BLF represents the frequency that, if injected into the input of the receiver, will place the least stress on an uplink signal tracking system in the receiver and permit the tracking system to quickly establish receiver lock using an efficient frequency sweep strategy. This advantageously improves the use of costly ground station time.  
      In the presence of the uplink signal, the present invention enables the user at the ground station to remotely determine the uplink frequency at the receiver, to maintain the uplink frequency at a desired frequency in the presence of Doppler shifts, and roughly estimate the Doppler velocity without the use of a ground-based Doppler measurement.  
      In another application, the ground station combines the telemetered successive counter values with a telemetered phase-locked loop static phase error (produced by the receiver tracking system) in order to monitor variations in the BLF over time. This allows the ground station to maintain the uplink frequency near the BLF and to ascertain what uplink frequency will quickly lock-up the receiver if the uplink signal link, between the ground station and spacecraft, is broken and must be re-established. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages will become more readily apparent from the detailed description of the invention accompanied by the following drawings, in which:  
       FIG. 1  is a block diagram of an example system in which the present invention can operate.  
       FIG. 2  is an example transponder configuration of a spacecraft depicted in  FIG. 1 .  
       FIG. 3  is a block diagram of example portions of the spacecraft and a ground station depicted in  FIG. 1 , relevant to the present invention.  
       FIG. 3A  is a block diagram of an example arrangement of a Frequency Converter and Tracker module of  FIG. 3 .  
       FIG. 4  is a flow chart of an example method of remotely estimating a rest or best lock frequency of a local station receiver, performed in the system of  FIG. 1 .  
       FIG. 5  is a flow chart of an example method of performing, in the system of  FIG. 1 , an uplink signal acquisition.  
       FIG. 6  is a flowchart of an example method of remotely monitoring an uplink signal frequency provided to a local station. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Definition  
      The term “receiver frequency” used herein denotes a frequency f u , at which a receiver operates, and can mean either (i) an uplink frequency when an uplink frequency is present and the receiver is in the locked state, or alternatively, (ii) a rest frequency or BLF of the receiver when the receiver is unlocked, e.g., when the uplink frequency is not present.  
      System  
       FIG. 1  is a block diagram of an example system  100  in which the present invention can operate. In a first operational arrangement of system  100 , depicted in  FIG. 1 , a ground station  102  transmits an uplink signal  104  to a spacecraft  106 , such as a satellite, for example. Spacecraft  106  transmits a downlink signal  108  to ground station  102 . Relative motion between ground station  102  and spacecraft  106  will typically cause Doppler shifts in uplink signal  104  and downlink signal  108 , as would be apparent to one having ordinary skill in the relevant art(s). Thus, uplink signal  104  has a frequency f t  at ground station  102  and a Doppler shifted frequency f u  at spacecraft  106 , while downlink signal  108  has a frequency f d  at spacecraft  106  and a Doppler shifted frequency f r  at ground station  102 . Spacecraft  106  may trace a trajectory relative to ground station  102  such that the uplink signal  104  and downlink signals  108  experience no Doppler shift, i.e., frequency f t =frequency f u , and frequency f d =frequency f r . In an X-band uplink and X-band downlink system, for example, f t  might be 7.18185 GigaHertz (GHz) and f d  might be 8.4380 GHz, although any combination of radio frequencies might be used.  
      Ground station  102  includes a transmitter  110  for transmitting uplink signal  108  through an antenna  112 . Ground station  102  also includes a receiver  114  for receiving downlink signal  104  through antenna  112 . A controller  118 , coupled with transmitter  110  and receiver  114 , controls ground station  102 , and performs methods in accordance with the present invention, as will be described below.  
      Spacecraft  106  includes a receiver  120  for receiving uplink signal  104  through an antenna  122 . A transmitter  124  transmits downlink signal  108  through antenna  122 . Receiver  120  and transmitter  124  together represent a spacecraft transceiver. An oscillator  126  provides a reference signal  128 , having a reference frequency f o , to both receiver  120  and transmitter  124 . The arrangement of spacecraft  106  depicted in  FIG. 1  is referred to as a transceiver configuration. In the transceiver configuration, transmitter  124  derives downlink signal  108  independent of the phase and frequency of uplink signal  104 .  
      In the presence of uplink signal  104 , receiver  120  operates in a locked state, wherein the receiver tracks uplink frequency f u . Receiver  120  generates a count signal  132 , including count values N 1  and N 2 , indicative of a ratio of uplink frequency f u  to reference frequency f o  (i.e., f u /f o ), and provides count signal  132  to a telemetry system  134 . Telemetry system  134  encodes count signal  132  into a telemetry stream  136 . Transmitter  124  transmits telemetry stream  136  to ground station  102  through downlink signal  108 . Receiver  114  of ground station  102  recovers count signal  132  from downlink signal  108  and passes the same to controller  118 . Controller  118  determines, that is, estimates, uplink frequency f u  based on count signal  132 . In this manner, system  100  determines uplink frequency f u  at spacecraft  106 . While  FIG. 1  shows the counts  132  to originate within the receiver, they may, in principle be generated within the transmitter or in any other subsystem where the necessary signals are present.  
      The first operational arrangement of system  100  described above assumes spacecraft receiver  120  operates in the presence of uplink signal  104 . In a second operational arrangement, receiver  120  operates in the absence of uplink signal  104 , that is, uplink signal  104  is not present. In the second arrangement, receiver  120  operates in an unlocked (i.e., free-running) state, and thus settles to a rest or best-lock frequency (BLF).  
      The BLF corresponds to the uplink signal frequency f u  that, if present, would cause receiver  120  to transition from the unlocked state to the locked state in a minimum amount of time and with a minimum amount of frequency pull-in. For this reason, the term “frequency f u ” as used herein is synonymous with the term “receiver frequency” and denotes either (i) uplink frequency f u  when the uplink signal  104  is present, or alternatively, (ii) the rest frequency or BLF of receiver  120  when the receiver is not locked to the uplink signal, e.g., when the uplink signal is not present. The receiver frequency is the frequency at which receiver  120  operates.  
      In the second operational arrangement (uplink signal not present), receiver  120  generates count signal  132  indicative of a ratio of the BLF to reference f o . Spacecraft  106  transmits downlink signal  108  to ground station  102 . Ground station  102  recovers count signal  132  and passes the count signal to controller  118 . Controller  118  determines the BLF based on count signal  132 .  
       FIG. 2  is an example transponder configuration  202  of spacecraft  106 . In transponder configuration  202 , oscillator  126  provides reference signal  128  to receiver  120 ′ only. Receiver  120 ′ produces a reference signal  206  that is phase and/or frequency locked to uplink signal  104 , and provides reference signal  206  to transmitter  124 ′. Transmitter  124 ′ generates downlink signal  108 ′ such that it is also phase and/or frequency locked to reference signal  206 , and thus uplink signal  104 .  
      In addition to satellite systems, such as system  100 , the present invention may be implemented a terrestrial system wherein ground station  102  and spacecraft  106  are replaced with a base station and a terrestrial mobile and/or fixed terminal, respectively. Such a base station may be arranged in either a transceiver configuration or a transponder configuration.  
       FIG. 3  is a block diagram of example portions of spacecraft  106  and ground station  102 , relevant to both the first operational arrangement (uplink signal present) and the second operational arrangement (uplink signal not present) of present invention described above. As depicted in  FIG. 3 , spacecraft receiver  120  includes a Frequency Converter and Tracker module  302  coupled to a Counter and Latch module  304 . Module  302  includes signal processing modules configured to (i) frequency down-convert uplink signal  104 , and (ii) phase and/or frequency-lock to, i.e., track, uplink signal  104 , when the uplink signal is present. In the absence of uplink signal  104 , module  302  operates at the BLF of receiver  120 .  
      Frequency Converter and Tracker module  302  includes parallel frequency multipliers  310   a  and  310   b  for frequency multiplying uplink/BLF frequency f u  and f o  by values A and B, respectively, to produce frequencies A·f u  and B·f o  respectively. Following parallel multipliers  310 , a combiner  312  combines, e.g., adds or subtracts, the multiplied frequencies from multipliers  310 , to produce a first linear combination signal  314  having a frequency equal to A·f u +B·f o  where B may be negative. In a similar manner, parallel frequency multipliers  320   a  and  320   b , and a frequency combiner  322 , operate on reference frequencies f o  and f u  to produce a second linear combination signal  324  having a frequency equal to C·f u +D·f o , where D may be negative. Values A, B are linearly independent of values C, D. As an example of these parameters, for an uplink frequency f u , of 7.18185 GHz and a frequency reference f o , of 30.6 MegaHertz (MHz), appropriate values of the parameters might be A=1/208, B=44/39, C=0, and D=1, resulting in signals  314  and  324  having frequencies of 5048 Hertz (Hz) and 30.6 MHz, respectively.  
      Telemetry system  134  generates a measurement trigger at a fixed, known time within each downlink telemetry frame. Telemetry system  134  uses the measurement trigger to (i) derive a reset signal  334 , and (ii) enable a subsequent latch signal or pulse  330 , which the telemetry system provides to Counter and Latch module  304 . The measurement trigger may be derived from other sources, including commands telemetered from ground station  102  to spacecraft  106 , fixed or scheduled timers, events local to spacecraft  106 , and so on. The measurement triggers should occur frequently enough or on a known schedule so that there is little or no ambiguity in latched values N 1 , N 2  due to counter roll-over in counters C 1 , C 2 . Measurement triggers that do not occur at fixed, known points within each telemetry frame enable subsequent latch signal  330  but do not provide reset signal  344 . Once a latch in module  304  has been enabled (discussed below), latch signal  330  is asserted at a beginning of a next cycle of count signal  314 .  
      Counter and Latch module  304  includes signal processing elements configured to produce count values N 1 , N 2 , of count signal  132 , responsive to linear combination signals  314 ,  324 , latch signal  330 , and counter reset  334 . Counter reset  334  resets counter C 2  of module  320 . Subsequently, counters C 1 , C 2  count cycles of respective linear combination signals  314 ,  324 . Latches L 1 , L 2  of module  320  simultaneously latch respective count values or counts N 1 , N 2  of counters C 1 , C 2  responsive to latch enable  330 , and provide the count values to telemetry system  134 . Telemetry system  134  downlinks count values N 1 , N 2  to receiver  114  of ground station  102 , via downlink signal  108 . In an embodiment, counters C 1 , C 2  and latches L 1 , L 2  are implemented in digital logic, and count values N 1 , N 2  are digital values. Additional embodiments and/or details directed to generating count values N 1 , N 2  on a spacecraft are given in the following U.S. Patents, which are incorporated by reference herein in their entireties: U.S. Pat. Nos. 5,995,039; 5,745,072; and 6,650,279.  
      Receiver  114  derives count values N 1 , N 2  from downlink signal  108  and provides such values to controller  118  of ground station  102 . Controller  118  produces an estimate  368  ({tilde over (f)} u ) of uplink frequency f u , or alternatively, BLF f u , based on recovered count values N 1 , N 2  and an predetermined estimate {tilde over (f)} o , of reference frequency f o . Predetermined estimate {tilde over (f)} o , may be stored, for example, in a memory module  370  accessible by controller  118 .  
      The above-described operations repeat continuously over time to produce a series of recovered count values N 1 , N 2  and corresponding frequency estimates {tilde over (f)} u . In an embodiment, controller  118  includes a differencer module  360  configured to (i) subtract successive values of N 1  from each other to produce count difference values ΔN 1 , and (ii) subtract successive values of N 2  from each other to produce count difference values ΔN 2 . This subtraction process accounts for the fact that one or both of counters C 1 , C 2  may have “rolled over” (gone from a maximum count to zero) between latched values as a result of a limited number of bits in counters C 1 , C 2 . This process of “unrolling” counters C 1 , C 2  can be performed on the basis of known information about the frequencies of linear combination signals  314  and  324  and a relationship of these signals to the timing of the telemetry frames. After the above-described differencing and unrolling is completed, an estimator  362  determines estimate {tilde over (f)} u  based on count differences ΔN 1 , ΔN 2  and predetermined reference frequency {tilde over (f)} o .  
       FIG. 3A  is a block diagram of an example arrangement  374  of Frequency Converter and Tracker module  302  of  FIG. 3 . Arrangement  374  includes dividers  375   a - c  to divide reference frequency f o , and thereby produce respective frequency divided reference signals  376   a - c . Successive frequency down-converters or mixers  377  and  378  frequency down-convert uplink frequency f u  to a signal  380  (frequency f dc ), responsive to (i) signal  376   a , and (ii) a feedback signal  379 . A phase and/or frequency detector  382 , a loop filter  384  and a Voltage Controlled Oscillator (VCO)  386  operate together as a phase and/or frequency tracking loop for tracking signal  380 . VCO generates feedback signal  379 . A divider  388  and subsequent mixer  390  operate on VCO output  379  to produce signal  314 .  
      When uplink signal  104  is present, the tracking loop tracks signal  380 , i.e., is locked to signal  380 , and thus receiver  120  operates in its locked state. In contrast, when uplink signal  104  is not present, the tracking loop is free-running, and thus receiver  120  operates in its unlocked state. In this condition, the tracking loop settles to a rest frequency, that is, the tracking loop will cause VCO  386  to oscillate at its rest frequency. Given the architecture depicted in  FIG. 3A , the rest frequency of the tracking loop implies, i.e., corresponds to, an uplink frequency f u  referred to herein as the BLF, as would be apparent to one of ordinary skill in the relevant art(s) given the present description.  
      Method Flow Charts  
      Methods of remotely monitoring (i.e., determining or estimating) a local receiver frequency that may be performed in system  100 , for example, are now described in connection with  FIGS. 4-6 . In the methods, spacecraft  106  is considered a local station and ground station  102  is considered a remote station. Thus, ground station  102  remotely monitors the frequency at which local receiver  120  operates. That is, ground station  102  remotely monitors receiver frequency f u  (the uplink frequency, or alternatively, the BLF). Each of the methods described below may be performed with both the transceiver ( FIG. 1 ) and transponder ( FIG. 2 ) configurations of spacecraft  106 .  
      Remote Estimation of Local Receiver BLF  
       FIG. 4  is a flow chart of an example method  400  of remotely monitoring, i.e., estimating, rest or best lock frequency f u  of local station receiver  120  (referred to as the local receiver), performed in the second operational arrangement of system  100 .  
      In a first step  404 , spacecraft receiver  120  operates in the absence of an uplink signal. Thus, receiver  120  operates in its unlocked state. That is, receiver  120  operates at its rest or BLF f u .  
      In a next step  406 , modules  302  and  304  of receiver  120  operate together to derive data, e.g., count values N 1 , N 2 , indicative of a ratio of BLF f u  to reference frequency f o . Step  406  includes the following further steps:  
      (i) telemetry system  134  generates counter reset  334  and latch signal  330 ; (ii) Frequency Converter and Tracker module  302  produces linear combination signals  314 ,  324  from frequencies f u , f o ; and  
      (iii) Counter and Latch module  304  counts cycles of signals  314 ,  324  occurring between pulses of counter reset  334  and latch signal  330  (i.e., over an elapsed time period), to produce count values N 1 , N 2  respectively.  
      Step  406  repeats to produce successive sets of corresponding count values N 1 , N 2 .  
      In a next step  408 , spacecraft  106  transmits or downlinks data, i.e., count values N 1 , N 2  (e.g., successive sets of count values N 1 , N 2 ), to ground station  102 .  
      In a next step  410 , ground station  102  receives the data transmitted in step  408 . Steps  408  and  410  collectively represent a step  411  of telemetering count values N 1 , N 2  from spacecraft  106  to ground station  102 .  
      In a next step  412 , ground station receiver  114  recovers count values N 1 , N 2  from signal and provides the same to controller  118 . Controller  118  determines estimate {tilde over (f)} u  of BLF f u  based on recovered count values N 1 , N 2  and predetermined estimate {tilde over (f)} o  of receiver reference frequency f o . In an embodiment, step  412  includes the following further steps:  
      (i) differencer module  360  subtracts successive values of N 1  from each other to produce count difference values ΔN 1 ;  
      (ii) differencer module  360  subtracts successive values of N 2 , corresponding to the successive values of N 1 , from each other to produce count difference values ΔN 2 ; and  
      (iii) estimator  362  determines frequency estimate {tilde over (f)} u  based on count differences ΔN 1 , ΔN 2  and predetermined reference frequency {tilde over (f)} o .    
      Estimator  362  determines frequency estimate {tilde over (f)} u  in accordance with the following expression: 
 
= {tilde over (f)}   u   ={tilde over (f)}   o ( B−Dr )/( Cr−A ), 
 
      where r=ΔN 1 /ΔN 2 .  
      In this manner, both ratio r (i.e., ΔN 1 /ΔN 2 ) and the corresponding ratio N 1 /N 2 , are indicative of frequency {tilde over (f)} u .  
      Uplink Signal Acquisition  
       FIG. 5  is a flow chart of an example method  500  of performing an uplink signal acquisition, performed in system  100 , for example.  
      First step  400 , corresponding to method  400 , is described above in connection with  FIG. 4 . First step  400  includes estimating BLF f u  of receiver  120 , to produce estimated BLF {tilde over (f)} u .  
      In a next step  504 , controller  118  determines a frequency f t  that, when transmitted from ground station  102  to spacecraft  106 , will be received at spacecraft  106  at or at least near to estimated BLF {tilde over (f)} u  (e.g., frequencies f t  and f u  may differ by a Doppler shift or offset caused by relative motion between ground station  102  and spacecraft  106 ). Exemplary methods related to determining and tracking such a Doppler shift are given in U.S. Pat. Nos. 5,995,039, 5,745,072 and 6,650,279.  
      In a next step  506 , ground station  102  transmits uplink signal  108  at the frequency f t .  
      In a next step  508 , spacecraft  106  receives uplink signal  108  at the Doppler-shifted frequency {tilde over (f)} u  (i.e., at the estimated BLF).  
      In a next step  510 , receiver  120  captures and locks to the uplink signal frequency {tilde over (f)} u  (i.e., receiver  120  transitions from its unlocked state to its locked state).  
      Remote Estimation of Uplink Frequency at Local Receiver  
       FIG. 6  is a flowchart of an example method  600  of remotely monitoring, i.e., estimating, uplink frequency f u  provided to local station receiver  120 , performed in the first operational arrangement of system  100 . Method  600  enables ground station  102  to remotely monitor uplink frequency f u  as received at spacecraft  108 .  
      In a first step  602 , spacecraft  106  receives and acquires uplink signal  108 .  
      In a next step  604 , receiver  120  operates in its locked state when tracking uplink signal frequency f u .  
      In the next step  606 , modules  302  and  304  of receiver  120  operate together to produce successive values of count values N 1 , N 2  indicative of the ratio of uplink signal frequency f u  to local receiver reference frequency f o .  
      In a next step  608 , spacecraft  106  transmits the successive values of count values N 1 , N 2  to ground station  102 .  
      In the next step  610 , count values N 1 , N 2  are received at ground station  102 .  
      In a next step  612 , ground station  102 , that is, controller  118 , estimates uplink frequency f u  based on recovered count values N 1 , N 2  and predetermined estimate {tilde over (f)} o  to produce estimated uplink frequency {tilde over (f)} u .  
      It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting the scope of the invention, but merely as exemplifications of the preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.