Method and apparatus for synchronizing communications in a satellite based telecommunications system

<u>FIELD OF THE INVENTION</u>

The present invention generally relates to a satellite based telecommunication system. More specifically, the present invention relates to synchronization of forward and return communications links between user terminals and earth stations.

<u>BACKGROUND OF THE INVENTION</u>

Satellite based telecommunications systems have been proposed for enabling user terminals to communicate with one another and with an existing public telephone switching network (PSTN). The user terminals interconnect with one another and with the public telephone network through earth stations strategically located at predefined geographic positions about the earth.

The proposed systems utilize a constellation of telecommunications satellites for relaying communications signals between the user terminals and earth stations. These communications signals pass along predefined channels uniquely assigned to each user terminal. Each channel includes a forward link along which an earth station transmits RF signals to a mobile terminal and a return link along which the user terminal transmits RF signals to the earth station. Each communications link passes through a corresponding satellite which functions as a "bent pipe" and retransmits all received communications to the corresponding earth station or user terminal.

Typically, satellite based telecommunications systems utilize one or more coding techniques to enhance the system capacity. For instance, the system may employ frequency division multiple access (FDMA) coding, time division multiple access (TDMA) coding, code division multiple access (CDMA) coding, or any combination of FDMA, TDMA, and CDMA coding. In general, to maximize system capacity, the earth station synchronizes in frequency and timing each return link assigned thereto in order to prevent interference between transmissions emitted from different user terminals. To maintain synchronization, the transmissions from multiple user terminals must be emitted at predefined times to ensure that the transmissions are received by a common earth station simultaneously. Variations in timing result as the distance varies between a coverage satellite and a transmitting user terminal. By way of example, as the distance increases between a satellite and transmitting user terminal, the transmission arrives later in time at the earth station. Divergently, as the distance decreases between a satellite and transmitting user terminal, the transmission arrives earlier in time. To account for such range variations, the user terminals are controlled to retard and advance the starting times for data transmissions (i.e., frames of communications data) to ensure that frames from multiple user terminals assigned to a common earth station arrive at the earth station simultaneously.

In addition, each earth station controls user terminals assigned thereto in order to ensure that the return link remains centered about an assigned carrier frequency (i.e., sub-band) as received at the earth station. The center frequency of each transmission from a user terminal experiences frequency changes due to the Doppler effect. The Doppler effect occurs due to the fact that a satellite continuously moves relative to a transmitting user terminal. The perceived carrier frequency of the receiver increases as the satellite moves toward a user terminal and decreases as the satellite moves away from the user terminal. If not corrected, Doppler induced frequency shifts and timing misalignment create co-channel interference between transmissions received from multiple user terminals. Thus, the user terminals continuously adjust the carrier frequency and timing of outgoing transmissions to ensure that the perceived carrier frequency and timing at the assigned earth station receiver equals the assigned carrier frequency and timing for the user terminal. A need remains for accurately maintaining the forward and return links between an earth station and user terminal.

Moreover, a need remains to facilitate handovers. Throughout transmission, satellites continuously orbit the earth. The user terminals may also move. Consequently, a communications link with a user terminal may need to be transferred or handed over from one earth station to another earth station. When a handover occurs, the user terminal must be changed to a new channel in which it establishes new forward and return links between the user terminal and the new earth station. This adjustment includes changing the timing of the user terminal to align with the new earth station's reference time. The user terminal transmitter must also shift to a new carrier frequency. A need exists to facilitate quick and reliable handovers by minimizing the time needed to achieve synchronized forward and return links with the new earth station.

Moreover, past systems have been unable to provide earth station sharing wherein multiple earth stations communicate with a user terminal at the same time through a common satellite. Earth station sharing requires timing and frequency alignment at the common user terminal of RF signals from both earth stations. Such alignment is complicated due to the presence of differing earth station to satellite path links and Doppler variations. Also, existing systems have been unable to provide return link synchronization with earth station sharing in which user terminals commmunicate in a common subband through a common satellite to multiple earth stations. To do so, the received communications signals must be aligned in time and frequency at any earth station receiving such signals.

A need remains within the industry for an improved telecommunications system capable of maintaining synchronization between multiple user terminals and sharing earth stations. It is an object of the present invention to meet this need.

<u>SUMMARY OF THE INVENTION</u>

It is an object of the present invention to provide return link synchronization to accurately align multiple user terminals in time and frequency, at a common earth station notwithstanding the differing user to satellite ranges and Doppler shifts.

It is another object of the present invention to provide forward link synchronization with earth station sharing between multiple earth stations which transmit communications to a common user terminal at the same frequency.

It is a corollary object of the present invention to provide forward link synchronization at a single user terminal which receives transmissions from multiple earth stations through a common satellite notwithstanding differences in the earth station to satellite range and Doppler effect.

It is yet a further object of the present invention to provide return link synchronization between a user terminal and multiple shared earth stations.

It is a corollary object of the present invention to align, in timing and frequency, at each of a plurality of earth stations transmissions from a user terminal transmitted at a common frequency through a common satellite to the plurality of earth stations.

These and other objects are provided by a method and apparatus for maintaining synchronous return and forward communications links between a set of user terminals and at least two earth stations communicating with the user terminals via a common satellite in a satellite based telecommunications system. Each user terminal maintains a closed synchronization loop with its assigned earth station. The user terminals update their internal timing and frequency based on the received error offset signals in order to maintain a synchronous return link with the associated earth station. In an alternative embodiment, one of the earth stations is designated as a master earth station while the remaining stations are designated as slave earth stations. The slave earth stations monitor at least one user terminal assigned to the master earth station. Based upon a timing and frequency of the monitored return link, the slave earth stations update their internal reference timing and frequency, in order to remain synchronous with the return link between the monitored user terminal and the master earth station. The slave earth stations maintain synchronous forward links by requesting synchronization update information from the monitored user terminal. Upon receiving a request, the monitored user terminal calculates a difference in timing and frequency between RF signals received from the requesting slave and RF signals from the master earth station. The user terminal transmits this difference along its return link. The slave earth station, while monitoring the return link, updates its forward timing and frequency generators to maintain its forward links synchronous with those of the master earth station.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

<ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 generally illustrates a satellite subsystem having two earth stations communicating with separate user terminals.</li><li>Fig. 2 illustrates an exemplary data structure for a frame of communications data, along with an exemplary synchronization waveform for use therein.</li><li>Fig. 3 illustrates a block diagram of the components of a user terminal and earth station utilized in connection with return link synchronization.</li><li>Fig. 4 illustrates an alternative embodiment for maintaining return link synchronization between multiple earth stations with respect to a common user terminal.</li><li>Fig. 5 illustrates a block diagram of a timing and frequency control subsection of the slave earth station illustrated in Fig. 4.</li><li>Fig. 6 illustrates a flow process carried out when achieving return link synchronization.</li><li>Fig. 7 illustrates an alternative embodiment in which synchronization is maintained within forward links between a common user terminal and master and slave earth stations.</li><li>Fig. 8 illustrates the process carried out during forward link synchronization according to the embodiment of Fig. 7.</li></ul>

<u>DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT</u>

Fig. 1 generally illustrates a sub-section of a satellite based telecommunications system including a satellite 10, earth stations <b>12</b> and <b>14</b> and user terminals <b>16</b> and <b>18.</b> Earth stations <b>12</b> and <b>14</b> include antennas <b>20</b> and <b>22</b> for tracking the satellite <b>10.</b> Additional antennas <b>24</b> and <b>26</b> are provided for tracking a second satellite <b>28.</b> Earth station <b>12</b> communicates with user terminal <b>16</b> via a forward link <b>30</b> and a return link <b>32.</b> The forward and return links <b>30</b> and <b>32</b> pass through the satellite <b>10</b> which functions as a bent pipe to retransmit all communications received. Earth station <b>14</b> communicates with user terminal 18 via forward and return links <b>34</b> and <b>36</b>, respectively. The earth stations and user terminals pass communications data and command information along the forward and return links in sequential discrete frames.

Fig. 2 illustrates an exemplary format for a communications frame <b>38</b>. The frame <b>38</b> may include a header section <b>40</b> followed by a data section <b>42</b>. The data section <b>42</b> may include telecommunications data and/or commands, while the header section <b>40</b> may include a synchronization field having synchronization information necessary to maintain synchronous communications along the forward and return links <b>30-36</b>. The header and data sections <b>40</b> and <b>42</b> may be configured in a variety of formats. For instance, the data section may include one or more PN codes, CDMA codes and similar signature waveforms which uniquely identify user terminals to which or from which the data is directed. Communications data may be modulated with the signature waveform of the user terminal transmitting or intended to receive the data. A frame <b>38</b> transmitted from a user terminal <b>16</b> only contains communications data related to that user terminal and accordingly, the data section <b>42</b> includes data modulated with the single corresponding signature waveform. A frame <b>38</b> transmitted by an earth station may include communications data sets directed to multiple user terminals. If so, the data section <b>42</b> is assembled by superimposing multiple data signals upon one another. Each data signal is modulated with a corresponding unique user terminal signature waveform to provide a composite communications data signal.

The header section <b>40</b> may include a sync field <b>44</b> configured to hold one or more synchronization waveforms. As explained above for the data section <b>42</b>, frames transmitted by a user terminal include a single unique synchronization waveform. Frames transmitted from earth stations may include multiple synchronization waveforms superimposed upon one another within the sync field <b>44</b>. Each synchronization waveform corresponds to a signature waveform and communications data set in the data section <b>44</b>.

The preferred embodiment of the present invention utilizes synchronous communications links in the forward and return directions. Communications signals are considered "synchronous" when the transmitter is controlled to ensure that a frame of communications data is received at a predefined point in time and at the nominal carrier frequency. With reference to Fig. 1, the forward link <b>30</b> from earth station <b>12</b> is considered to be synchronous since the earth station <b>12</b> transmits superimposed communications data for multiple user terminals 16 and 18 in a single frame. The overlapping data is aligned or "synchronized" within the communications data section <b>42.</b> The return links <b>32</b> and <b>36</b> are considered synchronous when multiple user terminals, such as terminals <b>16</b> and <b>18,</b> are controlled to transmit frames of communications data such that both frames are received simultaneously by the earth station 12 and synchronous with respect to a reference time and frequency.

Returning to Fig. 1, the earth stations <b>12</b> and <b>14</b> include modulator and demodulator banks <b>50</b> and <b>52</b> respectively. The demodulators in bank <b>52</b> operate to separate communications data for each desired user terminal from the composite communications frame. The modulator bank <b>50</b> operates to combine, into a composite frame, communications data for multiple user terminals.

Single Earth Station Return Link Synchronization

Next, the discussion is turned to Fig. 3 which illustrates the components of a user terminal <b>60</b> and earth station which cooperate to maintain a synchronous return link <b>64</b> therebetween. While not illustrated in Fig. 3, it is understood that the return link <b>64</b> progresses via a satellite. The user terminal <b>60</b> includes a demodulator <b>67</b> which locks onto and tracks an incoming RF signal received from forward link (FL) <b>65</b>. The demodulator <b>67</b> demodulates the incoming RF signal to obtain encoded frequency and timing error offset signals therefrom. The encoded frequency and timing error offset signals are embedded in the incoming RF signal as explained below. The demodulator <b>67</b> effects demodulation based on an estimated RF carrier frequency and an estimated timing (e.g., code chip timing) with respect to the user terminal's reference oscillator (not shown). The reference oscillator generates reference signals upon which all modulation and demodulation are based. The demodulator estimated frequency and timing are output along lines <b>73</b> and <b>75</b>, respectively, to transmission frequency and code calculation modules <b>77</b> and <b>79</b>, respectively. The demodulator <b>67</b> outputs the encoded frequency and timing error offset signals to frequency and code loop filters <b>74</b> and <b>76</b>, respectively. Optionally, the error signals may represent bit streams. The frequency and code loop filters <b>74</b> and <b>76</b> may output average values for a plurality of received error offset signals. The outputs of the loop filters <b>74</b> and <b>76</b> are delivered to the transmission frequency and code calculation modules <b>77</b> and <b>79,</b> respectively. The frequency and code calculation modules <b>77</b> and <b>79</b> combine corresponding input frequencies and chip timing codes to correct for Doppler shifts and range variations between the satellite and the user terminal. Optionally, the frequency and code calculation modules <b>77</b> and <b>79</b> may combine corresponding input signals to correct for error within the modulation/demodulation reference oscillator of the user terminal. By way of example only, the frequency and code calculation modules <b>77</b> and <b>79</b> may produce sums of, or differences between, corresponding inputs. The frequency and code calculation modules <b>77</b> and <b>79</b> output the frequency and coding signals to the oscillator <b>70</b> and code generator <b>68</b>, respectively.

The user terminal <b>60</b> also includes a signal modulator <b>66</b> which transmits communications frames (structured as shown in Fig. 2). The modulator <b>66</b> operates at a carrier frequency defined by the oscillator <b>70</b> and at a chip timing defined by the code generator <b>68.</b> The oscillator <b>70</b> may adjust its output carrier frequency based on the output of transmission frequency calculation module <b>77</b>. Optionally, the modulator <b>66</b> may combine outgoing communications data with a signature waveform produced by the waveform generator <b>68</b>. A modulated combination of the data and signature waveform may be transmitted in the communications data section <b>42</b> of a frame. In addition, the waveform generator <b>68</b> may produce a synchronization waveform which is inserted into the sync field of the header section <b>40</b> of the frame <b>38</b>. The synchronization waveform may be part of, or separate from, the signature waveform. The generator <b>68</b> produces the sync and signature waveforms based on the output of the transmission code calculation module <b>79</b>. These components cooperate to retard or advance the transmission starting time for each frame <b>38</b> and to define the central carrier frequency and corresponding sub-band within which the communications frame <b>38</b> is transmitted over the return link <b>64</b>.

As explained above, it is necessary to continuously adjust the timing and carrier frequency in order to maintain synchronized communications. The user terminal and earth station of Fig. 3 maintain synchronization through a closed loop control process whereby the earth station <b>62</b> continuously provides, via the forward link <b>65,</b> timing and frequency update information. The earth station <b>62</b> includes at least one demodulator <b>80</b> to demodulate the composite communications signal within the communications section of each frame <b>38.</b> The demodulator <b>80</b> outputs the communications data transmitted by user terminal <b>60</b> based on the signature waveforms imbedded within the received RF signal.

While the incoming RF signal may include multiple data and sync waveforms from an equal number of user terminals, the following explanation is only provided with respect to a single user terminal. The following process and structure merely need be repeated for the portion of the RF signal corresponding to other user terminals.

The earth station <b>62</b> includes timing and frequency discriminators <b>82</b> and <b>84.</b> The timing discriminator <b>82</b> compares the sync waveform from the sync field <b>44</b> of each frame with a timing reference signal to obtain a timing offset error therebetween. The timing offset error represents the amount by which the synchronization waveform in the received sync field varies, in time, from the timing reference signal. This offset error represents the amount by which the user terminal must advance or retard its timing to properly align subsequent frames with the reference time of the earth station <b>62.</b> The code timing discrimination may be effected through auto correlation of the reference and received timing signals, such as through calculating the dot product of the received and reference signals and the like.

A frequency discriminator <b>84</b> compares the received synchronization waveform with a frequency reference signal to determine the amount by which the received signal in return link <b>64</b> has shifted from the assigned carrier frequency (i.e., assigned sub-band). Frequency discrimination may be effected through a variety of known techniques, such as by converting the received and reference signals to the frequency domain and comparing phase shifts therebetween. The frequency discriminator <b>84</b> generates a frequency error offset representing the amount by which the user terminal <b>60</b> must shift its carrier frequency to properly align the return link <b>64</b> within its assigned sub-band centered about its assigned carrier frequency.

The timing and frequency error offsets are transmitted via a forward link <b>65</b> (through a satellite not shown) to the user terminal <b>60</b>. The timing and frequency error signals are delivered to code and frequency filters <b>74</b> and <b>76</b>. The filters <b>74</b> and <b>76</b> may receive multiple error signals through return link <b>65</b> and separately average multiple timing and frequency error signals. The averages of the timing error signals are supplied to the code generator <b>68</b>. The average of the frequency error signals are supplied to the oscillator <b>70</b>. The oscillator <b>70</b> and generator <b>68</b> adjust the carrier frequency and timing by an amount equal to the received error offsets. In this manner, the user terminal <b>60</b> retains a synchronized return link <b>64</b> with its associated earth station <b>62</b>.

The foregoing path between the user terminal <b>60</b> and earth station <b>62</b> represents an active closed loop architecture to correct for propogation delays and Doppler effects unique to the transmitting user terminal <b>60</b>. This timing and frequency correction process is performed independently for each user terminal communicating with the earth station <b>62</b>. In this manner, all frames arriving at the earth station are aligned in time and frequency.

Return Link Synchronization With Earth Station Sharing

Figs. 4-6 illustrate an alternative embodiment in which return link synchronization is maintained between multiple earth stations.

A system controller (not shown) assigns one master earth station to each satellite <b>106</b>, while all other earth stations covered by the satellite operate as slave earth stations with respect to this common satellite. As satellites orbit, they cover different earth stations. The master earth station is reassigned for a satellite each time the satellite's coverage area moves beyond its previous master earth station. Once assigned, an earth station may remain as the master earth station for a given satellite until the coverage area passes beyond the master earth station. An earth station may be assigned as the master earth station based on several criteria, such as geographic location, duration within the satellite's field of view (coverage area), access to the public switching telephone network, and the like. Once a master earth station is assigned to a satellite, all other earth stations entering and leaving that satellite's field of view are slaves. This assignment is performed separately with respect to each satellite. Thus, while in an overlap region between multiple satellites, an earth station may be a slave with respect to one satellite and simultaneously a master with respect to another satellite.

Fig. 4 illustrates master and slave earth stations <b>100</b> and <b>102</b> which have been assigned in connection with a common satellite <b>106.</b> The master earth station <b>100</b> receives communications from a user terminal <b>104</b> via a return link <b>108</b>. The slave earth station <b>102</b> continuously monitors the return link <b>108</b> (as illustrated by dashed line <b>110</b>).

The satellite <b>106</b> transmits RF signals received from user terminal <b>104</b> along the feeder links to all earth stations in the satellite's field of view. While the user terminal <b>104</b> may only retain a forward link <b>108</b> with the master earth station <b>100</b>, the RF signals are transmitted by the satellite toward all slave earth stations. By monitoring return link <b>108</b>, the slave earth station <b>102</b> is able to intercept RF signals traveling to the master earth station <b>100</b>. As explained below in connection with Figs. 5 and 6, the slave earth station <b>102</b> updates its internal timing and frequency reference signals to maintain synchronization with the return link <b>108</b> of the user terminal <b>104</b>.

Fig. 5 illustrates a subsection of the slave earth station <b>102</b> which operates in connection with shared return link synchronization. The RF signals passed along return link <b>108</b> are received on line <b>110</b> and supplied to timing and frequency discriminators <b>116</b> and <b>118</b>. The discriminators <b>116</b> and <b>118</b> compare the received communication signal on line <b>110</b> with reference signals from timing and frequency signal generators <b>112</b> and <b>114</b> to obtain timing and frequency offset error signals which are output at lines <b>120</b> and <b>122</b>. The timing and frequency offset error signals are delivered to switches <b>124</b> and <b>126</b>. When switched to a user terminal forward link synchronization position in the direction of arrows A, the timing and frequency offset error signals are delivered along lines <b>128</b> and <b>129</b>. As explained above in connection with Fig. 3, the offset error signals are delivered along a forward link (<b>65</b> in Fig. 3) to resultant user terminals to synchronize the return link <b>64</b>. However, when performing earth station shared return link synchronization, the switches <b>124</b> and <b>126</b> are rotated in the direction of arrows B to connect with input lines <b>130</b> and <b>132</b> which deliver the offset error signals to corresponding return link timing and frequency signal generators <b>112</b> and <b>114</b>. The timing and frequency reference signal generators <b>112</b> and <b>114</b> are updated based upon the incoming error offset signals on lines <b>130</b> and <b>132</b> such that the signal generators produce timing and frequency reference signals synchronized with the incoming timing and frequency received on line <b>110</b>.

The timing and frequency reference signal generators <b>112</b> and <b>114</b> control the timing and carrier frequency of outgoing frames transmitted by the user terminal <b>113</b> along a return link <b>111</b> to the assigned slave earth station <b>102.</b> Thus, as the generators <b>112</b> and <b>114</b> adjust to align with the received timing and frequency of the RF signal from user terminal <b>104,</b> they similarly align return link <b>111</b> from user terminal <b>113.</b>

Fig. 6 illustrates the return link synchronization process. At step <b>200</b>, the user terminal <b>104</b> transmits a communications frame along the return link <b>108</b> (Fig. 4) to the master earth station <b>100.</b> This communications frame is monitored by the slave earth station <b>102</b> via link <b>110</b> (at step <b>202</b>). Next, the slave earth station calculates the timing and frequency offsets (within discriminators <b>116</b> and <b>118</b>) between the received frame and the slave earth station's reference timing and frequency signals (step <b>204</b>) from generators <b>112</b> and <b>114.</b> At step <b>206,</b> when the switches <b>124</b> and <b>126</b> are moved in the direction of arrows B, the slave earth station updates its reference timing and frequency generators <b>112</b> and <b>114</b> to be synchronous with the received RF signal of return link <b>108</b>.

It is understood that, once the slave earth station adjusts its return link timing and frequency reference generators, the slave earth station subsequently adjusts return link synchronization of all user terminals (<b>113</b>) assigned thereto. In this manner, the subsystem of earth stations and user terminals serviced by satellite <b>106</b> are synchronized with respect to a single master earth station <b>100</b>. This synchronization sequence is carried out by first synchronizing the return link <b>108</b> of the user terminal <b>104</b> with the master earth station <b>100.</b> Thereafter, the slave earth station <b>102</b> adjusts its timing and frequency return link reference signals to be synchronous with the return link <b>108</b>. Subsequently, the slave earth station <b>102</b> instructs the user terminal <b>113</b> assigned to the slave earth station <b>102</b> to adjust its return link timing and frequency in order to be synchronous with the updated timing and frequency reference signal generators <b>112</b> and <b>114.</b> Ultimately, the return link user terminal <b>113</b> assigned to the slave earth station <b>102</b> becomes synchronous with the return link <b>108</b> to master earth station <b>100</b>.

Forward Link Synchronization With Earth Station Sharing

Fig. 7 illustrates an alternative embodiment in which forward links from multiple earth stations <b>72</b> and <b>76</b> in a common satellite's <b>74</b> field of view are synchronized. The earth stations <b>72</b> and <b>76</b> transmit to the user terminal <b>70</b> via forward links <b>78</b> and <b>80.</b> The user terminal <b>70</b> transmits along a return link <b>90</b> which is received by earth stations <b>72</b> and <b>76</b>. While the embodiment illustrated in Fig. 7 shows forward and return links between earth stations <b>72</b> and <b>76</b> and the common user terminal <b>70</b>, the user terminal <b>70</b> maintains a single communications link at any given instant. In the example of Fig. 7, the user terminal <b>70</b> and earth station <b>72</b> maintain forward and return communications links <b>78</b> and <b>90</b> until the user terminal <b>70</b> is handed off to earth station <b>76</b>. Upon completion of the handoff, user terminal <b>70</b> only maintains forward and return links <b>80</b> and <b>81</b> with the earth station <b>76</b>. While forward and return links <b>78</b> and <b>90</b> are maintained with earth station <b>72</b>, the earth station <b>76</b> monitors the return link <b>90</b> (as shown by line 91).

As explained above in connection with Fig. 4, the earth stations are configured as master and slave earth stations <b>70</b> and <b>72</b>, respectively. While two earth stations are illustrated, it is understood that any number of earth stations may be within the field of view of the common satellite <b>74.</b> All earth stations which communicate via satellite <b>74</b> may be considered to represent an earth station sharing subsystem within which a single earth station is assigned as the master station and all remaining earth stations are assigned as slave stations with respect to communications links passing through satellite <b>74</b>.

As explained above, each earth station may communicate with more than one satellite. Accordingly, an earth station may represent a master station with respect to a first satellite while operating as a slave station with respect to a second satellite.

In the example of Fig. 7, station <b>72</b> has been assigned as the master earth station and as such, maintains a closed synchronization loop (as explained above in connection with Figs. 1-3) with the user terminal <b>70.</b> Thus, the master earth station <b>72</b> adjusts the timing and frequency of the user terminal <b>70</b> until synchronous with the master earth station's timing and frequency reference signals. At predefined times throughout processing, the slave earth station <b>76</b> transmits a synchronization update request along feeder link <b>80</b> to the user terminal <b>70.</b>

The user terminal <b>70</b> includes two receivers and at least one transmitter. Thus, the user terminal <b>70</b> may receive RF signals upon two separate forward channels. The user terminal <b>70</b> receives the update request from slave earth station <b>76</b> via the second receiver, without interfering with the forward link <b>78</b> with master earth station <b>72.</b> The request instructs the user terminal <b>70</b> to perform a differential timing and frequency measurement between the forward links <b>78</b> and <b>80</b> of the master and slave earth stations <b>72</b> and <b>76.</b>

To effect this measurement, the user terminal <b>70</b> passes a communications frame received upon forward link <b>78</b> to a master demodulator <b>82</b> and passes a communications frame received upon forward link <b>80</b> to slave demodulator <b>84</b>. The master demodulator 82 compares the received master synchronization waveform with a stored synchronization waveform to determine timing and frequency differences therebetween. The slave demodulator <b>84</b> compares the received slave synchronization waveform with a reference synchronization waveform to obtain timing and frequency offset signals therebetween. Optionally, these timing and frequency differences may simply represent the timing and frequency error offset signals (as explained above in connection with Fig. 3).

The demodulated master and slave timing and frequency waveform signals are passed to a comparator which obtains differences therebetween. The differential timing and differential frequency obtained by comparator <b>88</b> are passed to modulator <b>86</b> and transmitted upon the return link <b>90.</b> The slave earth station <b>76</b> monitors the return link <b>90</b> to obtain this differential timing and frequency information along line <b>91.</b> Thereafter, the slave earth station <b>76</b> adjusts its forward link timing and frequency reference signals based on the differential timing and frequency measurements received on line <b>91</b>. In this manner, signals passed along forward link <b>80</b> become synchronized at the user terminal <b>70</b> with signals passed along forward link <b>78</b>.

The slave earth station <b>76</b> includes a modulator <b>92</b> which transmits along link <b>80</b>. The modulator operates based on a carrier frequency generated by the frequency generator <b>94</b> and based upon a timing signal generated by the timing generator <b>96</b>. The frequency and timing generators <b>94</b>, <b>96</b> are set based on the differential timing and frequency measurements transmitted by the user terminal <b>70</b> and monitored along link <b>91</b>.

According to the above embodiments, forward and return link synchronization with earth station sharing are achieved by assigning one earth.station as the master and the remaining earth stations viewed from a common satellite as slaves. The slave earth stations align their forward and return links relative to the master earth station's forward and return links to maintain synchronization at the common user terminal. While multiple common user terminals may be used, only one of the common user terminals need be monitored by the slave earth station to maintain synchronization with the master earth station. This process is repeated at a rate consistent with the dynamic effects being tracked and the required level of accuracy.

While some preferred embodiments of the invention have been described in detail, these are to be considered exemplary only and not as limitations on the scope of the invention.