Abstract:
A single cable inter-facility link is provided using an IDU-ODU telemetry interface employing an encoder in the status transmit circuitry and in the control transmit circuitry which encodes the appropriate data by integrating Manchester encoding, a preamble and postamble, and on-off keying to create a unique data packet scheme which is compatible with any existing inter-facility link protocol and which does not interfere with the DC power or normal data traffic. Together with a simple receiver structure implementing an adaptive threshold detector that decodes the telemetry data, any kind of information can be communicated between the IDU and ODU over a single cable. Control of the ODU by the IDU via a bi-directional half-duplex or full-duplex telemetry interface allows the IDU to control the ODU settings.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    The present application claims benefit of the filing date of U.S. provisional application No. 60/471,110 filed on May 16, 2003, the entire content of which is incorporated herein by reference. 
     
    
     
       STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
           [0003]    REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.  
           [0004]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0005]    This invention relates to VSAT (Very Small Aperture Terminal) telecommunication systems and in particular to the architecture of the receiving and detecting functions for the antenna and receiver for receiving signals from a satellite.  
           [0006]    For many modem communication terminals, the system architecture consists of an Outdoor Unit (ODU) and an Indoor Unit (IDU). The ODU typically performs the front-end radio and antenna functions exposed to the environment, and it provides an interface to the IDU. It is designed to withstand the more severe environmental outdoor conditions. The IDU typically performs the modem and networking functions and interfaces to the user and customer premise equipment. The IDU is physically located in more benign operating conditions than the ODU. The inter-facility link, namely, the physical connection between the IDU and ODU, may vary, depending on the functional partitioning between the two subsystems. However, it is not uncommon for the inter-facility link to consist of a single coaxial cable. Transmit and receive signals as well as a frequency reference signal (typically 10 MHz) are conventionally multiplexed on the same cable, along with a DC supply voltage needed to power the ODU. However, ODU to IDU control communication that is designed to be communicated through baseband techniques can only be effected on a second cable or cable set. This significantly increases the complexity and cost of a set of units.  
           [0007]    As data rates continue to increase and unit costs continue to decrease it becomes increasingly necessary to have some sort of communication link between the IDU and the ODU. By allowing the IDU to monitor data such as the ODU transmit power, as well as other status information the IDU can make adjustments to compensate for the ODU&#39;s performance, thus relaxing the requirements for the ODU performance and lowering the cost of the ODU.  
           [0008]    It is desired that the ODU to IDU communication be effected on the same cable as other signals, thus greatly simplifying and reducing the cost of the inter-facility link (IFL). However integrating these five elements on a single cable has proven to be a longstanding problem, and no simple, cost-effective method for doing this exists although many attempts have been made. Communicating across the IFL through baseband techniques offers the simplest approach and most affordable hardware. However, conventional baseband IDU/ODU communication techniques cannot be used when a DC voltage is present on the cable, so a second cable has been required to implement this method. Known methods of transmitting the IDU-ODU data that avoid this problem include common analog and digital modulation techniques. These approaches have drawbacks because analog modulation, such as AM or FM, requires calibration to obtain and maintain proper performance over temperature variations and long-term component aging. Digital modulation techniques, such as BPSK can produce good performance without calibration, but they generally require a complex implementation with automatic gain control (AGC) and timing recovery.  
           [0009]    Another technique for IDU/ODU communication is known as the Digital Satellite Equipment Control Bus (DiSEqC) standard, a published standard developed by EUTELSAT. Under this bus standard, communication on the inter-facility link is achieved with a pulse width keyed data bit symbol of 1.5 ms duration with the pulse tone at 22 kHz. One problem with this system is its relatively low data rate (limited to 667 bps). The system is quite complex due to the needed bus structure (designed so that several ODUs can be connected to a single IDU), not to mention the complexity of the pulse width keying demodulator.  
           [0010]    What is therefore needed is a simpler, more cost-effective, easy to implement and easy to mass-produce method for producing high data rate IDU/ODU communication via a single inter-facility link which can coexist with normal data transmit and data receive traffic, the reference signal, and the ODU DC supply voltage.  
         SUMMARY OF THE INVENTION  
         [0011]    According to the invention, a single cable inter-facility link is provided using an IDU-ODU telemetry interface employing an encoder in the status transmit circuitry and in the control transmit circuitry which encodes the appropriate data by integrating Manchester encoding, a preamble and postamble, and on-off keying to create a unique data packet scheme which is compatible with any existing inter-facility link protocol and which does not interfere with the DC power or normal data traffic. Together with a simple receiver structure implementing an adaptive threshold detector that decodes the telemetry data, any kind of information can be communicated between the IDU and ODU over a single cable. The main utility of the ODU-IDU telemetry interface is to allow the IDU to monitor the output power of the ODU as well as other “status” information. Control of the ODU by the IDU via a bi-directional half-duplex or full-duplex telemetry interface according to the invention allows the IDU to control the ODU settings, such as transmit and receive frequency control words, transmit enable/disable (on/off) switch, transmit attenuation settings, and receive IF gain adjustment settings. The invention defines a telemetry interface that provides this functionality as a subset of the overall architecture leading to a low cost solution for ODU control by the IDU.  
           [0012]    The ODU to IDU telemetry interface includes and merely requires relatively simple encoding and decoding hardware to implement. The telemetry interface according to the invention achieves the desirable qualities of digital modulation (high data rates and good performance over time and temperature) for an inter-facility link without the complexity previously required in a digital implementation. It thus allows the implementation of both a continuous transmission system and a flexible burst transmission system with timing synchronization between the ODU and ODU, as well as fast settling frequency-hopped transmission, slow frequency hopped transmission and fixed frequency transmission.  
           [0013]    These are some of the advantages of this invention.  
           [0014]    1. By providing that the IDU constantly monitor and control the output power of the ODU, requirements for ODU gain variation; gain flatness, temperature compensation, absolute output accuracy, inter-facility link cable variations over frequency; IDU analog output gain variations, ripple, VSWR ripple, attenuator step size, attenuator accuracies can all be relaxed, thus allowing for more affordable parts to be used in the construction of the hardware. It also allows more flexibility in installations. Since the IDU will automatically compensate for any attenuation that occurs in the inter-facility link, a larger range of inter-facility lengths is allowed.  
           [0015]    2. Monitoring other status information from the ODU greatly enhances the service provided. This allows the IDU to notify the service provider of necessary preventative maintenance and to automatically schedule service. This includes scenarios where a graceful degradation has occurred, where the terminal is still providing acceptable service performance but has reached a point of degradation where the performance is not at the specification limits. Equipping the IDU with a simple mechanism that monitors OPldB allows it to determine whether the ODU transceiver or boost amplifier needs to be field replaced or undergo preventative maintenance. Solid state power amplifier end of life degradation and fan failures which characteristically have been shown to have an effect on MTBF&#39;s are monitored. Unit-by-unit automatic monitoring and reporting also enhances the service offering, allowing automatic notification to the service provider of any replacements or repairs that need to be made. Thus, overall system reliability achievable through better preventative maintenance is a benefit realized by the end user.  
           [0016]    3. Unwanted transmissions are also minimized. Fault detection and fault isolation of the ODU by the IDU is also possible over the telemetry link. Regulatory agencies require that a transmitter be disabled when a transmission fault is detected. The ODU monitors local faults and reports these to the IDU. The IDU then automatically turns off the transmit circuitry in the event that a fault is detected.  
           [0017]    4. Multiplexed signal interface reduces inter-facility link cabling requirements and interconnect resulting in a simpler and lower cost terminal when installed and operated over multiple cable interfaces. This invention also takes advantage of low cost cabling performance by staying at frequencies easily transmitted over low cost cable.  
           [0018]    5. Telemetry digital modulation and demodulation design simplifies hardware and lowers parts cost by allowing for use of readily available analog components. Because the ODU transmitter uses Manchester encoding, no precision or calibrated parts are required for demodulation and decoding in the IDU. Thus the encoding, decoding, modulation and demodulation can be implemented with a small parts count using readily available low cost parts.  
           [0019]    6. Scalable design is easily modified to provide for supporting data links at a variety of rates. Demodulation and data synchronization do not need to be modified for different size packets and data rates.  
           [0020]    This invention will be better understood by reference to the following detailed description in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a the top level block diagram showing an IDU and ODU connected by a single Inter Facility Link (IFL).  
         [0022]    [0022]FIG. 2 is a high level circuit diagram of an ODU in accordance with the invention.  
         [0023]    [0023]FIG. 3 is a high level circuit diagram of an example of status transmit circuitry in accordance with the invention.  
         [0024]    [0024]FIG. 4 is a diagram of one embodiment of telemetry logic used in accordance with the invention to generate packets of data that will be sent to the IDU.  
         [0025]    [0025]FIG. 5 is a diagram of one embodiment of a Control Receive Circuitry in accordance with the invention.  
         [0026]    [0026]FIG. 6 is a high level block diagram of one embodiment of an IDU Circuit in accordance with the invention.  
         [0027]    [0027]FIG. 7 is a diagram of one embodiment of a Status Receive Circuit according to the invention.  
         [0028]    [0028]FIG. 8 is a diagram of one embodiment of a control transmit circuit in accordance with the invention.  
         [0029]    [0029]FIG. 9 is a diagram illustrating one possible way to define telemetry messages sent over the IFL.  
         [0030]    [0030]FIGS. 10A, B and C are illustrations of how in the frequency domain this invention (methods A and B) differ from the prior art EUTELSAT method.  
         [0031]    [0031]FIG. 11 illustrates a definition of Manchester Encoding as well as how sample packets are defined in the present invention.  
         [0032]    [0032]FIG. 12 is a timing diagram illustrating how the envelope detector and comparator in the IDU serve to demodulate the data received from the ODU in accordance with the invention.  
         [0033]    [0033]FIG. 13 is a timing diagram for examples of the telemetry logic and demonstrating how packets are created.  
         [0034]    [0034]FIG. 14 is a diagram illustrating a way to organize packets so that a bi-directional telemetry link can be established between the IDU and ODU. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    [0035]FIG. 1 shows the top level layout of a satellite ground station transceiver control circuit  10  with an IDU  12  and an ODU  14  connected by a telemetry interface using a single physical Inter Facility Link (IFL)  16  operative according to a telemetry interface protocol as herein explained and referred to herein as a telemetry interface. The IFL  16  carries Status Telemetry, Control Telemetry, Data Transmit, Data Receive, Reference Signal and DC Voltage. The IDU  12  has an IDU multiplexer  100  coupled to status receive circuitry  101  and control transmit circuitry  102 . The IDU multiplexer  100  also provides data receive  202  to the ground station, handles data transmit  204  from the ground station, a reference signal  206  from the ground station, and DC voltage  108 . The ODU  14  has an ODU multiplexer  200  that is coupled to status transmit circuitry  103  and to control receive circuitry  104 . The ODU multiplexer  200  also handles data receive  210  from the satellite (not shown), data transmit  212  to the satellite, a reference signal  214  and DC voltage  216 . These features are shown in greater detail in FIG. 2. According to the invention, the telemetry interface minimizes required circuitry in the ODU  14  without over-complicating the receiver of the IDU  12 . The telemetry interface utilizes a “digital” amplitude modulation scheme that may be at any frequency that does not interfere with other information on the IFL  16 , (for example, an 85 MHz carrier for an L-band IFL). In operation a digital symbol “1” is transmitted over the IFL  16  by enabling a tone and a digital symbol “0” is achieved through nulling the tone. Any type of information can be sent over the telemetry interface, i.e., according to the telemetry interface protocol. However, there are two major categories that are mainly used, as described herein.  
         [0036]    [0036]FIG. 2 shows the circuitry of a typical ODU  14  in accordance with the invention. It has Status Transmit Circuitry  103  having the function to measure RF output power and convert this data as well as other status information into packets that can be sent over the IFL  16  to the IDU  12 . The ODU  14  also has Control Receive Circuitry  104  having the function to receive packets from the IDU  12 , to decode the packets into the constituent commands, and to pass this information to the appropriate circuitry of ODU  14 . Further it has multiplexer  200  having the function to combine all signals onto a form that can be carried on the IFL  16 . Still further it has a Data Transmit Chain  218  coupled to receive the data transmit signals  212  and having the function to frequency shift those signals to be transmitted from the intermediate frequency (IF) to the higher transmitted radio frequency (RF). Still further it has an ODU control module  220  coupled between the Status Transmit Circuitry  103  and the Control Receive Circuitry  104 . Its use is only under operational Method B to schedule packet send and packet receive of the control and status telemetry in order to avoid packet collision. The ODU  14  has what is herein labeled ODU Circuitry  222  having the function of reading and responding to incoming Control Commands from the Control Receive Circuitry  104  and to pass on to the Status Transmit Circuitry  103  selected extracted status information to be transmitted to the IDU  14  (FIG. 1). An RF coupler  224  coupled to receive the RF signals from the Data Transmit chain  218  is coupled to an antenna (not shown) and to the Status Transmit Circuitry  103  to relay the transmitted RF signal to both the antenna and the Status Transmit Circuitry  103 .  
         [0037]    [0037]FIG. 3 shows an example of Status Transmit Circuitry  103  in accordance with the invention. A diode detector  110  is coupled to receive RF and DC from the RF coupler  224  (FIG. 2) and measures the DC level of ODU output power. It feeds its output to a low pass filter  111  operative to minimize out of band noise in the DC measurements. The output is coupled to pre-digitization circuitry  112 , which is operative to adjust the output power voltage to match the input signal interface specifications of an M-bit analog to digital converter ADC  113  to which its signal is coupled. The output of the ADC  113  is coupled to an M-bit look-up table in a PROM or the like  114 , which is used for converting a voltage reading to the digital domain representing output power. The digital output is provided as parallel bit lines to a multiplexer  115  that is operative under the supervision of a status transmit control circuit  116 , which determines whether output power or other status data (input to the Status Control Circuitry  103  from elsewhere will be transmitted to the IDU  12  (FIG. 1). The N-bit status data is typically padded with lead zeros to match the format of the M-bit output power data. The multiplexer  115  feeds digital M-bit data to a telemetry logic circuit  117  (FIG. 4), which formats the digital data into packets. Its output packets are supplied to an on/off modulator  118 , which employs a local oscillator  119  of for example 85 MHz to convert the digital data packets into frequency pulses. The output is fed through a bandpass filter  120  to suppress spurious artifacts before being fed to the ODU multiplexer  200  (FIG. 2).  
         [0038]    The first type of data sent to the IDU  12  by the ODU  14  over the telemetry interface, i.e, according to the telemetry interface protocol via the IFL  16 , is output power. A simple diode detector  110  in the ODU  14  can be used to measure the output power in accordance with typical industrial practice. As shown in FIG. 2, the RF coupler  224  provides the signal to be transmitted to both the antenna (not shown) and to the diode detector  110  (FIG. 3) with good matching and minimum loss. The diode detector  110  converts the RF power to an analog voltage as detected from the output from the coupler. The diode detector  110  output is low pass filtered in order to remove the higher order undesired products. The low pass filter  111  is designed so that the reading is capable of settling in time to adequately provide a valid output value representative of the output power level in real time. The filter design is especially important for frequency hopped and burst transmission systems. The filtered output signal is then conditioned such that the interface to the M-bit ADC  113  is within the input signal interface specifications of the ADC  113 . The circuitry required to do this may be as simple as a resistor divider network, or it may include operational amplifiers in order to provide a DC bias offset and signal level scaling. The filtered voltage is then sampled using the M-bit ADC  113 . The M-bit data is then passed through the lookup table PROM  114 . The M-bit ADC digital word is an address value that addresses a PROM table for the representative data value. The PROM  114  is used to minimize measurement errors such as coupler offset errors and errors due to temperature changes as well as distinguish the data as an output power reading. The PROM  114  is calibrated to convert the M-bit data into N-bit data, where N is greater or equal to M. The N-bit data will represent the output power of the ODU as well as define the data as a power reading, distinguishing it from other types of data.  
         [0039]    The second type of data sent over the telemetry interface is status information. Status information can be inserted at any time into the data stream of the telemetry interface. The IDU  12  distinguishes status information from output power information by its N-bit value. A few N-bit values are reserved to represent different status information messages.  
         [0040]    The N-bit data is processed through the telemetry logic circuit  117  (FIG. 4 for details) to create data packets  126  (FIG. 11) that can be modulated and transmitted across the IFL  16  via the MUX  200 . In the telemetry logic circuit  16 , the N-bit data (either output power or status) is encapsulated with a 2-symbol preamble  130  and a 1-symbol postamble  132  (see FIG. 11). The IDU  12  utilizes the preamble/postamble to detect word synchronization. The preamble  130  is a digital “1” (or 2-symbol duration tone) while the postamble  132  is a digital “0” (or 1-symbol duration null). N-bit words are Manchester encoded (as shown in FIG. 11) to differentiate data from the preamble/postamble, as well as to improve synchronization between the IDU  12  and ODU  14 . Manchester encoding guarantees a bit transition at the center of each data symbol. A digital “1” bit is represented with a rising edge transition and a digital “0” bit is represented with a falling edge transition (see FIG. 11).  
         [0041]    This example circuit  117  operates using a 10 MHz clock. The telemetry logic utilizes a 5-bit modulo 18h counter  140  for timing control. Data alignment/control is provided by simple decode logic of the 5-bit counter output. Shown in the circuit  117  is an 8-bit ADC  142  followed by an 8-bit parallel to serial 8-bit register  144 . Manchester encoding (an XNOR gate  146 ), and parity generation (XOR gate  148 ) are also provided. The exact control circuitry for the ADC interface (ADC Clock and 8-bit shift register controls) will vary depending on the specific ADC selected. The circuit  117  in FIG. 4 assumes an 8-bit parallel ADC that takes approximately 2.2/sec (22 10 MHz clock cycles). See FIG. 13 for a complete example of a timing diagram for the ODU  14 . (In an embodiment where the ADC has an internal output register, then a more appropriate circuit (i.e., one having fewer gates) could utilize an 8:1 multiplexer (of approximately 21 gates) instead of the illustrated 8-bit serial shift register (of approximately 64 gates) as well as utilize combinational logic for parity generation, i.e., eliminating the need for DFFs.)  
         [0042]    The data packets of the example system are then passed through the On-Off Modulator  118  (FIG. 3) that uses an 85 MHz carrier (which may be of any frequency that does not interfere with other information on the inter-facility link  16  and yet stays within L-band bandwidth limitations). This On-Off keying creates a form of “digital” amplitude modulation. A digital symbol “1” is achieved by enabling a tone and a digital symbol “0” is achieved through muting the tone to form a tone-modulated data packet. The tone-modulated data packet is then passed through the band pass filter  120  centered at the carrier of 85 MHz to reduce spurs and increase efficiency before being passed to the MUX  200 .  
         [0043]    Referring to FIG. 5, Control Receive Circuitry  104  in accordance with the invention is in the ODU  14 . Its principal function is in its digital capture logic  318 . It employs preprocessing as follows: An X MHz bandpass filter  319  centered at the control transmission frequency to eliminate noise from the IFL  16  is coupled to an envelope detector  320  to convert the pulse modulated signal to a digital signal, which in turn is coupled to a lowpass filter  321  to eliminate high frequency noise from the signal, and which in turn is coupled to a comparator  322  to distinguish between on and off levels and to normalize the signal to match the input requirements of digital capture logic  318 . There is provided a threshold detection circuit  316  which in a specific embodiment is an automatically calibrated threshold unit at the input to the comparator  322  to track the Manchester encoded input. In a specific embodiment, it is a long time-constant RC circuit referenced to ground. The example digital capture logic utilizes the following: a delay  323 , a Telemetry state machine  24 , a preamble detector  325 , a packet capture module  326 , and an “integrate and dump” module  327 . The output of the comparator  322  is fed through a digital lowpass filter  328  to the preamble detector  325  and the packet capture module  326 , which uses the preamble detector output for synchronization to provide signal to the integrate and dump module  327 , the output of which are control commands that are clocked out by the telemetry state machine  325  in accordance with input configuration data and the appropriately delayed epoch information. The digital lowpass filter  328  includes an analog to digital filter and counter, which outputs a true or false (a one or a zero) representative of a simple majority vote over a period of odd-numbered samples, for example five samples, in a half clock cycle. The preamble detector  325  thus simply reads the digital values produced by the digital lowpass filter  328 .  
         [0044]    Referring to FIG. 6 and to expand on FIG. 1, the IDU  12  is shown in a high level block diagram in accordance with the invention. The IDU  12  comprises status receive circuitry  101  to receive and decode the telemetry data sent from the ODU  14 , control transmit circuitry  102  to send commands to the ODU  14  (which is used when implementing a bi-directional telemetry link), an IDU controller  428  to interpret telemetry status data received from the ODU  14  and to generate commands to be sent via the control transmit circuitry to the ODU  14 , and multiplexer  100  to combine the signals onto the IFL  16 . The digital capture logic used in the status receive circuitry  101  and the control receive circuitry  102  (see FIG. 5 and FIG. 7) utilizes a clock that operates at a much higher rate than the symbol rate to process the input signal (approximately 10 times the symbol rate).  
         [0045]    Referring to FIG. 7, the front end of the status receive circuitry  101  looks much like the front end of the control receive circuitry, with a bandpass filter  428 , an envelope detector  429 , an analog lowpass filter  430  and a comparator  431 . The comparator  431  has a threshold circuit  416  that in a specific embodiment uses an automatically calibrated threshold unit to track the Manchester encoded input. (This can be a simple RC circuit.) The first block in the digital capture logic  418  of the status receive circuitry  101  is a (digital-type) low pass filter (LPF)  427 . In this case, the LPF  427  is a simple majority vote over five samples. The preamble detect module  434  determines packet synchronization. The preamble detect detects a 1-symbol duration “0” followed by a 2-symbol duration “1”. Note: Because the preamble detect continuously runs, the ODU reference can be fairly coarse (10×5 accuracy). The preamble detect is designed to account for transfer and filtering distortions. Once packet synchronization is achieved, the logic will capture the input data. The input signal is sampled at a much higher rate than the input symbol timing. Therefore, once packet synchronization has been achieved it is easy to determine the center of each bit.  
         [0046]    [0046]FIG. 13 shows the IDU timing diagram. To account for timing errors and waveform distortion only the center portion of each bit half is to be used to determine bit value during any sampling period. That is, the center samples of the first half of the bit time is subtracted from the center samples of the second half of the bit time. The sign bit of the resultant value determines the receive bit value (0 or 1). The next step is to accumulate multiple N-bit values. This accumulation is performed by the integrate and dump function of integrate and dump module  436 . The accumulate/dump process starts accumulation at the “Start CMD” or Start Command issued by the telemetry state machine  433  and integrates (adds) until the “End CMD” is received. Only input packets that exceed the programmed signal threshold and that are identified as output power data are accumulated. When the “End CMD” is received, the accumulated value is divided by the number of N-bit inputs accumulated. The averaged value is then placed into an output buffer (not shown) for post processing. FIG. 5 and FIG. 7 each illustrate suitable digital capture logic  318  and  418 . The actual start and end commands are design specific, and so details thereof are subject to engineering choice as determined by each system designer.  
         [0047]    The example telemetry link is designed for bi-directional communication with two exemplary methods in mind. In method B, special circuitry in the ODU  14  and in the IDU  12  is used to schedule packets sent and received from the IDU  12  (see FIG. 2, ODU Control and FIG. 6 IDU Controller). Data packets are sent and received in a synchronous format so that collision of the data packets does not occur. In method B, data packets are sent initially from the ODU  14  to the IDU  12  at power up. The IDU  12  must first obtain data packet synchronization with the incoming ODU telemetry packets before it can send telemetry packets to the ODU  14 . The IDU  12  is programmed to align the outbound packets to start at the middle of the stop symbol of the incoming packet generated by the ODU  14 . The ODU  14  is programmed to be required to detect the incoming packet and not start transmission of the outgoing packet until the middle of the incoming packet&#39;s stop bit time. This is necessary only in the case of a bi-directional link where the IDU  12  sends commands for the ODU  14  to perform using the same carrier frequency as used to send data from the ODU  14  to the IDU  12 .  
         [0048]    Referring to FIG. 9, an example of data packet scheduling is shown in accordance with the invention. In this case an 8 bit word is used, and the lowest  8  values for 8 different status values are reserved. The upper  247  codes are reserved for power readings. The IDU  12  can use a table like this to interpret the difference between a power reading and a status message. Another way to achieve this bi-directional telemetry link is with a full-duplex connection as in method B. In this method more bandwidth is allocated for the IDU to ODU communication and the transmit and receive occur at different frequencies. An example is to use 85 MHz as the carrier frequency for sending data from the ODU  14  to the IDU  12  and to use 2 Hz as the carrier frequency for sending data from the IDU  12  to the ODU  14 . This extreme separation means that both signals are simultaneously dealt with in the multiplexer.  
         [0049]    [0049]FIG. 8 shows an example of a control transmit circuit  102  in accordance with the invention. Its similarity to the elements in the status transmit circuitry  103  are evident. This circuit has the following: telemetry logic circuit  537 , which accepts 8-bit commands from the IDU Control and outputs Manchester Encoded Packets; an on-off modulator  538 , which accepts the packets and in turn pulse width modulates them in accordance with the selected local oscillator frequency and method; a bandpass filter  539  centered at the selected local oscillator frequency to eliminate noise.  
         [0050]    [0050]FIGS. 10A, B and C illustrate graphically how in the frequency domain the present invention (according to methods A and B) differ from the existing Eutelsat method. Whereas in Eutelsat, a status and control band is centered at 22 kHz, here status and control are either in a pair of bands at 80 MHz and 200 MHz or in a single 85 MHz band.  
         [0051]    [0051]FIG. 12 shows how the envelope detector and comparator in the IDU  12  serve to demodulate the data received from the ODU  14 . The sample packet stream (Line A) produces envelope detected input (Line B) with a comparator input (Line C) to yield comparator output (Line D).  
         [0052]    [0052]FIG. 14 shows a possible way to organize packets so that a bi-directional telemetry link can be established between the IDU  12  and ODU  14 . In this case, both packets could be modulated with the same carrier frequency and multiplexed in time alternating between an ODU packet stream  550  interrupted by insertion of an IDU packet stream  552 .  
         [0053]    The invention has been explained with reference to specific embodiments. Other embodiments will be evident to those of ordinary skill in the art. It is therefore not intended that this invention be limited, except as indicated by the appended claims.