Abstract:
A single software defined radio handles both AIS and ORBCOMM communications. A software defined software defined radio detects incoming signals and resolves whether they are AIS or ORBCOMM signals. The signal is directed to a processor in which an algorithm is selected in correspondence with the type of signal which has been recognized. The algorithm extracts intelligence when receiving or encodes intelligence when transmitting. The present software defined radio switches from the ORBCOMM mode to the AIS mode automatically as required in order to maintain a mandatory duty cycle in both the AIS and ORBCOMM modes as defined by regulations, and provides user configurable communications capabilities over both the AIS and ORBCOMM networks in a low-cost, integrated, hardware implementation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/224,961 filed Jul. 13, 2009, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present subject matter relates to software defined radio, and more particularly to a radio capable of operation with multiple protocols. 
     2. Background 
     Two significant forms of radio communication are referred to as AIS and ORBCOMM. 
     AIS or Automatic Identification System is a short range coastal tracking system used on ships and by Vessel Traffic Services (VTS) for identifying and locating vessels by electronically exchanging identification, position, course, and speed data with other nearby ships and VTS stations. Class A AIS radio is required under an international convention to be fitted aboard international voyaging ships with gross tonnage (GT) of 300 or more tons, and all passenger ships regardless of size. Class A AIS radios are significantly more expensive than other classes of AIS radios and have a robust set of capabilities. 
     Other classes of AIS radios are Class B and receive (RX) only. These are simpler, and are used in fishing vessels and leisure craft. They have a smaller set of capabilities. A Class B transponder provides both AIS reception and transmission at a fraction of the cost of a conventional class A transponder. The Class B AIS radio requires use of Digital Selective Calling (DSC) channel  70  for channel management. Additionally, a GPS antenna is included. The RX only AIS radio does not include a calling channel. 
     In order to prevent overloading of available bandwidth, Class B transmissions are restricted to 2 watts. This limits range for vessels using Class B AIS to a range of about 5 to 10 miles. At the present time, almost all Class B units use software defined radio. The transmitted signal is a standard AIS data stream at 9600 bps using Gaussian Minimum Shift Keying (GMSK). Generally, an AIS radio is equipped with a serial interface acceptingRS-232 and/or NMEA formats. 
     ORBCOMM satellites are low Earth orbit communications satellites, operated by the United States satellite communications company Orbcomm, Inc. As of 2008, 44 such satellites were in orbit. The ORBCOMM Satellite Communication System is a wide area, packet switched, two-way data communication system that utilizes constellations of the ORBCOMM satellites and earth station gateways. These satellites relay digitized data in the vicinities of 137 MHz and 150 MHz. A terrestrial ORBCOMM radio communicates with a satellite. A nominal ORBCOMM radio may comprise a single board microprocessor based VHF transceiver capable of transmitting and receiving messages in cooperation with the Orbcomm Satellite Communication System. ORBCOMM customers access the gateway and thus, the satellite, via dial up circuits, the Internet, or X.25 protocol access systems. The ORBCOMM radios transmit between 148.00 and 150.05 MHz at 5 to 10 watts using 2400 bps Symmetric Differential Phase Shift Keying (SDPSK) modulation and receive downlink 4800 bps SDPSK modulated signals between 137.0 and 138.0 MHz. They access the satellite via an ORBCOMM proprietary acquire-communicate TDMA/FDMA protocol. 
     The AIS and ORBCOMM radios operate on diverse frequencies and use different forms of signal modulation. The data structure for packets in each system is different. If a ship or other communications platform wishes to use both AIS and ORBCOMM communications, the operator must buy separate AIS and ORBCOMM radios. 
     SUMMARY 
     The present subject matter provides for a single radio to process both AIS and ORBCOMM communications. Briefly stated, in accordance with the present subject matter, there is provided a software defined radio that detects incoming signals and resolves whether they are AIS or ORBCOMM signals. The signal is directed to a processor in which an algorithm is selected in correspondence with the type of signal which has been recognized. The algorithm extracts intelligence when receiving or encodes intelligence when transmitting. The present radio switches from the ORBCOMM mode to the AIS mode as required in order to maintain a mandatory duty cycle in both the AIS and ORBCOMM modes as defined by regulations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present subject matter may be further understood by reference to the following description taken in connection with the following drawings: 
         FIG. 1  illustrates employment of AIS and ORBCOMM communications in a maritime context; 
         FIG. 2  is a chart illustrating an AIS data packet; 
         FIG. 3  consists of  FIGS. 3   a  and  3   b , which are charts illustrating an ORBCOMM data packet; 
         FIG. 4  is a block diagram of an AIS/ORBCOMM radio constructed in accordance with the present subject matter; 
         FIG. 5  is a block diagram of an AIS Class B/ORBCOMM radio constructed in accordance with the present subject matter; 
         FIG. 6  is a flow chart illustrating the architecture of software for operating the present Class B/ORBCOMM radio and also illustrating operation of the radio. 
         FIG. 7  is a flow chart illustrating the architecture of software for operating the present AIS RX only/ORBCOMM radio and also illustrating operation of the radio; and 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the present subject matter, a radio is provided which efficiently and cost-effectively combines AIS and ORBCOMM functionality. In order to better understand the present subject matter, prior art AIS and ORBCOMM radios are discussed. 
       FIG. 1  illustrates employment of AIS and ORBCOMM communications in a maritime context. Ships  10 , e.g., ships  10 - 1 ,  10 - 2 , and  10 - 3 , communicate AIS data. Alternatively, one of the ships  10  may carry a receive only (RX only) AIS radio. AIS data may include a ship&#39;s call sign, name, navigation-related information, and location and speed information. Additionally, ships  10  interact with a shore station  12 - 1 . 
     A ship  10 , e.g., ship  10 - 2 , may need to communicate with a remote shore station  12 - 2  via ORBCOMM radio. The communication is sent via a satellite  16  in a satellite constellation  18 . Each satellite  16  is in an orbit  20 . This context is but one example of an application of the present subject matter, and does not indicate any limitation on the context in which communications discussed herein may be employed. 
       FIG. 2  is a chart illustrating an AIS data packet  120 . The data packet  120  comprises a preamble  122 , start flag  124 , data  126 , frame check sequence (FCS)  128 , end flag  130 , and a buffer  132 . 
       FIG. 3 , consisting of  FIG. 3   a  and  FIG. 3   b , is a chart illustrating an ORBCOMM data packet.  FIG. 3   a  illustrates a downlink packet structure  160  comprising fifty segments  162 . The segment  162 - 0  is a synchronization segment, followed by 49 information segments  162 - 1  through  162 - 49 . As seen in  FIG. 3   b , each information segment  162  comprises twelve bytes  166 . The bytes are designated  166 - 0  through  166 - 12 . Byte  166 - 0  identifies the packet type. Bytes  166 - 1  through  166 - 10  are data or payload bytes. Bytes  166 - 11  and  166 - 12  provide checksums. 
     AIS and ORBCOMM radios have distinct protocols, and they operate at different frequencies. If a user of a Class B AIS radio or an RX AIS only radio wants to use both protocols, the user must buy a separate ORBCOMM radio. The present subject matter allows an AIS radio user or an ORBCOMM radio user to have the functionality of both protocols at only an incremental increase in cost or even virtually no increase in cost. 
       FIG. 4  is a block diagram of an AIS/ORBCOMM radio  200  constructed in accordance with the present subject matter. The particular diagram of  FIG. 4  represents one form of hardware. The radio  200  receives and transmits signals from an antenna  202 . The radio  200  operates in a half duplex mode. In the AIS/ORBCOMM application, transmission periods are short compared to receiving periods. The antenna  202  is coupled to a front end  204 . The front end  204  is coupled to a processing section  206 . The front end  204  is a section in which incoming signals and outgoing signals are converted to and from baseband frequency at which the processing section  206  operates. The processing section  206  may communicate with a user terminal  208  via a data bus  210 . The user terminal  208  may comprise a personal computer or a maritime display console. The user terminal  208  may be a source of command signals. 
     The received signals may be coupled through a front end filter  220  and a low noise amplifier  222 . The front end  204  includes an AIS signal path  228  and an ORBCOMM signal path  230 . 
     The radio is connected to the received signals from the first signal receiving path  228  or the second receiving signal path  230  by switch  236 . The AIS and ORBCOMM receiving signal paths  228  and  230  include bandpass filters  236  and  238  respectively. The filters  236  and  238  couple signals to a mixer  240 . The mixer  240  receives a second input from a frequency synthesizer  242 . The mixer  240  provides an output signal at a preselected frequency to the input of an analog to digital converter (ADC)  244 . The ADC  244  provides a digital signal to the processing section  206 . 
     The AIS and ORBCOMM bandpass filters  236  and  238  are selected to have center frequencies each corresponding to the receive frequency associated with the network. In the present illustration, the bandpass filter  236  is associated with AIS frequencies, and the bandpass filter  238  is associated with ORBCOMM frequencies. 
     For RX only AIS, transmission is done over the ORBCOMM channel only. For generality, both AIS and ORBCOMM transmission is shown in  FIG. 4  and described here. Transmission is done over first and second transmission signal paths  268  and  270 . A digital to analog converter (DAC)  272  receives a digital output from the processing section  206 . A mixer  274  receives inputs from the DAC  272  and the frequency synthesizer  242 . The converted frequency is coupled via a transmission amplifier  276  to a switch  280 . The switch  280  selectively connects the transmitted signal to the transmission signal path  268  or  270 . First and second bandpass filters  284  and  286  are connected in the first and second signal paths  268  and  270  respectively. The bandpass filters  284  and  286  each provide an output signal to the antenna  202  when connected for transmission. The passbands of the filters  284  and  286  are selected to correspond respectively to a frequency utilized for protocol of the signal path. In the present illustration, the bandpass filter  284  is associated with AIS transmission. The bandpass filter  286  is associated with ORBCOMM transmission. 
     The frequency synthesizer  242  receives a control signal from the digital processor  206 . The control signal commands a first or a second state of the first and second switches  236  and  280 . The control signal also commands the state of frequency synthesizer  242 . Each state of the frequency synthesizer  242  corresponds to the provision of AIS or ORBCOMM frequencies. 
     The processing section  206  may comprise a digital signal processor (DSP)  300 . The DSP  300  has an input terminal  302  and an output terminal  304  coupled to the filters  244  and  272  respectively. In practice, the input terminal  302  may be a set of pins on a digital device rather than a discrete terminal. The DSP  300  also communicates with the user terminal  208  via data bus  210 . First and second software stacks comprise an ORBCOMM software stack  312  and an AIS software stack  314 . In the present description, a software stack is a set of programs that work together to produce a result. A software stack may include an operating system and its applications, particularly a group of applications that work in sequence toward a common result or any set of utilities that work as a group. In the present embodiment, the software stacks comprise software defined radio processors. 
     Software defined radio routines are known. See, for example, Mark Cummings, Todor Cooklev,  Tutorial: Software Defined Radio Technology  25th International Conference on Computer Design, ICCD 2007, 7-10 Oct. 2007, Lake Tahoe, Calif., USA, Proceedings. IEEE 2007, ISBN 1-4244-1258-7. These software stacks provide their outputs to the output terminal  304 . The selection of software stacks and is made by a selector  320 . In order to command a mode or modes, the selector  320  responds to an interrupt detector  322 . 
     A number of different means for providing a command input to the interrupt detector  322  may be provided. A timer  330  may be connected to provide a periodic control signal in order to assure that the AIS mode is commanded for at least the duration of time periods required by regulations. The interrupt detector  322  is also responsibly coupled to a mode selector control  332  and an emergency communication and distress signal control  334 . The mode selector control  332  may be commanded from the user interface  208 . 
       FIG. 5  is a block diagram of an AIS Class B/ORBCOMM radio constructed in accordance with the present subject matter. In  FIG. 5 , components corresponding to those of  FIG. 4  are provided, but an additional channel for DSC is required for emergency communications and channel management. In the hardware implementation form shown in  FIG. 5 , ORBCOMM and DSC will share a common digitized receive channel, and the AIS channel reception will occur via a separate baseband channel. In  FIG. 5 , components corresponding to those of  FIG. 4  are provided. Receive components  402  through  444  of  FIG. 5  correspond to components  202  through  244  of  FIG. 4  respectively. They operate similarly, with the exception that the shared receive channel contains ORBCOMM and DSC vice ORBCOMM and AIS. Components  500  through  532  correspond to components  300  through  332  respectively. They also operate similarly, with the exception that the DSC software stack replaces the AIS software stack. The AIS receiver is accommodated by separate receive signal path  450 , an AIS bandpass filter  452 , a mixer  454 , synthesizer  456 , an analog to digital converter  458 , an input terminal  540 , and an AIS software stack  542 . Transmit components  268  through  286  correspond to components  468  through  486 . They operate similarly. 
       FIG. 6  is a flow chart illustrating the architecture of software for operating the present Class B/ORBCOMM radio and also illustrating operation of the radio. In  FIGS. 6 and 7 , operational blocks are either explicitly referred to by reference numeral as blocks within the description or simply have the reference numeral following after a descriptive clause. Operations need not take place in the order described unless logically required. Hardware components described below are illustrated in  FIG. 5 . 
     Operation begins at block  610  with power on. Configuration parameters are read,  612 . The radio is tuned to first and second AIS frequencies,  614 . Receiving, transmitting and packet processing is performed in accordance with AIS class B requirements,  616 . 
     Also, operation proceeds to block  620  where the system determines if it is time to tune to DSC in accordance with Class B requirements. A type signal may be provided by the timer  530  ( FIG. 5 ). If so, the software stack is set to the DSC mode,  622 . The first synthesizer  442  tunes to DSC channel  70 ,  624 . DSC received packets are processed,  626 . This process occurs until there is a timer interrupt  628 , at which point the radio is configured for ORBCOMM operations. Operations will continue in ORBCOMM mode as packets are processed and received and transmitted in accordance with the ORBCOMM protocol  632 . This continues until a timer interrupt is received  634  to return the radio to DSC operation,  622 . 
       FIG. 7  is a flow chart illustrating the architecture of software for operating the present AIS RX only/ORBCOMM radio  300  and also illustrating operation of the radio. The software architecture of  FIG. 7  defines interconnections in a processor. The software architecture also defines the program for operation on a digital processor. In the following description, where a reference to both follows a sentence, it refers to the operating block at which the recited operation is illustrated.  FIG. 7  may be viewed as having an AIS operating routine  702 , an ORBCOMM operating routine  704 , and a multimode operating routine  706 . These designations are simply for convenience in description. They are not intended to be rigorous descriptions, and do not limit the present subject matter. Hardware components referred to are illustrated in  FIG. 4 . Interrupts discussed below may be provided as described with respect to  FIG. 4 . 
     Operation is initiated at terminal  710  with power being turned on. The non-volatile configuration parameters are read in a digital processing unit,  712  (e.g., processor  300 ). The processor  300  determines whether the radio has been configured for AIS, ORBCOMM, or multimode,  714 . In the AIS mode, the hardware is configured for AIS and AIS software stack  314  is selected,  716 . In absence of an ORBCOMM interrupt  726  or a manual mode selection interrupt  770  the processor  300  operates in accordance with the AIS stack,  718 . Alternatively, an ORBCOMM interrupt may be provided,  726 . In this situation, the hardware is configured for ORBCOMM and software  312  is selected,  722 , and the processor  300  will operate in accordance with the ORBCOMM stack until finished with the ORBCOMM command,  724 . A manual modem selection interrupt  770  can also remove the radio from AIS only operation,  718 . When this occurs, the processor  300  determines which operating mode has been selected,  772 . When AIS is selected, the processor  300  returns again to configure the radio for AIS operation,  716 . 
     If the mode sensor senses an ORBCOMM signal, and the ORBCOMM detection has been made at  714  or  772 , the software stack  312  is selected,  728 . ORBCOMM processing continues,  730 , until a mode selection interrupt is provided,  732 . The selected mode is determined,  734 . When AIS mode has been commanded, operation is routed to block  716 . Otherwise, operation is routed to block  740  where the combined ORBCOMM and AIS multimode operations commence. In this configuration, the processor prepares the radio for combined network operations by first configuring the radio for AIS operations,  740 . A range sensor determines if AIS mobile contacts are “in-view,”  772 . If so, the signal is provided to receive and process AIS packets,  774 . The radio continues to run in the AIS configuration,  774 , as long as a determination is made that AIS mobile contacts are in-view,  772 . If an ORBCOMM or timer interrupt is provided,  748 , then the radio is set to the ORBCOMM configuration  750  and remains in the ORBCOMM configuration until ORBCOMM operations are completed, and then operation returns to the AIS configuration,  740 . If a mode selection interrupt  756  is provided while in AIS configuration,  774 , the mode is determined,  762 , and the radio is routed to the applicable mode,  716  or  728 . 
     If AIS contacts are not in-view while in multimode,  772 , then the radio is configured for ORBCOMM,  746 , and processing occurs until an interrupt is provided,  758 . If a mode selection interrupt  736  occurs while processing ORBCOMM packets  758 , then the mode is determined,  762 , and the processing is routed to the applicable mode,  716  or  728 . If a timer interrupt  768  occurs while processing ORBCOMM packets  758 , then the radio is reconfigured to AIS,  740 , in order to check if AIS contacts have moved into range  772 . 
     Commands may also be introduced, as from the user interface  208  while in any of the operating modes,  702 ,  704 ,  706 . ORBCOMM packets may be processed and sent or received per the ORBCOMM serial interface specifications via the user interface. Mode selection interrupts can also be generated via the user interface, and ORBCOMM and mode selection interrupts can also be generated through manual switches on the radio. 
     AIS transmissions and DSC reception (Class B) signals require a minimum duty cycle in order to comply with safety requirements. In the present system, ORBCOMM operations can be suspended in accordance with ORBCOMM regulations in order to permit AIS transmissions and DSC reception. The system stores ORBCOMM data, and processing resumes when the AIS transmission is completed. 
     Thus, the scope of the embodiment should be determined by the appended claims and their legal equivalents, rather than by the examples given.