System and method for decoding automatic identification system signals

• US 8,374,292 B2
• Filed: 10/29/2010
• Issued: 02/12/2013
• Est. Priority Date: 06/08/2007
• Status: Active Grant

• Current Assignee: Com DEV International Limited (Honeywell International Inc.)
• Original Assignee: Com DEV International Limited (Honeywell International Inc.)
• Inventors: Peach, Robert
• Primary Examiner(s): Liu, Shuwang
• Assistant Examiner(s): HUANG, DAVID S
• Application Number:
  US12/915,699
• Publication Number:
  US 20110268229A1
• Time in Patent Office: 837 Days
• Field of Search: 375/150, 375/324, 375/343
• US Class Current: 375/343
• CPC Class Codes: G08G 3/02 Anti-collision systems
• CPC Class Codes: H04B 7/18513 Transmission in a satellite or space-based system

Abstract

Various embodiments are described herein for a system and method of detecting Automatic Identification System (AIS) signals in space and decoding these signals. In one aspect, a system for performing this function is described which includes a receiver configured to receive the plurality of AIS signals and pre-process the plurality of AIS signals to produce digital input data, and a processing unit configured to process the digital input data to identify one or more candidate AIS message signals based on Doppler offsets associated with the digital input data, determine corresponding Doppler offset estimates and time estimates of the one or more candidate AIS message signals, decode the one or more candidate AIS message signals to obtain corresponding message segments and validate the decoded message segments for proper AIS formatting.

Claims

1. A method for producing validated decoded Automatic Identification System (AIS) message segments from a plurality of AIS signals wherein a plurality of AIS signals are received at one or more satellites in space and pre-processed to produce digital input data corresponding to the received plurality of AIS signals, the method comprising:

processing the digital input data by correlating the digital input data with a plurality of predefined signals having different Doppler offsets to compute a plurality of corresponding correlation signals, wherein the plurality of predefined signals are generated by applying the different Doppler offsets to a predetermined AIS code sequence, and scanning the plurality of correlation signals for amplitude peaks exceeding neighbouring amplitude peaks by a predetermined amount in order to identify one or more candidate AIS message signals;

determining corresponding Doppler offset estimates and arrival time estimates for the one or more candidate AIS message signals;

decoding the one or more candidate AIS message signals in the digital input data to obtain corresponding message segments; and

validating the decoded message segments for proper AIS formatting to produce validated decoded AIS message segments.

2. The method of claim 1, wherein prior to decoding the one or more candidate AIS message signals, the method further comprises refining the group of one or more candidate AIS message signals to obtain a refined group of candidate AIS message signals and performing the decoding on the refined group of candidate AIS message signals, wherein refining the candidate message signals comprises:

determining whether any of the one or more candidate AIS message signals is repeated at a different Doppler offset estimate; and

removing all repeated candidate AIS message signals.

3. The method of claim 1, wherein prior to decoding the one or more candidate AIS message signals, the method further comprises refining the group of one or more candidate AIS message signals to obtain a refined group of candidate AIS message signals and performing the decoding on the refined group of candidate AIS message signals, wherein refining the candidate message signals comprises:

sorting the one or more candidate AIS message signals by their corresponding arrival time estimates; and

removing all remaining candidate AIS message signals that are overlapped on their high time side by a stronger candidate AIS message signal.

4. The method of claim 1, wherein before decoding the method further comprises filtering the one or more candidate AIS message signals by applying narrowband filtering centered on the Doppler offset estimate that corresponds to the one or more candidate AIS message signals.

5. The method of claim 1, wherein the method further comprises employing at least two antennas configured to receive the plurality of AIS signals, the at least two antennas being differentiated from one another in at least one of a spatial and a polarization manner, and before the decoding step the method further comprises combining the one or more candidate AIS message signals received by one of the at least two antennas with corresponding one or more candidate AIS message signals received by at least one of the other antennas by employing a phase-shift that maximizes the amplitude of the combined one or more candidate AIS message signals relative to any overlapping signals, and performing decoding on the combined one or more candidate AIS message signals.

6. The method of claim 1, wherein the method further comprises de-correlating the validated decoded message segments to obtain modified digital input data, and performing the processing, determining and decoding steps on the modified digital input data.

7. The method of claim 1, wherein prior to decoding the method comprises determining a more precise Doppler offset estimate and arrival time estimate for the one or more candidate message signals.

8. The method of claim 7, wherein the method further comprises employing a Viterbi decoder configured to decode GMSK modulated signals in the decoding step.

9. The method of claim 1, wherein at least one decoded message segment is associated with a given ship, and after decoding the method returns to the processing step which further comprises:

correlating the digital input data with a plurality of predefined signals that correspond to a subsequence of the at least one decoded message segment associated with the given ship; and

determining whether the digital input data is a candidate AIS message signal associated with the given ship.

10. A system for producing validated decoded Automatic Identification System (AIS) message segments from a plurality of AIS signals wherein a plurality of AIS signals are received at one or more satellites in space and pre-processed to produce digital input data corresponding to the received plurality of AIS signals, the system comprising:

a processing module configured to process the digital input data by correlating the digital input data with a plurality of predefined signals having different Doppler offsets to compute a plurality of corresponding correlation signals, wherein the plurality of predefined signals are generated by applying the different Doppler offsets to a predetermined AIS code sequence, scan the plurality of correlation signals for amplitude peaks exceeding neighbouring amplitude peaks by a predetermined amount in order to identify the one or more candidate AIS message signals, and determine corresponding Doppler offset estimates and arrival time estimates for one or more candidate AIS message signals;

a decoder configured to decode the one or more candidate AIS message signals in the digital input data to obtain corresponding message segments; and

a validation module configured to validate the decoded message segments for proper AIS formatting to produce validated decoded AIS message segments.

11. The system of claim 10, wherein the system further comprises a refining module configured to refine the one or more candidate AIS message signals to obtain a refined group of candidate AIS message signals and wherein the decoder module is configured to perform decoding on the refined group of candidate AIS message signals, and wherein the refining module is configured to refine the one or more candidate AIS message signals by determining whether any of the one or more candidate AIS message signals is repeated at a different Doppler offset estimate;

and removing all repeated candidate AIS message signals.

12. The system of claim 10, wherein the system further comprises a refining module configured to refine the one or more candidate AIS message signals to obtain a refined group of candidate AIS message signals and wherein the decoder module is configured to perform decoding on the refined group of candidate AIS message signals, and wherein the refining module is configured to refine the one or more candidate AIS message signals by sorting the one or more candidate AIS message signals by their corresponding arrival time estimate;

and removing all remaining candidate AIS message signals that are overlapped on their high time side by a stronger candidate AIS message.

13. The system of claim 10, wherein the processing module is further configured to filter the one or more candidate AIS message signals by applying narrowband filtering centered on the Doppler offset estimate that corresponds to the one or more candidate AIS message signals.

14. The system of claim 10, wherein the system further comprises at least two antennas configured to receive the plurality of AIS signals, the at least two antennas being differentiated from one another in at least one of a spatial and a polarization manner, and wherein the processing module is further configured to combine the one or more candidate AIS message signals received by one of the at least two antennas with corresponding one or more candidate AIS message signals received by at least one of the other antennas by employing a phase-shift that maximizes the amplitude of the combined one or more candidate AIS message signals relative to any overlapping signals, and wherein the decoder is configured to perform decoding on the combined one or more candidate AIS message signals.

15. The system of claim 10, wherein the system is further configured to de-correlate the validated decoded message segments to obtain modified digital input data, and wherein the processing module is further adapted to process the modified digital input data.

16. The system of claim 10, wherein the processing module is further configured to determine a more precise Doppler offset estimate and arrival time estimate for the one or more candidate AIS message signals prior to decoding.

17. The system of claim 16, wherein the decoder comprises a Viterbi decoder configured to decode GMSK modulated signals.

18. The system of claim 10, wherein the system is further configured to associate at least one decoded message with a given ship, correlate the digital input data with a plurality of predefined signals that correspond to a subsequence of the at least one decoded message associated with the given ship and determine whether the digital input data is a candidate signal message associated with the ship.