The Coast Guard plans to operate an Automatic Identification System (AIS) Digital Selective Calling (DSC) based transponder system as part of the Ports and Waterways Safety System (PAWSS) in the lower Mississippi River. The AIS uses two duplex channels for ship-to-shore and shore-to-ship digital data transmissions and a simplex channel for channel management. The duplex channels are identified as AIS working channels.
The PAWSS relies on a combination of voice and AIS working channels in the VHF maritime mobile band to provide signal coverage in the Vessel Traffic Service Area (VTSA). The proposed coverage area for the VTSA encompasses the Mississippi river starting at river mile 255 to the sea buoy located at Southwest Pass and also includes an area in the Gulf of Mexico for ships approaching the sea buoy. Other waterways such as the Mississippi River Gulf Outlet and the Industrial Canal are also included in the proposed VTSA. A contractor has installed government owned equipment at five tower sites along the river to provide the coverage for both the voice and data channels. Each tower provides coverage for part of the VTSA. Before the system can reach operational status, the coverage of the RF portion of the system must be measured and documented.
The Coast Guard funded NTIA to perform coverage tests on the system to examine and evaluate the adequacy of the RF coverage of the channels used for voice and AIS data transmissions. The tests were performed August 4-10, 1999 by NTIA ,Coast Guard, and SETA personnel with vans carrying the necessary test equipment. Two vans were used to make measurements on top of the river levee every three miles. One van went upriver on top of the levee and the other van went down river on top of the levee. For measurements taken in the southern part of the area covered by the tower located at Venice, a ship was used to take measurements down to the sea buoy and out into the Gulf of Mexico.
The objective of these tests was to verify the signal coverage in the VTSA. This was accomplished using quantitative and qualitative methods.
For the quantitative methods, the strength of the signal that was received from each of the five towers on simplex voice channels 11 and 14 was measured. The signal strength of the shore-to-ship side of the duplex AIS working channels 90 and 94 was also measured.
For the qualitative methods, Channel 70 functions were measured by noting the time it took for the transponders to switch to the AIS working channel. For qualitative voice measurements, hand held and fixed mount radios were used to establish communications with personnel at the Vessel Traffic Center (VTC) for each tower on channels 11 and 14 via land lines. The communications were qualitatively rated as good, poor, or bad. To prevent emissions from adjacent towers affecting the tests, each tower was tested with the other four turned off.
During the tests, it was observed that other authorized users were present on the voice channels, the ship-to-shore side of the AIS working channels, and possibly channel 70. These other users could degrade the quality of the voice measurements and effect the transponder’s ability to switch to the AIS working channel. In addition, loading on channel 70 due to the test configuration may have affected the transponders ability to switch to the working channel.
The results of the tests for each of the five towers are represented by signal strength graphs of the voice and AIS working channels, AIS observations, and voice communication observations. The test results show that there are RF coverage gaps for the voice and AIS data services that require corrective measures to ensure that the Coast Guard PAWSS can operate in the proposed VTSA. These corrective measures may include adding base stations at additional tower locations, moving existing base stations to other tower locations, and/or changing the orientation/height of the antennas on towers at existing at base stations.
The results and conclusions of this report can also be used by the Coast Guard to develop coordination procedures with Public Correspondence System operators, so that both of their systems can operate in the VHF maritime mobile band with a minimal amount of mutual interference.