Canadian Coast Guard

PART 6

LORAN-C NAVIGATION SYSTEM

TABLES

1GULF OF ALASKA LORAN-C CHAIN GR1 7960

2CANADIAN WEST COAST LORAN-C CHAIN GR1 5990

3U.S. WEST COAST LORAN-C CHAIN GR1 9940

4NORTH CENTRAL U.S. LORAN-C CHAIN GRI 8290.

FIGURES

1NORTH AMERICAN COVERAGE OF LORAN-C CHAINS

2LORAN-C COVERAGE DIAGRAM – GRI 5990

3CANADIAN WEST COAST CHAIN COVERAGE DIAGRAM

4INDEX AND CHARTLETS SHOWING ADDITIONAL SECONDARY FACTOR

CORRECTIONS

A. Loran-C Chain Coverage

Figure 1 shows the North American coverage of Loran-C while Figure 2 shows further details of the Canadian West Coast and suggested rates and master-secondary pairs to use in particular areas. Figure 3 shows the individual coverage pattern provided by the Canadian West Coast Chain. The following notes pertain to Figure 2:

Note 1:The dividing lines between the Loran-C rates do not necessarily mean there are no other suitable Loran-C station pairs which could be used to safely navigate in an area. For example, while it is recommended to use 5990 XZ (i.e., Williams Lake - Shoal Cove - Port Hardy on Rate 5990) in the Hecate Strait area, coverage also exists there for 5990 XY. It is simply estimated that 5990 XZ provides better coverage in this area.

Note 2:The position repeatability of the 5990 XZ signals in this area may degrade to 1/3 nm due to geometric considerations. Theoretically 7960 XZ should be used in this area, but this is not recommended due to weak signal strength of the distant master station at Tok, Alaska.

B. Chain Details

Tables 1 through 4 give technical details of chains that provide coverage in waters off Western Canada and proximity.

C. Loran-C Coordinate Converters

Listing of vectors from the Loran-C coordinate converter position to the true position.

D. Loran-C Receiver Latitude/Longitude Corrections

Today’s Loran-C receivers are equipped with microprocessors which are designed to internally compute the latitude and longitude coordinates of the receiver, based on the Time Difference (TD) readings, and directly display these values. This reduces the need to possess Loran-C charts, though it is still recommended they be procured.

The latitude/longitude computation may be based upon a pure seawater path. This leads to errors if the Loran-C signals from the various stations involve appreciable overland paths since the speed of the signal will decrease by varying amounts, depending on the nature of the earth’s surface over which it is passing. Loran-C operates by measuring the difference in arrival times of the signals from the different stations in the Loran-C chain, and thus any unforeseen variation in the speed of a signal will result in an error in the latitude/longitude reading. Note that when the receiver is being used in the time difference mode (time difference readings being used to manually plot lines of position on a Loran-C chart), these errors are minimal and the system should be accurate to within 1/4 nautical mile. This is because the Loran-C lattice on a nautical chart has already been adjusted to allow for the signal variation as it travels over land.

It is recommended that mariners’ using the latitude/longitude feature of their receiver check the manufacturer’s operating manual to determine if corrections are necessary and how they may be applied to compensate for overland paths in order to obtain a greater fix accuracy. The correction can be applied in either of two forms: (i) insertion of a correction when the vessel is at a known location, or (ii) the insertion of a correction factor that is determined from a table or chartlet. The latter is called an Additional Secondary Phase Factor (ASF) correction, and the chartlets in Figure 4 can be used to ascertain the numeric value to apply. These corrections will normally be valid only within 50 to 100 miles of the location at which the correction was inserted because of the changing effects of land mass on the Loran signals in the different areas.

E. Cautionary Note - Fishing Near the B.C.- Alaska Coastal Boundary

Canadian fishermen using the Loran-C system for navigation must exercise caution when operating the near “A-B” Line, the coastal boundary in the waters separating B.C., and Alaska.

You should be particularly aware of the signal phase problem discussed in Section D. A receiver being used in the latitude/longitude mode can experience errors of up to several miles if ASF corrections have not been applied. In some receivers the corrections are applied automatically within the receiver, while in others the corrections must be applied manually, be it while located at a known spot or by the addition of ASF corrections.

Note that some of the ASF corrections shown in this publication stop short of the A-B line, and caution must be exercised when operating in this area.

F. Waypoint Navigation Cautionary Note

Mariners are cautioned that an error can exist between the waypoint navigation information provided by their Loran-C receiver and the desired straight-line track plotted on a chart. A straight line course plotted between two waypoints on a mercator chart is a rhumb line, defined as a line on the earth’s surface cutting the meridians of longitude at the same angle. The course and distances displayed by a microprocessor-based Loran-C receiver, used in the waypoint mode, are normally computed for a great circle track, not a rhumb line. In the northern hemisphere, a great circle track between two waypoints lies to the north of a rhumb line joining those same waypoints.

This offset distance, or error, is a maximum when sailing East-West at a latitude of approximately 45 degrees, decreasing to zero at the equator and at the North and South Poles. It also decreases to zero as your track becomes North-South, regardless of the latitude. As an example of the offset error possible, a journey from St. John’s, Newfoundland, to the Lands End area, England, a distance of roughly1850 nm, would have a maximum offset of approximately 140 nm when comparing a rhumb line and a great circle track between the two places. The rhumb line versus great circle path offset becomes a danger only if the navigator has not laid off a great circle course on a Gnomonic chart, ensuring the vessel will pass clear of all navigational dangers.

G. Loran-C System Status Information

Up-to-date Loran-C status information is available by telephoning:

Loran-C Chain/Rate Phone Number

West Coast U.S./9940(510) 437-3232*

Canadian West Coast/5990(709) 454-3129Control/Monitor for Canadian Loran C Operations

Gulf of Alaska/7960(510) 437-3232*

North Central U.S./8290(510) 437-3232*

* This service is available only between the hours of 0700-1530 Pacific time on normal work days.

H. Loran-C NOTSHIPs

Loran-C Notices to Shipping (NOTSHIPS) concerning the status of Loran-C signals in the coastal waters off Western Canada and the immediate proximity are broadcast from the following Marine Communications and Traffic Services Centres (MCTS):

Note that these broadcasts may only be made from those MCTS Centres located in the general area where the Loran-C signal normally exists.