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Chapter 5 - Determining Your Position
Press the L/L TDS menu key several times to display either the particular pair of TDs you want to use, or the loran-derived latitude/longitude.
NOTE:
If you want the 958 to display loran position coordinates from a connected loran, the loran must support the standard NMEA 0183 “RMA” output data sentence structure. Without this output capability, the 958 won’t receive position data from the loran.
Checking GPS status
To display GPS coordinates and signal status, press the CHECK
GPS menu key. The 958 continues to navigate using loran, and will display loran data the next time you display the
LORAN POSITION screen.
Viewing loran status
To check the quality of signals being received by your loran, press the LORAN INFO menu key from the LORAN POSITION screen. The SNR for each loran transmitting station is displayed graphically on the LORAN INFO screen.
Setting loran GRI
To change the four-digit loran Group Repetition Interval (GRI), press the SET GRI menu key on the LORAN POSITION screen.
For the 958 to accurately convert the loran’s TDs to latitude/
958 Operator’s Manual, Rev. A Page 67
Chapter 5 - Determining Your Position longitude position fixes, the GRI must correctly match the GRI being used by your loran receiver.
Viewing loran warnings
Three possible loran warning indicators obtained from any
Northstar 800 series loran receiver are displayed to the right of each TD. (These indicators are available only from Northstar
800 series lorans.)
Table 5: Northstar 800 loran warning indicators
Indicator
SNR
BLNK
CYC
LOCK
Meaning
Signal-to-Noise Ratio is low—use caution
Coast Guard is transmitting a blink signal, indicating a probable transmitter problem
The receiver has detected a possible cycle slip
The loran has locked onto the loran track point (this isn’t a warning indicator, but an indication of normal performance)
The L/L indicator to the left of a TD designates it as one of the two TDs that the loran receiver has chosen to use to calculate latitude and longitude.
The F/L indicator to the left of a TD indicates that it has been forced to be used to calculate latitude and longitude.
Returning to GPS navigation
To return to using GPS signals for navigation, repeat the steps shown for selecting loran in “Selecting and displaying loran” on page 66, but specify GPS instead of loran.
Using Phantom Loran
If you have lists of fishing spots and other waypoints that you recorded as loran time differences (TDs), the 958 can automatically convert its GPS coordinates into TDs—in real time, as you travel—letting you display your current position as
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Chapter 5 - Determining Your Position
TDs. The accuracy from this conversion is generally better than 500 to 1500 feet.
This feature is called Phantom Loran, which helps you move from using loran to using GPS. Phantom Loran lets you display your position as TDs, or enter waypoints as TDs. You can use the 958 to simulate the full operation of a loran receiver, using all of the 958’s navigation functions as if it was actually receiving loran signals.
These calculated TDs won’t exactly match the positions of previously recorded TDs obtained directly from actual loran signals. In most areas, TD errors shouldn’t exceed one microsecond. However, larger errors are possible in areas with poor configuration, or for which the 958 doesn’t have accurate, factory-programmed ASF correction points.
Selecting and displaying Phantom
Loran
To display position coordinates as Phantom Loran TDs:
1. Press the STAR key to display the OPTIONS/SERVICE
INFO screen.
2. Press the NAVIGATION OPTIONS menu key, then highlight the NAVIGATION SOURCE option.
3. Press the EDIT menu key, then press the CURSOR PAD to select PHANTOM LRN.
4. Press the ENTER key, then press the RETURN menu key.
5. Press the CHART key to display the GPS POSITION screen.
6. Press the CHECK LORAN key to display the PHANTOM
LORAN screen.
958 Operator’s Manual, Rev. A Page 69
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