Figure 4: Position Variances With Uncorrected Gps - NorthStar 961 Reference Manual

Chapter 5 - Understanding Position Coordinates

Figure 4: Position variances with uncorrected GPS

Viewing loran TDs
Some readers may be used to navigating with a loran receiver;
if you're an experienced mariner, you may already have
fishing spots and other waypoints that you recorded as loran
TDs. For those readers who are unfamiliar with loran TDs, they
represent the measured amount of time—in millionths of a
second—between loran signals received from the master loran
station and the signals received from the secondary loran
stations. These signals provide a basis for a navigational
system that's totally independent of the GPS system.
The 961/962 automatically converts GPS coordinates to loran
TDs, which can be viewed on the CHART screen at any time
(as well as other screens). This conversion is known as
Northstar's Phantom loran feature. With Phantom loran, you
can display your position as TDs, or enter waypoints as TDs.
The approximate TD values are displayed that identify two or
more lines of position on an Loran-C chart. Also displayed is
the Group Repetition Interval (GRI), the specific interval at
which the loran stations transmit groups of pulses. Generally,
you can expect the accuracy from this conversion to be better
than 500 to 1,500 feet.
961/962 Operations and Reference Manual, Rev. A Page 5-9