Figure 4: Position Variances With Uncorrected Gps - NorthStar 961X Operation And 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 fish-
ing spots and other waypoints that you recorded as loran TDs.
For those readers who are unfamiliar with loran TDs, they rep-
resent the measured amount of time—in millionths of a sec-
ond—between loran signals received from the master loran
station and the signals received from the secondary loran sta-
tions. These signals provide a basis for a navigational system
that's totally independent of the GPS system.
The 961 automatically converts GPS coordinates to loran TDs,
which can be viewed on the 961's CHART screen at any time
(as well as other screens). This conversion is known as North-
star's Phantom loran feature. With Phantom loran, you can
display your position as TDs, or enter waypoints as TDs. The
961 displays the approximate TD values 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 Operations and Reference Manual Rev. D Page 5-9