NorthStar 958 Operator's Manual page 194

Page 186
repeatable accuracy has served harbormasters and fishermen
so well over the last 25 years.
Loran, however, isn't as proficient as GPS in absolute accuracy,
or when visiting a spot for the first time. Loran transmits at a
relatively low frequency (100 kHz—similar to that of an AM
radio). Your ability to get an accurate position fix is directly
related to the following factors:
•
your distance and line geometry from the loran
transmitters and monitor stations
•
the weather, topography, and ionospheric conditions
between you and the transmitters
In these two areas GPS consistently excels when it comes to
absolute accuracy. GPS provides an absolute accuracy better
than 100 feet nearly everywhere in the world, under virtually
any conditions. This far surpasses the 600-foot absolute
accuracy that's achievable from a good loran receiver under
ideal conditions.
If you're used to your Northstar loran's repeatable accuracy
always returning you to within a boat length of a lobster trap
or buoy, you won't necessarily see this same performance with
uncorrected GPS. Instead, you could be as far as 200 feet from
your expected position. This is especially apparent when
you're displaying the 958's CHART screen, where you can see
your vessel's plotted position appear to wander around within
an area up to 200 feet in diameter (see figure below).
958 Operator's Manual, Rev. A