This can be due to the random error induced in the GPS position, known as Selective Availability and affects every GPS. The error is completely random and is imposed by the American Department of Defence as a security measure, GPS manufacturers have no control over this. A measure of the magnitude of this error is the HDOP, the larger the figure, the more inaccurate the position. The HDOP also includes other errors due to satellite grouping, timing errors etc...

Having said all this, the error will generally be better than 100 - 150 feet for the majority of the time. You should always check the displayed boat position with the actual boat position, if there is a discrepancy this could also be due to the variation in Chart Datums and the datum used in the GPS - most use WGS84, if there is a difference, use the position offset correction function.
 
 

The LORAN must first be initialised. An estimated position (within 60 miles) must be entered. In most cases the "Auto GRI" mode will select the proper GRI chain and Secondaries for you. In some areas it will be necessary to enter the GRI and Secondaries manually. Check with other LORAN users in your area or the Coast Guard. An Earth ground (such as a Dynaplate) may have to be added if the signal to noise ratio is low. If excess noise is present in the power supply, a noise filter may have to be installed.
 
 
 

Yes. Raytheon and Apelco GPS receivers will navigate to LORAN TD waypoints by converting them to Latitude/Longitude. In addition, the Raytheon Nav298 and Nav398 models can convert Latitude/Longitude to simulated LORAN TD's.
 
 
 

You can use a system called differential GPS, to improve the accuracy. The differential system uses fixed ground stations which transmit a signal that corrects some of the errors in the standard GPS, improving accuracy to within a few metres.

The Raystar 114 receiver combines a GPS receiver with a differential beacon receiver to produce position fixes to 5m accuracy.