LORAN - GPS - DGPS DIFFERENCES

Several terms need to be defined in order to understand the differences between these navigational systems. ABSOLUTE ACCURACY refers to the availability of a navigational device to determine your exact latitude and longitude coordinates on the surface of the earth, as compared to a known position as determined from a nautical chart or other reference. REPEATABLE ACCURACY refers to the ability to return to a position whose coordinates you have previously recorded with the same receiver. SELECTIVE AVAILABILITY (SA) is the intentional altering of GPS satellite signals by the U.S. Department of Defense to guard against hostile forces using GPS as targeting data against U.S. interests. The U.S. Department of Defense's position is that SA will remain in effect indefinitely.

LORAN is an acronym for LOng RAnge Navigation. LORAN has an absolute accuracy of only about 600 feet in a strong signal coverage area and in favorable weather conditions. However, since the 1970s, LORAN users have relied on that systems ability to repeatedly get them back to a previously recorded location with good accuracy. LORAN's repeatable accuracy was superior to anything else commercially available at the time. In very good coverage areas, LORAN could get you to within 50 feet of your previously visited location. LORAN has poor absolute accuracy, since it transmits at a relatively low frequency (100 kHz), and due to electrical interference caused by the distance and bearing from the LORAN transmitters, other electronic transmissions (TV, microwave, etc.) and the weather between the receiver and the transmitters. This electronic signal distortion (which changes daily) is a major reason why exact mathematical conversions between LORAN and latitude/longitude coordinates are almost impossible to achieve. The LORAN system was scheduled to be shut down on December 31, 2000 due to federal budget concerns, but recent lobbying by navigational and aviation interests in Washington appears to have been successful in keeping the LORAN-C system operational until at least 2005.

GPS is an acronym for Global Positioning System. GPS is a satellite-based navigation system provided by a network of 24 satellites in six separate orbital paths. Each satellite circles the earth about twice every day, traveling at almost 1.8 miles per second. The major advantage of GPS is its all-weather, worldwide availability and absolute accuracy. The signals are transmitted at an extremely high frequency (1.575 gHz or about 1.5 billion cycles per second). This makes GPS signals impervious to fluctuations caused by weather. However, the interference caused by SA effectively reduces GPS's absolute and repeatable accuracy to approximately 330 feet. Therefore, uncorrected GPS with SA in effect is inferior to the repeatable accuracy of LORAN. But this level of accuracy for GPS with SA was too low for U.S. Coast Guard requirements for harbor navigation, so the U.S. Coast Guard developed a system of differential corrections that enabled them to effectively correct for the SA errors.

These differential corrections are generated by comparing the position of fixed reference stations with satellite positions and calculating correction factors. These differential corrections are broadcast by marine radiobeacons to Differential GPS (DGPS) navigational receivers. With DGPS, anglers are guaranteed absolute and repeatable accuracy of 6-15 feet (50% of the time) and 30 feet (95% of the time). In addition to accurate positioning information, DGPS provides a critical "integrity monitoring" which enables users to be notified within seconds that there are problems with the signal from a satellite. These integrity monitors tell the DGPS receiver not to use the problem satellite. DGPS systems provide the best accuracy are completely unaffected by weather and have global coverage. The only major limitation to using DGPS receivers is that coverage from the transmitting beacons is not complete in all portions of the U.S. However, consumers can purchase differential correction transmissions from commercial providers if they are planning to navigate in areas where the DGPS beacon signals are weak.

DGPS receivers are more expensive than GPS receivers. However, there exists some confusion among consumers between these two vastly different types of navigational receivers. Many GPS receivers are marked as ‘Differential Ready’ or ‘Differential Capable’, and these have been confused with true DGPS systems. This is particularly true of the popular handheld receivers. These types of GPS receivers must be connected to another external differential receiver and separate antenna in order to function as a DGPS unit. True DGPS systems are commonly referred to as "integrated DGPS".

There is a large amount of confusion among anglers between these two vastly different types of navigational equipment, especially since current models of LORAN, GPS and DGPS receivers will indicate positions in both LORAN time delays (TDs) and latitude/longitude. However, all receivers must use mathematical formulas to calculate these "phantom coordinates". As mentioned earlier, there is a certain level of electrical interference with LORAN signals which varies according to weather and the distance from the LORAN transmitter to the receiver. This interference obviously changes daily and must be accounted for when converting LORAN coordinates to latitude/longitude coordinates. In 1988 the U.S. Defense Mapping Agency (DMA) published a series of corrections called "Additional Secondary Factors" (ASF) which reduce the error in calculating conversions from LORAN TDs to latitude/longitude coordinates. However, DMA states that these ASFs should not be used for conversions of locations within ten (10) nautical miles of the shore, due to the large variability in the amount of electrical interference. Additionally, the amount of electrical interference is constantly changing as more microwave, television, radio and other types of electrical transmitters become operational in all areas. These are some of the reasons why it is almost impossible to accomplish an exact mathematical conversion between the two types of coordinates, even using apparently sophisticated computer conversion programs with the most recent ASFs available.

For the fastest and most accurate relocation of a geographical position, the angler should use the type of navigational receiver with which the coordinates were originally obtained. Many pamphlets, atlases and books containing lists of fishing locations supply both types of coordinates, but the user does not know which type of coordinate was originally taken at the location and which coordinate has been mathematically converted. Many people have also "converted" their old TDs to latitude/longitude coordinates using their receivers or computer software programs. These "converted" latitude/longitude coordinates are, in many cases, more inaccurate than the LORAN TDs from which they were calculated. Due to the conversion difficulty discussed above, it is extremely important that anglers determine how any coordinates they are using were obtained. If the coordinates are TDs, you need to determine whether they are "raw" or "ASF-corrected". Due to conversion errors, many frustrating hours have been spent trying to relocate a LORAN generated position using converted position data with GPS receivers or DGPS systems.

Another method which has been used to "convert" one set of coordinates to the other is to use a nautical chart with a LORAN overlay. The position using either the overlay or the latitude/longitude scales, and the corresponding coordinate is determined. However, theses conversions also are not accurate, as the LORAN overlays on the nautical charts were positioned with an accuracy of only 0.25 of a nautical mile (1,519 feet). Therefore, even the most careful interpolation (reading coordinates from a chart) may produce a substantial error. The only truly accurate way to convert LORAN TDs to latitude/longitude coordinates is to relocate a LORAN position using a LORAN receiver, and then simultaneously take a reading with a GPS receiver or DGPS system. This latitude/longitude position can then be used in the future to relocate the position with either a GPS receiver or DGPS system. However, remember that the repeatable accuracy of GPS is less than that of LORAN, so there really isn't anything to gain unless you are using a DGPS system. The advantage of obtaining this type of accurate conversion is that any other DGPS system using the latitude/longitude coordinate will be just as accurate as the original system. It is worthy to note that this type of accuracy is not possible between LORAN receivers. The TDs to latitude/longitude coordinates shared between LORAN receivers without correcting for differences between the manner in which the different LORAN receivers adjust for electrical interference will likely to enable the angler to return to the exact same location.

Hopefully the above explanations will assist anglers in relocating their favorite fishing spots as quickly and safely as possible, so that more time can be spent actually catching fish.

 

Related Books of Interest:

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