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ContinuousWave: Small Boat Electrical
HDS: Phantom LORAN
|Author||Topic: HDS: Phantom LORAN|
posted 04-20-2012 12:35 AM ET (US)
With a recent software update my Lowrance HDS-8 gained a feature that allows it to compute the time delays of LORAN lines of position based on the latitude and longitude of the cursor when in CHART mode. There is no mention of this feature in the literature, and I searched for the terms "phantom Loran HDS" to see if I could learn more about it. No one explained how it worked, so I had to figure it out for myself. However, I did find several comments about a large inaccuracy in the predicted LORAN time delays. One boater said they were "off by five miles." I could not comprehend how a device as sophisticated as the Lowrance HDS could add a feature that was so inaccurate, and I investigated the accuracy myself.
With the Obama administration having shut off the LORAN system, there were no LORAN signals available to use, nor did I have a LORAN receiver to get some LORAN time delay numbers for comparison, but I did have some old NOAA charts that were marked with LORAN time delay overlays. I dug out a chart of Lake Erie from a few decades ago. On the chart I located three positions which were at the exact intersection or close to the intersection of time delay overlay lines. I noted the LORAN time delays for those objects, and then I went to the HDS.
On the HDS I enabled Phantom Loran, selected the proper chain, the 9960 group repetition interval, and selected the two stations to use for the delays. Then I moved the cursor over the charted position of the objects as shown on the INSIGHT digital chart cartography of the HDS, and I noted the Phantom Loran predicted time delay values: Here are the results:
Bouy near Chickenolee Reef; 9960-X, 9960-Z
These values were in good agreement. The position of the buoy may have moved from the old paper chart to the new digital chart. The variation was only a matter of a few hundred feet.
Shore object near Bolles Harbor; 9960-W; 9960-Z
These values were way off. The predicted value is about a mile away. Something has gone wrong here.
Detroit River Light; 9960-W; 9960-Z
The Z-value is close, but the W-value is again rather far off.
My conclusion is that accuracy of these predicted LORAN time delays probably varies quite a bit depending on the location and its relationship to the various stations being used. I did not see any error of the magnitude of "five miles" as was suggested. I think if one had some old LORAN time delay numbers, the Phantom Loran could get you in the vicinity of the original location, but not right on the spot.
posted 04-20-2012 06:54 AM ET (US)
Do you recall when the last chart with the overlays were published? It does not make sense to me to include software for charts that were pretty dated. The later LORAN units had conversion software from the factory that did the LOP to L/L conversion.
posted 04-20-2012 07:59 AM ET (US)
The chart I used was NOAA 14830 WEST END OF LAKE ERIE, 18th Edition, June 1983, LORAN-C OVERPRINTED and issued almost 30 years ago. I don't know when NOAA stopped publishing the charts with LORAN information--but that's a darn good question.
posted 04-20-2012 08:57 PM ET (US)
Looking over NOAA Chart 14830 I noticed that three lines of position from LORAN time delays cross nicely at the flashing red on the Southwest tip of South Bass Island. I compared the charted values of LORAN time delays with the computed values of LORAN time delays made by the HDS. The results are very good:
Flashing Red at SW end of South Bass Island
Charted values----HDS vales
The X and Z values are almost exactly the same. The W value is off just one microsecond, a nominal error. This comparison shows the HDS computed time delays to be closely correlated with the NOAA chart overlays.
posted 04-21-2012 12:53 PM ET (US)
Jim--Interesting. I recall an interesting anomaly regarding LORAN accuracy; seasons affected the accuracy. They had to adjust the signal between the winter and the summer, due to the deciduous trees. Might be OWT [possibly an acronym for "old wives tale"]. Regards--Don
posted 04-21-2012 01:09 PM ET (US)
Don--I had not heard of that effect. LORAN used very low-frequency (0.1-MHz) ground wave signals. The speed of propagation of the signals is not precisely constant. This same problem occurs in a space-based navigation system and is due to variations in the thickness and consistency of the ionosphere. In this way, LORAN and GPS are precisely alike.
posted 04-21-2012 05:02 PM ET (US)
I always heard that the absolute accuracy of LORAN was poor, but repeatability was excellent. Also, I'm at a loss of the usefulness of this feature since LORAN has gone away, can you give me some of the benefits?
posted 04-21-2012 06:24 PM ET (US)
I imagine the usefulness of the Phantom LORAN feature of the HDS devices would be most appreciated by some old fishermen who found a list of great fishing locations that was notated with LORAN time delays.
The LORAN system was approximately 80-percent finished with modernization of the system which would have enhanced its accuracy and usefulness to be on a par with available space-based systems, but with the enormous advantage of almost complete immunity to jamming and the ability to be useful indoors and inside buildings--not to mention a cost that is about one-onehundredth of space-based system and a location of all the resources on our own soil, not in outer space. The Obama administration, against the advice of almost all technical advisory committees, shut down LORAN and began to dismantle the system. The dismantling of the system has cost more than it would have cost to finish the modernization. This ridiculous action is a good example of what happens when non-technical government executive-branch political appointees are allowed to make budgetary decisions about long-term infrastructure technical projects. When a government runs on the basis of how many votes can be bought with each dollar spent, incomprehensible actions like this are the result.
Fortunately, there has been a resurgence of interest in modernization of LORAN, and at the moment there is a test program underway to further explore restoration of LORAN as a position, navigation, and timing service. Our modern technological society cannot live without position, navigation, and timing services with ultra-precise information. An enhanced and modernized LORAN is now seen as a very important alternative to the billion-dollar-per-satellite approach of GPS.
However, there is a certain Luddite mentality that only know enough about electronic navigation to read their position off a GPS, and these folks all seem to think that LORAN is outdated. Nothing could be farther from the truth. LORAN is needed more than ever, according to experts in the field of position, navigation, and timing.
posted 04-22-2012 01:02 PM ET (US)
There's been comments before comparing the accuracy (both absolute and repeatable) between both systems. What some forget was that in many cases, the comparison was an apples and oranges situation; i.e. the comp's were between an older LORAN unit and a new GPS.
Of course there was going to be a significant variance, favoring the GPS unit!
I also recall that there could be a difference in readings between the manufacturers of the LORAN units of the day depending on which algorithm they embedded in their software (I never realized there was more then one way to convert TD to L/L until reading about it). That may have accounted for some of the absolute errors.
tmann45 - I've seen the repeatability in action with a King (I think...it's been a while!)unit mounted in our Revenge in the mid 1980's. I would find that most of the time the comparison between readings prior to leaving our slip and returning varied no more then .02 of a lat/lon.
And that was over 25 years ago....
So how accurate are the new GPS units? Well, I got assigned to assist our IT department on what is called the Continuous Operating Reference Station (CORS for short) project. It is a grant offering from the fed's to the states and passed on to local counties to improve the accuracy of surveys. Here's how it works:
In our county, we installed five CORS units. They are passive receivers mounted on a solid aluminum pole, encased in a concrete tube. The tube goes down 20ft into the ground, and extends 10 feet about the surface, with the pole extending another five to eight feet terminating with an antenna that looks and is the size of 1/2 a basketball.
Once the system is installed, NOAA's National Geodetic Survey group comes and plots with their equipment, the absolute known L/L to the top of the antenna. They told me that it's plotted to 1/10,000 of L/L. When I asked to give me some sort of reference, the guy said it's about the diameter of a dime (knowing that in the real world, that dime can become oblong depending on distance from the equator). Close enough for me.
The antenna feed goes to a server mounted in a building close by, and data from the server across the 'net to our data center where it's stored and accessible by local survey companies to increase the accuracy either real time in the field, or by post processing their recorded data.
The CORS data gives them the difference between what is the known plotted absolute, and where the CORS antenna thought it was, based on GPS readings.
For those interested in more:
Regards - Don
posted 04-22-2012 08:37 PM ET (US)
For what it's worth, I have at least one L. Superior chart from the late 90's that is LORAN overprinted.
I didn't check all of them.
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