GPS: True or Magnetic

[Stu Jackson]

XTE

In addition to earlier responses, just following the arrow on your GPS can really lead to trouble. That's the arc route that's been mentioned.One of the easiest features to understand, and that is included on even the most basic GPS (I have one of those!) is Cross Track Error (XTE).It says how far off the course you are (this is an important concept) between the time you first pressed Go To and your destination. As you move forward, that point gets further and further behind (unless you keep updating Go To as you sail along). Without updates, all the GPS is doing is to continue to head you to your original destination from where you currently are. But remember where you were when you first said Go To to your GPS. It's still working on that line in defining your XTE.Another way to meld your GPS with basic navigation skills is very much like radar (without the Starpath computer program).Go out on a nice clear day and practice both together, so that way you'll have confidence in both sources of navigational input and information.

 

[Vic]

Dan ...

of course the real reason to run the positions to the chart is to make sure whoever is on the helm knows where he is in relationship to any kind of obstructions down the line. I don't care what chart plotter you use ... it always seems to be better to have a chart on the right page at the Nav station. That's especially true when someone forgets to go back to the scale that has the important stuff to display on the chart plotter ... and you sail merrily along not knowing what's in front of you.We came back from Lucaya, Bahamas to Port Canaveral on Wed. and used the boat's B&G at the nav station, with the garmin 162 duck taped up at the wheel. Took 33 hours to go south and 23 to come back.Coming back north we corrected {guessed} a bit to the west to sail a nice beam reach and were set[?] back to the GPS course.and of course the other thing is that the GPS displays the magnetic heading in numerals big enough to see easily ... that's important when you wear trifocals.Vic

 

[Daniel Jonas]

Navigational objective

Vic, Well put. The whole concept is making sure you are where you want to be, and if not, at least not where you don't want to be.Also good comments from others. Electronic systems all have their challenges. As pointed out, XTE provides useful information if you don't keep pressing the GoTo function, otherwise you are on an arc. The secret is understanding exactly how the system works. I believe that you are more prepared to recognize a problem if you actually understand the underlying navigational concepts.Its been many years ago but I once spent some time flying an aircraft that had an autopilot that went into a more sensitive mode for approaches based on an expected sequence of events. It also could be cycled back into a normal mode (not ideal for an approach, and not annunciated) based on subsequent inputs that would seem inocuous.With all the gee whiz stuff these units can do, I wonder how many of us have the 180 page manuals memorized enough to quickly sort through a problem or opportunity when time or manuevering room is short.Having said that, I need to go review the Raytheon RL70CRC manual to find out why an alarm keeps bothering me when I get back to the dock, yet, does not seem to be reflected in anything associated with the alarm menu.Dan Jonas (S/V Feije II)

 

[JR]

tech upgrades

After sailing all over Lake Huron for over 30yrs with just a compass, knot meter and watch doing DR plots, I finally bought a Loran about 10 yrs ago....wow...whole new game! Then just as I had Loran just about figured out, I changed boats. The Loran unit with the 'new' boat was junk! I replaced it with a Garmin75 handheld GPS. I love this unit! I always use magnetic so I have a compass backup...I still do the DR thing primarily...and use statute miles since Great Lakes charts are in stature miles....makes life MUCH SIMPLER!

 

[Doug Rodrigues]

I'd go with the magnetic

After factoring-in magnetic variation from your charts, you'd rely on the magnetic heading of your compass. If you had true set on the GPS, you would constantly be thinking something like, "Okay, 020 true really means 036 on the compass, so I'll maintain 036 to track 020." OR "Okay, if I have to do a 180, I'll add 200, subtract 20, and add 16 for the variation to match the GPS." Wouldn't it be much simple to have both mag compass and GPS operating the same way? Anyway, this is just my personal opinion as an instrument rated airplane driver who also sails.

 

[Clyde]

It doesn't matter, its user’s choice.

Since the GPS is considered a backup to onboard charts and compass, how you use your GPS is up to you. The GPS should never be used as your primary navigational aid. On sailboats, they should be considered as a backup since the primary purpose of the GPS is for US military use. Because of the present situation with Iraq, the GPS accuracy for civilian use can be selectively degraded from 3 meters to 100 meters or even randomly changed or become completely unavailable in certain areas. For sailing, having an accuracy of 100 meters should not greatly affect you if you use your paper charts and compass in conjunction with your GPS chart plotter. The change in accuracy mostly likely would affect taxicabs and cars that use them to get around major cities. There is a story on the Internet about the German automobile cub telling their members to be prepared for the lost of accuracy.Fair Winds.Clyde