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#21
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As others have pointed out, it's all there on my web-site, but you have to piece it together. http://williams.best.vwh.net/avform.htm Many thanks for a great page. Truly a great piece of work. I have looked at it and found the mag var code. I am just surprised that they vary so much depending what model is been used. What model is the best? And also it seems that by the poles the calculation are somewhat wrong, is it just my imagination? Sims |
#22
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"Mogens Beltoft" writes:
We use GPS receivers in our gliders, and most of them can find the wind direction and speed automatically by detecting that you have made a full 360 degree turn (happens often for gliders when searching for thermals), and analysing the 360 degree turn you have made for drift direction and speed. And I'm certain that the information is used to display a wind-corrected heading. So which GPS receivers do this for you? We use Pocket-Nav http://www.cambridge-aero.com/pocketNAV.htm and others. That seems to be an application that runs on an external computing device, not something that a GPS receiver itself provides. Interesting, but different. Dave |
#23
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Ed,
I second the compliment on your site. I have found it useful several times. John Bell www.cockpitgps.com |
#24
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Dave,
A GPS-only unit has no way of knowing what your current heading is in the first place. I agree with this statement. I haven't heard of a GPS receiver that will tell you what heading to fly or sail to compensate for wind or current. Neither have I. It can tell you how much to turn to align your heading with the desired course, but that assumes no wind or current. In practice, you'll have to make adjustments to that heading, while watching the cross-track error. I agree that a GPS will not tell you an absolute heading to fly, but it will tell you a relative heading. The GPS will give you a value of TRACK and it will give you a value for BEARING. The idea is to adjust your heading so that TRACK matches BEARING. Many GPS receivers have a data field called TURN --this is just the difference between TRACK and BEARING. If the GPS indicates a TURN of 5 degrees left, adjust your heading 5 degrees left. If you follow TURN or match TRACK and BEARING, you should track directly to the active waypoint. I agree that the GPS could care less what your heading is. Example: If you have a TRACK of 85 and a BEARING of 80, the TURN would be L5. Turn 5 left so that TRACK matches BEARING. Let's say that you have done that and the TRACK and BEARING are now both 80. The GPS has no idea if you are heading 065 for a left crosswind, 95 for a right crosswind, or 080 for no wind. If you were to follow a heading to match the GPS value of BEARING, a curved path would result from a crosswind or cross current. This is often refereed to as homing and is considered bad technique. For large turns the change in heading does not always match the change in TRACK. For example, if you were flying north into a northerly wind and turned to 090, the change in track would be more than the change in heading. The headwind would change to a crosswind. However for small adjustments, the change in heading is proportional to the change in track. There is a slight variation in this technique for panel mounted aviation receivers. The difference is that they usually do not offer TURN, but do offer a field called Track Angle Error, TKE. TKE is the difference between TRACK and DTK or COURSE (synonyms). TKE gets you parallel to the course as opposed to directly to the active waypoint. In this case, cross-track error is necessary. However by turning so that TKE is zero will result in a constant cross-track error. It may then be necesary to adjust the heading a little more to reduce the cross-track error to zero. The method that you describe of adjusting your heading to manage cross track is not incorrect. However, I thing that the ability to get an exact heading that compensates for wind or currents by comparing the TRACK to BEARING is one of the most powerful features that GPS has to offer aviators and boaters. I have more details in the navigation section of either of my online books at www.cockpitgps.com. --John Bell |
#25
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"John Bell" writes:
I agree that a GPS will not tell you an absolute heading to fly, but it will tell you a relative heading. The GPS will give you a value of TRACK and it will give you a value for BEARING. The idea is to adjust your heading so that TRACK matches BEARING. Many GPS receivers have a data field called TURN --this is just the difference between TRACK and BEARING. If the GPS indicates a TURN of 5 degrees left, adjust your heading 5 degrees left. If you follow TURN or match TRACK and BEARING, you should track directly to the active waypoint. That's correct, with a couple of caveats. The first is that in an aircraft at least, you normally fly a heading based on the directional gyro, since it gives you a stable reading. The GPS TRACK doesn't reflect heading changes instantly, while the DG does. So you maintain a heading, then see what happens to the track, then possibly adjust the heading. The second is that this method takes you directly to the destination, no matter where you are now, no matter how much you've drifted off your original course. Sometimes that's fine, but sometimes you want to regain your originally-planned ground track (e.g. it takes you between mountains, or avoids submerged rocks, or avoids a restricted area). For these cases, what you really want is to look at cross-track error (the deviation from your planned route) and get back on the planned track. The method that you describe of adjusting your heading to manage cross track is not incorrect. However, I thing that the ability to get an exact heading that compensates for wind or currents by comparing the TRACK to BEARING is one of the most powerful features that GPS has to offer aviators and boaters. Why is it not correct? Keeping the cross-track error zero takes you directly to your next waypoint, factoring in any wind or current correction that is necessary, and it lets you follow your originally- planned ground track. It's the best you can do. It's like following a VOR radial, except that the virtual "VOR" can be placed anywhere (it's just a waypoint), and the "needle deflection" vs. track error has constant gain all along the route. The method of matching track to bearing will also take you directly to the destination if you never get off course. But once you do get off course, this method takes you along a new "direct" path to the next waypoint. This is *faster*, but it isn't always safe. Navigating to minimize cross-track error is safer, but potentially slower. Both result in exactly the same ground track if you never get off the planned route. You can say that either is better than the other, depending on circumstances. I don't see how you can say one method is "not correct". Dave |
#26
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Dave,
I actually agree with your points. In fact, many of the points that you make are ones that I make in my chapters on navigation in both of my texts at www.cockpitgps.com. The only thing that I substantially disagree with you is in that I disagreed with you in the first place. In answer to the comment: "I don't see how you can say one method is 'not correct'." I had actually written that your original post was "not incorrect." I apologize for the double negative, but my intent was to emphasize that I was not contradicting your original post so the double negative seemed appropriate in context. I feel confident in my ability to navigate with a GPS. However, I have never bragged about my writing abilities. My point is that although old methods of navigation based on cross track error are still correct, the GPS gives a unique capability to more accurately find a very precise heading to compensate for winds and cross currents because of its ability to sense an actual track. I think that cross track error is a very useful and at times essential piece of data. I see the method that you describe and the method that I describe as being complimentary, not contradictory. --John Bell, www.cockpitgps.com |
#27
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"John Bell" writes:
The only thing that I substantially disagree with you is in that I disagreed with you in the first place. In answer to the comment: "I don't see how you can say one method is 'not correct'." I had actually written that your original post was "not incorrect." Oops. I was in a hurry, and misread that as "not correct". My fault. Dave |
#28
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Dave Martindale wrote:
"Mogens Beltoft" writes: We use GPS receivers in our gliders, and most of them can find the wind direction and speed automatically by detecting that you have made a full 360 degree turn (happens often for gliders when searching for thermals), and analysing the 360 degree turn you have made for drift direction and speed. And I'm certain that the information is used to display a wind-corrected heading. So which GPS receivers do this for you? We use Pocket-Nav http://www.cambridge-aero.com/pocketNAV.htm and others. That seems to be an application that runs on an external computing device, not something that a GPS receiver itself provides. Interesting, but different. The GPS receiver is just an standard "mouse", so NMEA data is analysed by PocketNav to provide the information. /Mogens |
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