Why Integrate the Autopilot ?

[DotDun]

Originally Posted by Stu Jackson

Good discussion.

Question: could you please DEFINE "track mode?"

I think we all understand steering to a waypoint, and I agree with Mark that it's rare to use nav aids for waypoints, 'cuz you can hit them (!) and only mobos do that, right?

But on my GPS, "tracK mode" means, to me and in the manual, a line of "cookie crumbs" as to where I have been, not where I am going.

My GPS (Garmin GPSMap 76Cx) does waypoints, XTE (digitally and the old highway thing) and lots of other features. But it doesn't "project" (i.e., predict) "tracks," it only saves them

Please help us understand.

Thanks.

PS --- I sail in tidal waters, so I understand current and drift, and am old enough to know and employ "traditional" navigation.

This question has nothing to do with integration, just wanting to understand by definitions which may be different between what my GPS tells me a "track" is and not understanding what you mean by that phrase.

When you set the GPS/Chartplotter to 'goto' a waypoint, you then set your AP to 'track' that waypoint. Your AP will then steer the boat on a straight line to the waypoint.

[Stu Jackson]

Originally Posted by DotDun

When you set the GPS/Chartplotter to 'goto' a waypoint, you then set your AP to 'track' that waypoint. Your AP will then steer the boat on a straight line to the waypoint.

OK, I understand. Let's take that a step further if you will.

My GPS, when set to a waypoint, will tell me the COG, heading (not necessarily the same as the boat's heading - I think we all understand the differences, GPS heading vs. bow of boat), and XTE (whether digitally or by the highway thing, as I've said).

However, many GPSs will show the "line" from the original point of entry, and leave it there, and NOT continue to use the changing boat position on this "track line" as you call it.

On my GPS, I can use the "Recalculate" feature to RESET that original point to my current position to the same waypoint without re-entering that waypoint.

This is very useful in sailing with currents.

It appears "track" may mean different things to different skippers.

Is this what you mean?

'Cuz track on my GPS, still, is where I have BEEN, NOT where I am going.

That's always, to me, the next waypoint.

Sure, the GPS is "tracking" where I am and where I want to go, but, please forgive me, I'm an engineer, and specificity means a lot to me, so does the application of individual words.

Are different GPSs different? Do they use different terminology?

Thanks.

[Ex-Calif]

Originally Posted by Stu Jackson

Good discussion.

Question: could you please DEFINE "track mode?"

Not to answer for DotDun and this may not be the point he is trying to make but some GPS like the Garmin 78 for example will navigate a previously saved track. As opposed to other models where you can just save and view them.

As for me, I'm all for technology use and I am all for knowing the underlying skill. In this case navigation.

Maybe the world can be divided into pre and post space travel. Technologies existing since the space race have made endeavors previously considered "too risky" less risky to those who may choose to undertake them.

Everything from navigation, to communication, weather information to boat reliability is improved and made easier.

I think some of the traditionalists must have an underlying ulterior motive for their objections. I mean I really can't see being so invested in some stranger on the internet that I will make impassioned pleas and arguments against passage making until the person has 30 years of boating knowledge, can rebuild their boat on the water, use a sextant like Captain Cook and understands how to make an SSB work. The argument that these folks are "less safe" to me is a red herring.

Coastal and harbor boaters (where the risk to me exists) have always had a mix of good ones and bad ones.

Here is what I can guess it is in partiality.

Poor, smart people used to make ocean crossings in small boats - often boats they made or heavily outfitted themselves. With enough study, learning and skill this could be done safely and made the person unique. It was one area where poor smart people held a big advantage over dumb rich people.

Today rich people can load a perfectly adequate production boat with tons of electronics, gain a little skill on how to use them and sail across oceans. I think it just pisses some people off.

I don't ever intend to learn celestial navigation - unless I get bored and decide to as an interest. I will never get my Ham license. I will likewise never learn to shoe a horse, make candles or weave cloth.

Does that mean I shouldn't be able to ride a horse, read at night or wear clothes?

Maybe GPS vs. sextant threads would be more useful if we spent less energy telling people not to use GPS and more energy telling people how to successfully use GPSs.

The question that this thread originally poses, "Why integrate the autopilot" seems obvious to people experienced with GPS and autopilots. Because it is a huge workload reliever for anyone sailing short handed.

The argument that GPS is dangerous is futile - Any self steering system left unattended can steer a boat into trouble, including windvane and tiller lashing.

[Stu Jackson]

Dan, I understand. But for my GPS, following a previous track, which can be done, requires one to watch the GPS and manually change the AP to, uhm, "keep on track" [sic] because that track was where you had BEEN, and is NOT associated with a "next" waypoint. Any GPS that shows this track feature should do this, as you said.

However, my question is is: Does an integrated ap follow a previous track (and by track I mean MY definition, not Dot's)>

Thanks.

[IslandHopper]

There are many valid reasons why many shipping companies (mine included) forbid Interfacing GPS and AP, in fact I have never worked for one that allows it.....

[StuM]

Originally Posted by Stu Jackson

Dan, I understand. But for my GPS, following a previous track, which can be done, requires one to watch the GPS and manually change the AP to, uhm, "keep on track" [sic] because that track was where you had BEEN, and is NOT associated with a "next" waypoint. Any GPS that shows this track feature should do this, as you said.

However, my question is is: Does an integrated ap follow a previous track (and by track I mean MY definition, not Dot's)>

Thanks.

""What we've got here is failure to communicate"

Two different but equally correct usages of the word "track".

One refers to plots of historical GPS positions. The other refers to an AP following a route from a start position to a specific destination point.

Just accept that the same word can have different meanings in different situations.

[StuM]

Originally Posted by Ex-Calif

The question that this thread originally poses, "Why integrate the autopilot" seems obvious to people experienced with GPS and autopilots. Because it is a huge workload reliever for anyone sailing short handed.

The argument that GPS is dangerous is futile - Any self steering system left unattended can steer a boat into trouble, including windvane and tiller lashing.

After 392 posts in the subject, someone finally sums it up with a simple, clear statement. Thank you.

[DotDun]

Originally Posted by Stu Jackson

Dan, I understand. But for my GPS, following a previous track, which can be done, requires one to watch the GPS and manually change the AP to, uhm, "keep on track" [sic] because that track was where you had BEEN, and is NOT associated with a "next" waypoint. Any GPS that shows this track feature should do this, as you said.

However, my question is is: Does an integrated ap follow a previous track (and by track I mean MY definition, not Dot's)>

Thanks.

I'm a communication protocol person. I figure out how things work by watching the communication between the various pieces. IOW, I reverse engineer things. A GPS/chartplotter, at the minimum, sends the AP bearing to waypoint (and declares either magnetic or true), distance to waypoint, and cross track error. The AP will start the 'track' by turning the boat to BTW. The GPS/chartplotter will continually update the AP with XTE, either right or left of track and how far off the track. The AP will turn the boat until XTE equals 0. The AP will continue until DTW = the preset 'arrival' distance (~ a few hundred meters).

So, if you tell your GPS to follow a previous track, I would hope you would be very close to that track before engaging the AP.

[sms18]

I have Morgan 41 Sailboat, 1980, and survey noted that that steering pressure is 14 lbs, should be 20. How this is dangerous, what should I do to fix this?

[Ex-Calif]

Originally Posted by Stu Jackson

Dan, I understand. But for my GPS, following a previous track, which can be done, requires one to watch the GPS and manually change the AP to, uhm, "keep on track" [sic] because that track was where you had BEEN, and is NOT associated with a "next" waypoint. Any GPS that shows this track feature should do this, as you said.

However, my question is is: Does an integrated ap follow a previous track (and by track I mean MY definition, not Dot's)>

Thanks.

Yes - The G78 - for example - allows one to pull a previously saved track, select "track mode" and the gps will navigate that track automatically without user intervention. Well technically that's not true. The GPS only provides steering information. A helmsman or coupled autopilot still needs to steer the boat.

The mode that only paints a course line from current position to the next waypoint is called "go-to" mode or I have heard it called "direct" mode. I think this is the mode many people are "concerned" about because this mode will plot a course through sandbars, into reefs over islands and through buildings.

I have seen this mode work two ways - One is the magenta line is continuously updated from current position to entered waypoint (single waypoint). The second is as you describe, a line (course) is painted from current position to waypoint and is not updated. In a way this is really route navigation. The GPS simply creates a "temporary" waypoint at current location.

In the first mode of operation you won't get XTE information because technically you are never of course

A "route" requires 2 or more waypoints. a line is drawn between them and then you get XTE. CTS now, may not equal course or bearing as the GPS is also trying to get you to steer to eliminate the XTE - way out from a waypoint this may not be noticeable but angle off a waypoint by 500 yards or sa and watch CTS as you approach the waypoint, at some point it will be advising a CTS 90 degrees off the course line, until you pass the waypoint - if it's the last waypoint it will want you to go back, if there is a next waypoint and it is reasonably far away you will get a CTS reasonably close to Brg. & Course (One could argue CTS should be called HTS)

The definitions are pretty standardized. Interestingly Garmin has the same definition for course and bearing (they use the definition for bearing below) in the G78 manual - which I disagree with.

To me course is the magnetic heading between two waypoints - i.e. the direction of the line.

Bearing is the current magnetic direction to the waypoint.

Not to overbeat a horse but if I have a course of 090 between two waypoints say 10nm apart and I am half way there and a mile right of course I could have:

Course 090
Bearing 085 (to know the real number I'd have to math and I am lazy)
CTS - 080

Track to me is always where I have been - the breadcrumbs behind me. You could be "tracking a course" but I would never say that on a boat. I would say

"I am navigating a course, I have 1nm cross track error. The bearing to the waypoint is 085 and I need to steer 080 to intercept the course."

If I knew I was in a pretty nasty current - the left to right one one that got me 1 nm right of course I might add, "We are being set pretty hard, make the heading 075."

If you said that to anyone who uses GPSs they would understand you.

However if Otto is steering, he is likely figured out a heading that eliminates the XTE before it starts.

In an earlier post I erroneously stated that should these conditions exist Otto would sail a parabola. I stand corrected, but close hauled to a waypoint Otto could have a heading that pinches and eventually luff the sails as he steers to overcome the set. If the above example was a port tack close hauled passage, you may not make the waypoint.

Then you need Mark's (colemj's) integrated autopilot and GPS that calculates VMG, laylines, current set, keel efficiency and future windshifts so it can auto-beat to windward. I think this mode is called Otto-beat

[Stu Jackson]

Track to me is always where I have been - the breadcrumbs behind me. You could be "tracking a course" but I would never say that on a boat. I would say

"I am navigating a course, I have 1nm cross track error. The bearing to the waypoint is 085 and I need to steer 080 to intercept the course."

If I knew I was in a pretty nasty current - the left to right one one that got me 1 nm right of course I might add, "We are being set pretty hard, make the heading 075."

Dan, that's pretty much what I would do if I was "going back along a previous track."

I did another thing this morning thanks to this topic: RTFM!

My Garmin GPSMap 76Cx has what Garmin calls a "TracBack" feature, for navigation back over a previously recorded track. I'd never used it before (have had this unit for seven years). That's because I do what Dan suggested, plus I never use routes, always waypoint to waypoint manually, so I never felt the need to employ it. I'll give it a try.

While "failure to communicate" may have been the perception of someone, I think what both of us were trying to do was to nail down specific definitions. You now know what happens when two engineers get into a discussion!!!

Thanks again, very helpful.

[DotDun]

The term "track" in the AP world means to follow the BTW and XTE values sent from the GPS/chartplotter towards the next waypoint. I used the term "track" as that is the industry norm on AP UIs.

"Track" may have a different meaning to you.

[sy_gilana]

Just a follow up to my previous post about the difference between APA and APB.

Here is a quote from NMEA data

"
APB - Autopilot format B is sent by some gps receivers to allow them to be used to control an autopilot unit. This sentence is commonly used by autopilots and contains navigation receiver warning flag status, cross-track-error, waypoint arrival status, initial bearing from origin waypoint to the destination, continuous bearing from present position to destination and recommended heading-to-steer to destination waypoint for the active navigation leg of the journey.
Note: some autopilots, Robertson in particular, misinterpret "bearing from origin to destination" as "bearing from present position to destination". This is likely due to the difference between the APB sentence and the APA sentence. for the APA sentence this would be the correct thing to do for the data in the same field. APA only differs from APB in this one field and APA leaves off the last two fields where this distinction is clearly spelled out. This will result in poor performance if the boat is sufficiently off-course that the two bearings are different.
$GPAPB,A,A,0.10,R,N,V,V,011,M,DEST,011,M,011,M*3C where: APB Autopilot format B A Loran-C blink/SNR warning, general warning A Loran-C cycle warning 0.10 cross-track error distance R steer Right to correct (or L for Left) N cross-track error units - nautical miles (K for kilometers) V arrival alarm - circle V arrival alarm - perpendicular 011,M magnetic bearing, origin to destination DEST destination waypoint ID 011,M magnetic bearing, present position to destination 011,M magnetic heading to steer (bearings could True as 033,T)
"

It would appear that the situation I explained in my post might be equipment dependent.

The point is....Be careful that your installation DOES steer by XTE and not by BTW.
or if it does steer by BTW then draw a new line on your chart and check it.

My answer to the OP question remains, Because its a great tool, it can allow you to sail better
BUT there is a caveat of caution, you must remain situation-aware and verify that
your gear is doing it's job, and practice using it in benign conditions, so that
you are not faced with a crisis when fatigued, or in tough conditions.

[Stu Jackson]

Originally Posted by DotDun

The term "track" in the AP world means to follow the BTW and XTE values sent from the GPS/chartplotter towards the next waypoint. I used the term "track" as that is the industry norm on AP UIs.

"Track" may have a different meaning to you.

Understood. But, it's not a different meaning to me, it is a different meaning to the makers of GPSs, seems different to the makers of the autopilots. Kinda like the incorrect definitions of headings & bearings Dan mentioned in the Garmin literature. Don't any of these guys read Chapman's or Dutton's?

Thanks for straightening that out. (pi)

[DotDun]

Originally Posted by sy_gilana

It would appear that the situation I explained in my post might be equipment dependent.

The point is....Be careful that your installation DOES steer by XTE and not by BTW.
or if it does steer by BTW then draw a new line on your chart and check it.

Are there APs that ignore XTE? Do you know of any?

AFAICT, XTE has been in NMEA since the beginning, I'm struggling to imagine an AP manufacturer that would support NMEA connections and ignore XTE.