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GPS groundtrack/heading


stevec
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Hi.

I’ve just ordered a Garmin Foretrex 401 (price has come down to £124.00 on Amazon). I’m trying to get my head around how GPS units cope with the the fact that our heading is almost always different to our ground track? If I take the extreme example of flying into a 30mph headwind, heading north, (no I’m not about to do that), my ground track will be southerly at say 10mph. As the unit expects the display to be facing the direction of travel (as in a car) then it will show South at the top of the compass. Will I need to orientate the unit relative to my ground track for it to make sense? How will this affect the bearing pointer?

The unit does have an electronic compass, but this is notoriously unreliable, and in any case I think it is over-ridden by the GPS compass over a certain speed.

Also, what is the best way of entering waypoints into it, other than manually by Long/Lat? I can see ways of downloading waypoints and tracks to Google Earth or other software, but doing it vice-versa seems a lot more complicated.

Thanks, Steve

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Ok. I needed to do my own homework and so I think I can answer my question!

I’ve overcomplicated things because I used to fly PPL using ADF and VOR which you have to relate back to magnetic compass headings headings.

With the magnetic compass turned off (it’s hopeless anyway), the “Heading” displayed on the gps is not really heading but track over the ground. So if I need to navigate to a waypoint, I can either choose to fly to a course line (fixed track from selected position) or a bearing line (a line which varies and gives a straight line to waypoint from wherever you are). The advantage of the gps derived heading to fly (track over the ground) is that it is the same as the course, or initial bearing and so automatically compensates for wind drift. In which case ignore your magnetic compass as you may be "pointing" in a completely different direction to where you are flying (depending on wind).

At least I think that’s the case?

This is a useful article:

http://www.aeroexperiments.org/GPSsoar.shtml

Now that I’ve got that straight, I’ll probably leave the thing on the ground and just enjoy flying!

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Hi Steve.

I have found trying to fly a compass heading to achieve accurate navigation is almost impossible on a PPG on anything but still calm conditions. I use a traditional compass for gross error checks making sure i am pointing approx the correct direction but need to study map to ground features to achieve accurate track. Good to practice for competitions and enjoyable. Often I am lazy and use the "go to" on the GPS.

Regards

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  • 4 years later...

Crikey, I didn't realise you guy's were so into precision navigation :shock:

I always thought you just picked a line feature and followed it ( with a GPS just pointing you in the appropriate direction by means of a pointer needle ) :arrow:

I might need to dig out some of my archaic GPS units & throw them away as you guys Are now using better kit than our GA Jockey's :roll:

Please tell me that Paramotor pilot's don't use GPS for Cloud flying or VFR on Top cruising?

I am perfectly aware that there are now microlight pilots who have progressed to In Cloud Flying on the belief that an Android Phone with a Artificial Horizon app will keep them upright & that a GPS will get them to the destination.

Android Artificial Horizon Apps do work as a " Get out of Jail " if used sensibly, but fail miserably if used as a means to fly in cloud on a pre plan mission.

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There are a handful of pilots who's 'thing' is navigation and distance flying.

Most will navigate as you suggested above.

Most will not fly in clouds but again, there are a handful of people that do unfortunately :-( The stability thing is less of an issue due to the pendulum stability that we enjoy. (not condoning it, just saying)

Having now flown for 5 hours IFR in cloud in a heli, I recon I spent about half of that time wondering if anyone else was in there with me that was not on radar. Scary monsters stuff that is.

SW :D

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I was hoping you weren't going to say that Paramotors do operate in Cloud but I was expecting the answer you gave.

I watch that video & think how lucky and how stupid that paraglider pilot was, so I can't understand why anyone would knowingly go into cloud.

I know that every style of flying has Renegades, been that myself but I am wiser now & worry that I may find a VFR paramotor in cloud whilst I am IFR in a GA aircraft.

http://youtu.be/J2iOgY_f92g

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OK now starts what may be a long line of question's that I need to ask before I can start into the World of Paratriking.

On the subject of Groundtrack/heading I am confused as to whether a Paratrike would adopt the same process as a 3 axis aircraft does?

To quantify my question : If I was flying from Boston to Marshland @ 35mph and the wind was 283/49 my Track Magnetic would be 147degrees but my Heading Magnetic would be 222degrees with my TAS of 35mph becoming a 45mph Groundspeed in a 3 axis aircraft.

Would the same figures be correct whilst flying the same routing in a Paratrike?

( I have used 283/49 as the wind to overemphasise the drift component for the sake of the question ).

75 degrees of drift being laid off is a crazy figure I know, but it does show up my question succinctly.

Or :

The question is, How does a Paratrike lay off any kind of drift?

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You will find that the triangles of velocity calculations are next to useless for flight planning on a Paratrike :-) As suggested above in detail by Pete the effect is identical but almost useless to plan against.

The main reason being, a tiny difference in the weather has a big effect on course. When we flew lands end to JOG we used a good old map and mark one eyeball. :-)

SW :D

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  • 1 year later...

In the set up menu activate COURSE DEVIATION INDICATOR (CDI). You can set the amount of drift, when activated the CDI will have a deviation measurement bar horizontally across the compass page.  The CDI indicator is a vertical line which moves to depict course deviation and where the intended track lies relatively to your current position.  If for example you drifted left the CDI line will show the bottom of the line swing out to the right depicting you are now left of track.  As you apply a right turn correction and begin to intercept the correct ground track again the bottom of the line will swing back towards the centre.

This is a very useful tool, if you had planned a course from one way point to another which was to say take you through a gap between two high features with a distance of 400m between them.  If you have drifted off course the GPS will continually update the course to take to get to the waypoint, but this new heading may now become controlled flight into terrain in bad weather.  By using the CDI you can course correct as soon as possible and know that it will take you safely between the intended path between the features.  Although no one advocates flying in cloud or poor visibility, the CDI will at least follow the intended ground track until you get out of it.  For those PPL guys this is the same system as flying radial on the VOR system. 

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  • 3 months later...

The fundamental issue which everyone has obliquely referred to, but not explicitly stated, is the relatively low airspeed of PG/PPG/Paratrike aircraft.

The triangle of velocities calculation is correct for the given input parameters. Flying a fixed wing aircraft at 70kts in winds of 10-15kts, a pilot should check location every few minutes. If the predicted track does not achieve the desired heading, then a correction must be made. This might involve a radio call for a local weather report, followed by recalculation with the new parameters.

If you are flying PPG at 25kts in winds of 10-15kts, the potential for heading errors due to changes in the wind is greater, so there will be more requirement for updating calculations.

The GPS is able to update the calculations quickly. You could do the same using your slide rule (or calculator for the young 'uns), but it would take up much of your attention.

Incidentally, ideally you wouldn't fly to a compass reading directly. If you use the compass once to identify a landmark on the correct heading, you can then keep aiming for that landmark. This effectively corrects for wind drift, although it does not result in the most efficient flight path.

P.S. A landmark is a fixed object on the ground, not a boat at sea, or (as I once overheard) 'that cloud that looks like a cat'. :-)

 

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