Wouldn’t it be more useful for ATC to know your track than your heading so they can approximate better where you’ll be in the future? So back to the original question: is asking for heading simply an artifact of all planes not being GPS equipped? If you wanted to separate traffic that is climbing or descending (with shifting winds) wouldn’t the track be more accurate than the heading? <<<
I’m not educated on ATC ways, but based on my personal experience the answer would be an clear no. It would be a pointless extra step for them to translate every command they give into an actual ground track heading. When he asks your heading, it’s usually not so he can tell what your destination is, or how accurate you are flying…he has your flight plan and knows you have GPS based on filing slant golf…rather it is usually in anticipation of giving you a course change calculated in degrees relative to the heading of other traffic. An experienced controller takes into account your plane type, known winds and can figure out quickly that if he needs you to turn X degrees right or left (ie, relative to other targets on the screen), then that mean X degrees plus/minus Y degrees for wind. He simply makes a quick calculaion or guestimate, and gives you the new heading, based on the present heading you gave him. The actual ground track magnetic or true heading is irrelevant to the process. In fact, if you fly the GPS number, you will likely be off his desired course.
For example, assume you are tracking (ground and compass)with a strong direct headwind at 270 degrees, and the controller needs you to make a ground track change (and relative heading) to the right by 20 degrees. He calculates that a compass turn of 15 degrees will put you right on the course he needs you to be on, so he orders you to turn to 285. Now you have a left crosswind and so are ground tracking at 290, i.e. right 20 degrees. If you turn to 285 on the GPS, you would be five degrees off of the direction he needs you to go.
While using only ground track reference might at first seem simple from the cockpit, from the standpoint of the radar room it would be complex. in order to ensure all lanes are operating on the same relative terms, for each command the controller would have to get out a protractor, find magnetic north on the radar screen, get another tool to allow him to slide the protractor over to the image of your plane on the screen, then try to quickly figure out what the desired magnetic ground track would be, before your plane slips away to another location on the screen. Meanwhile, he may be monitoring a dozen other planes! It would never work!
On the other hand, a controller could express every command simply as turn right or left so many degrees, eg right 155, or left 170 degrees, instead of giving headings. In this case, GPS works fine. But think about it…that leaves you with the task of adding the degrees to your present heading, and at no point has the actual heading been verbally expressed over the radio. Things would quickly fall apart using such a system.
The only way that I can see that a magnetic ground track based ATC system would work would be if all planes were guaranteed to have GPS, AND the radar scope indicated actual ground track for each airplane , AND some sort of checking system (like RAIM checks) that compares planes’ GPS readings to radar readings.
I think the controllers must take some personal pride in how accurate their assigned headings are. I fly in the Washington DC area and am under control all the time. ATC knows I have GPS, but frequently give me a heading to my next waypoint, a VOR, or my home airport, when they could just as easily tell me to go direct. I make a point of flying the assigned heading precisely, rather than cheat with the GPS. It’s amazing to me how right on target ATC is almost every time, even when I start 20 miles or more away from the target