Earlier this year there were stories in the news about how the north magnetic pole is moving faster than modelled, and that this was causing problems for GPS navigation.
I thought GPS just used the positions of satellites in orbit, rather than anything to do with compasses. Can anyone explain in simple terms why a GPS system needs to know where the magnetic north pole is?
Does the GPS indicate that a ship should use its magnetic compass to go on a bearing of “25 degrees west of magnetic north” or something like that? In which case the figures could be wrong.
Internet crap. Click Bait. It doesn’t matter if it’s true as long as people click.
GPS units are positioned relative to GPS Satellites. GPS Satellites are positioned relative to GPS ground stations. Hence: GPS units are positioned relative to GPS ground stations.
The position of magnetic north matters to some people. It moves around quite a lot. It’s got nothing to do with GPS. It’s like saying GPS needs to be updated because you got a Democrat Governor in your state.
I imagine that due to continental drift, the ground stations move relative to each other and relative to your GPS unit. I imagine that the GPS system has to be updated due to continental drift. Fortunately, continental drift is smaller than the accuracy of GPS anyway, so I imagine that doesn’t matter very much.
Ships normally (?) use a gyroscopic compass for the autopilot. Having the magnetic pole in the wrong place would confuse people (near the north pole), not ships.
I think this is basically it. AIUI, GPS operates entirely without knowledge of magnetic north. It’s great for positional accuracy, but if/when it ever craps out (power/electronics failure, hostile interference, reactivation of selective availability), it sure is nice to have a good ol’ magnetic compass as a backup tool. NOAA has this to say about the World Magnetic Model:
When you’re far from the pole, the distance between expected magnetic north and actual magnetic north is relatively small. when you get to high latitudes, the distance matters more. If your GPS navigational system craps out and you’ve fallen back to depending on magnetic north to know where you are and where you’re headed, being off by a few miles can make the difference between a safe journey and a shipwreck.
FWIW, NOAA’s explanation about measuring travel direction seems a bit odd. After all, a compass doesn’t measure travel direction either; it just tells you which way you’re facing.
Of tangential interest: Satellite compasses are available that can tell which way a ship is pointing without any movement. They do this by employing an array of two or three GPS receivers spaced several feet apart.
GPS doesn’t use magnetic north at all. But the location and navigation system on your phone, which has GPS as one component and which most folks mistakenly call as a whole GPS, has magnetic north as another component.
GPS tells position very well, but it only tells position. It can, with time, tell what direction you’re going, by comparing where you are at different times. But that’s a slow process, and it can’t be used at all when you’re stopped to tell what direction you’re facing.
A compass can tell that. So phones use a combination of GPS and a compass (and a whole bunch of other techniques) to tell you where you are and what direction you’re facing.
The section you quoted says a compass is used to determine immediate heading, i.e. which way you’re facing.
What they are saying is, when the vehicle is making a turn, GPS isn’t fast enough to display course (travel direction) or heading in real time. But a compass can still display the heading in real time, which is much better than nothing.
But note that the OP’s two examples deal with ship & aircraft - which care about navigation mostly when in motion.
And on a vehicle the size of either of these, it would be practical to have at least two separate GPS receivers, to give accurate heading at all times.
Which is a bit outdated - GPS receivers that update 5 to 10 times a second are becoming common.
This won’t help if the vehicle is moving very slowly. GPS can only measure direction if you’ve moved far enough in that direction to measure the difference in position.
Incidently, I saw that chart a while back, and hadn’t realized that magnetic north rate of drift was as fast as it was even before recent decades. They’ve been doing updates to compass variation figures on regular basis. Probably not a big deal if they have to update it more often. BTW, look at the projected path. For now, it’s moving towards the true pole - compass variation has been getting less for most locales. The magnetic field is getting slightly weaker, too. At least a flip is still believed to be a few thousand years into the future.
With strong emphasis on very. Modern receivers are capable of reporting direction of motion fairly accurately at speeds greater than, say, 10 feet per minute.
Ah, you’ve seen me running with my GPS watch.
But on a ship rolling on the sea, surely the margins of error for direction would be huge over short periods of time (unless you’re triangulating multiple receivers)?
If you cared about roll-induced error, you could use a gyroscope input and correct for roll in software.
I don’t see how. 10 feet per minute is 2 inches per second. If you want to determine the direction you moved in the last second, to an accuracy of 5 degrees, you need a position measurement with a precision of 2 inch * tan(5 degree) = 0.17 inch.
GPS doesn’t use position measurements to calculate speed; it measures doppler shift of the GPS signal.
It’s pretty common in literature on the subject to breakup “GPS System” into some sort of space segment, control segment and user segment. Different authors do it differently but the main point is that the user end is usually included. When you include the user end you then also have to include datums, projections and navigation.
I thought most commercial GPS receivers didn’t have this capability but perhaps I was mistaken. But still, if a receiver measures each vector component of velocity to 10 ft/min accuracy, you need to be moving 10x that speed to determine your course direction to 5 degree accuracy.
I’m getting in over my head here, but this PDF file speaks of the ability (in theory if not in practice) for GPS systems to measure velocities with accuracy on the order of a few centimeters per second (see second page of document), which is on par with your 10 ft/min spec for accurately assessing course direction.