Ummm. no. All I have to do is turn it on. It shows north.
Hey, I like the nav systems in new cars. It’s sad. But I do. I’m a map maker. I can fly into any city in the US and have my Garmin or an in car system and get where I need to go.
It’s about 11 degrees west here. But I don’t see how it’s relevant.
Right - it’s a GPS receiver. Which means it listens to precisely-timed signals from a constellation of satellites and from this information calculates position. Not heading, unless the receiver is moving, or has information from an additional sensor.
Two? GPS is one - what’s the other?
Right - but how does this bear on the OP’s question?
A GPS receiver is receive-only - it never sends anything to a satellite.
Almost everyone seems to think that this can’t be done with GPS signals alone. Do you disagree? If so, let us know how this works.
I’ve noticed my TomTom cheats. It assumes I’m following the route it suggests, and just locks me to the road. If I take an exit it isn’t expecting, it will show me on the highway for a bit, pointing in the road direction for a few seconds, until it notices I’m too far off the route and moves me. It’s like I’m on rails.
No. A GPS receiver (without any other type of sensor) cannot tell which way it is pointed. GPS works by measuring the time difference between signals from multiple satellites. But it doesn’t know which direction the signal comes from, because the antenna is not directional.
The only reason your GPS unit always displays north is because it does have a magnetic sensor.
GPS doesn’t work when you are surrounded by dense tree cover or tall buildings. And tunnels, of course. That’s why high-end car navigation systems also have gyros and also connects to the car’s wheel sensor. This allows dead reckoning even when the GPS signal is not available. Some have accelerometers as well.
Nothing is sent to the satellite. The GPS receiver is just that, a receiver. It calculates the position (lat/lon) based on time difference between signals from different satellites. And I believe it also measures the Doppler shift to calculate velocity (speed and direction of movement). But it has no way of knowing the orientation of the receiver or the vehicle, unless it has a magnetic compass built in.
Stupid question, but couldn’t the unit just remember the direction you were traveling in before you stopped?
Too see if your GPS has a compass system, hold it flat and move along a straight line. As you move turn the unit 90 or 180 degrees in your hand and see what the display does. That will show you …
I think the problem with that approach is, the last movement made by the car may be a small correction for parking, i.e. a sharp low-speed turn. It would be difficult to measure the final orientation of the car by just looking at the GPS signal.
Okay, if you say so - but how? As a GIS professional explain how it works. As everybody else has noted you need something more that the actual GPS satellite signals to determine direction in a **stationary ** satnav system.
For a GPS to determine heading, it must move, unless supplemented by another technology. Maps aren’t GPS, accelerometers aren’t GPS, and compasses aren’t GPS, although they may be part of GPS units. They’re external data. In all, maybe I’m guilty of not understanding you now. I seem to hear you saying that pointing north is your heading, whereas the heading is really the direction of travel or potential travel (when you’re stopped). GPS alone can’t determine heading without motion. In any case, a compass points north, but doesn’t tell you what your heading is; you need to know how to use the compass. Maybe what you’re saying is that upon power up, your unit points north regardless of the orientation of your unit? That makes sense, I guess, especially if it’s intended for car use, since cars presumably always have a heading, even when stopped. As a hiker, heading changes would drive you crazy unless there were some built-in fuzzy logic to prevent constant heading updates (or like in my portable unit, I just tell it I’m a pedestrian and it ignores heading altogether).
GPS per se is just a fancy digital clock, which is why you need multiple satellite locks to establish your position in space and increase accuracy.
Oh, forgot to mention. I drove a Zephyr with an OEM navigation system most of last year. When I took the ferry across the St. Clair River, it accurately showed my heading the entire trip. The variation of heading across the river was approximately 180°. As you know, when crossing a river the direction of motion and the direction the car/boat is moving are not always coincident. Even such, the nav system always marked the car’s forward pointing direction (not direction of motion) as the heading, despite the fact that the motion tracked laterally. That is, the little arrow always pointed towards where the car would move if I were to drive it, but did track the physical position across the river.
I think that you folks may be right. I have an appointment to go to this morning, but will check on my Garmin later.
I plan to put it on a flat level surface turn it on and check north. Turn it off, and spin it turn it on and check north again. Perhaps it does have a magnetic compass too. I’ll have to check the manual on that one.
And yes, I know the unit sends nothing up to the sats. Bad wording on my part.
And technically, what you get is the course or track, i.e. the direction of travel. Heading refers to the orientation, which may not be the same as the direction of motion (e.g. for airplane flying through crosswind, or a handheld GPS receiver held at an angle).
Yes, thanks for distinctly pointing that out – I kind of described it in terms of the ferry, but not as succinctly.
Sorry, I didn’t notice your last post before I posted my reply, didn’t mean to imply your explanaiton was unclear…
Heading could be derived by GPS alone. If the system had high resolution (high accuracy not required) and antennae near the front and rear of the car, and a means to switch between them, (or redundant receiver circuits) it could determine the heading via the difference in the position of the two antennae.
Not that it is done that way, just pointing out that it is indeed possible using only GPS, without magnetic or inertial sensors.
ETA: Heading vs. track information is very useful to boats or aircraft, as the difference can be used to infer currents or winds that are otherwise troublesome to measure.
lets try this again.
Okydook. I hit enter and wasted my previous post.
I spoke too soon and did not think the OP’s question through. I started to think about data and not the real question.
From my Garmin manual
So it has a basic magnetic compass in it.
I suspect that the heading is stored in some sort of ROM. And it is updated as you move using data stored in the unit.
What is an Electronic compass?
OK, in the car systems I am familiar with the following items are included.
A single GPS antenna. (Sorry Kevbo just one not two)
A GPS reciever
A yaw rate sensor I referred to it as a gyro before, it is not in fact a gyroscope, but a solid state unit that tells the difference between headings.
A compass.
A map database either on a hard drive or DVD based. (early cars were CD-Rom based)
A feed from the Transmission Control Unit for gear position (sticks use back up light switch)
A vehicle speed sensor signal
Why all of this stuff?
GPS antenna and receiver. To determine position and track as you drive.
Compass to determine heading.
TCM feed and vehicle speed, and Gyro. For dead reckoning when GPS is not available. Also GPS does not respond well to very small changes in direction or distance.
Picture this:
If I park in an underground parking garage, I have no GPS sats available. Yet when I start my car and program a destination, it will show the map, and my track starting at the nearest road. I will hear a message that says, "Route guidance will begin when you join the highlighted route. The cursor will be shown in the correct relative position to the road, as the system recalls where it was shut off last (more on this later) As I drive around and around in the underground parking, the system shows my car turning and moving due to the feed from the TCM, The vehicle speed, and the compass.
Now if the unit is powered down for a long period of time (battery disconnected for storage) the system will forget where it is. On a Volvo the default location is somewhere in Queens New York a couple of blocks from the docks. I have no idea why this is, but when the system is confused this is where it runs home to.
Even though it has it’s location all fucked up, the heading is still correct. North is still shown as north. This is due to the on board compass.
To get the car to re-orient, I have to pull outside where I can get a clear shot at at least 3 sats and wait a bit. Some times driving around can also help. (it will maybe pick up more sats) Depending on location it can take up to 15 minutes for the system to re-orient. During this entire time the on board compass is correct.
What’s fun is to drive around during that time and program in your address to find out just how far you are from Queens NY, and how long it will take to drive back “home”. 
I hope this clears up some of the confusion.
If you had multiple antennas, it may be easier to use the phase difference to calculate which direction the radio signals are coming from.
Actually I’ve always wondered how difficult it would be to implement this method. Does anyone know if it’s been done?