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How does my Subaru BRZ track me before it has satellite lock?

My car's built-in GPS is well known among owners for being slow/failing to acquire/losing GPS position when starting out. But I've discovered that that without a destination set and before acquiring position, it can track me leaving home for a few turns, despite my trying different departures to fool it. But then then gets lost and gives up within half a mile or so.

How does it do this? Since I usually leave the house the same way, I've tried leaving in the other direction and making different choices at subsequent intersections. It follows me correctly for a short way but then it gives up until it acquires satellites.

I do know that it has a speed input from the car, but it would need some directional input to supplement that - compass, gyro, or even steering input, but all of it seems a bit beyond. I get the choice of a speed input to improve dead-reckoning during signal losses, but directional inputs seem a bit exotic and a frivolous expense (given that ground inputs + map still only gets me 1/2 a mile further than no supplemental information).

I'm not asking for speculative answers or clever designs (that's another question for another forum :) but whether anyone knows what this car actually does. I believe the head-unit is a Fujitsu-10.


  • Tim 1456 Points
    Even though you asked for no speculative answers, I'm going to offer one. I have no idea exactly which model GPS you have, but many made by Fujitsu Ten have a gyro. It probably doesn't work well in the scenario you described as a loss of signal is a bit different of a scenario from not having one yet. I suspect if your GPS lost signal while you were in the middle of a trip it would also "give up" after a similar amount of time.
  • Boyd 1725 Points
    edited November 2016
    Well you said that it has "dead reckoning", and what you describe is the definition of dead reckoning.

    And if you don't want speculative answers, you came to the wrong place! :O)
  • Dead reckoning requires some kind input. No inputs? Well, that's where the term "dead" comes from. :) Ded(uced) Reckoning is based on velocity, time, and direction. Ya gotta have all three, and that is the basis of my question.

    - Velocity:
    I know the head unit gets a speed signal from the car. My Garmin uses the last calculated speed when the signal drops (e.g., in a tunnel); it doesn't even try before it first acquires one.

    - Time: Pretty much any CPU can deliver a time base; only relative time is needed ("How long have I been moving at velocity V?" in traditional navigation; and is good enough for tunnels and other temporary signal loss, in car navigation. Apparently the nav-unit in my car tries to integrate V over time as a refinement.

    - Direction: Where does that come from? Without it all we can know is how much accumulated distance we've covered, whether we've driven directly away from the starting point or circled the block several times.

    - The GPS navigator has another trick up its sleeve: It has a map of roads so it can assume a lot about where I must not be, which the traditional navigator can't do. But it still can't know which turns I've taken without some kind of direction input. Even GPS doesn't give my heading; it can only deduce which way it's traveling when it's moving. In a "Start from parked, no signal" situation, it can't even know my direction of departure. I have a small parking area and always park nose-out, so it could learn my initial habits. But I almost always drive out to the right so I've driven out to the left to test this situation - and it knows. I've varied my subsequent route away - and it knows. If it can Ded Rec with a map to restrict its choices, and either direction or turn inputs, it should be able to do better than I could do with only a watch and a compass, but it doesn't. It gets lost in a few minutes, then sits in a field somewhere waiting for satellites.

    Tim: A gyro (plus knowledge of driver's habits) could help here. It wouldn't give my starting direction, but an assumption based on my habits could. That may be what it is doing. Perhaps I'll park on the street overnight once, so I can, with a "cold" GPS, depart crossways to my usual starting direction and see what it does.

  • Boyd 1725 Points
    edited November 2016
    I think these functions are all handled my the chipsets in current phones for example. They have an accelerometer that can tell the speed and a triaxial compass that tells heading. Do you have some sort of connectivity also (my VW has something called "CarNet")? If so, then cell signals can also be used for location.

    Maybe I'm missing something, but none of this seems too far fetched. I had a TomTom GO 920 many years ago and it had dead reckoning capability that used an internal accelerometer. It "sort of" worked in situations where GPS signal was lost, but not very accurate. I think modern chips would be much better at this.
  • No connectivity other than satellite that I know of. But you're right; it clearly gets more inputs from somewhere, than my old Garmin does. Just that the difference between what it does with those (or what I think those must be...) falls short of what seems like it should be able to do. That in itself is a clue - I just don't know how to interpret it yet. :)
  • Boyd 1725 Points
    edited November 2016
    Garmin started putting accelerometers and/or triaxial compasses in the Nuvi quite awhile ago, but only in the high-end models. I believe the first was the Nuvi 3790. When was that… 2010 maybe?? It was discussed quite a bit in the forums at the time, and some people claimed it was used to show your vehicle heading more accurately than a satellite fix could. I remember someone gave an example of swerving around in a parking lot and watching the Nuvi screen rotate more responsively than the once-per-second satellite fix.

    I had a 3790 and was never so convinced about this myself however. The accelerometer is how the first iPhones could switch between landscape and portrait view, but I think Garmin used the triaxial compass to do the same thing. They added a compass to the iPhone several generations ago, perhaps the iPhone 5?

    Anyway, I think these techologies are cheap widely and used now, probably all available in a single chip.
  • "Garmin started putting accelerometers and/or triaxial compasses in the Nuvi quite awhile ago,"
    And that's probably what this built-in does, but with those, it's ded-rec should be better - almost as good, if not as good, as with full GPS information, since in most cases it can use the map to limit or correct its errors.
  • Tim 1456 Points
    JRobert said:

    but with those, it's ded-rec should be better - almost as good, if not as good, as with full GPS information

    It could be, yes. I suspect it isn't something the GPS companies feel is worth prioritizing their engineering resources for.
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