kalman filter to improve the gps data received from an iPhone
in [DSP]
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From: andreivig on 25 May 2010 10:28 Hi, I am currently working on an navigation application for an iPhone device. The GPS data that I receive is not very accurate and I want to use a kalman filter. The problem is that I don't really know how to build the kalman equations for x and y coordinates. What kind of model should I use?
From: Tim Wescott on 25 May 2010 11:29 On 05/25/2010 07:28 AM, andreivig wrote: > Hi, > > I am currently working on an navigation application for an iPhone device. > The GPS data that I receive is not very accurate and I want to use a kalman > filter. The problem is that I don't really know how to build the kalman > equations for x and y coordinates. What kind of model should I use? Without having some sort of independent measurement or estimate of motion, there's not much that a Kalman filter can do for you. If you know the phone isn't moving then the best you can do is also simple: just average a whole bunch of position readings. If you have no guarantee that the phone is staying still then you need more data. Usually this is done with a 6-axis inertial measurement unit (three accelerometers and three gyros). But you need very accurate devices -- you can spend thousands of dollars on your IMU and still have it be insufficient for all but specialized uses. Worse, the IMU with electronics but without batteries occupies at least a dozen cubic inches, much more than an iPhone, and that's without the considerable battery pack you'd need to carry it around. Combining GPS with odometry and a compass may work, but would be awkward. What resources does the phone give you beyond the GPS? I understand that there is some inertial measurement going on, but my understanding is that it is more for gesture recognition, which really doesn't put it in a class to work for this sort of thing. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: claudegps on 25 May 2010 12:00 [cut] > What resources does the phone give you beyond the GPS? I understand > that there is some inertial measurement going on, but my understanding > is that it is more for gesture recognition, which really doesn't put it > in a class to work for this sort of thing. There should be just a 3 axis accelerometer, a very cheap/small one. You can't get much help from that... :) Best way may be to improve the GPS reception using A-GPS and try have a good visibility of the sky.
From: Tim Wescott on 25 May 2010 12:10 On 05/25/2010 09:00 AM, claudegps wrote: > [cut] >> What resources does the phone give you beyond the GPS? I understand >> that there is some inertial measurement going on, but my understanding >> is that it is more for gesture recognition, which really doesn't put it >> in a class to work for this sort of thing. > > There should be just a 3 axis accelerometer, a very cheap/small one. > You can't get much help from that... :) > > Best way may be to improve the GPS reception using A-GPS and try have > a good visibility of the sky. Not just an accelerometer. Where you really need help with the combined IMU/GPS solution is in the gyro, which has to be good relative to the speed at which the earth-relative acceleration vector on the IMU and antenna is changing (i.e. the more jerk there is, and the more different directions it occurs, the better off you are). You _could_ get by with a cheap set of gyros, but only if you had a lot of guaranteed motion of the platform -- and even then a cheap gyro is a heck of a lot less cheap than a cheap accelerometer! -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: claudegps on 25 May 2010 13:41
On 25 Mag, 18:10, Tim Wescott <t...(a)seemywebsite.now> wrote: > On 05/25/2010 09:00 AM, claudegps wrote: > > > [cut] > >> What resources does the phone give you beyond the GPS? I understand > >> that there is some inertial measurement going on, but my understanding > >> is that it is more for gesture recognition, which really doesn't put it > >> in a class to work for this sort of thing. > > > There should be just a 3 axis accelerometer, a very cheap/small one. > > You can't get much help from that... :) > > > Best way may be to improve the GPS reception using A-GPS and try have > > a good visibility of the sky. > > Not just an accelerometer. Where you really need help with the combined > IMU/GPS solution is in the gyro, which has to be good relative to the > speed at which the earth-relative acceleration vector on the IMU and > antenna is changing (i.e. the more jerk there is, and the more different > directions it occurs, the better off you are). I understand. I was referring that the iPhone, as far as I know, does not have gyro but only the accelerometer. Not sure about the magnetometer... I think some Nokias have it, but no gyro around. Take into account MEMS accelerometer are in 3$ range for 1k pcs... gyro are at 20$ !! :) (very rough pricing.... just to give an idea) > You _could_ get by with a cheap set of gyros, but only if you had a lot > of guaranteed motion of the platform -- and even then a cheap gyro is a > heck of a lot less cheap than a cheap accelerometer! Having a complete set of inertial measurements could certainly help. I think that if those hardware would be available, then the iPhone SW (or the GPS sw... wherewere the main KF runs...). should already be able to do a tightly coupled integration of the data for the best performance. PS: sorry if my bad english may lead to misunderstandigs :P |