Benefits of newer units
1 : time. Sometimes my pharos unit takes forever to get its initial lock. At least it used to when I ran it on a laptop in the passenger seat. seems faster now that im on a powered usb hub
2 : Accuracy. It's pretty accurate already but at times it simply doesn't know where I am. I turn left and it says I'm still going straight or says offroute because it thinks im in a field somewhere. Mostly happened with MS streets n trips
3 : Ammount of satellite connections. With newer units you can connect to an insane ammount of satellite. 11!. The most i've ever gotten on my older gps unit is 7. an averge is 4.
4 : Sirf III provides better building penetration. IE you'll get a signal in places where you didnt get it before. With my current unit its odd for me to get a lock while in my garage with the door closed. But with Sirf III I have gotten 8 satellite lock.
2.) Never had that problem. Ever.
3.) I usually get 7 or 8. What would the advantage of 11 satellites really be over 7 or 8? As long as you have 3, you are good. The rest are backup really. But if 8 satellites get blocked, where are you driving! :)
4.) In my garage door closed I get a lock. Not sure how many satellites but I think it is only 3 or 4. But I know where I am in my garage! ;) Do you get a lock in a parking garage? I lose a signal when I go into a parking garage, but since it is all cement and steel, I figured it was normal.
By comparing the signal from 11 satellites, the receiver has a larger "reference" to determine if any of the signals deviate significantly from the mean. Many variables that can affect GPS signal path length, including upper atmospheric conditions and the angle of the satellite above the horizon (the lower the angle, the more atmosphere the signal must go through before reaching your receiver). So, if your 3D fix is based on only 3 satellites, and the signal path length from one of those satellites is excessively long, your position will be in error, and the GPS reciever won't have any way of determining the error exists. Similarly, 11 satellites would provide even better resolution than 7 or 8, although I don't know how MUCH better.
FYI, from wikipeida:
Multilateration is commonly used in civil and military surveillance applications to accurately locate an aircraft, vehicle or stationary emitter by measuring the time difference of arrival (TDOA) of a signal from the emitter at three or more receiver sites.
If a pulse is emitted from a platform, it will arrive at slightly different times at two spatially separated receiver sites, the TDOA being due to the different distances of each receiver from the platform. In fact, for given locations of the two receivers, a whole series of emitter locations would give the same measurement of TDOA. Given two receiver locations and a known TDOA, the locus of possible emitter locations is a hyperboloid (a surface approximately shaped like two cones joined at the points). In simple terms, with two receivers at known locations, an emitter can be located onto a hyperboloid. Note that the receivers do not need to know the absolute time at which the pulse was transmitted - only the time difference is needed.
Consider now a third receiver at a third location. This would provide a second TDOA measurement and hence locate the emitter on a second hyperboloid. The intersection of these two hyperboloids describes a curve on which the emitter lies.
If a fourth receiver is now introduced, a third TDOA measurement is available and the intersection of the resulting third hyperboloid with the curve already found with the other three receivers defines a unique point in space. The emitter's location is therefore fully determined in 3D.
In practice, errors in the measurement of the time of arrival of pulses mean that enhanced accuracy can be obtained with more than four receivers. In general, N receivers provide N-1 hyperboloids. When there are N > 4 receivers, the N-1 hyperboloids should, assuming a perfect model and measurements, intersect on a single point. In reality, the surfaces rarely intersect, because of various errors. In this case, the location problem can be posed as an optimization problem and solved using, for example, a least squares method or an extended Kalman filter.
Additionally, the TDOA of multiple transmitted pulses from the emitter can be averaged to improve accuracy.
Well yes more satellites is more accurate, but the inherent inaccuracy of GPS is the 1Hz signal. It is too slow to make the more satellites really that more accurate.
Lets say at t=0, you are at point A on a road which is the centre lane of a 3 lane road driving north. At t=1 which is 1 second later, you will have traveled some distance x north. That distance is the error due to the 1Hz signal. Now take that distance and draw a circle with radius x, and you have the position that you could be in at any time before it samples again. The faster you are going the more error there can be. So you could be at A + x, A - x in the horizontal and vertical planes.
The only advantage of more satellites in my eyes would be for speeds less than your average error in meters per second. So for this unit I think it advertises 2m-3m accuracy. So unless you are going less than 5-7mph, the error is negligent.
Does my bassackwards way of thinking sorta make sense? I know what I mean, but hard to put to words...
I realize that some GPS pucks may not update the data stream any more frequently than 1/second, but that doesn't mean the timing signal is beng received only 1/second.