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Minimum sats. for position lock-60Cx?


dhatw
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I have spoken to 2 different techs at Garmin and both say that you only need 3 satellites to get a location.

I have had my 60Cx wait for 4 bars on more than one occasion before it gives a position. It even shows virtually 3 full bars but will hold on for that 4th satellite. Anyone else notice this? Garmin has no answer.

 

Thanks in advance.

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That's interesting that the Garmin Techs say you only need three sats to get a location. You can read this in Garmin's own GPS guide for beginners (get it here: http://www.garmin.com/manuals/GPSGuideforB...ners_Manual.pdf): "to determine postion using pseudo-range data, a minimum of four satellites must be tracked". Maybe the Garmin techs should read through the Garmin beginner's guide?

 

There's also a good explanation of why the fourth satellite is necessary at Trimble.com: read through the GPS tutorial at http://www.trimble.com/gps/index.shtml.

 

I have spoken to 2 different techs at Garmin and both say that you only need 3 satellites to get a location.

Edited by Michael Cook
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I have a Garmin 60CX and and 60CX and neither one of them will get a lock until they have 4 satellites. I looked at the manual and it says 3, but the only thing I can figure is that there must be a setting somewhere to select 2D fix. Maybe then it would only need 3 since I think the 4th satellite give you elevation.

As mentioned above I know with Magellans 3 sats will give you 2D and 4 sats will give you 3D. I just wanted to make sure that the Garmin techs did not know what they were talking about. Just kidding if they are reading this. I also asked the last tech about almanacs and he said he never heard of it and they never sent anything down about almanacs to him. Oh well, the 3 or 4 sats is not a huge deal as I usually get more anyways, I just wanted to make sure there was nothing wrong with my unit-my 60Cx that is.

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I don't have my GPSr here with me, but IIRC, you can get a 2D lock with just 3 satellites on a 60C if you tell the GPSr where you are to start with. If you have it set to search for your starting location on power-up, it needs the 4th satellite.

 

All the GPSr knows about any satellite is exactly how far away it is. If you think about it in a spacial context, that information describes a sphere, and the receiver know that it is somewhere on that sphere. With two satellites, it can find the intersection of two spheres, which describes a circle. Three satellites describes two point on that circle. Since the Earth usually lies on only one of those two points, you would think that the GPSr could use the geoid of the Earth to find its location. Actually, it will do that, but it uses the hypothetical WGS84 geoid to rule out the outlying point. It still doesn't know its actual altitude above the geoid until it gets its 4th lock.

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I don't have my GPSr here with me, but IIRC, you can get a 2D lock with just 3 satellites on a 60C if you tell the GPSr where you are to start with. If you have it set to search for your starting location on power-up, it needs the 4th satellite.

 

All the GPSr knows about any satellite is exactly how far away it is. If you think about it in a spacial context, that information describes a sphere, and the receiver know that it is somewhere on that sphere. With two satellites, it can find the intersection of two spheres, which describes a circle. Three satellites describes two point on that circle. Since the Earth usually lies on only one of those two points, you would think that the GPSr could use the geoid of the Earth to find its location. Actually, it will do that, but it uses the hypothetical WGS84 geoid to rule out the outlying point. It still doesn't know its actual altitude above the geoid until it gets its 4th lock.

 

Yeah, what Sputnik 57 said. Anyway, I guess I have an answer to my question. There is nothing wrong with my GPSr. Thanks for all your imput.

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It happens to be like this:

 

There are four unknowns, X, Y, Z and time. To solve for these four values, you need four equations, in this case represented by satellites.

 

If only three are available, then the GPS can manage anyway, by making the often fairly correct assumption that your elevation (Z) is the same as it was last time it was on. Thus only three unknowns remain to find.

The fact that you often don't get any lock with three satellites, is that usually, the reason that you can get only three is that buildings or trees are obstructing the view of the sky. Hence, the satellites that still can be received, are probably pretty high on the sky, and grouped rather tight together, or perhaps more or less on a line. The latter often happens in a street, where there are high buildings, since you can only see a line of the sky.

This kind of satellite arrangement will be so unfavorable, that the GPS will find the DOP value too high, and thus defer the positioning until it gets the distance from one more satellite. Then, it may actually be a 2D solution you get, in spite of having four satellites, since two of them may be too close to each other to be useful as anything but one single satellite.

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I had a 60CS and now a 60CSx. This is just a general impression, but it seems to me that the 60CS would give a 2D position when it had acquired 3 satellites, but that the 60CSx usually waits till it has has 4 or more and has a 3D position. The acquisition logic is probably quite a bit different in SiRF chip, but it also could be just that it is acquiring a lot more satellites faster than the 60CS did. I believe I've also seen the 60CSx go into a 2D mode when the number of satellites dropped to 3, but I haven't tested for that specifically; its hard to get it to drop to just 3.

 

Last year before the 60/76 "x" models came out I was reading some information on the SiRF logic, and it was pretty interesting: evidently, if the number of satellites drops to 2 or even 1 it will continue tracking making assumptions about your movement. In my own case, while I was doing some "tunnel" tests I noticed that when it suddenly lost all satellites it would sometimes snap to the road (even though I had Lock on Road Off) and continue along at the last speed for a while.

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With my older gps's they state that if you want a better postion with a 2d fix you can input the elevation. This confirms apersson850's take on things. He's a smart guy and worthy of listening to as an expert anyway.

As the above post points out, the new 60cx with the sirf chip does not do the 2d fix thing but waits for a 3d fix. That's what I've seen too. Sirf chips act differently in many ways. I've read that in some units they defer to static navigation in tough spots. In otherwords, they lock postion and don't navigate, but I know this is disabled in many units, hopefully ours.

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A GPSr will only give you a position after it has determined it can not immediately track more satellites. For example, If you are in a canyon and your GPSr can only track 3 satellites after say... a minute of being on, it will make do with 3 sats.

 

I am pretty sure that while driving to work on more than one occasion my 60Cx had 3 sats solid and coould not get a solid fix on any more and after a while it displayed "poor satellite reception use with GPS off?".

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One sat enables the gps to know how far away it is from that one satellite (valid points are on a sphere of that distance). Two Sats define a second distance away from that sat so now the only valid places the GPS could be is the intersection of the two spheres (this is a circle, 2 dimensions). The third satellites sphere now will intersect that circle in two points and so with three sats there are two possible points the GPS could be. Usually one point is non-sensical (like 30 miles below the surface of the earth) leaving only one that can be the true position. A fourth satelite eliminates the ambiguity of the two points.

 

My 60csx seems to always require 4 to get a good lock. I think my older units did it with three.

 

In THEORY you only need 3 points to get a fix in ANY type of navigation. More is better so I suspect that the microprocessor is programmed to use a min of 4.

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Yes, in theory knowing the distances from 3 points will give you a fix. But how does the GPSr know the distances from the three satellites? It does this by measuring the time each signal takes to get from the satellite to the GPSr. If the GPSr had a clock with atomic precision inside it, 3 satellites would suffice. But it doesn't, so it needs the data from a fourth satellite. This is all explained at http://www.trimble.com/gps/index.shtml. Look specifically at this page: http://www.trimble.com/gps/howgps-timing2.shtml.

 

Once the 60CSx has got a fix using four satellites (3D fix), it can continue giving a reasonable estimate of position with only three satellites (2D fix), by assuming that the altitude doesn't change.

 

In THEORY you only need 3 points to get a fix in ANY type of navigation. More is better so I suspect that the microprocessor is programmed to use a min of 4.

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It happens to be like this:

 

There are four unknowns, X, Y, Z and time. To solve for these four values, you need four equations, in this case represented by satellites.

 

If only three are available, then the GPS can manage anyway, by making the often fairly correct assumption that your elevation (Z) is the same as it was last time it was on. Thus only three unknowns remain to find.

The fact that you often don't get any lock with three satellites, is that usually, the reason that you can get only three is that buildings or trees are obstructing the view of the sky. Hence, the satellites that still can be received, are probably pretty high on the sky, and grouped rather tight together, or perhaps more or less on a line. The latter often happens in a street, where there are high buildings, since you can only see a line of the sky.

This kind of satellite arrangement will be so unfavorable, that the GPS will find the DOP value too high, and thus defer the positioning until it gets the distance from one more satellite. Then, it may actually be a 2D solution you get, in spite of having four satellites, since two of them may be too close to each other to be useful as anything but one single satellite.

 

Great explanation! This is precisely what's going on.

(Ok, one minor correction: GPSs operate in a Cartesian coordinate system where the origin is the center of Earth. So Z is technically not your elevation, but if you assume an elevation you can add another equation for X, Y and Z that restricts possible solutions.)

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It happens to be like this:

 

There are four unknowns, X, Y, Z and time. To solve for these four values, you need four equations, in this case represented by satellites.

 

If only three are available, then the GPS can manage anyway, by making the often fairly correct assumption that your elevation (Z) is the same as it was last time it was on. Thus only three unknowns remain to find.

The fact that you often don't get any lock with three satellites, is that usually, the reason that you can get only three is that buildings or trees are obstructing the view of the sky. Hence, the satellites that still can be received, are probably pretty high on the sky, and grouped rather tight together, or perhaps more or less on a line. The latter often happens in a street, where there are high buildings, since you can only see a line of the sky.

This kind of satellite arrangement will be so unfavorable, that the GPS will find the DOP value too high, and thus defer the positioning until it gets the distance from one more satellite. Then, it may actually be a 2D solution you get, in spite of having four satellites, since two of them may be too close to each other to be useful as anything but one single satellite.

 

Great explanation! This is precisely what's going on.

(Ok, one minor correction: GPSs operate in a Cartesian coordinate system where the origin is the center of Earth. So Z is technically not your elevation, but if you assume an elevation you can add another equation for X, Y and Z that restricts possible solutions.)

 

Here's a visualization of what he means

 

Owell it didn't work when i submitted it.

Edited by Train_Man
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