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Why the need to be stationary to get a quick fix?


Fuchsiamagic
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While approaching a cache site in my car today, I suddenly remembered that my GPSr wasn't switched on. I turned it on, left it near the windshield and kept driving. It took a good five minutes to lock onto it's position.

 

I know we are always told to stand still with it in order for it to lock on to the satellites quicker, and in most circumstances, it usually does this in less that 30 seconds, quicker if it's near the same spot where it was last used.

 

Why does this happen? After all, the satellites up there are all whizzing round us at quite a speed, so why does my GPSr need to be stationary with respect to the earth in order to get a quick fix? (if you will pardon the expression).

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While approaching a cache site in my car today, I suddenly remembered that my GPSr wasn't switched on. I turned it on, left it near the windshield and kept driving. It took a good five minutes to lock onto it's position.

 

I know we are always told to stand still with it in order for it to lock on to the satellites quicker, and in most circumstances, it usually does this in less that 30 seconds, quicker if it's near the same spot where it was last used.

 

Why does this happen? After all, the satellites up there are all whizzing round us at quite a speed, so why does my GPSr need to be stationary with respect to the earth in order to get a quick fix? (if you will pardon the expression).

The satellites are barely moving relative to your position. They are in geosynchronous orbit. If you could see them from the ground, they would not appear to be "whizzing around us" - rather, they would appear to be stationary.

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While approaching a cache site in my car today, I suddenly remembered that my GPSr wasn't switched on. I turned it on, left it near the windshield and kept driving. It took a good five minutes to lock onto it's position.

 

I know we are always told to stand still with it in order for it to lock on to the satellites quicker, and in most circumstances, it usually does this in less that 30 seconds, quicker if it's near the same spot where it was last used.

 

Why does this happen? After all, the satellites up there are all whizzing round us at quite a speed, so why does my GPSr need to be stationary with respect to the earth in order to get a quick fix? (if you will pardon the expression).

The satellites are barely moving relative to your position. They are in geosynchronous orbit. If you could see them from the ground, they would not appear to be "whizzing around us" - rather, they would appear to be stationary.

Nope, they are not geostationary (the WAAS ones are, though). They're in a polar orbit. I believe the orbits were carefully designed as to give coverage over most or all of the world for at least part of the day before the entire constellation of satellites was launched and operational.

 

As to the OPs question, your GPS knows the path of the satellites and it knows the signal timing received from those satellites, but it can't know what you're doing if you're moving around. You're adding another variable into the equation.

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Actually, the biggest factor in how long it took to get a fix this time was the fact that you had driven some distance from the place you had the last fix. The fact you were also moving simply complicated it a little bit more.

 

The reason it is suggested that you stand still when looking for a fix is not the length of time it might take to get a fix, as long as you are near the place you had your last fix. If my last fix was when I drove into my driveway and turned off the receiver, the next time I use it in the car I can turn it on just as I start to back out of the driveway and it makes no difference whether I sit there or start driving down the street .. it will take the same amount of time to get a fix. The reason for the recommendation of not moving is that the longer you stand still, the more accurate your fix can become because the receiver (some receivers) can "average" the location readings over a period of time.

 

Increased accuracy, might be useful when geocaching as you approach "ground zero" (it really depends on whether the person who placed the cache also had a really accurate "averaged" reading .. we'll never know because there is no requirement for cache placers to say whether they "averaged" the reading and/or used WAAS, so nobody includes that info).

 

High accuracy (within reasonable limits) is not terribly relevant for vehicle navigation.

 

...ken...

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Also depends on how long the receiver has been off. This too affects the validity of the almanac data which the receiver uses to locate the satellites -- so it can download the detailed ephemeris data which allows it to get a fix. To oversimplify a bit, almanac data is good for several months. Ephemeris data is good for about thirty minutes. Moving more than a few hundred miles or losing (reasonably) accurate time also invalidates the almanac.

 

This is why a warm start (after turning the unit off briefly) gets a very quick fix -- almanac and ephemeris data are still good. If the ephemeris data is invalid but the almanac is good, you get a cold start -- which simply involves downloading the ephemeris data from satellites which can be readily located. If the almanac data is bad, the receiver will have to search for satellites, download the almanac data (from any one of them), and then download the ephemeris data from each individual satellite it intends to use. When you need almanac data (when the unit is new, after a firmware update, after a reset, or when it's been off for a long time), the typical recommendation is to let the unit sit still with a clear view of the sky for 20 minutes. Some receivers allow you to speed up the process of acquiring the almanac by allowing you to specify your approximate location on the map.

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While approaching a cache site in my car today, I suddenly remembered that my GPSr wasn't switched on. I turned it on, left it near the windshield and kept driving. It took a good five minutes to lock onto it's position.

 

I know we are always told to stand still with it in order for it to lock on to the satellites quicker, and in most circumstances, it usually does this in less that 30 seconds, quicker if it's near the same spot where it was last used.

 

Why does this happen? After all, the satellites up there are all whizzing round us at quite a speed, so why does my GPSr need to be stationary with respect to the earth in order to get a quick fix? (if you will pardon the expression).

 

Simply put, it's easier to zero in on a non moving target. Here is where I can be utterly wrong. When getting a fix the GPS assumes where you are (which is where you were when you shut off the GPS last), checks that postion against a quick look at the satalite. No match, it makes a calculated guess and checks again. If you are moving the guesses take longer to zero in.

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While approaching a cache site in my car today, I suddenly remembered that my GPSr wasn't switched on. I turned it on, left it near the windshield and kept driving. It took a good five minutes to lock onto it's position.

 

I know we are always told to stand still with it in order for it to lock on to the satellites quicker, and in most circumstances, it usually does this in less that 30 seconds, quicker if it's near the same spot where it was last used.

 

Why does this happen? After all, the satellites up there are all whizzing round us at quite a speed, so why does my GPSr need to be stationary with respect to the earth in order to get a quick fix? (if you will pardon the expression).

 

Simply put, it's easier to zero in on a non moving target. Here is where I can be utterly wrong. When getting a fix the GPS assumes where you are (which is where you were when you shut off the GPS last), checks that postion against a quick look at the satalite. No match, it makes a calculated guess and checks again. If you are moving the guesses take longer to zero in.

 

According to information that I looked up on the Joe Mehaffey and Jack Yeazel website (there is a link to the website from geocaching.com) they say that nobody is 100% sure why it takes longer to achieve an initial fix if you are moving but they think that it involves sending the GPS data thru a Kalman filter...which I think is similar to what Renegade Knight posted above.

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Changing positions relative to the satellites because you are moving around has nothing to do with it. The satellites orbit the earth at 11,000 miles and complete an orbit in just under 12 hours. (Reference link) If I’ve done the math correctly, that means that the satellites are moving at about 5800 miles per hour around the earth. Depending on where you are on earth, the earth’s surface may be moving in the direction of rotation at up to 1000 miles per hour (at the equator). So even when you are stationary on the surface of the earth, the position of the receiver with respect to the satellites is changing at thousands of miles per hour. Any velocity you might add by moving on the surface of the earth is insignificant by comparison.

 

twolpert got pretty close to the correct reason. He just didn’t give the whole thing. The following quote from this page explains why it takes longer to lock if you’re moving. The article was written about an old model of receiver, but the information regarding ephemeris data is valid for any receiver.

 

If power is cycled OFF on a GPS receiver, then fairly immediately ON, then the Ephemeris data is still "fresh", and lock-on will be very quick since the GPS does not have to collect new Ephemeris data. This is called a "warm" start. As noted above, "fresh" can mean up to several hours depending on information contained in the Ephemeris data packet for a particular SV. On the other hand, if the Ephemeris data is "stale" (as can be determined from data within the Ephemeris data packet) then the GPS will collect another Ephemeris data packet for any SV for which it has stale data. (Actually, the GPS is always trying to collect fresh Ephemeris data.)

 

If less than 3 SVs have "fresh" Ephemeris data when a unit is first turned ON, lock cannot occur until at data from at least one additional SV must be acquired before the GPS can lock. Note: With SOME Garmin models, when the GPS is powered OFF twice without getting a lock, the GPS software "throws out" any Ephemeris data it may have assuming it is stale.

 

The Ephemeris data comes in 3 packets. The packets are linked together by an "issue of data" identification which, if the receiver properly checks the IOD, will ensure that the packets are all from the same data set. In this way collection order does not affect the outcome. This allows Garmin receivers (and others) to be able to "piece together" these three data packets from two or three data transmissions in case part of a packet stream is received damaged. This ability results in the fastest possible lock under adverse conditions.

 

If you are "motoring along" lock can take MUCH longer than if you are "at rest" and out in the clear. This is because the GPS must receive a COMPLETE Ephemeris data packet ERROR FREE before it will make the signal block "solid" indicating "lock" with a given SV. Momentary tree blockage, making a turn and momentarily losing signal from a given SV, multipath from buildings, mountains, etc. can all cause errors which will cause the GPS to have to start over collecting one or more data messages which are essential to lock.

 

Actually, the biggest factor in how long it took to get a fix this time was the fact that you had driven some distance from the place you had the last fix.

My understanding has always been that this is insignificant unless the distance is several hundred miles. Given that the satellites are orbiting 11000 miles above you, moving a few miles doesn’t change your position relative to them much at all.

 

The reason it is suggested that you stand still when looking for a fix is not the length of time it might take to get a fix, as long as you are near the place you had your last fix.
I respectfully disagree. It is for the reason stated in the last paragraph of the web page quote above: the potential interruption in the reception of ephemeris data.

 

If my last fix was when I drove into my driveway and turned off the receiver, the next time I use it in the car I can turn it on just as I start to back out of the driveway and it makes no difference whether I sit there or start driving down the street
If the next time you use your car is soon enough that the unit has fresh ephemeris data for enough satellites to calculate a position, that would be true. If not, it may take longer to gather the ephemeris data and acquire a fix. My own experience bears this out.
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Well thanks to everybody for your replies. I understand a bit better now, but I was surprised that so many people have different ideas as to what the correct answer is! The most accurate interpretation seems to have come from those in the satellite industry who know what they are talking about, so many thanks to them.

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