Significance Of Antenna Sizes

[Wooden Bystander]

Reluctant to be a spendthrift, I've been carefully shopping around for an appropriate GPS unit. It seems that the Geko 201 might be sufficient, and I want things to be affordable as possible.

 

However, I've read that the etrex has greater problems under thick foliage than most GPSrs because of its smaller size necessitating a smaller antenna. Since the Geko is smaller still, I imagine it would have greater trouble still.

 

Is the difference between smaller and larger antennas under thick foliage significant? I'm aware that all units will have difficulties in such circumstances, but naturally I want to avoid making the problem much worse than it needs to be. I don't want to end up spending half the Geko's value on a re-radiating external antenna with a portable power source when I could have used that extra money for a better unit.

 

edit: Note that I live in Australia where we don't have access to such things as WAAS. If this needs to be taken into consideration, feel free to do so.

Edited August 31, 2004 by Wooden Bystander

[+briansnat]

My wife has a Gecko 201 and uses it under heavy tree cover all the time, with few problems. If it does lose a lock, its only for a moment. However if the unit is held correctly (flat, with the face pointing skyward) its rare that it loses a lock. I have had similar experience with my eTrex.

 

If you don't hold the unit correctly, you will probably lose your lock.

[+strikeforce1]

I'm aware that all units will have difficulties in such circumstances, but naturally I want to avoid making the problem much worse than it needs to be.

This may not be the specific answer you are looking for, but it will help. Before heading into heavy cover, use a $5.00 compass to take a bearing to the waypoint. Up here, where the trees have heavy cover, it's almost a norm to use a compass, when sat. lock is lost. If the first bearing doesn't put you on it, try a second and or third. It's called "triangulation". By having at least two lines intrersecting, this way, you should be within 6 ft. of your waypoint. Hope this will help, and save you some money . Happy Caching. SF1

[+CompuCash]

There are two different types of antennas use in the GPS receivers.

 

The flat type or plate antenna must be held such that the surface of that plate is held horizontal to present its greatest area to the satalites (toward the sky).

 

The newer type antenna is referred to as the quad coil. These antennas must typicallly be held such that the receiver is held upright or vertically. The coil antenna is perported to have a higher gain and better reception under cover of trees or clouds.

 

I have a Magellan which uses the quad coil antenna and have had no trouble at all under tree cover.

 

I hope this helps explain the difference and how they are held and used.

[thorin]

My wife has a Gecko 201 and uses it under heavy tree cover all the time, with few problems.  If it does lose a lock, its only for a moment.  However if the unit is held correctly (flat, with the face pointing skyward) its rare that it loses a lock. I have had similar experience with my eTrex.

 

If you don't hold the unit correctly, you will probably lose your lock.

I want to echo Brian's experience exactly. Over the last 6 months I haven't had any problems with my Geko 201 maintaining a lock once establish no matter how thick the tree cover is.

 

The newer type antenna is referred to as the quad coil.

Isn't it called "Quad Helix" ? (Picture a strand of DNA or a ladder with 4 side "rungs" instead of 2)

 

Thorin

Edited August 31, 2004 by thorin

[+CompuCash]

My wife has a Gecko 201 and uses it under heavy tree cover all the time, with few problems.  If it does lose a lock, its only for a moment.  However if the unit is held correctly (flat, with the face pointing skyward) its rare that it loses a lock. I have had similar experience with my eTrex.

 

If you don't hold the unit correctly, you will probably lose your lock.

I want to echo Brian's experience exactly. Over the last 6 months I haven't had any problems with my Geko 201 maintaining a lock once establish no matter how thick the tree cover is.

 

The newer type antenna is referred to as the quad coil.

Isn't it called "Quad Helix" ? (Picture a strand of DNA or a ladder with 4 side "rungs" instead of 2)

 

Thorin

Oops - you are correct with the teminology - sorry if I missled any one - my understanding is that it is a quad coil arrangment but I might have that wrong.

[thorin]

Don't worry about it, I'm going from memory here too. I just seemed to recall reading "Quad Helix" all over the place <shrug>

[+Renegade Knight]

The quad helix is better under tree cover. The patch (like in your gecko) is better in the "urban jungle". Neither has enough of an edge to where the antanea should be the deciding factor on the GPS.

[+DavidMac]

I used to have an Etrex, which I usually had pretty good luck with even under tree cover. Once, I got to test this out while trying to mark a waypoint in rough terrain under damp tree cover with both the vista and a StreetPilot III (which I think has a quad-helix antenna, but don't quote me on that. Sat coverage on the vista was decent, while the streetpiolt's performance under canopy was qyestionable. My 60CS, which I believe has a quad helix antenna (the documentation says "quadrifilar" which I think is the same thing- hope I have my terminology straight!) is a wh0L3 tvv0 1nch3s l0ng3r!!!! but only gets slightly better coverage in the woods than either unit, but not really enough to make a noticeable difference.

Edited August 31, 2004 by DavidMac

[+CompuCash]

I used to have an Etrex, which I usually had pretty good luck with even under tree cover. Once, I got to test this out while trying to mark a waypoint in rough terrain under damp tree cover with both the vista and a StreetPilot III (which I think has a quad-helix antenna, but don't quote me on that. Sat coverage on the vista was decent, while the streetpiolt's performance under canopy was qyestionable. My 60CS, which I believe has a quad helix antenna (the documentation says "quadrifilar" which I think is the same thing- hope I have my terminology straight!) is a wh0L3 tvv0 1nch3s l0ng3r!!!! but only gets slightly better coverage in the woods than either unit, but not really enough to make a noticeable difference.

That is where I got it -

 

"quadrifilar"

 

bifilar refers to a coil wound with 2 wires - quadrifilar is a coil wound with 4 wires

[thorin]

Ok so are there actually 3 standard antennas in use then?

 

Thorin

[+as77]

No, there are only two. The patch and the quad helix.

 

The Geko 201's antenna is not worse than that of the Etrexes. I think it's the same antenna. But the Geko is a newer unit and I read somewhere that because of the better software, it actually keeps the lock better than the Etrexes.

[+Nurse Dave]

Okay, I have to throw out the opinion of anyone who says "I've never lost lock under trees" or "I've never had a problem under heavy tree cover". Then you haven't been in heavy tree cover!!! Jeesh, if you can't see the sky at all, then that's heavy tree cover and no matter what kinda antenna you have you can't get a signal. And for those of you that are going to argue about that, then I'll wait to hear how you get WAAS in your basement.

[+briansnat]

Okay, I have to throw out the opinion of anyone who says "I've never lost lock under trees"  or "I've never had a problem under heavy tree cover".  Then you haven't been in heavy tree cover!!!  Jeesh, if you can't see the sky at all, then that's heavy tree cover and no matter what kinda antenna you have you can't get a signal.  And for those of you that are going to argue about that, then I'll wait to hear how you get WAAS in your basement.

This photo was taken last week. Was it at night? Nope, it was taken about 3 pm on a bright, sunny day...under a leaf canopy so heavy it seemed like dusk. In fact it was so dark my flash fired. My wife's Gecko and my eTrex both had a solid lock at the cache site.

 

 

And what our region's leaf canopy looks like from above. Our units work just fine down there.

 

No, I don't get WAAS in my basement. Don't even get a lock, but I have gotten a lock with the unit on my living room coffee table, with the blinds closed. Not often, but it does happen.

Edited September 1, 2004 by briansnat

[+EScout]

Its not the size of the antenna, its the gain patten. Patch and Quadrifilar Helix antennas are omni directional (circular polarized for satellites.) The quad helix is made of 2 full wave loops, twisted updward (the currents in the 2 loops are in phase quadrature.) Quad helix antennas have more gain toward the horizon versus the patch antenna, and are better in cover such as trees that block some of the signals overhead.

[+Team Shibby]

We should also remind all Magellan owners that their units are losing signal just as much as the Garmin owners are, the only difference is Magellan continues to average the information it has obtained since its last lock on the satellites.

 

If you are concerned about getting a unit that will work best in your area, your best bet is to go with something with a Quad antenna AND the ability to accept an external antenna. Almost all external antennas sold today are "patch" antennas so you will be getting the best of what both have to offer.

 

Just my $0.02...

 

Kar of TS!!

[+briansnat]

Its not the size of the antenna, its the gain patten.  Patch and Quadrifilar Helix antennas are omni directional (circular polarized for satellites.)  The quad helix is made of 2 full wave loops, twisted updward (the currents in the 2 loops are in phase quadrature.)  Quad helix antennas have more gain toward the horizon versus the patch antenna, and are better in cover such as trees that block some of the signals overhead.

And as I understand it, patch antennas are slightly better at picking up sats directly overhead, which gives them an advantage in canyons, along cliffs and among tall buildings.

 

This being said, the advantages of each antenna are minor and shold not be the primary consideration when buying a GPS.

Edited September 1, 2004 by briansnat

[BassoonPilot]

Okay, I have to throw out the opinion of anyone who says "I've never lost lock under trees" or "I've never had a problem under heavy tree cover".

There has been a substantial amount of good information presented in this thread, and of course you are entitled to discount any opinion that doesn't coincide with your own.

 

Hey, aren't you also the guy that "doesn't like Sara Lee?"

[thorin]

Okay, I have to throw out the opinion of anyone who says "I've never lost lock under trees"  or "I've never had a problem under heavy tree cover".  Then you haven't been in heavy tree cover!!!  Jeesh, if you can't see the sky at all, then that's heavy tree cover and no matter what kinda antenna you have you can't get a signal.  And for those of you that are going to argue about that, then I'll wait to hear how you get WAAS in your basement.

You're definately entitled to your opinion but it does not discount everyone elses experience(s).

 

Thorin

 

Edit: Corrected spacing.....

Edited September 1, 2004 by thorin

[+Thot]

I have to ask this naive question. How do you know if you’ve “lost lock?”

 

I use a Magellan SporTrak Pro and if I’ve ever lost lock I didn’t know it. I check the EPE regularly when the target is wandering, and I’ve seen it quite large in heavy tree cover (forest). But, the distance to target (which I use the most) never stops changing as I move. Does some kind of warning (beeps, notice on the display, etc) notify you that you’ve lost satellite signals?

Edited September 1, 2004 by Thot

[+CompuCash]

I have to ask this naive question.  How do you know if you’ve “lost lock?”

 

I use a Magellan SporTrak Pro and if I’ve ever lost lock I didn’t know it.  I check the EPE regularly when the target is wandering, and I’ve seen it as high as 75 feet in heavy tree cover (forest).  But, the distance to target (which I use the most) never stops changing as I move.  Does some kind of warning (beeps, notice on the display, etc) notify you that you’ve lost satellite signals?

 

I have the same gps - I love it -

 

if you look at the sat receiver screen - if you have only 1 or 2 sats with low signal strenght you can consider yourself unlocked - requires 3 sats to get a basic position - 4 to get altitude.

 

even at 75' you have a pretty poor chance of finding a cache. The generally accepted min. error in postion is considered 30' and you have an error greater than that. That is, when you arrive at the posted coordinates for a cache, your best accuracy is considered to be 30'.

 

Edited September 1, 2004 by CompuCash

[+Thot]

if you look at the sat receiver screen - if you have only 1 or 2 sats with low signal strenght you can consider yourself unlocked - requires 3 sats to get a basic position - 4 to get altitude.

I understand, but using that criteria you have to deduce/suspect you've lost enough satellite data to have useful information. The word lock sounds like a definite thing that’s either on or off. Somehow the way people talk about “losing lock” it sounds you either have it or you don’t. It doesn’t sound like, “Well the quality and quantity of satellite signals suggests I may not be getting useful data.”

 

You can check the book but I think what your are saying is feet is really and estimated position error of .075  seconds which roughtly translates to 75 times 6'.

My unit displays the EPE in feet. Some of these values can be changed in setup. It’s been too long for me to remember if this is one I changed from the default.

 

even at 75' you have a pretty poor chance of finding a cache.  The generally accepted min. error in postion is considered 30' and you have an error greater than that. 

I agree, but “large errors” sounds different to me than “loss of lock.”

 

(Note that I edited my post to remove the 75’ because I came to doubt that was an accurate number. It's the last one I remember, not necessarily the largest.)

 

That is, when you arrive at the posted coordinates for a cache, your best accuracy is considered to be 30'.

I’m sorry, I didn’t follow that.

Edited September 1, 2004 by Thot

[4x4van]

I don't agree with the popular held belief (mainly by Garmin owners) that the Magellan doesn't tell you when it loses "lock" and instead continues to average. The averaging feature and the satelite lock are two entirely different things.

 

Magellans don't "average" unless they do have lock and you are standing still (argue with that all you want, but Magellan actually "tells" you when it is averaging, shown on the position screen). The averaging feature is useful for getting slightly better coordinates while standing still, but is not in effect if you are moving (again, it "tells" you when it is averaging, and that only occurs when standing still).

 

As for losing "lock", the second my ST Pro drops to only 2 satelites (or 3 in 3d mode), it warns me that GPS coverage is poor, and the EPE field (among others) goes blank! Once it regains a 3rd (or 4th, in 3D mode) satelite, position info comes back. That certainly doesn't sound like the unit is "guessing" or "averaging", as Garmin owners would like you to believe.

 

Back on topic, both the quad helix and the patch antennas have their strong and weak points, neither is so much better so as to make the other obsolete.

[+Thot]

As for losing "lock", the second my ST Pro drops to only 2 satelites (or 3 in 3d mode), it warns me that GPS coverage is poor, and the EPE field (among others) goes blank! 

How does it warn you? Do you have to be looking at the EPE field and see it go blank?

[+tirediron]

Just a further thought on the terms "tree coverage" and "canopy". In my experience, the GPSr's ability to hold lock under canopy has a lot to do with the type of canopy. I can walk through an alder forest where there is barely any sky visible at all, and maintain a decent lock with either my Legend of my 76CS, but walking under what appears to be equally heavy cover in a predominately fir forest, both units will lose lock, albeit the Legend more quickly and for longer.

 

My personal opinion is that this relates to both the height of the canopy as well as the the quantity and size of branches. Trees with smaller, light branches but large leaves make a less physically dense canopy than a small leafed (needeled) tree with large heavy branches like a Doug fir.

[+EScout]

I agree with 4X4Van. I have owned and used a Meridiana and Legend for over 1.5 years. I know when my Merdian does not have lock on at least 3 sats because it gives a warning. Under heavy cover, indoors, in a car, in a boat (under cover), the Meridian works much better, based on the numbers of satellites received and locked. The only place I have not had a lock on at least 3 sats with the Meridian, is indoors, although most of the time it does get at least 3.

 

Much of this I attribute to the gain pattern of the Meridian's antenna (it may have a better receiver also.) Satellites overhead are much closer and are received more easily so it usually benefits to have more gain at lower angles.

[4x4van]

As for losing "lock", the second my ST Pro drops to only 2 satelites (or 3 in 3d mode), it warns me that GPS coverage is poor, and the EPE field (among others) goes blank! 

How does it warn you? Do you have to be looking at the EPE field and see it go blank?

A warning pops up on the screen, as well as an audible alarm. You have to go into the menu to "alarms/Msgs" and set the "GPS coverage" alarm to "on". I believe that the default setting is off, but I'm not sure.

[+CompuCash]

As for losing "lock", the second my ST Pro drops to only 2 satelites (or 3 in 3d mode), it warns me that GPS coverage is poor, and the EPE field (among others) goes blank! 

How does it warn you? Do you have to be looking at the EPE field and see it go blank?

A warning pops up on the screen, as well as an audible alarm. You have to go into the menu to "alarms/Msgs" and set the "GPS coverage" alarm to "on". I believe that the default setting is off, but I'm not sure.

that's nice - did not know that -

 

thanks -

 

I watch the compass screen mosty - when I get close or the compass starts jumping around I switch to the position screen. This tells me direct coords. If things get strange or I am marking a waypoint then I switch to the satalite screen to be sure I have good signal. If you try to set a waypoint and don't have 4 sats for 3D you get a warning that it will be a 2d waypoint only and have to confirm.

Edited September 1, 2004 by CompuCash

[+CompuCash]

 

You can check the book but I think what your are saying is feet is really and estimated position error of .075  seconds which roughtly translates to 75 times 6'.

 

My unit displays the EPE in feet. Some of these values can be changed in setup. It’s been too long for me to remember if this is one I changed from the default.

 

you got there before I took that part out - you are correct - it is feet - checked the manual. not sure how you got that quote because there was a lot of time in there. could be you did not refresh your page.

Edited September 1, 2004 by CompuCash

[+Renegade Knight]

...How does it warn you? Do you have to be looking at the EPE field and see it go blank?

A magellan does keep some kind of moving average. This is different from averaging your location. The moving average is what causes the boomerang effect some magellan ownens have before they learn to compensate for it.

 

A sport track pro out of the box will keep the moving average going even if it's lost lock. You can adjust the settings so that it beeps when it's lost lock. By default the pro's don't. I have had others tell me that different magellans don't have the same default setting as the Pro.

 

Since I use a GPS V I can't tell you how to change the setting. Only that you can. Night Stalker has a pro and changed his settings, then after getting tired of all the beeping changed it back.

[+Thot]

A warning pops up on the screen, as well as an audible alarm.  You have to go into the menu to "alarms/Msgs" and set the "GPS coverage" alarm to "on".  I believe that the default setting is off, but I'm not sure.

That's the answer. Thanks. The alarm was off by default, as you said. That's why I've never been aware of a loss of signal.

 

It's on now.

[+Renegade Knight]

I don't agree with the popular held belief (mainly by Garmin owners) that the Magellan doesn't tell you when it loses "lock" and instead continues to average. The averaging feature and the satelite lock are two entirely different things.

 

Magellans don't "average" unless they do have lock and you are standing still (argue with that all you want, but Magellan actually "tells" you when it is averaging, shown on the position screen). The averaging feature is useful for getting slightly better coordinates while standing still, but is not in effect if you are moving (again, it "tells" you when it is averaging, and that only occurs when standing still).

 

As for losing "lock", the second my ST Pro drops to only 2 satelites (or 3 in 3d mode), it warns me that GPS coverage is poor, and the EPE field (among others) goes blank! Once it regains a 3rd (or 4th, in 3D mode) satelite, position info comes back. That certainly doesn't sound like the unit is "guessing" or "averaging", as Garmin owners would like you to believe.

 

Back on topic, both the quad helix and the patch antennas have their strong and weak points, neither is so much better so as to make the other obsolete.

Driving a twisty mountain road using the "Rubberband map" and having spotty reception due to tree cover will teach you that Magellans do have some kind of moving average they use.

 

After watching my GPS V and Night Stalkers Sport Track Pro give conflicting directions on that mountain road, because of how they operate: the difference between the "Moving Average" of the Magellan (call it anything you want) and the Lack of one on the Garmin was apparent. When all was said and done my GPS V was giving me the correct direction and the Magellan wasn't.

 

However in another situation that illistrates the same thing. The cache was on top of a small cliff. The base of the cliff which was the access point was under heavy tree cover. My GPS V would get under that tree cover, lost lock and give me no useful information. The Magellan's (Two of them that day) both kept up their moving average trick and said the cache was on top of the cliff about 15' from the face. The Magellans were right.

 

After a lot of experimending the GPS V and the ST Pro were roughly comporable in gaining lock or losing it in real world conditions.

 

YMMV.

[+Thot]

I may never get to test that alarm. In an attempt to test it I went into a closed bathroom in the center of my house. There were three walls between me and the outside in all directions. Above there's the ceiling, an HVAC unit, the roof decking and the asphalt shingled roof.

 

I stood there about a minute to see if I'd lose the signal, but didn't. This was while holding the unit flat. It's supposed to be held vertical.

 

When I raised it to vertical the unit got 4 satellites solid and a partial other. The EPE was 30 feet

Edited September 1, 2004 by Thot

[+Thot]

The Magellan's (Two of them that day) both kept up their moving average trick and said the cache was on top of the cliff about 15' from the face.  The Magellans were right.

I may not understand what you're saying, but it sounds like you're saying the Magellans continued to track your movements after they no longer had satellite information. If so, I don’t see how that’s possible. Without satellite signal it has no way to know which way you’re moving.

Edited September 1, 2004 by Thot

[+Renegade Knight]

...I may not understand what you're saying, but it sounds like you're saying the Magellans continued to track your movements after they no longer had satellite information. If so, I don’t see how that’s possible. Without satellite signal it has no way to know which way you’re moving.

It's the moving average thing. Magellan will take it's best guess and keep you on your heading even without a lock.

 

So when we turned on that twisty road without a lock, the Magellan gave directions like we didn't turn. Which led to a debate about who's GPS was correct which led to learning a little about the differences between the two brands.

 

Maybe moving average is not the correct term, but there is reason the Magellan did that and the Garmin didn't. In similar circumstancs the Garmin just told me I had no lock and that was that

[thorin]

...I may not understand what you're saying, but it sounds like you're saying the Magellans continued to track your movements after they no longer had satellite information.  If so, I don’t see how that’s possible.  Without satellite signal it has no way to know which way you’re moving.

It's the moving average thing. Magellan will take it's best guess and keep you on your heading even without a lock.

 

So when we turned on that twisty road without a lock, the Magellan gave directions like we didn't turn. Which led to a debate about who's GPS was correct which led to learning a little about the differences between the two brands.

 

Maybe moving average is not the correct term, but there is reason the Magellan did that and the Garmin didn't. In similar circumstancs the Garmin just told me I had no lock and that was that

In addition to the point RK is trying to impress upon you alot of newer GPS also include a magnetic compass so even if the road was twisty it would still approximate your avg. speed plus bearing and continue to provide a location to the best of it's ability without a lock. Obvious your avg speed changes as you travel so the further you've travelled on the train the more accurate it would be. It's kinda a very stripped down version of the inertial guidance system like a plane uses.

 

Thorin

Edited September 2, 2004 by thorin

[+NightPilot]

The 'moving average effect' is called dead reckoning. Many units have it, including the Trimble in my helicopter. If it loses signal, it assumes you keep going in the same direction at the same speed. If you don't, it gets really confused about your position. It can work for you or against you, as RK said, and you have to look at your position with some skepticism if you've lost signal.

[4x4van]

Driving a twisty mountain road using the "Rubberband map" and having spotty reception due to tree cover will teach you that Magellans do have some kind of moving average they use.

 

After watching my GPS V and Night Stalkers Sport Track Pro give conflicting directions on that mountain road, because of how they operate: the difference between the "Moving Average" of the Magellan (call it anything you want) and the Lack of one on the Garmin was apparent. When all was said and done my GPS V was giving me the correct direction and the Magellan wasn't.

 

However in another situation that illistrates the same thing. The cache was on top of a small cliff. The base of the cliff which was the access point was under heavy tree cover. My GPS V would get under that tree cover, lost lock and give me no useful information. The Magellan's (Two of them that day) both kept up their moving average trick and said the cache was on top of the cliff about 15' from the face. The Magellans were right.

 

After a lot of experimending the GPS V and the ST Pro were roughly comporable in gaining lock or losing it in real world conditions.

 

YMMV.

Actually, I believe that the "boomerang effect" of the Magellans have nothing to do with "averaging", and I also don't believe that the unit uses a "moving average". I believe what you are describing is more a symptom of slightly slower processing. In other words, there is a few second delay between the time that Magellan receives/processes information and the time that it actually displays that information. Because of that, your turn directions while on a windy road will appear to be off compared to the Garmin, which I believe is a bit faster in it's reporting. Also because of that delay, the "lost GPS fix" alarm probably occurs a few seconds after actual loss.

 

So the "boomerang effect", I believe, is due to a delay in reporting the processed info, rather than any "moving average", and that delay is only about 3-4 seconds, in my experience. I have also noticed it while driving on winding mountain roads. OTOH, I have occasionally gotten a "lost GPS fix" alarm when driving under a freeway overpass, even though it only took a few seconds to go under and come out on the other side. If the unit was using any kind of "moving average" and guessing at my position for those few seconds, it would not have reported the GPS fix loss.

 

'Course, I could be full of it and completely off-base!

[team_tar]

The 'moving average effect' is called dead reckoning.  Many units have it, including the Trimble in my helicopter.  If it loses signal, it assumes you keep going in the same direction at the same speed.  If you don't, it gets really confused about your position.  It can work for you or against you, as RK said,  and you have to look at your position with some skepticism if you've lost signal.

That's what I know too... and, just to be clear, also Garmin have this feature, at least the eTrexes. If I remember correctly, dead reckoning lasts for 30 seconds in the eTrex (or was it 15?).

The idea is that when the gps loses lock, it assumes that your direction and speed are the last ones it was actually able to record, and calculates your position accordingly (as Thorin pointed out, in some models steering variations are actually sensed during dead reckoning) . This makes sense if you are losing lock for, say, 10 seconds while passing in a small tunnel. It's reasonabily likely hat the tunnel will be straight, and that your speed will not change much in these 10 seconds, so that the GPS will be giving you a reasonabiliy likely estimate.

You tipically prefer this than a pop up or whatever blinking any time the GPS loses lock for 2 or 3 seconds.

 

Clearly, as longer the GPS "projects" your position, as less likely is that you are still moving in the same direction and speed. So after a while (the famous 30 secs) the GPS stops giving any guess.

This, at least in the eTrex series, means that it says "poor satellite reception" and blanks out compass a speed indications, interrupts tracklogging and so on. If you are looking at the satellite page, you will always be able to see if the GPS is actually tracking enough satellites: as soon as the locked satellites go below 3, the GPS is guessing.

 

Acaro of Team TAR

[Wooden Bystander]

Thanks.

[maddog1404]

I just recieved a Magellen 2000 and I have yet for it to find any sattelites. But maybe thats because I was trying it at night with some cloud cover

[+briansnat]

I just recieved a Magellen 2000 and I have yet for it to find any sattelites. But maybe thats because I was trying it at night with some cloud cover

Daylight (or lack of) and cloud cover shouldn't make a difference. Take it out to a field (your local ballfield should do) and let it sit about 15 minutes.

[+Nurse Dave]

Hmmm, I have a Meridian and I think this continuing to move you on your present course thing must be only with the sportracs. What I see it something like it must be using previous data and a non3D lock to extrapolate where I am. I'll have a great lock, then move under tree cover where I only get like 2 sats and it will continue to point me in the right direction even if I turn on a trail or stop. It isn't simply assuming I keep moving. If I turn the arrow swings around to continue to point to the cache so even without a perfect lock it knew I actually changed direction. That's not the same as assuming I kept walking in the same direction at the same speed.

[+briansnat]

I'll have a great lock, then move under tree cover where I only get like 2 sats and it will continue to point me in the right direction even if I turn on a trail or stop. It isn't simply assuming I keep moving.

 

Since a GPS uses TRIangulation to arrive at your position, how does your unit work with only two sats?

[peter]

Since a GPS uses TRIangulation to arrive at your position, how does your unit work with only two sats?

If GPS receivers actually did use triangulation then two satellites would be sufficient. The term comes from being able to navigate by taking a bearing on two reference points at known locations. The two known points plus the bearings from your location define a *triangle* and allow you to determine your position.

 

GPS actually works by simultaneously solving for the time and the distances from your location to at least four satellites (three if you assume your altitude is known). Four are needed since you need to calculate four variables (lat, long, altitude, and time).

 

But the receiver also measures the Doppler frequency shift of the satellite signals due to their relative movement and this data is used as an input in determining your speed.

In principle, the receiver could determine your horizontal speed and direction of movement based on the Doppler shift of two satellite signals. Then using your last known position the unit could make a decent estimate of how far you've moved and in what direction. Of course the accuracy would degrade over time until the unit again gets a proper satellite lock

 

No idea if Magellan or any other receivers actually try to use the Doppler data in this form of "dead reckoning," but it sounds feasible in theory and would explain Dave's observation.. It would be nice to know if Dave has the Platinum model in which case the pointer could also be turning based on the magnetic compass reading.

[+as77]

If GPS receivers actually did use triangulation then two satellites would be sufficient.

Not in three dimensions.

 

GPS works by determining your distance from at least 3 satellites whose positions are known. This is the same principle as triangulation.

[+briansnat]

If GPS receivers actually did use triangulation then two satellites would be sufficient.

Not in three dimensions.

 

GPS works by determining your distance from at least 3 satellites whose positions are known. This is the same principle as triangulation.

Then how does NurseDave's GPS work so well with only 2 sats?

Edited September 13, 2004 by briansnat

[+as77]

Then how does NurseDave's GPS work so well with only 2 sats?

What GPS is that?

If you have only 2 sats and nothing else, the position cannot be determined unambiguously. If a GPS does that then I would guess some dead reckoning is happening.

[peter]

If GPS receivers actually did use triangulation then two satellites would be sufficient.

Not in three dimensions.

 

GPS works by determining your distance from at least 3 satellites whose positions are known. This is the same principle as triangulation.

Yes, also in three dimensions. Having two reference locations whose positions in 3D space are known and the bearings (azimuth and altitude) to both from your location will precisely determine a triangle in 3D and would let you calculate your 3D position. That would be triangulation, but is certainly *not* what GPS does.

 

The principle of triangulation is not used by GPS - no angles are measured nor is any triangle defined. The only similarity is that both techniques use known reference locations to determine your previously unknown position, but the measurements and mathematical principles of the two approaches are different.

 

As I said before, four satellite measurements are required for GPS to work in 3D since the receiver must simultaneously solve for lat, long, alt, and time. Using three is only sufficient if we assume a fixed altitude (or if we had a properly synchronized atomic clock in the receiver).

[+as77]

It doesn't seem to be a "different mathematical principle" to me. Whether you construct a triangle using three sides or one side and two angles is not really that different.