Weak Satelite Signal

 

Lately, I have been having trouble acquiring full satellite signal on my Garmin Nuvi 255 and my wife's Nuvi 255W. This is on a clear day, out in a field, anywhere. Sometimes, whilst driving, my car icon "drifts" off the road and if in an urban area, onto another road. This is getting quite annoying, especially when the "other" street is a one-way. I have the latest firmware. Is this happening to anyone else?

Weak signal

I noticed that my 750, while showing a full signal bar, only had info on 9 satellites the other day. It still listed the accuracy as about 14-15 feet and appeared to work fine. This was after about 1/2 hour of operation. I think the bar indicates the degree of plotting accuracy, not the signal strength of the received signal.

Do you have full signal bars or only partial? How many satellites does your GPS have info on?

Jack j

Not yet

I have a 750 and 755 and have not noticed any problems. I'm going on a 7 hour road trip with the 755 tomorrow and will monitor the signal and to see if it drifts.

--
Nuvi 750 and 755T

Three is enough

The satellites are not necessarily spread evenly above the earth surface as they rotate in polar orbits. They may at time be less concentrated above your location. But anything with 3 satellites or more is enough for the GPS to pinpoint your location.

3 satellites?

I have read that it only takes 3 satellites to determine your position, but I have watched the satellite screen many times on my 755T while it is acquiring. It never locks in with 3 or 4. It locks in at 5. The initial accuracy is usually more than 100 feet. it then starts improving the accuracy as more satellites are found until it gets down to 14 to 25 feet.

--
Alan - Android Auto, DriveLuxe 51LMT-S, DriveLuxe 50LMTHD, Nuvi 3597LMTHD, Oregon 550T, Nuvi 855, Nuvi 755T, Lowrance Endura Sierra, Bosch Nyon

Signal Strength

alandb wrote:

I have read that it only takes 3 satellites to determine your position, but I have watched the satellite screen many times on my 755T while it is acquiring. It never locks in with 3 or 4. It locks in at 5. The initial accuracy is usually more than 100 feet. it then starts improving the accuracy as more satellites are found until it gets down to 14 to 25 feet.

I have noticed the same thing with my 255s. I thought the number of green bars equated to the number of sats since it turns from red to green at 3. However, mine needs more than 3 satellites to get a lock.

I have also noticed a definite decrease in satellite acquisition over the last few days.

--
><> Glenn <>< Garmin nüvi 2598

3 is not enough

jale wrote:

The satellites are not necessarily spread evenly above the earth surface as they rotate in polar orbits. They may at time be less concentrated above your location. But anything with 3 satellites or more is enough for the GPS to pinpoint your location.

Three satellites are NOT considered enough to calculate a location. The reason is that an inexpensive GPS receiver does not have the atomic clock that would be needed to accurately determine the time to the resolution needed for GPS calculations. So to make up for not having a few million dollars worth of atomic clock, the GPS receiver needs at least one extra satellite signal. With this extra signal the GPS can do some tricky match and compute what the actual precise time must be based on when roughly all 4 signals were received.

Some GPS units will try to give you a 2D fix with only 3 satellites, but this really only works at sea level, and any error would be at least as great as the local elevation, plus the other GPS errors. This can be huge in cities like Denver and even at lower elevations is a very significant error that would make turn warnings useless. I do not believe that the nuvi line will try to do 2D navigation at all.

While 4 satellites are "enough" for doing the GPS calculations, the error quality is pretty dependent of just where in the sky those 4 satellites are. With more than 4 satellites the gps can refine the calculation and give more accurate numbers. In general the more of the sky that you can "see" satellites in, the better the math will be.

I had noticed a significant

I had noticed a significant decrease in reception about a month ago (July I believe). I would suddenly loose reception even on a clear day. Lately I have not had issues.

Actually, 3 satellites are sufficient for latitude and longitude, 4 are required for elevation. See http://electronics.howstuffworks.com/gadgets/travel/gps.htm for a tutorial (more tutorials are available - just Google for lots more).

Three is not enough

jale wrote:

The satellites are not necessarily spread evenly above the earth surface as they rotate in polar orbits. They may at time be less concentrated above your location. But anything with 3 satellites or more is enough for the GPS to pinpoint your location.

Not very accurate or close to being your actual position, with a lock with only three birds.As you populate satellites you gain accuracy.
One more thing, does this look like a polar orbit to you?
http://www.how-gps-works.com/faq/q0113.shtml

Not to me!

--
Using Android Based GPS.The above post and my sig reflects my own opinions, expressed for the purpose of informing or inspiring, not commanding. Naturally, you are free to reject or embrace whatever you read.

Oops - this is not iridium

BobDee wrote:

...
One more thing, does this look like a polar orbit to you?
http://www.how-gps-works.com/faq/q0113.shtml

Not to me!

Sorry, my mistake. I was thinking of Iridium.

Maybe it's the fading satellites

I've been pretty good with

I've been pretty good with my satellite signals, minus this morning which took about 5 minutes of driving (also on a clear day) before grabbing a signal. Hopefully its only a short term issue for you.

760 Follows Road/Directions Even When I don't

Sometimes when I go into a parking lot my 760 plots the position as being on the nearby road. It's usually about 30 or 40 feet away. When I stop, it then plots a line in from the street to my actual location and another back out to the street. I've noticed similar behavior on exit ramps when I've decided not to take the exit. It seems to assume that I am following the calculated route and not freelancing. It recovers quickly, but I find the behavior strange.

Snap to road

@edwardw66 - You are experiencing the "snap to road" feature.

--
Alan - Android Auto, DriveLuxe 51LMT-S, DriveLuxe 50LMTHD, Nuvi 3597LMTHD, Oregon 550T, Nuvi 855, Nuvi 755T, Lowrance Endura Sierra, Bosch Nyon

possible update..

might be time for a software update

--
WE LIVE IN THE LAND OF THE FREE, ONLY BECAUSE OF THE BRAVE! IN GOD WE TRUST

For your reading pleasure

I came across this site awhile back and it has a lot of info about GPS satellites. Note that while we often state polar orbit, the current flock of birds are in a semi-polar orbit as explained in this article.

http://www.kowoma.de/en/gps/orbits.htm

--
"There's no substitute for local knowledge" nüvi 750, nüvi 3597

3 or 4 needed?

ewillner wrote:

I had noticed a significant decrease in reception about a month ago (July I believe). I would suddenly loose reception even on a clear day. Lately I have not had issues.

Actually, 3 satellites are sufficient for latitude and longitude, 4 are required for elevation. See http://electronics.howstuffworks.com/gadgets/travel/gps.htm for a tutorial (more tutorials are available - just Google for lots more).

You are right and wrong. hehehehe

3 satellites will determine your position as one of two possible spots. One spot is on the surface of the earth while the other is in outer space. But it takes 4 to narrow your position down to only one spot. 4 are also needed to give you your altitude above mean sea level.

Jack j

Interesting that you bring

Interesting that you bring this up, I had the same issue all of a sudden on my 205w, I am now wondering if there's been issues with the WAAS system as that is what truely gets you down to the high accuracy. I finally RMA'd mine to Garmin after a few days of emails back and forth (they're exceptionally helpful by the way) and after finding no resolution they had me send mine back and promptly sent me a new rebuilt unit (which I received today) I think for me it started sometime in July as well (I sat for nearly a month before finally sending my unit off for RMA) and literally I was driving on a local freeway here in the Seattle area and all of a sudden it kept saying "recalculating" and the car was jumpin all over the screen, when I went into the signal screen it was showing accuracy of 500 - 600 ft, which is way off from the normal 9-15 that I normally get, for several days after that I could never get better than around 50-70 ft accuracy no matter where I was.

A good GPS tutorial

Here's a good GPS tutorial:

http://www.trimble.com/gps/index.shtml

you don't know how GPS works

jackj180 wrote:

You are right and wrong. hehehehe

3 satellites will determine your position as one of two possible spots. One spot is on the surface of the earth while the other is in outer space. But it takes 4 to narrow your position down to only one spot. 4 are also needed to give you your altitude above mean sea level.

Jack j

Three satellites would be required if your GPS receiver had a multi-million dollar atomic clock. In that case you could indeed calculate a position with 3 birds, assuming it was accurately synced to a global standard in some way. (GPS throws away the solution that is in space). But in the real world the clock in a GPS receiver is not accurate enough to give meaningful solutions with only three satellites. So rather than expect users to have clocks accurate to a millionth of a second or better, GPS used a 4th satellite in the calculation calculate to make up for insecurities in the receiver's clock.

Three can indeed be used for crude 2D navigation, but it is very inaccurate to do this at anything except sea level. The nuvi does not do 2D navigation, and thus needs 4 satellites at a minimum.

It's really a shame that you use hehehehe in a post giving incorrect information.

What a shame

Sorry about the hehehehe but the info is NOT incorrect. Check out how your GPS works. Yes, it needs an accurate time base but that doesn't come from the 4th satellite. The GPS does not throw away the solution in space. If it did then it couldn't give you elevation, it would throw away anything that wasn't at sea level.

1 satellite places you somewhere on the surface of a sphere. 2 satellites define 2 spheres, you are on a circle where the spheres intersect. 3 satellites defines three spheres and you are at either of two places where the three spheres intersect. 4 satellites define 4 spheres and you are at the only place where the 4 spheres intersect. A satellite is at the center of each sphere. The GPS defines these spheres by knowing exactly when a signal is sent from the satellite and then measuring the length of time it takes for the signal to reach the receiver. The time correction factor is computed using the time delay from several satellites, knowing exactly where each of the satellites was when the time pulse was transmitted and comparing each of the delays vs. the satellite's position to come up with a correction factor to apply to the local clock.

Jack j

This answers my question

jackj180 wrote:

...
The time correction factor is computed using the time delay from several satellites, knowing exactly where each of the satellites was when the time pulse was transmitted and comparing each of the delays vs. the satellite's position to come up with a correction factor to apply to the local clock.
Jack j

I've been wondering how the internal clock in the GPSr could be set so accurately that it could measure the differences in received times from the various satellites. As there would be a delay in receiving any time hack, the GPSr would always be a bit off. Your explanation of the calculated correction factor make this all come together.

--
"There's no substitute for local knowledge" nüvi 750, nüvi 3597

My wife's unit improved, but

My wife's unit improved, but mine is getting worse. I commute 88 miles each way to and from work, I turned on my unit in Bayonne, NJ, and did not get a signal until I was in Elizabeth, passing through 2 counties on the NJ Turnpike with no traffic data. The thing is becoming paperweight. How did you exchange your unit and how long did you have it?

NJ wife's unit OK, husbands unit failing badly. Bada Bing!

eungkeupsil wrote:

My wife's unit improved, but mine is getting worse. I commute 88 miles each way to and from work, I turned on my unit in Bayonne, NJ, and did not get a signal until I was in Elizabeth, passing through 2 counties on the NJ Turnpike with no traffic data. The thing is becoming paperweight. How did you exchange your unit and how long did you have it?

-=-=-=-=-=-=-=-=-=-=-

Please check for evidence of arsenic, hemlock, rat poison or remnents of a NJ cheesesteak sandwhich on your nuvi. That may be the problem. wink

--
Garmin DriveSmart 61 LMT-S, Prev.GPSs: Drive61 LM, nuvi 3790LMT, 755T & 650, GPSIII+, SP 2610, 250W; Magellan 2200T; Originator of GARMIN NUVI TRICKS, TIPS, WORKAROUNDS, HINTS, SECRETS & IDEAS http://bit.ly/GARMIN-TNT

Software Update

eungkeupsil wrote:

Lately, I have been having trouble acquiring full satellite signal on my Garmin Nuvi 255 and my wife's Nuvi 255W...

Yes, my 255W was doing the same. Garmin just came out with a fix. Use the Garmin WebUpdater, there's an update for the software. Once I updated it, I haven't had any problems.

--
"People say nothing is impossible, but I do nothing everyday" ~ Winnie the Pooh

External Antenna

I have had the same problem at times with my 350 and 750 when traveling. Do the external antenna's really improve reception at all? Any one with some good input would be appreciated.

--
JG - Nuvi 2460

External Antenna is well worth it.

I have a 760 and have always had a lag problem with the internal antenna finding satellites. Normally would have to travel 3-5 blocks once I pulled out of the garage. With the external antenna, I usually sync to satellites before I back out of the driveway. Accuracy also improved greatly from an average of 30-40 feet to less than 10.

weak signal

my c330 lost reception a week ago last friday. I have used the external antenna laying on the dashboard next to the windshield for about 3 years with out a problem. now the external antenna has to be on roof of the car for it to work. anyone have any ideas?

my 765t gets a sweet signal,

my 765t gets a sweet signal, I haven't really had any issues at all. I even get signal inside the front of my house, not in the basement though.

My 260 does the same things

My 260 does the same things with lost sats. It has been rebooted several times and after a couple days does it all over again, I will contact Garmin and see what happens.
Will check for updates but, last week there was nothing.

Virgo53

--
Virgo53 Nuvi 780/265W

Antenna Gone Bad?

wayne1939 wrote:

my c330 lost reception a week ago last friday. I have used the external antenna laying on the dashboard next to the windshield for about 3 years with out a problem. now the external antenna has to be on roof of the car for it to work. anyone have any ideas?

Maybe one idea. Your external antenna has developed a problem. Try the GPS with the outside antenna disconnected (take it outside the car if necessary for the experiment). Let it lock onto a bunch of satellites then watch your sat screen as you plug in your external. If the number of locked on birds changes for the worse then you've found the problem.

Cheers

--
Nuvi 760 & 660, Streetpilot, GPS III, GPS 10X