Are GPS useless in big cities, ie. NY

Tall buildings and tree cover will block the GPS signals making GPS reception in these areas iffy at times. A unit with a high-sensitivity receiver will do a better job at receiving the signals in these areas.

I would not say that the units are useless, but it is a limitation.

I have had problems in NYC but have found adding an external antenna helps. I put it on the roof and had fewer lost signals. I have not had has many problems in cities with smaller buildings, such as Philadelphia.

It really seems to depend on the receiver. Most newer receivers deal with multi-path signals better than ones made more than a year ago. I have a newer receiver and in Seattle I only occasionally have problems. Its not perfect but it is usable.

If you're in the middle of a block with tall buildings, you might have trouble getting a signal.

It might help a little bit if you go near an intersection.

You happen to be walking around the worst area in the nation for obtaining a GPS signal. I have had no trouble in downtown Chicago.

I own that C550 unit also. It happens that I used it in Boston last week. Saw the satellite reception fail once, briefly, traveling east on Boylston somewhere between the Pru and Hancock tower.

Inside a car, under the windshield, my guesstimate is that the GPS can see roughly 1/3rd of the sky, so relatively few of the satellites can be received even in flat desert terrain. Some GPS units allow installation of an external antenna on the car roof, which increases the sky fraction to near 100%. Tall buildings reduce the fraction. The satellites are not necessarily distributed evenly across the sky, so yes the slot canyon arrangement of tall buildings in Manhattan might easily reduce your reception below 3 satellites.

Trees immediately overhead will also kill satellite reception. Any veteran geocacher is very familiar with this effect. Walking directly under trees along the sidewalk or in say Central Park will do it.

Some industrial GPS receivers use a fixed land-based signal source to augment the satellite sources. Hasn't happened yet for consumer GPS, as best I can tell.

I've had it happen with both my unit and permanent installed unit. But I found if I just drove (or walked) into the clear I got the signal back. It really freaked me out in downtown Boston when it happened the first time and I was late to pick up some one from the airport. I had no clue which way to go. So I just drive but it got a lock back.

Each generation has gotten better at sensitivity - and I've had no problems in Houston or Chicago - much better than previous generations of GPS'.

beside not having a direct line of sight to satellites, GPSr might also received multi-path signals from a single satellite due to the signal being bounced off buildings after buildings.

The most expensive units have what is called dead reckoning. If they temporarily lose the signal, a gyro inside the unit keeps you on track. This was one of the reasons I purchased the Garmin 7500. It works flawlessly. I used to lose the signal in downtown Toronto. Not anymore.

I was able to pick up signals in Manhattan when I was there last month with my Nuvi 370. I didn't use it extensively, so I don't know how consistent the signal coverage is. However, taxis in NYC are now being fitted with GPS. The taxi drivers are not too thrilled about it, as it tracks their movements.

Related to this thread, the cabbies also complain that their cabs go out of sight of the dispatcher. This causes cabs to be dispatched much farther away from the customer. Customers may have jumped into another cab by time the dispatched cab arrived.

I too have the c550. Earlier this year I took a trip to NYC and walked around Manhattan. I lost the signal once. If I remember right it was around the NYSE building. That seemed to be the only hiccup that day.

I have lost a signal while driving in downtown Fort Worth, during a rain storm, with not so tall buildings.

I surmise that the main problem in any given city with tall buildings would be directly related to the individual position of each gpsr at any given moment in relationship to the location of the satellites at that very same moment and thence the signal being blocked by the buildings.

I have found that the best solution and one that works extremely well for me is ....
to move out and stay away from all cities with tall buildings, or medium buildings, or small buildings. Unless summonsed to court....for jury duty or other such duties of citizenship.

Is multipath signals...there's much more 'noise' in the form of reflected signals from the satellite caused by the environment...and the GPSr has to sift out what's good from what's not.

Hi guys,

I drive for a living in NYC. Using the C330 without the external antenna, the signal fades in and out but it’s bearable. Adding the additional in car antenna and the signal strength is stronger then ever.

I was in Boston the other day (live in Suburb) and I too had signal lost on my C550. I'd like to use this for geocaching instead of my Garmin Legend but it doesn't have the best type of software for this. I'd like Garmin to have this as a feature in their automotive GPS units, ie. compass.

I have a geocache myself (check zip 02052 and "glove cache".

Some additional experiences with the C550.

In Boston tunnels (MassPike and Storrow Drive), it extrapolates. My guess is that it knows the car's speed as of signal loss, and uses that info to estimate current position in the tunnel.

If it contains a gyro or accelerometer, I would be pleasantly surprised.

If it does not contain a gyro or accelerometer, then estimation error will accumulate in a long tunnel. You could likely see a visible and discontinuous correction when you exit the tunnel and the GPSr re-acquires satellites.

One thing that all geocachers learn very quickly. The GPSr determines both heading and compass orientation by comparing consecutive positions, meaning it contains no internal compass. This has implications for folks who carry their C550 while on foot. Any stop will cause the C550 to lose both compass orientation and your heading ... unless it extrapolates those too.

Common sense applies here. If you don't have an unobstructed view of the sky, neither does your gps. The gps tries to use as many satellites as possible to create triangulations. The fewer it tracks, the worse the fix and "difference of position".

An additional external antenna on the outside of your vehicle is the best fix for "city eclipse". It takes away the obstruction of tracking satellites through your vehicle roof and the perils of the city. My c330 has occasional problems in Philadelphia because of the buildings and trees in the historic district (but so does my Sirius receiver). "It's all about being able to see the sky."