Gpsr's Versus Cell Phones

[+SBPhishy]

I was curious about this. I have a GPS, and I have a cell phone. My cell phone gets good reception anywhere (more or less). In my house, car, most big buildings, etc.

I realize that there are many antennas everywhere, depending on your cell phone service, that are used to keep the reception clear. Are satelites used at all to aid in REGULAR cell phone reception, or are they only used for SATELITE phones.

 

My main question is that, why does my cell phone get good reception, where my GPS can not. Is it just because of the land-based antennas? That was the only answer I could come up with.

 

Thanks!

[+Enspyer]

There is a lot more commercial incentive to make the cell phones work anywhere.

 

When I first read your topic I thought it was someone asking if there was any difference in a GPSr vs. cellphone by way of function...

[+wildearth2001]

Are satelites used at all to aid in REGULAR cell phone reception, or are they only used for SATELITE phones.

To my knowledge, No. As for the other question, I also want to know...

[+wildearth2001]

When I first read your topic I thought it was someone asking if there was any difference in a GPSr vs. cellphone by way of function...

When I first read the title I thought you were asking about GPS vs. LBS

[+Criminal]

You don't cache where I like to cache. There is NO cell service before you ever get out of your car. Way out in the Olympics, you are on your own.

 

Apples and hot dogs.

[+SBPhishy]

Well, I'm glad no one knows the answer right off. I have thought about it before, and was just curious why GPSr's can be so "touchy" about their signals. Bad weather, bad signal, wooden ceiling in the house, no signal.

 

If they could make GPS's like cell phones as far as reception, that would be awesome!

 

I realize that there are definitely places where cell phones dont work at all. I, however, live in a decent size city, so even in the mountains around here, they work just fine.

Edited February 29, 2004 by SBPhishy

[+Sparky-Watts]

Two completely different types of transmission and reception....cell phones are microwave which will bounce around a lot more objects.....and the cell towers are generally much closer together (depending on where you live, of course) so you have more of them to radiate a signal than you do satellites for GPS signals....

[+TeamK-9]

I think the ground attenas have a lot to do with cell phones working so well. I mean, GPS signals have to fight through the atmosphere, and then through buildings, trees and other obstructions, and the signal to start out isn't even as strong as a phone signal...

[+SBPhishy]

That seems to sound right. Thats what i thought anyways.

[+Wacka]

I have trouble getting good cell reception in my apartment! I go outside- it's fine.

[+TeamK-9]

It's actually probably your actual cell phone, and not the system. Like my phone, I'll be sitting in my school auditorium and it won't work worth crap, but I borrow a friend's newer phone and it's perfect.

[ucmike]

as has been said, your cell phone is working from the network of land based transmitters transmitting a different type of signal. the frequency and several other factors would account for the cell signal getting into buildings, etc. most newer cell sites have a gps receiver mounted near the transmitters, i think it uses the gps signal to triangulate for 911 capable phones.

[+paul.blitz]

The answer is very simple: the amount of power you will receieve from a cellphone transmitter (when in a covered area, of course!) is FAR higher than the tiny signal you get from a GPS satellite.

 

Whilst the actual power transmitted is probably not *hugely* different, the "working reception area" of the cellphone system is just a few miles, compared to the several HUNDREDS of miles (probably more like thousands!) over which the GPS signal needs to be received.

 

Paul

[+GeckoGeek]

In addtion to the other comments, GPS units are delibertly desgined to have limited reception to keep from being fooled by bounced signals. Bounced signals for cell phones is no big deal. It may allow you coverage where you otherwise couldn't get it. But with GPS, a bounced signal will result in a false postion reading since the distance the signal travels is a critical part of how the system works.

 

 

(flying without a spell checker....)

[+Sparky-Watts]

In addtion to the other comments, GPS units are delibertly desgined to have limited reception to keep from being fooled by bounced signals. Bounced signals for cell phones is no big deal. It may allow you coverage where you otherwise couldn't get it. But with GPS, a bounced signal will result in a false postion reading since the distance the signal travels is a critical part of how the system works.

 

 

(flying without a spell checker....)

Dang, and I was gonna use that defense for my next DNF...

[+Confucius' Cat]

All radio signals VHF and above are generally line of sight. They manage to get into buildings by multiple bounces. The distance factor is the main ingredient in in-building coverage. In order to get in-building coverage, you need a very high level signal so that the random bounces can penetratee through openings in the building.

 

Due to the law of inverse squares, the signal strength diminishes VERY RAPIDLY as the receiver moves away from the transmitter. Thus a very strong signal many miles away will be received more poorly than a comparatively weak signal close by.

 

Cellular is an "interference limited" system, meaning the limit of distance is set by the point where other signals override the desired signal (as opposed to the more familiar "noise limited" -such as gps signals- where the limit is set by the noise level of the environment because there is no undesired signal to compete with it).

 

Because it is interference limited, cellular requires MUCH stronger signals locally. This is accomplished by using low towers, high power transmitters, highly directional antenna patterns, and a LOT of sites.

 

Thus the bottom line is with cellular, in a saturation coverage area, you are receiving a signal from just a mile or two away- any further from the cell site and you won't have much in-building coverage unless the building in question has an in-building distribution system.

 

The satellites for GPS are several hundred miles away so their signals don't penetrate very well. The GPS system is designed for open air use and was never intended to penetrate buildings.

 

Re: GPS on cellular

The newer phones have GPS receivers built in for the purpose of transmitting location on 911 calls, but it don't work yet in most places.

 

GPS is also used to synchronize the timing of signals from the cell sites and clocking of t1 circuits. It is absolutely mandatory in CDMA systems due to the nature of the spread-spectrum signal requiring VERY precise synchronization.

[+SBPhishy]

Re: GPS on cellular

The newer phones have GPS receivers built in for the purpose of transmitting location on 911 calls, but it don't work yet in most places

 

I was looking at Nextel phones a while back and I remember seeing a menu for GPS. I thought it was strange and had no idea what it's use would be. A cell phone that you can cache with? I guess not... Someday I'm sure though.

[+Confucius' Cat]

Yeah, it kinda inhales massively. They put the GPS chip in the phone, give you the option of turning it on or off for other purposes besides 911, and than they don't give you a position readout or waypoint capability.

 

They could easily do so much more with it.

 

I have to wonder if it works at all. I know it doesn't in my area (tested with PSAP). I am curious just how well it receives the GPS signals. Indeed it doens't need to be as accurate as a geocaching GPSr but it would still be cool even to have a rough readout.

 

OBTW my company makes the chip. Just saw a news release on it a month ago. It's really cool and all on one chip about the size of a dime... just add battery.

Edited February 29, 2004 by ChurchCampDave

[+NightPilot]

To answer the original question directly, although it has been answered indirectly, no, cell phones do not use satellites at all, unless the trunk circuit you hit on a long-distance call happens to be via satellite. The cell phone only talks to the nearest tower directly. After that it's impossible to tell, just like with an old-fashioned phone. After the signal goes to the nearest central office, the path is unpredictable.

 

Another reason for the poorer signal from GPS is the frequency. The characteristics of electromagnetic waves are greatly affected by their frequency, and the higher the frequency, the more difficult it is to penetrate anything. Light waves are just extremely high-frequency electromagnetic waves, just like radio waves, but they're completely stopped by a thin board. Infrared waves, just slightly shorter, can travel through some wood, and easily through metal, but very slowly. GPS signals are extremely high-frequency radio waves, and are much more easily blocked than cell-phone frequencies, which in turn are more easily blocked than FM radio, which are more easily blocked than AM frequencies, etc, etc. HF band frequencies can actually be bounced off the ionosphere, and there are prediction tables that can be used to determine the best frequency to use between 2 sites at any time and date, depending on the height of the ionosphere, which moves rather predictably, as long as sunspot activity is predictable.

 

Also, the cell phones don't need to receive GPS satellite signals. It's done through the cell towers. The towers know exactly where they are, and they receive the GPS signals just for the timing info. Using the timing info from the GPS system, and the signals from the cell phone, the towers can determine where the phone is, and send its position to it if it is properly equipped. Few are currently, but it's coming, for a variety of reasons. Government agencies want to know where the phone is more than the owner wants to know where he is. It's primarily for 911 service, but that's certainly not the only reason.

Edited March 1, 2004 by NightPilot

[+Team GPSaxophone]

GPSr's only receive signals, they don't transmit.

Cell phones do both.

 

GPS satellites orbit the earth at roughly 11,500 miles away.

Cell phones couldn't transmit a signal that far without giving you brain tumors (some conspiracy theorists think they already do, so do you really want to increase the power on one?)

 

GPSr's receive signals from as many satellites as are available

Cell phones only talk to one tower at a time, then handoff when you get closer to another one.

[+pater47]

Big difference. My GPS shows me where I am. My cell phone ringing is usually somebody asking "where are you?"

[+WalruZ]

Yep. The birds are thousands of miles up there and they don't transmit with all *that* much power. The signal your GPS receives is so weak as to be barely different from the noise in the environment. This is one (of many) reasons it actually takes a bit to lock on. Not only does the GPS have to solve many equations in parallel to sort out what time it actually is (based on the times that the birds are transmitting), and thus calculate your position, but it has to deal with bad data. If the GPS receives some corrupted data it just throws it out and keeps listening. Since it doesn't take much to hose the signal (say, leaves), this is why you keep switching from bird to bird as you walk around. The GPSr listens to more than 4 birds, it just chooses the 4 that it likes the best. If one goes odd, say because you're standing in front of it, it just switches to the next best.

 

Your cell phone uses alot more power, and if you're more than about a mile away she no workee (sometimes she no workee anyway.) This is why you should not duct-tape it to your head. Well, that and also as a fashion statement it is SO 90s.

[+PDOP's]

Yep. The birds are thousands of miles up there and they don't transmit with all *that* much power

For those who want to know.

[+haggaeus]

[...]

Cell phones only talk to one tower at a time, then handoff when you get closer to another one.

 

But they are aware of the other stations and the strength of signal from them, so it could be possible to do some triangulation? It would be cool to use a cellphone as a GPS replacement.

[+Stunod]

I was looking at Nextel phones a while back and I remember seeing a menu for GPS. I thought it was strange and had no idea what it's use would be. A cell phone that you can cache with? I guess not... Someday I'm sure though.

My Nextel phone has GPS and you CAN cache with it. There are some very neat Java apps you can download to utilize the GPS signal.

[+SBPhishy]

sweet. i shoulda got one!

[+Stunod]

sweet. i shoulda got one!

Note I said "CAN be used", not that it can be easily used. It is slow to update position (I think it is only a 1 channel receiver). I'm not ready to trade in my MeriPlat yet.