All About GPS
GPS stands for Global Positioning System. GPS was invented in the '70s, and was originally designed for military applications. The system was started in '78 and completed in '94. Today there are hundreds of thousands of civilian users, in addition to the military uses we see from time to time on CNN.
GPS works because of the NAVSTAR system, a system of 24 satellites which orbit the earth at an altitude of about 12,000 miles. Each satellite orbits the earth in 12 hours. These orbits are designed so that at any given time anywhere on the earth you are guaranteed to be able to "see" at least 4 satellites at once. It is said that there is another complete NAVSTAR system orbiting the earth, blacked out. This is so that if an enemy shot down the current NAVSTAR system, the military can replace it in seconds with all new satellites, all in different orbits. The individual satellites broadcast low-power 50 watt radio signals telling you the precise time when the signal left, and the precise position of the satellite. If your GPS unit can see at least three satellites, then by triangulation it can tell where on the earth it is. If it can see four satellites, it can also tell its elevation. Your GPS unit works by noting the time when it received the individual signals. Using the speed of light, it can then calculate how far it is from each satellite. The reason it takes four satellites is that one extra signal is needed to correct the clock in your GPS unit. We're deep into relativity here: your GPS unit must locate itself not only in space but also in time. The satellites each contain atomic clocks, so they know what time it is.
With one satellite and a clock, you can determine your distance from the satellite, which places you on a sphere centered around the satellite. This is not a perfect sphere, it has some imprecision due to relativistic time effects, inaccuracies in the clocks, inaccuracies in the satellite orbit, and various problems with signal reception. Two satellites gives you two imperfect spheres, which places you on the intersection of the two spheres, an imperfect circle. Three satellites places you on the intersection of two imperfect circles, which is two fuzzy points. We're pretty close now, because one of those points is correct, and the other is most likely inside the Earth or somewhere high in space. Remembering that the clock in your GPS is not anywhere close to atomic accuracy, using a fourth satellite you can correct your clock inaccuracy to reduce the fuzziness and chose which of the two points is correct.
The GPS civilian broadcasts are "line of site," which means the radio signals are blocked by buildings, mountains, bridges, etc. Clouds are not a problem. Your GPS unit will not work indoors, underground, or under water. The satellite broadcasts are encrypted. A civilian unit is made to decrypt enough of the signal to have accuracy within about ten feet. Military units can do somewhat better. The Pentagon has a switch to instantly change the encryption, so that in time of war they can instantly turn off all civilian GPS units. Also, ground stations can broadcast signals which override the civilian GPS system and shut it down. For this reason, when you get near certain military installations your GPS will quit working. The DOD didn't spend $25 billion to help enemies attack us.
When you first turn on your GPS unit, it has to locate satellites. Each GPS unit contains an almanac which tells it where all 24 satellites are supposed to be at any given time. However, this almanac is imperfect. Because the Earth is not a perfect sphere, and there's a lot of other stuff around like the Moon, the Sun, and Jupiter, the satellite orbits vary a bit in ways that cannot easily be calculated. Also, from time to time DOD ground control will tell an individual satellite to fire its rockets and change its orbit a bit. So, it can take your GPS as much as a minute to find itself when you first turn it on, or if you turn it off and then fly across an ocean. After the GPS has found itself and corrected its satellite almanac, it can update its position in well under a second.
As you and the satellites move around, the accuracy of your GPS changes. When the satellites happen to be close to each other, your GPS unit has reduced accuracy. Also, the satellite broadcasts can bounce off of buildings or rocks, so that your GPS hears an echo of the original broadcast. This is called multipath, and is another source of error. If a satellite is very low, just over the horizon, then the satellite broadcast must travel through a lot of atmosphere to get to you. Radio signals travel slower in atmosphere than in space, so this change in the speed of light causes errors. Your GPS unit is designed to try to compensate for each of these errors, but at times it can all get to be a bit much. Most of the time, you will find your GPS is very accurate.
There is a whole extra navigation system called WAAS, Wide Area Augmentation System. This is a system of 25 ground stations set up in N.America which broadcast a NAVSTAR type signal, except the ground stations never move so they know exactly where they are. If your GPS is WAAS enabled and you are within range of one of these ground stations, your accuracy improves. There are currently two geo- synchronous satellites to rebroadcast this WAAS info, one over each coast, and a third is planned to cover the central states. Also, there is yet another system called DGPS, Differential GPS. Again, this is based on ground stations with a known position broadcasting corrections to the satellite signals.
When selecting a GPS, you have a few decisions which should be made before you get serious. Do you want something hand-held that's useful when backpacking, or is a larger dash-top unit ok? Do you want a good street-level map of the entire US available? Would you like your GPS to be able to do routing, that is to find you directions to get from where you are to where you are going? Finally, are you interested in travelling off-road, either by hiking, backpacking, 4wd, or boating? Today, you cannot get a single GPS unit that will do everything, you have to pick.
Some people buy good mapping software, for example by National Geographic or DeLorme, and plug an inexpensive GPS into their laptop. The laptop is now your interactive map, and the GPS unit is just a passive device which supplies info to the laptop. You might wonder why laptops don't have GPS built-in. This is an example of the immaturity of this technology. A GPS meant primarily to plug into a laptop or to simply tell you where you are in latitude / longitude can be purchased for under $100. Currently portable GPS units to not have enough memory to hold all the street and topographical information for the US. A complete digital map of the US is about 4 gigabytes, and requires a DVD or about seven CDs. So, a laptop-GPS combination is considerably more powerful than a stand- alone GPS. You can get a Pharos PCMCIA GPS unit for your laptop, about $155 including MicroSoft Streets and Trips.
Some people prefer a hand-held unit, typically about 6" tall, 2" to 3" wide, ½' to 1½" thick, 5 to 10 oz. These are the best choice for backpacking or hiking. Some of these units have downloadable databases that let you prepare your GPS in advance with highly detailed maps of where you are going. Some can auto-route, finding you a route from where you are to where you are going. The maps may be street oriented or topographically oriented.
Some people are only interested in using their GPS in a car. They can choose a unit which is 3" tall, 6" wide, 4" thick, and up to 5 pounds. These units typically cannot accept topographic data - another sign of the immaturity of this market.
The two largest companies in this field are Garmin and Magellan. There are many other companies making inexpensive GPS units, but they do not have capabilities to download detailed map info and compute routes. With the other brands of GPS, I think you would have to have a map with you at all times.
I use a Magellan Meridian Color with a 256 meg SD ram and MapSend Topo. Using a ram greater than 64meg in the Meridian series requires you also have a SD reader on your computer. I paid $353 for all of it.
There's a lot of sites on the internet which purport to give info / advice on GPS units. Here's the only one I found that seemed non-commercial: GPS Information.net.
Radar Detectors
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| The Bel RX 65 / 55 is about $330 / $230. | The Passport 8500 is about $250. | The Valentine 1 is about $400. |
There are $100 radar detectors which do an adequate job of protecting you. The Bel Express and Whistler have detectors which are better than the cheap competition, but not as good as the Vector / Escort / Valentine. Ka warnings come in about a two miles, instead of a four miles. Police radar only works at distances of about 1/4 mile, so this is still ample warning for all but the most serious mileage eaters.
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| The Bel Express 925 is about $100. | The Whistler 1748 $100. |
I use a Bel 926.
Everything you could ever hope to learn about using radar and laser detectors is at RadarTest.com and Speedzones.com.
All About Radar Detectors
Here are some basic radar detector facts. Police radar guns operate in one of three frequency bands, called X, K, and Ka. The Ka band is much harder to pick up than the other two; it is Ka performance that separates the men from the boys in radar detectors. X band is left over from the Nixon era, and shared with grocery store's door openers, etc. There are fewer then 3,000 X band radar detectors left in the US, and no one is buying new ones. Don't worry about it. Currently, most radar detectors in service are K band, but police are only buying new Ka band guns. Within a few years, all police will be using either Ka or laser. If it weren't for Ka, you could buy any $19 radar detector and be just fine. There are a lot of radar detectors based on antennas made in Korea. None of these detectors do a decent job of picking up Ka. Sorry, the simple fact is that for adequate protection, you're going to have to pay decent money for a US made unit.
Current Bell, Escort, and Valentine radar detectors can pick up Ka at over 4 miles in a straight line, but can only manage about a quarter mile around a corner. A new police radar, the Bee III with its "Pop" mode has proven to be invisible to most radar detectors - the Bel and Escort pick this up. Michael Valentine has written up a long diatribe noting that the BEE III users manual says that pop mode readings are not evidence quality. I think this means he's unable to figure out how to detect it. Tests confirm that the Valentine-1 does not handle pop radar well. Unfortunate: this guy used to be really good. Mr. Valentine is technically correct, legally pop mode readings are not admissible in court, but if the cop lies about how he used his gun you're toast. The good news: there are actually very few of these pop mode radars in use in the US, only about 1% of the active units.
RDs are illegal to use or possess in Washington DC, Virginia and Canada outside of British Columbia, Alberta, and Saskatchewan. It is illegal in the entire US for an 18 wheel truck to use a radar detector. In these areas you will sometimes run into a cop with a device called a VG-2. This device is a radar detector detector. It works by looking for what's called the intermediate mixing frequency which all super-heterodyne radar detectors use. Sorry, that was engineer speak. VG-2s can spot a typical RD at about 300 yards. They can spot a Valentine 1 at about 4 feet, so as long as you don't run over the cop, you should be ok. Many RDs claim to be shielded against the VG-2, but their performance does not always back up their claims.
There's a new radar detector detector from Australia, the SMS Spectre II RDD. The Spectre is in use at least in Ontario, Canada, and in Texas. If you're detected by the Ontario police in Canada, you are subject to confiscation and a $1000 fine. Virginia cops are more to the point: they like to run over your radar detector with their car, then hand it back to you. As of 4/04, every radar detector including those made by Beltronics, Escort, and Valentine is visible to this device at a distance of 1000 - 1500 feet. The Spectre can't positively pinpoint a specific vehicle that contains a radar detector. It merely provides "reasonable suspicion" that a device in the area has activated the RDD. If you are the only vehicle in the area, an officer may stop you decide he has probable cause to search your vehicle. The only way to beat the Spectre is to turn your radar detector off.
Police sometimes have a pocket-sized radar detector detector, which they use after they have stopped you and are talking with you. Solution: turn off your radar detector at the first sign of being pulled over.
Some people make claims about jamming radar. This can be done in a technical sense, but it requires something that can broadcast several watts of power. I have a friend who's done it, but the device was the size of a VCR and used about 100 watts. It was also wildly illegal. Lotsa fun, though. The radar jammers you can buy don't broadcast anything, they "mix up and reflect" the incoming signal. They don't work. Car and Driver has never seen one work, and as an electrical engineer, I'm here to tell you it would drop my jaw if one did. There are kits offered from time to time in the back of Popular Electronics and Popular Science so that you can build your own radar jammer. It's illegal for a US citizen to possess or use a device which broadcasts at police radar frequencies. The FCC would raid and shut down any company which actually made a real live radar jammer, which is why you can only buy a kit. So, you've been warned, now go make your own decision.
Police laser guns work by
reflecting a laser beam off your car and computing your speed from that. Most of a car or motorcycle is not very reflective
to a laser gun. By far the brightest thing on a car is the license plates, which are now made reflective for just this purpose.
The hope with a laser detector is that you'll pick up beam fringe or scatter before the gun gets a reading on you. While you're
at it, you should probably also hope for world peace, honest politicians, and to win the lottery. By the time a RD has picked
up a laser beam, it's nearly certain that the cop has already got your speed, so a laser detector could mostly also be called
a ticket detector. Laser detectors currently don't work past about 3/4 of a mile. The closer you get to a policeman with a
laser gun, the smaller the laser beam spread, and the less likely your laser detector will pick it up. At 300 feet, a policeman
can get your speed with a laser gun and your detector won't go off, regardless of brand. At 500 feet, only a Valentine-1 and
Passport will go off. At 1000 feet, most laser detectors will tell you that you just got a ticket. The latest development is a
laser gun attached to a video camera, so the cop can show a movie of you speeding to a judge. You're not going to detect this
in time, and you're not going to beat it in court.
It is possible to buy a working laser jammer. Three actually work: the Bel LaserPro 904, $250, the Blinder, $350, and the Escort Shifter ZR3, $450. The rest don't work. California, Minnesota, Oklahoma, Nebraska, Virginia, Washington D.C. and Utah have specifically outlawed the sale, possession, or use of laser jammers. But, they have to catch you first. The Blinder doesn't light up unless it detects a laser gun. All three light up at frequencies that your eye cannot see.
Some police have radar in their cars that works while they're driving. These guys just leave the radar on all the time. It's really easy to spot these guys with a RD, pretty much any unit will pick them up at about a mile, whereas they can only see you at a few hundred yards. The threat here is if they are behind you. Unless you have a Valentine 1, your only hope is to catch the back scatter of their radar off another vehicle, or a building, or a sign. If you're the only car around driving through Nebraska, you're hosed.
The motorcycle cops often have instant-on radar guns. They "point and click" and only put out a radar beam for a fraction of a second. It's harder to pick up these guys because they're not broadcasting so much. Also, these cops live to write guys in Porsches who have radar detectors, so they like to hide over hills and around curves, where your chance of picking them up before they pick you up is much smaller. If the cop is smart enough, nothing will save you.
Radar / GPS mounts
CycoActive has a zillion different GPS mounting systems.