Portable GPS Navigation Systems

The next best thing to having your own local guide is becoming more affordable and feature packed

If you often travel to new places, have trouble reading maps, or simply hate to ask for directions, you might want to consider a portable GPS navigation system. Once you give it a destination, the system can plot out a route, deliver spoken directions, and display each turn as you drive, or in some cases, walk. Most units let you choose guidance options that include plotting the shortest, fastest, and even a toll-free route. An internal database also includes common points of interest such as gas stations and ATMs, and the nav system can route you to the nearest one. You can even choose a nearby restaurant by the type of food.

Although not always as easy to use as the in-dash systems available on many new vehicles, portable systems are catching up. Features like internal, rechargeable batteries and pre-loaded North America map databases are now commonly included on budget systems. More premium features such as real-time traffic reporting are becoming available on more affordable units, although those often require additional hardware.

Portables have the distinct advantage of being easy to move from car to car, enabling a family to share a unit or lend it to others. Their low weight and small size are well suited to long-distance business travel and vacations by plane (for use in a rental car when you arrive), or to walking and biking tours (for use as a handheld device for).

And with prices from about $250 to $700, portable units are much less expensive than the typical price tag of up to $2,000 for an integrated, in-dash factory system.


WHAT'S AVAILABLE

Many manufacturers offer units that will get you where you need to go, and more are entering the market as sales continue to skyrocket. Systems from Garmin, Magellan, and TomTom have historically scored highest in our ratings. Those navigation-focused companies now face competition from Delphi, Harmon-Kardon, and Rand McNally--none of which were available for our previous test. Our latest report also includes units from Alpine, Audiovox, Cobra, Lowrance, and Navman.

Units have decreased in size since the first portables came on the market, but too small a device is hard to read or operate. We find that the common 3.5-inch screen, measured diagonally, is a good compromise between portability and usability. We found the Gamin nuvi 660 to be particularly easy to use with its wide-screen format, while remaining portable.

Special features like photo viewing, video player, hands-free calling, and live traffic reporting are available in an increasing number of units.

TOP SELLERS

Garmin StreetPilot c330 Vehicle GPS Navigator

Garmin has introduced two new in-vehicle GPS receivers for use in your car or truck, the StreetPilot c320 and Street Pilot c330. We'll be discussing both in each description as both are virtually identical with one key difference. The c320 comes with Mapsource City-Select award-winning software on CD (for installation into a notebook or desktop computer) and uses a 128MB SD card which allows you to download a particular region for GPS mapping as you drive. The c330, on the other hand, comes pre-loaded with highly detailed MapSource City Select street data of the entire United States, Canada, and Puerto Rico. The database features an industry-leading five million-plus points of interest?including hotels, restaurants, gas stations, ATMs, and attractions. So, if you do lots of long-distance continental driving, the c330 may be the better value for you. If you usually stick to driving within a section of the country, the c320 might just be the best for you. Both are exceptional units but you must choose which is best for you. Either way, you can be assured that you'll get where you want to go easily and comfortably. Choose between a three-dimensional navigation view or the more traditional ?bird's eye? overhead view Each boasts distinctive and sleek styling?the only buttons are a rotary volume knob and an on/off switch. The device is secured by a suction-mount cradle that fastens to the windshield for easy portability among vehicles. Users can customize their unit's appearance with an array of colored faceplates (sold separately). WAAS-enabled 12 parallel channel GPS Built-in patch antenna; MCX-type connector for optional external GPS antenna connection Unit dimensions - 4.4? W x 3.2? H x 2.8? D

TomTom ONE Portable GPS Vehicle Navigation System

Coming to the United States after making a splash across Europe, the TomTom ONE is a complete portable GPS vehicle navigation device with an incredible range of features. A sleek design, comprehensive wireless capabilities, and a sharp display combine to make the TomTom ONE a vital companion on the road. Like TomTom's other navigation systems, ONE gives you coverage in all fifty of the United States....

Garmin Nuvi 370 Pocket Vehicle GPS Navigator and Personal Travel Assistant with European Maps

The Nuvi370 GPS Navigator and Personal Travel Assistant builds upon the best selling features of the Nuvi family while adding pre-loaded map coverage of North America and Europe. Nuvi370 also offers Bluetooth wireless technology for hands-free calling when paired with compatible phones. Users simply tap Nuvi's screen to answer calls and then speak directly into the unit's built-in microphone. The Nuvi370 is packaged with Garmin's GTM 12 FM TMC traffic receiver, which allows users to avoid traffic tie-ups by simply pushing a button to calculate a new route. The Nuvi370 comes preloaded with highly detailed City Navigator NT road maps for North America and Europe. This map database features points of interest, including hotels, restaurants, gas stations, ATMs and attractions. The map data is provided by NAVTEQ, a world leader in premium quality mapping. Over 1GB of internal memory for storage of supplemental maps, MP3s and audio books Configurable vehicle icons allows users to select a fun, customized car-shaped icon Audible and visual navigation instructions and warnings Offers a three-dimensional mapping perspective or 2-D overhead view High-sensitivity GPS receiver for improved performance and reception Built-in lithium ion battery Battery life - 4-8 hours depending on usage Simplified PC connectivity using USB mass storage with access to either the SD card slot or to the unit's internal memory directly from your PC desktop With the POI loader program, users can set up proximity alerts for school zones, safety cameras, create custom POIs and more Unit Dimensions - Width 3.87 x Height 2.91 x Depth 0.87 inch Unit Weight - 5.1 ounces

Garmin StreetPilot c340 Traffic-Ready Vehicle GPS Navigator

Package Includes: StreetPilot c340, windshield mount, USB cable, Lithium-ion battery The new Garmin StreetPilot c340 provides in-car navigation that's simple to use and offers use-friendly options and control. The c340 gives you turn-by-turn directions and turns text into speech -- and if you miss a turn, it will automatically calculate a new route. Follow directions with a color-coded map and 6 million different points of interest. 3.5 LCD display with backlit touch screen Offers 3D mapping or 2D overhead view Built-in 12V power adapter/speaker for external power in your vehicle Integrated suction cup mounting system, for easy adjustment & quick release Lithium-ion battery for trip planning - approx. 4-8 hours battery life USB interface Dimensions(WxHxD) - 4.4 x 3.2 x 2.8 Weight - 9.4 oz.

Garmin Nuvi 350 Pocket Vehicle GPS Navigator with Maps for North America

What if one device could help you navigate anywhere in the U.S. and Canada, while offering travel tips, storing your favorite tunes and photos, providing translation assistance, and more? That device is here, and it's not much bigger than a deck of cards. The Garmin n¼vi 350 is set to revolutionize what we expect from a GPS navigation device, or from any device for that matter....

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HOW TO CHOOSE

None of the navigation systems we've tested is perfect. They don't substitute for local knowledge, and all databases had minor errors. But a portable system will usually get you there, guiding you on the way and providing you with peace of mind when you are traveling to unfamiliar areas. The highest-rated models make it especially easy to enter destinations and they give the most helpful directions.

Navigation is the priority. We recommend focusing first on how well the system works for navigation, using the ratings to prioritize nav features and map database coverage, over entertaining, nonessential extras.

Built-in battery convenience. Look for a unit with a built-in battery that will operate for at least four hours on a charge, especially if you want to use it for walking. While all systems include a plug for your car's 12-volt outlet, a built-in battery leaves you the option to use the power port for another device, such as a cell phone, and it eliminates cord clutter. A battery also enables you to pre-load a route before you enter the vehicle. Some models are also packaged with a traditional AC plug for in-home use and recharging.

Text-to-speech capability. A system that speaks street names rather than says simply "turn left" can help you negotiate an unfamiliar area and is especially useful in urban driving, where streets can be close together. It is a handy feature that reduces the need to take your eyes off the road to scan the on-screen map.

Avoiding traffic. A system with traffic-reporting capability can be helpful if you travel a lot in congested cities that have good traffic-monitoring coverage. Between the map and detour functions, the system can help you to route around traffic-slowing problems. But just like with traditional radio traffic reports, there are some weaknesses in the nav-system-based services, specifically related to the timeliness of the report.

Modern features. A full-featured model can effectively upgrade an older car with features like Bluetooth hands-free telephone capability, MP3 player, an iPod connection, and an FM transmitter.

Travel gear. If you travel outside the United States, look for a unit that offers maps for navigating overseas. Most will function in the U.S. and Canada, but some models, such as the TomTom Go 910, will also work in Europe and other countries. Of the tested group, only the budget-priced Magellan RoadMate 2000 is pre-loaded with maps limited to the 48 contiguous states.

Size matters. The more portable the unit, the better--especially if you frequently pack it in a suitcase. Some are no bigger than a wallet and weigh less than 7 ounces, while others are as large as a paperback book and weigh considerably more--two pounds or more.

Mounting. Most models mount to your windshield using a suction cup attached to either a ball-in-socket, rigid, or gooseneck-type arm. We find the rigid arms are better at holding the units in place, especially over bumps. (Note: Windshield mounting is prohibited in California and Minnesota.)

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed and direction.

Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr. John Walsh, a key decision maker when it came to the budget for the GPS programParkinson, B.W. (1996), Global Positioning System: Theory and Applications, chap. 1: Introduction and Heritage of NAVSTAR, the Global Positioning System. pp. 3-28, American Institute of Aeronautics and Astronautics, Washington, D.C.). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year, including the replacement of aging satellites, and research and development. These costs are funded by the US and GPS is free for civilian use as a public good.

GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Navigation signals

Each GPS satellite continuously broadcasts a Navigation Message at 50 bps giving the time, an almanac and an ephemeris. The almanac consists of coarse orbit and status information for each satellite in the constellation; a complete almanac transmission takes 12.5 minutes, and is responsible for the long initial acquisition process when a new receiver is first turned on. The ephemeris gives the satellite's own precise orbit and is transmitted every 30 seconds. The almanac assists in the acquisition of other satellites, while an ephemeris from each satellite is needed to compute position fixes using that satellite. The ephemeris is updated every 2 hours and is valid for 4 hours.
The time needed to acquire it is a significant element of the delay to first position fix when a receiver is switched on after having been off for several hours.

Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise (P) code, which is usually encrypted and reserved for military applications. The C/A code is a 1,023 chip pseudo-random (PRN) code at 1.023 million chips/sec so that it repeats every millisecond. Each satellite has its own C/A code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency. The P-code is a 10.23 megachip/sec PRN code that repeats only every week. When the "anti-spoofing" mode is on, as it is in normal operation, the P code is encrypted by the Y-code to produce the P(Y) code, which can only be decrypted by units with a valid decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user. Frequencies used by GPS include

* L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code.
* L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
* L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.
* L4 (1379.913 MHz): Being studied for additional ionospheric correction.
* L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2008.
Calculating positions

The coordinates are calculated according to the World Geodetic System WGS84 coordinate system. To calculate its position, a receiver needs to know the precise time. The satellites are equipped with extremely accurate atomic clocks, and the receiver uses an internal crystal oscillator-based clock that is continually updated using the signals from the satellites.

The receiver identifies each satellite's signal by its distinct C/A code pattern, then measures the time delay for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number as the satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorangehttp://gge.unb.ca/Resources/HowDoesGPSWork.html.

The orbital position data from the Navigation Message is then used to calculate the satellite's precise position. Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. When four satellites are measured simultaneously, the intersection of the four imaginary spheres reveals the location of the receiver. Receivers known to be near sea level can substitute the sphere of the planet for one satellite by using their altitude. Often, these spheres will overlap slightly instead of meeting at one point, so the receiver will yield a mathematically most-probable position (and often indicate the uncertainty).

Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism: if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite. In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators. RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective.
Accuracy and error sources

The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about 1% of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only the GPS C/A signal.

Position accuracy can be improved by using the higher-chiprate P(Y) signal. Assuming the same 1% bit time accuracy, the high frequency P(Y) signal results in an accuracy of about 30 centimeters.

Electronics errors are one of several accuracy-degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy.

GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset (19 seconds) with International Atomic Time (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC, which as of 2006 is 14 seconds. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, at the current rate of change of the Earth's rotation, is sufficient to last until the year 2330.

As opposed to the year, month, and day format of the Julian calendar, the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6 1980 and the week number became zero again for the first time at 23:59:47 UTC on August 21 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation messages use a 13-bit field, which only repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137.