While these location-sharing services have some interesting possibilities, they also raise some disturbing implications for privacy — or maybe it just seems that way if, like me, you’re over 35 years old. Lately I’ve been testing a cellphone location-sharing service that I found simple, useful and non-creepy enough that I can imagine people thirtysomething and older using it.

Send a ‘Glympse’ to share your location

The free service is called Glympse, from a company of the same name that has designed it to share your location with friends and colleagues in small increments of time — glimpses, as the name implies, of your whereabouts. Glympse just released a test version of the service as an application for the G1, a phone offered by T-Mobile that runs Google’s (GOOG) Android operating system.

The company will release versions of Glympse for BlackBerry, Windows Mobile, iPhone and Nokia (NOK) phones in the coming weeks. Users can download the Glympse software onto G1 phones through Android Market, the online clearinghouse for applications for Android phones.

I used Glympse on an iPhone and a G1 and, for comparison, tried out a couple of other location-sharing applications, Google Latitude and Loopt. When you start the Glympse application, it identifies where you are on a map using a combination of location technologies in cellphones, including GPS satellites, Wi-Fi hot spots and phone towers.

Your location isn’t shared with others until you “send a Glympse” to someone. The software allowed me to send a Glympse with my location for selected chunks of time lasting anywhere from zero minutes to four hours. Picking zero minutes shared only my location at the moment of sending, while selecting four hours meant the recipients of my Glympse could track me for that period of time, wherever I went.

The sender of a Glympse can address it by entering a recipient’s email address, or a mobile-phone number for a text message. Recipients get a message with a link to a map on a Web page. A nice thing about the service is that it doesn’t require recipients to have the Glympse software, though the experience is richer when they do.

Sending a Glympse can be helpful anytime a friend, family member or colleague is expecting you. You can send a Glympse that lets a friend know you’re stuck in heavy traffic (although it’s wise to do that before you’re on the road to stay safe and, in some states, to avoid breaking the law). Parents can insist that teens send a Glympse when they go out for the evening.

Bryan Trussel, the CEO of Glympse, sent me several Glympses with short messages like “late lunch meeting” and “headed home” to explain where he was going. On the G1, I could see an icon representing Mr. Trussel moving around a map as he drove through the streets of Redmond, Wash., including his speed. I was also able to see traffic conditions, which would have been helpful if he had been running late. Glympse gave me a similar view of his location through a Web browser running on my PC. On the iPhone, the experience was more static, forcing me to reload a Web page to get a fresh view of Mr. Trussel’s whereabouts. Glympse says it will fix this so users of the iPhone can watch someone’s location in real time.

I encountered a bug with the service when I sent a couple Glympses to my own cellphone and that of a colleague — both BlackBerrys and neither of which had Glympse software. Both Glympses linked to high-level maps suggesting I was somewhere in North America, which wasn’t very helpful, even if it was technically accurate. The company couldn’t figure out what the problem was, and it eventually stopped happening for me too.

Once a Glympse expires, the service no longer tracks the sender’s location. It will show the sender’s last known location for 48 hours after the Glympse expires. Glympse may some day use your location information to target advertising to you, but the company isn’t doing that for now.

I’m far more comfortable with this form of finite location-sharing than the approach used by other services. Google Latitude lets you share your location to only a list of friends. It gives you the option of turning location sharing off or, through a “city level” option, it reveals only your general whereabouts. Loopt similarly allows you to set options to show, or hide your location from all or some online friends. A feature called Loopt Mix lists strangers you can send messages to in your general vicinity — many of whom seemed to be looking for romantic partners in my area.

Despite these controls, I easily forgot I was sharing my location with these two services. This might be fine for someone who enjoys being tracked down by friends during a night out on the town or is diligent about changing privacy settings when they want to go stealth.

I’m too lazy to manage my privacy so closely though. I don’t mind giving friends a look at my location, but only if I know the invitation isn’t open-ended.

• Email nick.wingfield@wsj.com.

Amazon (AMZN) launched a new service called Amazon Video on Demand, which allows users, for a fee, to watch any of 40,000 TV episodes or movies, in real time, on their Windows and Macintosh computers, and on specially equipped Sony Bravia TV sets. This service replaces an earlier Amazon video offering called Unbox.

I have been testing Amazon Video on Demand and I found it worked well, although it has some limitations. The user interface is clean and smart, the quality is good if you have a fast Internet connection, and there are some clever features. On the downside, it works poorly with the slow connections typical in places like hotels. And there are some studio-imposed limits on what content is offered and how you can view it.

To date, there have been three major models for legally getting TV shows and movies from the Web.

In one, best represented by Apple’s (AAPL) widely used iTunes store, the TV shows and movies are ad-free, but you pay for each. All of this content is downloaded to your Windows or Macintosh computer or your Apple TV set-top box for later viewing, even when you’re not connected to the Internet — though on Apple TV you can start watching while the material is being downloaded.

A second model, best represented by the studio-owned Hulu.com, presents movies and TV shows on a Windows or Macintosh personal computer free of charge, but requires you to watch commercials that can’t be skipped. The TV shows and movies you view on Hulu are “streamed” rather than downloaded, meaning they are meant to be viewed immediately, in real time, rather than stored for viewing when you’re not connected to the Internet.

The third online model is best represented by Netflix (NFLX), the popular DVD distributor. It offers a “Watch Instantly” streaming option on Windows PCs or on TVs equipped with a special set-top box, for a small portion of its large catalog of TV shows and movies. These videos are ad-free and don’t carry an individual charge, but require a monthly subscription fee.

The new Amazon Video on Demand service is a hybrid of these others. As on iTunes, the TV shows and movies it offers are ad-free and purchased individually, don’t require a subscription, and work on both Windows and the Mac, plus on one type of set-top box. In Amazon’s case, that’s Sony’s Bravia Internet Link, a $299 device that works only with Sony (SNE) TVs.

But, like Hulu and Netflix and unlike Amazon’s older Unbox service, the new Amazon Video on Demand service offers videos via real-time streaming. In many cases, it also allows downloading, iTunes-style, to Windows PCs (but not Macs) and to TiVo devices attached to a TV. The videos can’t be streamed in real time using a TiVo (TIVO).

Amazon’s streaming videos are viewed in any of the major Web browsers and don’t require any special software. I tested the new service on both Windows PCs and Macs, and on a Sony Bravia TV equipped with the $299 adapter box. In my tests, Amazon’s videos looked quite good over a fast Internet connection. However, on a typically lousy hotel Internet connection, the movies were often grainy and kept stuttering.

The new service doesn’t yet offer videos in high definition, something Apple just announced this week it is adding for some TV shows. Amazon says it is working on HD.

Amazon’s user interface for the new service is very nice. All of your purchased videos are available in a library stored on the company’s servers, so you can easily watch them again and again if they are purchased, or resume a partly watched rental. And the service remembers where you stopped watching a video and resumes it at that point, even if you started it on, say, a Dell (DELL), and resumed it on a Mac.

On the downside, the new Amazon service isn’t always simple or consistent. For instance, you can watch only two videos at a time, and not all titles can be either streamed and downloaded, or be either purchased or rented. Most rentals last 24 hours, but some differ.

Selection was OK, but not great. Because of studio policies, many current and recent movie hits aren’t available. There are gaps as well in the TV selection. For example, while iTunes offers the current second season of the excellent “Mad Men” series, Amazon has only the first season.

Prices also can be confusing. Amazon rents most movies for $3.99 and sells them for between $9.99 and $14.99. TV shows generally cost $1.99. But some titles carry different prices, albeit these are often lower.

All in all, Amazon Video on Demand is a good service for people who prefer paying for ad-free TV shows and movies, and is another strong step in the Internet’s rising competition with traditional TV.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.

Beginning this summer, as soon as next month, wireless Internet access will arrive in the passenger cabins of some commercial U.S. airliners.

On these Internet-equipped planes, any passenger with a Wi-Fi enabled laptop — or a cellphone with Wi-Fi — will be able to do almost everything he or she could do online at home or at the office. That includes surfing the Web, using email, having instant-messenger text chats, downloading and uploading files, and streaming video and audio.

In fact, I did all these things a few days ago on a test flight using the new system, called Gogo. During the flight from San Francisco to Denver, on a small test jet, I could operate online as if I were sitting at my desk, or in a Starbucks. I used Dell (DELL) and Apple (AAPL) laptops, a BlackBerry (RIMM), a Windows Mobile phone and an iPhone to perform all the most common online tasks, while soaring over majestic mountains and glorious national parks.

I sent and received emails on Microsoft (MSFT) Outlook and Apple Mail, including messages with hefty attachments. I conducted IM chats on AOL (TWX) Instant Messenger and Google (GOOG) Talk. Using all the major Web browsers, I called up dozens of Web sites, and watched video clips on Hulu and YouTube. I downloaded photos, songs, PDF files and Microsoft Office documents. I used all the Internet functions on the iPhone, and on the Wi-Fi-equipped BlackBerry and Windows Mobile phone.

One important caveat: Gogo is a data-only system. It doesn’t allow phone calls and will block all services that allow voice conversations to be made over the Internet.

Gogo will launch on three American Airlines (AMR) routes, likely in July. The first planes to use it will be American’s 15 Boeing 767s flying between New York and Los Angeles, San Francisco and Miami. Later in the year, Gogo will be available on all of Virgin America’s small number of routes, and possibly additional American routes, if the first deployment works well. It’s supplied to the airlines by a Denver-based company called Aircell, which says it is in negotiations to offer the Gogo service on several other major U.S. airlines by next year.

The Gogo service will cost a flat fee of $12.95 for flights of three hours or longer, and $9.95 for shorter trips. You log into Gogo as you would any commercial Internet service, registering on a special Web page. Aircell plans to allow advance sign-up, so you’d only have to enter an ID and password on the plane. No add-on software, hardware or cables are required.

A few Web functions will be offered free from Gogo, including access to the American Airlines Web site, to Frommer’s online travel guides and to a limited selection of articles from The Wall Street Journal.

Gogo isn’t the first in-plane Internet service. A few years ago, Lufthansa (LHA.MU) offered a satellite-based service from Boeing (BA), mainly on over-ocean flights, but it was canceled.

The service operates at respectable, if not blazing, speeds — similar to what you’d get on a cellular broadband service or a slow home DSL line. On my test flight, download speeds varied from 266 kilobits per second to about 1.4 megabits per second, with the most typical speeds hovering between 500 and 600 kbps. Upload speeds were between 250 and 300 kbps. I found that most of the tasks I tested, except for streaming video, felt smooth and normal.

Speeds could degrade on a large plane with scores of people online simultaneously. But Aircell claims it has the technology to make my experience representative for anyone doing common tasks, such as Web surfing and email. During my test flight, eight laptops and six Wi-Fi-enabled smart phones were using the system simultaneously. All registered decent speeds, except for a couple of minutes when the plane was crossing between the zones controlled by the company’s ground-based towers.

Aircell gets Internet access to the planes through a network of 92 towers scattered across North America. These essentially are cellphone towers, carrying a high-speed cellphone data signal, except that the Aircell antennas point up, into the sky. A receiver on the underside of the aircraft picks up the signal, which is then distributed through the plane via Wi-Fi.

The companies say Gogo is safe and won’t interfere with the plane’s operation. It is government-approved, and pilots can shut the system off should they deem it necessary.

Gogo has some limitations. The service plans to allocate its capacity so that low-bandwidth activities like Web surfing and email take priority over high-bandwidth ones like streaming video. That means you may find video to be slow and halting.

And Gogo is a North American, land-based service only. It won’t work over the oceans and, for now, it won’t work on other continents.

But for U.S. travelers who want to stay connected in the air, Gogo does the job.

• Find all of Walt Mossberg’s columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com. Email him at mossberg@wsj.com.

Starting this week, just such a smarter navigation box is hitting the market. Called the Dash Express, this $400 product looks a lot like units from better-known firms such as Garmin (GRMN) and Magellan. Like them, it uses GPS satellite signals to locate your car on an easily seen map, and to route you to destinations and places of interest, using both visual and spoken instructions.

But, unlike any other in-car navigation device I’ve seen, each Dash Express, from a Silicon Valley start-up called Dash Navigation, becomes part of a network, connected to the company via the Internet. Each device not only receives and displays information, but transmits it as well, acting as a “probe,” as Dash calls it, to measure local traffic speeds. This information is compiled by the company and then broadcast back to all other Dash units in your area, almost instantly painting streets on your map with color codes to indicate traffic speeds.

I’ve been testing a Dash Express in and around my home base of Washington, D.C., and, while it isn’t perfect, I like it a lot. If the company sells enough units to create a solid network, Dash could radically improve in-car navigation.

That “if” is the big catch with Dash — in order to get its special benefits, enough units must be sold in your city to feed the network with sufficient traffic data. According to the company, for most cities just “several hundred” units would be enough to provide more than half of the significant traffic data it requires for major roads during normal commuting hours.

Meanwhile, Dash, like some of its competitors, makes use of limited traffic data provided by a commercial vendor. This information, which mainly covers major highways, is presented as a dotted line on the Dash maps, to indicate that it may be stale. By contrast, fresh input from Dash’s own network is presented as a solid line.

For the next 30 days, the Dash Express will be available exclusively at Amazon.com (AMZN) for $399.99. You get three free months of Dash’s service, including traffic and other features, such as wireless map updates. After that, the service costs between $9.99 and $12.99 a month. That fee includes the cost of the Internet connection used by the Dash Express, which is achieved using both cellphone and Wi-Fi networks.

To test the Dash, I had to create a tiny two-car network. My colleague Katie Boehret and I each drove the same route in Dash-equipped cars, about 15 minutes apart. The route included everything from the smallest residential streets to large local commuter arteries to the jammed Washington Beltway.

Katie went first, and by the time I retraced her route, my Dash unit’s screen was ablaze with solid-color streets indicating the traffic speeds she had encountered: green for free-flowing traffic, yellow for moderate congestion, red for stop-and-go conditions. Even two-lane local roads, the kind where traffic data are almost never available, were colored in.

Once Dash begins selling, the company won’t rely much on the information provided by a single driver like we did. It will average and weight the information it receives, to eliminate odd results from especially fast or slow drivers, and to emphasize the newest data. Each Dash reading will time out after no more than 25 minutes, turning solid lines into dotted ones as a warning that the information may be old.

I did run into a couple of glitches during the test. For one small road Katie had traveled, I received no Dash data. And on the return trip, Dash tried to route me right into a Beltway traffic jam, even though its screen showed that area in red. The company is working a future feature, called My Route, that would allow savvy drivers to order the device to use the local routes they prefer, to avoid such jams.

Dash Express has a host of other nice features, explained online at dash.net. Instead of giving you one route to your destination, it offers three choices, one of which supposedly incorporates current traffic conditions. It allows you to type in a destination on a personalized MyDash Web page and have that address sent wirelessly to your Dash unit, ready to be selected. You could even have a colleague or friend send you a destination while you are driving, so you don’t have to pull over to type it in or, worse, try typing while driving.

And the Dash also connects to the Internet to perform searches for local businesses, and then routes you there. Plus, you can create your own lists of favorite places and points of interest, or share those created by others and send these to your Dash from the MyDash Web site.

Dash Express finally brings the power of the Internet, and of community information, to auto navigation. If it becomes popular, it could be a big deal.

• Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.

But for some folks, getting the most out of these players takes too much work or too much money. Converting CDs to music files takes time, as does selecting and downloading tracks from online music services, and synchronizing players with PCs. Creating great playlists also takes time and effort. Some people prefer the old radio model, where songs are programmed by somebody else and you just listen.

Plus, whether you download songs for 99 cents apiece, use a subscription download service for $15 a month, or sign up for satellite radio at $13 a month, the costs to keep your portable player filled legally can add up quickly.

So a new kind of portable player, one for more passive and budget-minded users, is slated to arrive late next month. It’s called the Slacker Personal Radio, and its name is meant to refer to people of any age who just want to sit back and listen instead of actively managing their music.

The new Slacker players will come in three models, ranging from $200 to $300, depending on capacity. But the music they play will be absolutely free, contained in preprogrammed Internet radio stations instead of individually selected songs and albums. The stations will be automatically refreshed with new tunes via a wireless connection built right into the device. You’ll have to be near a hot spot for these updates. But you won’t need a hot spot just to hear your music, because the songs are cached on the device. And you’ll never have to plug it into a computer.

The player is tied to Slacker’s free Internet radio service, slacker.com, which is already up and running, and allows you to listen to music via any standard Windows or Mac Web browser. Using the service, you can personalize your player by selecting from over 100 canned stations or by creating stations based around any of 10,000 artists. These stations will be beamed to your player wirelessly. You can even choose which stations are loaded onto your player before the company ships it to you.

The company, a San Diego-based start-up of the same name, hopes to make money eventually via advertising on the player, and by selling an optional paid premium plan that offers some additional features.

I’ve been testing a prototype of the chunky, black plastic Slacker player, which is dominated by a four-inch color screen. It has two redundant navigation systems: a touch-sensitive strip at the side of the screen and a wheel on one edge. It provides a rich listening experience, including album art and other photos, artist bios and album reviews. The sound is good, and the Wi-Fi wireless connection worked in both my home and office.

The two prototype Slacker units I tried, however, were hobbled by bugs and glitches that the company must expunge by the release date, which was originally slated to be this month. For instance, the players sometimes failed to wake up after going to sleep, requiring a reboot. The touch strip was unreliable. One player failed several times to connect to my account. Battery life is well below Slacker’s goal of 12 hours between charges. The company says it is aware of these problems, and pledges all will be fixed.

Slacker isn’t the only portable player to offer programmed Internet stations. The Rhapsody service offers similar, customizable Internet-based stations on a couple of players. And both the Sirius and XM satellite-radio networks offer portable players for listening to their stations, although the stations can’t be customized. But all of these players require monthly subscription payments, while Slacker’s stations are free.

The Slacker Radio

Because Slacker is based on Internet radio, it has some limitations imposed by the rules governing that format. For example, you can’t specify a particular song to play, or skip back to repeat a song. And you can skip ahead only six times per station per hour. Even if you create a station around a particular artist, the station will mainly be filled with artists the service considers similar. Songs by the artist you selected will be played only four times every three hours.

The player has a “heart button” for designating a song for frequent play and a “ban” button to eliminate the songs you hate.

If you’re willing to pay, or put in more effort, you can get additional capabilities. For example, Slacker players can hold and play some of your own songs, in addition to programmed stations, if you download a free Windows software program. And, if you sign up for the premium option at $7.50 a month, you get unlimited song-skipping, no ads, and the ability to save favorite songs on the device and play them as often as you like.

But the basic idea of Slacker is to make portable listening free of effort and of service charges. If the company can wring the bugs out of its new player and if its ads aren’t too annoying, that formula may appeal to some busy music lovers.

Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.

The program lets you view satellite and aerial photos of pretty much any spot on the planet. In big metropolitan areas in the U.S., Canada and Western Europe, you can locate, and zoom in on, individual buildings and houses, and see cars and trees. You can overlay streets onto these urban images, as well as markers indicating restaurants, hotels and more. In other places, you can make out only towns and large geographical features, like lakes.

The program rapidly fetches the images from the Internet and visually “flies” you from place to place around the globe. The process is so fluid it feels like a Hollywood stunt. For instance, if you’re staring at a bird’s-eye view of St. Mark’s Square in Venice and you type in your address in Boston, Google Earth will zoom out till you seem high in the sky, then rapidly “fly” you west across the Atlantic into the U.S., and then stop right over your house.

Google first released the program last week at earth.google.com. But demand was so high that the company’s servers were overwhelmed, so Google is intermittently turning off downloads. You may have to visit the site several times to download the software.

When you first try Google Earth, you’ll want to type in all the places you frequent and see how they look from the air. You’re also likely to call in family and friends to see how cool the program is, which is exactly what I did. I located my house, my office, my old college dorm and the house where I was raised. I wowed visitors by typing their addresses into the program and “flying” them to aerial views of their houses.

It’s an amazing demo and a great example of how much power computers and the Internet have put into the hands of average people. The trouble is, I’m not sure how practical Google Earth is for most people. In many cases, it’s better to look at a map of an area rather than a photo of rooftops. Driving directions can be clearer when superimposed on a map, rather than on a giant photographic landscape where specific features, like signs, can’t be seen in detail.

Google Earth, which runs only on Windows PCs with 3D-capable video cards, isn’t the first software of its kind. Satellite photos have been available online for years, and Google Earth is based on a product called Keyhole that the big search company purchased last year. Microsoft, which has been in this field for a long time, is working on a similar product called Virtual Earth, which is due out later this summer.

But Google Earth is the first free satellite-photo product I’ve seen that makes navigating the globe really easy, and that integrates local databases of businesses and other important sites so you can quickly locate them right on the aerial images. You can search for dry cleaners in Cleveland and quickly see a satellite map of that city with the dry cleaners marked on it.

When you play with Google Earth, you notice some odd things. For instance, the details of the White House roof have been blanked out, presumably to obscure security features and vulnerabilities. And all but the outline of the U.S. Capitol is deliberately obscured. But the Pentagon and CIA headquarters are shown crystal clear, in considerable detail.

Confusingly, Google offers another satellite map service besides Google Earth. The company’s Google Maps service, which runs in a Web browser instead of the special Google Earth software, uses the same maps as Google Earth, and it can alternately display a map or aerial photo of a location, something Google Earth can’t do. But Google Earth offers many features Google Maps lacks, and it allows you to zoom in much more on the photos than Google Maps does.

With Google Earth, you can add your own place markers, with custom icons, to the aerial photos. You can save lists of places and return to any of them with a couple of mouse clicks. The program also allows you to save, print or email snapshots of the aerial images you find.

One very cool feature allows you to get driving directions and see your route outlined on photos of the actual roads. You can then click on a VCR-like “play” icon to take a virtual drive along the route.

A big limitation of Google Earth and other similar programs is that they show only rooftops, which tell you little, not the fronts and sides of buildings. Google Earth tries to compensate by adding featureless 3D-drawn images of buildings in some big cities, but the effect isn’t great.

Microsoft plans to leap ahead on this issue. Its Virtual Earth product will include real photos of the fronts and sides of buildings, taken using planes equipped with multiple cameras that capture a 45-degree view. Google says it is working on the same thing, but it has announced no release date.

You may not use Google Earth every day, but it’s worth fooling around with just because it’s cool.

Write to Walter S. Mossberg at walt.mossberg@wsj.com