When checking the specs on a GPS device or GPS-enabled device, you’ll likely see two sets of letters that could be confusing to you: GPS and A-GPS. GPS, as you know, stands for global positioning system. The “a” in A-GPS stands for assisted, as in assisted global positioning system. So, what’s the difference?

A standard GPS works by communicating directly with satellites. These satellites orbit the earth and send down signals. In order to get a “fix” on your location, you have to be in contact with three or more satellites. Each satellite measures the time it takes your signal to reach the satellite and then triangulates your position using this information. This method is extremely accurate, and is typically very reliable on clear days where there is line of sight with the sky.

Assisted GPS also uses satellites to get a fix on your location. The difference is how it gets a fix on your location. Instead of trying to contact satellites directly, A-GPS networks use other resources to help it contact satellites. In most cases, this is a cellular network. That is, an A-GPS device first gets in contact with cell phone towers, which then help the device get a fix on a satellite. This lets you get a fix much more quickly and is more reliable in situations where a standalone GPS unit might have poor reception.

So which is better: GPS or A-GPS?

That depends. GPS units are more common as standalone units, such as car navigation systems and handheld GPS devices. But cell phones tend to rely on A-GPS. Cell phone A-GPS technology varies on other factors. For example, some smartphones use hybrid position systems that combine Wi-Fi positioning systems and cell-site triangulation which further improve the reliability and performance of the location services. However, some cell phones carry only A-GPS technology and cannot fall back on standalone GPS functionality. The latter type may actually be less reliable than standalone GPS in certain situations.

As such, when you’re evaluating GPS and A-GPS devices, it’s best to take them on a product-by-product basis. Not all GPS devices are created equally, nor are A-GPS devices. As a general rule of thumb, the recognized brand names, such as Garmin, TomTom and Magellan are safe bets. In terms of mobile phones, these vary widely within brands and carrier networks. Read plenty of user reviews, if the performance of the location services on a smartphone are high priorities for you.

So, remember: GPS works by contacting satellites directly. They work best in clear conditions, where there aren’t any mountains, valleys or structures blocking the line of sight with the sky. A-GPS works better in these situations, provided that they have decent cellular service coverage. And A-GPS is always better if it also has other technologies to fall back on. With this in mind, if you’ll be travelling internationally, a standalone GPS may be better, since some areas have less coverage for cellular service, and some international cellular networks are not compatible with devices that may work in your home country.