iOS (and other mobile operating systems for that matter) work very well with GPS. Photographs can be geotagged, nearby points of interest can be found, directions can be given, and so on. Part of the reason these devices work so well with location-based services is that they have GPS built right in–all the iPhone’s after the original have it and so does the 3G iPad/iPad 2. So what’s the point of an accessory GPS receiver for iOS devices?For fans of GPS (anything from casual mappers to vacationers) there a few reasons why an external receiver makes sense. The most logical is that not all the iOS devices have GPS built in–aside from the original (2G) iPhone, there are the WiFi iPads, and then all the iPod touch models. These work perfectly well with GPS, they just lack the on-board hardware.AdChoices广告Another reason to add GPS to your iDevice is that the internal receiver, like most internal receivers, isn’t that great. External ones might suck down more power, but they are typically more accurate and more consistent.A final reason, and admittedly a niche case, is when you are traveling internationally with your iPhone. If you keep your smartphone in airplane mode the internal GPS will not work, despite the fact that GPS transmissions aren’t part of your data plan. (GPS can’t be an airplane mode exception, like WiFi.)The Bad Elf receiver is a tiny device (about 1-inch by 1-inch) with a 30-pin connection on one side and Micro USB on the other. It plugs into your iDevice, just like your charger. When iOS sees it built-in logic recognizes the Bad Elf’s GPS signal as being more accurate then that of the on-board chip. The USB port is used for charging the phone/tablet/media player while GPS is active (the Elf is small but relatively power-hungry). Finally there is an LED indicator that tells the user when a signal is being acquired and when it’s locked on.It’s crucial that the receiver be compatible with all your favorite GPS apps. The good news is that the Bad Elf is–they have a long list of them on their site. The top picks like MotionX GPS, TomTom, Navigon, Magellan, RunKeeper, GeoCaching, and Nike+ are all there.The Bad Elf works rather well. With its MTK chipset the receiver more consistent than the iPhone 4’s GPS, but the real benefit is that it can add GPS to my WiFi iPad 2. Sure, at $99 I probably should have purcahsed the 3G model (a $129 upgrade) but this way I can remove the receiver and use it with other products, plus I can use GPS in airplane mode. If I really want to geek out, I can also add an in extension cable and place the Bad Elf a few feet away from my device.There are some issues with the Bad Elf, but they are relatively minor. First there is the price. At $99 this isn’t exactly an impulse purchase, unless you have something special planned for it. Battery life is an issues as well, because if the device isn’t plugged in via USB you’ll drain your battery in a matter of hours. In other words, an iPhone 4 will not get through a full day of GPS usage and I managed to use about 50% of my battery life in 4-5 hours. GPS lock time is a bit slow on the Bad Elf, but it’s usually under a minute from a cold start so it’s barely worth mentioning.Then you have the logistics, which are an unavoidable complication. Here is a scenario: you want to add GPS to your iPad. The Bad Elf will do that, but without a data connection you won’t be able to grab maps in real-time. That means you’ll need to preload maps, which requires an application that works with cached maps. Popular ones like OffMaps and MotionX GPS do this, but you’ll have to make sure you get maps that have sufficient detail and then make sure you cache them before going out. They can range from dozens to hundreds of megabytes so you’ll want a steady WiFi connection. You can record a GPS track without a map, but this won’t be helpful if you want to use the location data for navigation purposes.The Bad Elf with an iPad or iPod touch is great combination. There are some issues that have to be worked around, but if you are using the Bad Elf simply for getting a GPS signal it does a great job. There are complications, like battery life and caching maps, but those both can be worked around. Do keep in mind that the Bad Elf is Apple approved (it even has its own app) while some similar devices require jailbreaking to work.