I was bored today, so I decided to write this about GPSes...hoping it will get stickied for some of the oft-asked questions on this board.
So you find yourself wanting a GPS to take on a ride? First thing to be aware of is that generally, consumer-level GPS receivers fall into a few categories.
There are car GPS receivers.
There are general outdoor receivers.
There are fitness receivers.
There are specialty receivers.
Then there are cell phones.
I won't really address car receivers or specialty receivers (golf, dog tracking, etc) as they are probably the farthest from ideal. Some might work in a pinch, but if you really want a GPS, get something else. A general outdoor model can be had for $100 usually. Sometimes less.
I also won't really address a lot of specific models, as I'd prefer this topic to remain relevant as technology improves.
First, a little bit about how GPS works in the first place. A GPS does NOT send any signals to satellites. The GPS receiver ONLY receives signals from GPS satellites. The quality of this signal depends on a lot of things. It could be impacted by solar radiation in space (solar flares). Since the signal passes through the atmosphere, atmospheric conditions can play a role (clouds, rain, all sorts of things). Terrain plays a role, since most satellites are in the southern sky (northern hemisphere) or northern sky (southern hemisphere, for you kiwis, aussies, and others below the equator). If you are on the north side of a mountain in the northern hemisphere, that mountain will interfere with your reception because the mtn is between you and the satellite. Then there's signal scattering. This is usually from vegetation, but can also happen from terrain, buildings, or anything else only partially in the way. Basically, part of the signal from the sats gets through to your GPS, but some of that signal bounces off something before it gets there, adding confusion. Older GPS receivers could not process this very well, lending to a lot of dropped signals. Newer ones do a much better job of it and have fewer problems. The reason this is confusing to the GPS is because the satellites transmit the time from atomic clocks on board. If parts of the same signal arrive at different times telling the GPS they're both the same time, it presents a bit of a conundrum.
Furthermore, access to the GPS system only costs you the price of a receiver. There are no subscription fees. The government has the capability to scramble the signal (the original use was military after all) but wholesale scrambling of GPS signals was stopped in 2000 (selective availability, it was called). That's when GPS receiver tech took off. Different countries have their own GPS systems, and there are all sorts of technical details I won't go into. This is for newbies who don't have a clue.
GPS receivers come in many flavors with lots of different feature sets. First, I'll address what I call 'general outdoor receivers'. This type has a wide range of useability, from hunting, geocaching, hiking, mountain biking, paddling, all sorts of things. They are typically shaped to fit well in the palm of your hand, though various mounts are available aftermarket to attach them to nearly anything. Because they're shaped for handheld use, button layouts are conducive to that use, too, and these can get kinda big. Some have touchscreens and integrated cameras now, among other things. These features are fancy bells & whistles at this point. Other, navigationally important features include an electronic compass (2 or 3 axis), and a barometric altimeter.
As to how these options work, first I'll talk about the compass. An electronic compass allows the compass feature of the GPS to function more like a real compass. Without this feature, the compass can only tell you what direction you are headed WHILE YOU ARE MOVING. With an electronic compass, you can stop to use the compass, and see which direction is which (or see which direction you NEED to go). This is handy if you travel off-trail (mostly relevant for hiking and hunting, rarely for biking).
The barometric altimeter is a more sensitive way to determine altitude. Without this feature, the GPS calculates your altitude based on distances/directions to satellites. To do so, it requires a signal from at least 4 (assuming the geometry of the arrangement of the satellites in the sky is ideal...more than 4 if those satellites are clustered in one part of the sky). There is a pretty significant amount of error inherent in this method because all the factors that impact GPS signal quality impact this calculation, too. The barometric altimeter is a separate measuring device in the GPS that uses air pressure to determine altitude. Its accuracy is dependent on calibration frequency (did you calibrate it a month ago or right before you started your trip?) and weather. An incoming or outgoing weather system will cause the barometer to drift, showing slightly higher or lower elevations. However, calibrating more often at known elevations can minimize drift. What's extra useful here is that knowing this phenomenon happens, you can use it to predict the weather.
Pretty much all outdoor receivers have similar feature sets. Only a few features are unique to a certain brand or model, and the available features usually changes over time, too. Right now, the ability to upload aerial imagery is a pretty unique feature to one brand of GPS. Eventually, all brands will probably include this feature. Same with touch screens. Integrated cameras aren't unique to one brand, but not all brands have adopted them yet. Some brands will accept basemap data that is freely available. Some require proprietary maps. If considering a general outdoor GPS, look at the extra features like this, more than the basic GPS mapping functions. It's the extras more than anything that mean the difference between one GPS and another.
Now for fitness receivers. These types are mostly intended for folks who want to improve their training and keep track of races and such. So far, there are wristwatch style and bike-specific models, though I wouldn't be surprised to see some company come up with a new idea down the line. Most of these types of receivers sacrifice mapping capabilities (they still record a track or course of your activity) at the expense of fitness options. These often will display more 'metrics' on the ride, essential information for someone doing the same activity repeatedly and tracking improvement. Another option would be an included heart rate monitor. The advantage of including the HRM with the GPS is that you can track heart rate according to elevation or speed or any other number of metrics the GPS tracks. With separate devices, it is difficult to impossible to relate these two data sources. Some cycling specific receivers include an additional speed/cadence sensor. This sensor is not (currently) used for speed data unless there is no GPS signal. It is primarily used for cadence data, or to allow the GPS to be used indoors on a trainer. A limited number of fitness receivers include the mapping capability of a general outdoor receiver. This is good for people who intend to navigate (either roads or trails) in addition to tracking fitness. These receivers allow users to upload the same types of basemap data as general outdoor receivers.
Fitness receivers also have optional feature sets including a barometric altimeter like general outdoor receivers. This allows the GPS to report more accurate elevation values and report elevation gain/loss (which can be important to fitness users). A note on this, however, as it comes up often in the forums. Different software handles this data differently. Some software simply reports these elevations just as the GPS collected them. Some programs will apply 'corrections' to this data. Some programs throw it out entirely and report their own elevations based upon the elevations your position corresponds to on a digital elevation model. It's important to realize that these values can be quite different from each other, and to recognize that the developers are only trying to provide the closest estimates to actual elevation possible. Right now, there does not seem to be a single 'best' method, so we just have to live with the discrepancies. If you want to talk up your ride, it would probably be best to share the highest of the reported values to impress your buddies most. Otherwise, I'm not sure it matters that much.
The final type of GPS I will address is the cell phone. I'm only addressing it at all because with the advent of the iphone, it seems an awful lot of folks want to use it as a training computer. To me, a GPS in a phone is a better navigational tool for the city, not the woods. To use a phone GPS on the bike, you have two options. The first is to toss it into your pack and ride. This is probably safest for the device, assuming you at least put it in a ziploc for rain protection in the event of a sudden downpour. The other option is handlebar mounting. This typically requires a protective case for the phone plus the mount itself. It seems a great deal of options on the market are unsatisfactory, so choosing the right one can be difficult.
Phone GPSes increasingly use touch screens and have integrated cameras, also, like some newer general outdoor receivers. Phone GPS receivers, however, lack barometric altimeters, and only within the past few months has apple introduced an electronic compass. As far as I know, there are no optional heart rate monitors or cadence sensors that will work with a phone GPS.
Phone receivers are heavily dependent on software for data collection/reporting. Few if any have adequate included software, so a user must use an additional program (some cost extra) for GPS data. Reported battery life is also dependent on the program. Some programs seem to drain the battery rapidly, while others make better use of the battery. None give run-times as long as GPS specific receivers, so battery life is still a concern for all-day rides.
The quality of the phone GPS receiver's chipset is also questionable. Most use a combination of satellites AND cell phone tower triangulation to report your location, but provide questionable results when cell towers are not within range.
Lastly, depending on the program used, basemaps may not be available if cell reception is lost. While a specific GPS receiver is able to store basemap data, some cell phone GPS programs do not store basemaps, they simply use a live download from Google Maps or other service.
Despite the limitations, many still use cell phone GPSes on their bikes, and some have been able to overcome those limitations. However, contrary to a specific GPS that does not require service fees, a cell phone GPS requires a service plan that includes GPS and data access. If a user already has a cell phone with a GPS, it may be useful to try (taking into account the need to protect the device from rain, dust, and shock) before buying a specific GPS. However, a cell phone GPS has many limitations when compared to a specific GPS, and certainly more advanced users often want those additional features.
Software to use with your GPS data will get covered in another topic. Software is a HUGE topic and one person cannot cover it all.
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