Preparation for GPS receiver initialization

You really don’t need to know this technical information to operate your GPS receiver, but to start acquiring satellites to get an accurate location fix, a GPS receiver needs the following satellite data:

• A current almanac (rough positions of all the satellites in orbit)
• The GPS receiver’s current location
• The current date and time
• Ephemeris data (precise position of individual satellites)

If some or all the data is missing or out-of-date, the GPS receiver needs to get updated information from the satellites before it can accurately fix a current position. The types of data that are outof-date or missing determine how long the GPS receiver takes to initialize. If the GPS receiver is brand new, out of the box, several hundred miles away from where it was last used, or has been stored for a prolonged period of time, initialization will take longer.

How to Initialize Your GPS Receiver?

Your GPS now has power, so it’s ready to go, right? Well, almost. After you put batteries in your GPS receiver and turn it on for the first time, don’t expect it to instantly display your location. A GPS receiver first needs to go through an initialization process before it can tell you where you are. The type of initialization and the amount of time it takes depends on what information the GPS receiver has previously received from the satellites and when. The process is mostly all automatic, and you don’t need to do much as your GPS receiver starts up and begins to acquire satellites. Your GPS user manual may contain model-specific initialization information. To initialize a new GPS receiver, take it outside to someplace that has an unobstructed view of the sky (such as a large field or a park) and turn on the power. (You did install the batteries first, right?). After the start-up screen displays, the receiver will begin trying to acquire satellites.
It can take anywhere from 5–30 minutes for the GPS receiver to gather enough satellite data to get a position fix for the first time (usually more toward the 5 minutes end of the scale). Don’t worry; your GPS receiver isn’t going to be this slow all the time. After the GPS receiver is first initialized, it usually only takes 15–45 seconds to lock on to the satellites when you turn it on in the future. In order to speed up the location fix for the first time or when the GPS receiver has been moved hundreds of miles since it was last turned on, many GPS receivers have an option where you move a cursor on an onscreen map of the United States or world to show your general location. Providing a general location helps the GPS receiver narrow its search for satellites that are visible from your present location, speeding up the initialization process. Manufacturers often use the terms cold start and warm start to describe different GPS receiver start-up states and times. Unfortunately, their definitions of these terms can be different, which makes comparative information about start times not very useful. Just remember that under the same conditions, with a similar view of the sky and with their antennas optimally orientated, most modern GPS receivers generally take the same amount of time to acquire satellites and fix a location.
Most GPS receivers have a satellite status page that’s displayed while the receiver is acquiring satellites; see an example status page in Figure 5-1. This page typically consists of two circles that represent a dome of sky above your head. The outer circle is the horizon, the inner circle is 45 degrees above the horizon, and the center of the inner circle is directly overhead. The N on the page represents north.
Based on the almanac information, the GPS receiver shows the position of satellites within the circles, representing them with numbers. As a signal from a satellite is acquired, the number is highlighted or bolded. Underneath the circles are a series of bar graphs with numbers underneath them that represent signal strength. The numbers correspond to the satellites that the GPS receiver has located. The more a bar is filled in, the better the GPS receiver is receiving signals from that particular satellite. Try moving your GPS receiver to watch the satellite signal strength change. If signals are weak or you get a message about poor satellite coverage, move to another location and change the position of the receiver to better align it with the satellites that are shown onscreen. If you’re successful, you’ll see new satellites acquired, the signal strength increase, or both. The more satellites you acquire and the stronger the signals, the more accurate your receiver is. Holding the GPS receiver properly will optimize signal reception. If your GPS receiver has a patch antenna, hold it face up, parallel to the ground. If your GPS receiver has a quad helix antenna, hold it straight up so that the top of the receiver is pointing toward the sky.
After the GPS receiver gets enough information from the satellites to fix your location, the screen typically displays an Estimated Position Error (EPE) number. Based on the satellite data received, this is the estimated error for the current position. The smaller the number displayed, which will be in feet or meters, the more accurate your position.
Estimated Position Error (EPE) is a bit confusing. If you see an EPE of 20 feet, it doesn’t mean that you’re within 20 feet of the actual coordinates. You’re actually within up to two times the distance of the EPE (or even more) from the actual location. For example, if you have an EPE of 50, your location could be 1–100 feet of the actual coordinates. EPE is not a maximum distance away from the actual location; it’s only a measurement estimate based on available satellite data. To complicate things even further, different GPS receiver manufacturers use different formulas for determining EPE, so if you set three different GPS receiver brands next to each other, they all display different EPE numbers. Some manufacturers are conservative with their numbers, and others are optimistic. Don’t get too caught up with EPE numbers; just treat them as ballpark estimates — and remember, the smaller the number, the better.

More about GPS battery

After you select the type of batteries you’re going to use, you should be aware of some other issues when it comes to powering GPS receivers:

• Battery life gauges: In the GPS receiver’s setup information page, you can specify what type of battery you’re using, such as alkaline or NiMH. The battery type setting helps the GPS receiver make an accurate guess how long the battery will last. Remember that different battery types have different discharge rates. All GPS receivers also have an onscreen battery gauge that shows you how fully charged the batteries are. If you set the wrong type, the worst that will happen is that the gauge won’t be accurate. See how to extend battery life with some GPS receiver models in the sidebar, “Battery saver mode.” Always check the battery level of your GPS receiver before you head out on a trip and also remember to carry spare batteries. One way to tell which batteries are new or charged is to put a rubber band around the good ones. By feeling around in your pack or pocket, you can instantly tell which ones are fresh. Note: Rechargeable batteries discharge faster than alkaline batteries when they’re not in use, so if you haven’t used your GPS unit in a couple of months, don’t be surprised if those rechargeable batteries are dead or don’t have much life left in them.
• Cigarette lighter adapters: If you’re primarily using your GPS receiver in a car or truck, you can save on battery costs by powering the GPS receiver with a cigarette lighter adapter. These handy devices run a GPS receiver from your car’s electrical system. You can buy a generic version or one made for your model (sold by that GPS receiver manufacturer). Depending on the model, adapters cost between $20–$40, with the generic versions a bit cheaper than the manufacturer models. Cigarette lighter power adapters have straight or coiled cables. Although coiled cables are tidier, if your cigarette lighter isn’t close to the dashboard, a coiled cable can pull your GPS receiver off the dashboard if it’s not securely mounted. Adapters with straight cables don’t have this problem; you can tidy up any slack in the cable with a plastic zip tie.

Battery saver mode

Some GPS receivers have a battery saver mode that can greatly extend the life of your batteries. (Check your user manual to see whether your model has this feature and how to turn it on.) Normally, a GPS receiver processes satellite data every second and determines your speed and location. Based on this information, the GPS receiver predicts where you should be the next time it gets satellite data. If the prediction is close to your actual position and battery saver mode is turned on, the GPS receiver will start receiving satellite signals every five seconds or so instead of every second. In addition, some of the internal electronics are turned off during this wait period. Because a reduced amount of power is needed, the battery life is extended. The GPS receiver continues to access satellite data every five seconds until the predicted location isn’t accurate anymore, at which time it switches back to receiving data every second, starting the process over again. (Some GPS receivers provide you with a number of choices of how often satellite data is received. The more seconds, the more battery efficient the receiver is.)

GPS Battery Resources

You can really get geeky with batteries and powering your GPS unit. If you get a charge out of electricity, here are some links to nitty-gritty information sources that cover voltage, milliamperes, and GPS drainage rates:

• Battery drain for selected GPS receivers:www.gpsinformation.net/main/bat-5.txt. This site offers the lowdown on just how much juice different GPS receiver models consume.
• The Great Battery Shootout!: www.imaging-resource.com/ACCS/BATTS/BATTS.HTM. This site is more orientated to digital cameras (not GPS receiver-specific), but you’ll find some good data on how different types of batteries perform.
• Newsgroups: sci.geo.satellite-nav. Do a Google Groups search in this USENET newsgroup for batteries and be prepared to spend a couple of hours reading through educational (and sometimes controversial) posts.

When you check these sources, you’ll run into mAh, which means milliampere-hours. Most rechargeable batteries like NiMH have the mAh rating printed on their label. This rating is the battery capacity. Typically, the higher the mAh number, the longer the battery will last.

Becoming Familiar with Your New GPS Receiver

After considering all the options, making your list, checking it twice, and finding out which GPS receivers are naughty and nice, you’ve finally come to that blessed event where you’re the proud owner of a GPS receiver. But before you step out the door for a 100-mile wilderness trek or cross-country road trip, intent on relying on your new electronic gadget as a guide, be sure spend some time getting to know your GPS receiver.
A good place to start your GPS familiarization process is with the user manual. Many GPS receivers have a quick-start guide that gets you up and running in a matter of minutes. These guides are perfect for those impatient, got-to-haveit-now people; however, I suggest that you also take the time to read the full user manual. Otherwise, you could miss out on some important information contained in the full user manual.
In addition to the user manual, this section will also help you become familiar with your GPS receiver so you can get the most out of it. Obviously, because so many GPS receiver models are on the market, don’t expect to find detailed operating procedures for your specific model here: You need your user manual for that. What you can expect is basic information that applies to most GPS receivers, including some things most user manuals don’t mention. Based on a number of years of search and rescue experience, I can list numerous occasions when hunters and hikers thought that their GPS receiver was some kind of magic talisman that would prevent them from getting lost. And quite often when the search teams finally found them, they had no clue whatsoever how to properly use their GPS receiver. If you’re going to rely on a GPS receiver for navigating outside of urban areas, take the time to find out how to use it so the friendly, local search-and-rescue people don’t have to come looking for you. I’ll step off my soapbox now, thank you.

Road warriors

If you’ll use your GPS receiver primarily in a car or truck for road navigation, your feature criteria is a bit different than a GPS receiver designed primarily for outdoor recreational use. Although any GPS receiver with a base map or uploadable street maps can help you stay found on the road, some models are more suited to automobile navigation. And although some GPS receivers are designed primarily for automotive use, I prefer handheld GPS receivers that offer versatility because they can be used in a car or for outdoor recreation. Some important features to look for in a GPS receiver that you’re going to use for road navigation include

• Automatic route selection: A GPS receiver designed for street navigation allows you to find addresses, street intersections, and highway exits. Just enter where you want to go, and the receiver calculates the shortest or fastest way to get there. Pretty slick, isn’t it? The selected route is highlighted on the map screen, and your progress is displayed as you travel. If you encounter a traffic jam or other road problem, you can instruct the receiver to recalculate a new route from your present position and avoid the problem.
• Turn here directions: The GPS receiver lists all the streets and roads in your route at which you’ll need to make turns, including the street name, an arrow that points to the correct turning direction, how far ahead the turn is, and how long it’s going to take to arrive at the turn. The GPS receiver gives an audible or visual signal prior to when you need to turn.
• Points of interest: Maps that are used with road navigation GPS receivers have databases of information about gas stations, restaurants, freeway exits, hotels, attractions, entertainment, shopping, and emergency services along your route. These are dubbed Points of Interest (POIs); the GPS receiver can display information about specific POIs.
• External antenna support: Because the metal body of a car or truck might interfere with satellite signals, an external antenna might be required to connect to the GPS receiver. An external antenna also provides you with more mounting location options because only the antenna (and not the GPS receiver) needs to be mounted someplace with a clear view of the sky. Note: Some heated windshields can block satellite signals. In cases like that, you’ll probably need to use an external antenna with a magnetic roof mount.

If you’re a GPS road warrior, you’ll definitely want a 12-volt cigarette lighter adapter so you don’t go through a lot of batteries during a trip. And finally, if you’re more of an urbanite (versus an outdoors) adventurer, another option is to use a PDA, such as a Pocket PC or Palm with GPS hardware and street navigation software. Chapter 6 discusses the ins and outs of using PDAs with GPS.