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.
GPS receiver initialization nitty-gritty
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If some or all the data is missing or out-of-date, the GPS receiver needs to get updated information from the satellites before it can accurately fix a current position. The types of data that are out-of-date or missing determine how long the GPS receiver takes to initialize.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.
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.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.
Figure 5-1:
A GPS receiver satellite status page.
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.
Trimble planning software
During certain times of the day, you might have better satellite coverage than at other times because of the number of satellites that are in view and the position of a single satellite relative to the GPS receiver and other satellites in the constellation.
Trimble Navigation (www.trimble.com), one of the largest manufacturers of commercial and professional GPS receivers, has a free Windows program called Planning, shown in the figure here. Planning is designed for surveyors who need to know when the best time is to use GPS surveying instruments. Just enter the latitude and longitude coordinates of a location and the date, and Planning gives you information on
DOP: Dilution of Precision describes how accurate a reported GPS position is. The smaller the DOP number, the higher the accuracy.
Satellites:You can see how many satellites are in view if the sky is unobstructed, the optimum times of satellite visibility, and the satellite orbit paths.
You don’t need to be a surveyor to use this information. Knowing optimal GPS times is useful for all sorts of outdoor activities. For example, if you’re serious about geocaching, you can select the best time of day to look for caches when your GPS receiver gives you the most accuracy.
Planning is easy to use and works for any location in the world with all the GPS satellite information presented in graphs or lists.
To download Planning, go to www.trimble.com/planningsoftware_ts.asp?Nav=Collection-8425.
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.
