GPS Receiver – Initialization

GPS receivers can come in many shapes and sizes. The handheld GPS receiver, used for navigation in the wilderness, shares a common function with all GPS navigation systems. That is the data it downloads from the GPS Sat Nav system and how it uses that data to plot its location.

Every GPS navigation receiver downloads or receives three types of data from the navigation satellites. We’ll take a detailed look at all three.

Almanac Data

This is specific information about the orbits of all 24 GPS satellites in the Global Positioning System. Each satellite broadcasts this information. It takes approximately 12.5 minutes to download this information.

“Initialization” is the process of downloading this package of data. Once completed this information is good for six months. However if you’ve moved more than a couple hundred miles from the location where this information was last downloaded you’ll have to repeat this process.

To initialize your handheld GPS receiver, turn it on and leave it sit out in the open with a clear view of the sky for twenty minutes (to be on the safe side.)

Ephemeris Data

Each satellite also transmits very detailed data about its own orbit. It takes about 30 seconds per satellite to download this information. Most GPS receivers are capable of downloading the ephemeris data from multiple satellites at the same time.

This information is good for a few hours. If you turn on your GPS receiver occasionally throughout the day, this data won’t need to be reloaded.

PNR Code

“PNR” stands for “Pseudo Random Noise”. This is the actual signal all GPS receivers use to measure the transmission time delay from each navigation satellite. From that it calculates the actual distance to the satellite. With the distance from four satellites it gets a fix on its position and cross checks it three ways to get a precise location.

Acquisition Time

This is how long it takes for your handheld GPS receiver to calculate its position. This varies depending on how long it has been since it was last used.

More than 6 months- The unit must be initialized, that is, download the almanac data. This will take a minimum of twelve minutes.

6 months to 6 hours- this is a “cold start”. The GPS receiver must download the ephemeris data from all satellites. Acquisition will take 45 seconds to 2 minutes.

3 to 6 hours- this is a “warm start”. The GPS receiver must download the ephemeris data from 3-4 satellites. Acquisition time takes 15 to 30 seconds.

Less than 3 hours- this is a “hot start”. All satellite data is up to date. Acquisition time will only take a few seconds.

Again, the best practice when using your handheld GPS receiver in the field is to turn it on and check your location every couple of hours or so, in which case you won’t have to wait at all for your GPS receiver to acquire its location.

The author, Mike Wreggitt, is an avid sportsman, and has navigated extensively with topo map and Brunton compass, before discovering what a great tool the GPS receiver has become. To learn more about GPS receivers, GPS navigation, and much more, visit http://GpsForHiking.net.

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