How GPS Works —— The secrets of Magnetic GPS
It’s fun to uncover something that you aren’t supposed to know about — many software applications and movies on DVD contain what are known as “easter eggs.” However, imagine finding something that you aren’t supposed to find that turns out to be useful and informative. In this article, we are going to hunt for GPS easter eggs, delving into the secrets that lie behind the standard menus on your GPS unit to find useful features and diagnostic screens that can give you information about your system and troubleshoot problems that you might have.
There’s a lot of information to find — technical information, diagnostic
information, and more.
Let’s begin our tour of the secrets of Magnetic GPS.
How GPS Works
The basic principle behind GPS is straightforward: The GPS receiver picks up a signal from three or more of the satellites and then uses this information to calculate the distance to the satellites. This information is, in turn, used to determine a location on the globe where the receiver is at that time. This whole process is based on a system called trilateration.
GPS receivers provide an amazing amount of information. They come loaded with many navigation features, maps, tide data and much more, but nothing is more important than position data and the direction to a waypoint.
Levels Of GPS Service
Two levels of navigation and positioning are offered by the Global Positioning System: The Standard Positioning Service (SPS). and the Precise Positioning Service (PPS). The Precise Positioning Service is a highly accurate positioning, velocity and timing service that is designed primarily for the military and other authorized users, although under certain conditions can be used by civilians who have specialized equipment. The Standard Positioning Service offers a base-line accuracy that is much lower than the PPS, but is available to all users with even the most inexpensive receivers. As we will see, there are various techniques available that substantially increase the SPS accuracy, even well beyond that which is offered by the PPS.（waterproof GPS）
Published specifications for the Precise Positioning Service are:
• 17.8 meter horizontal accuracy
• 27.7 meter vertical accuracy
• 100 nanosecond time accuracy
Published specifications for the Standard Positioning Service are:
• 100 meter horizontal accuracy
• 156 meter vertical accuracy
• 167 nanoseconds time accuracy
GPS Setup: North or Magnetic North
Much of land navigation is based on the relationship to the North Pole; also known as “true north. The measure of degrees of direction in relation to true north is called “degrees true.” Maps are laid out in degrees true. Land features on a topographic map are in reference to degrees true. By that I mean the bearing from one mountain peak to another will be referenced in degrees true. The issue is that the magnetic compass’s direction information is in degrees magnetic. The angular measurement between degrees true and degrees magnetic is called declination. The hiker will need to compensate for declination.
What works for me is to use a magnetic compass that can be adjusted to report in degrees true. magnetic compass can be adjusted by simply turning two components while the Silva models come with a small flat screw driver to make adjustments. Check the owner’s manual for detail on how to adjust the compass. Importantly, the adjustable compass eliminates the requirement to calculate for declination. Do remember that the magnetic needle always points to magnetic north and the adjusting accounts for the angular measurement of declination. To keep my navigation simple I want my GPS（GPS ID Card）to provide the same directional information as my compass and map; degrees true. Because a new GPS receiver will be pre-set to report in degrees magnetic at the factory, the unit will need to be re-set to report in degrees true.
There’s a lot of information to find — technical information, diagnostic
information, and more.
Let’s begin our tour of the secrets of Magnetic GPS.
How GPS Works
The basic principle behind GPS is straightforward: The GPS receiver picks up a signal from three or more of the satellites and then uses this information to calculate the distance to the satellites. This information is, in turn, used to determine a location on the globe where the receiver is at that time. This whole process is based on a system called trilateration.
GPS receivers provide an amazing amount of information. They come loaded with many navigation features, maps, tide data and much more, but nothing is more important than position data and the direction to a waypoint.
Levels Of GPS Service
Two levels of navigation and positioning are offered by the Global Positioning System: The Standard Positioning Service (SPS). and the Precise Positioning Service (PPS). The Precise Positioning Service is a highly accurate positioning, velocity and timing service that is designed primarily for the military and other authorized users, although under certain conditions can be used by civilians who have specialized equipment. The Standard Positioning Service offers a base-line accuracy that is much lower than the PPS, but is available to all users with even the most inexpensive receivers. As we will see, there are various techniques available that substantially increase the SPS accuracy, even well beyond that which is offered by the PPS.（waterproof GPS）
Published specifications for the Precise Positioning Service are:
• 17.8 meter horizontal accuracy
• 27.7 meter vertical accuracy
• 100 nanosecond time accuracy
Published specifications for the Standard Positioning Service are:
• 100 meter horizontal accuracy
• 156 meter vertical accuracy
• 167 nanoseconds time accuracy
GPS Setup: North or Magnetic North
Much of land navigation is based on the relationship to the North Pole; also known as “true north. The measure of degrees of direction in relation to true north is called “degrees true.” Maps are laid out in degrees true. Land features on a topographic map are in reference to degrees true. By that I mean the bearing from one mountain peak to another will be referenced in degrees true. The issue is that the magnetic compass’s direction information is in degrees magnetic. The angular measurement between degrees true and degrees magnetic is called declination. The hiker will need to compensate for declination.
What works for me is to use a magnetic compass that can be adjusted to report in degrees true. magnetic compass can be adjusted by simply turning two components while the Silva models come with a small flat screw driver to make adjustments. Check the owner’s manual for detail on how to adjust the compass. Importantly, the adjustable compass eliminates the requirement to calculate for declination. Do remember that the magnetic needle always points to magnetic north and the adjusting accounts for the angular measurement of declination. To keep my navigation simple I want my GPS（GPS ID Card）to provide the same directional information as my compass and map; degrees true. Because a new GPS receiver will be pre-set to report in degrees magnetic at the factory, the unit will need to be re-set to report in degrees true.
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