Global positioning systems revealed

Added: 11-08-2010
Author: Ed Holdsworth
Category: GPS

Simple Explanation

Hand held GPS receiver calculates its position by first measuring the distance between himself and the 4 satellite Navigation Satellite Constellation (Four satellites are required as there are 4 known to be reckoned with, x, y, z, and time). This is done by measuring the time delay between transmission and receive all satellite signals, such as travel speed of the signal is known (near the speed of light).

In addition to satellite systems also deliver health information as part of the navigation message, rough information about the orbits of all satellites, and others in the system known as the Almanac and accurate information about satellite orbits, known as the ephemeris (satellite only transmits itself, but all satellite transmits ephemeris Almanac, which has extensive information systems).

The next step is to change the position calculated in the local frame memory, this is done through specific reforms geode.

Detailed

GPS satellites transmit signals from the two is one of the options available, which is known as the C / A (Coarse / Acquisition) Regulations, and is used for the Standard Positioning Service (SPS), and provide accuracy of 5 - 10m. The second is P (right) Regulation, which is used for precise Positioning Service (PPS), is reserved for U.S. military use and is encrypted to prevent fraud / FRAUDULENT used, this can give a few meters of accuracy. Each satellite C / A code that is different (which is produced by the Pseudo Random Number generator) so that each satellite can be uniquely identified and known about him from others.

P-code decrypted by using the code Y to produce P (Y) code. This P (Y) code can be decrypted only with the unit and finding the right decryption key.

GPS satellites broadcast signals at two carrier waves, known as L1 and L2;

L1 (1575.42MHz)

Navigation message, C / A Code and Encrypted P (Y) code

L2 (12227.60 MHz)

P(Y) Code

The dilution of Precision (DOP)

The DOP is considered as a sign of how best to present the geometry of visible satellites Constellation is counting for the solution space. The values used are appropriate indicators of how the current geometry of the solution space. High DOP worse when considered good, low DOP. DOP considered to enlarge the errors inherent in the solution space. The DOP also influenced by the structure that blocks the view to the sky.

There are a variety of DOP, each used to measure the quality of the satellite constellation for various purposes:

GDOP (Geometric DOP)

applies to complete the 3-D solution and time.

PDOP (Position DOP)

valid for 3-D position solution.

HDOP (Horizontal DOP)

Valid for 2-D (X & Y) position solution

VDOP (Vertical DOP)

Applicable to long-solutions

TDOP (Time DOP)

Valid for the time part of the solution

As head of the rule of thumb DOP between 1-6 which values are considered good when more than 6 is considered suspect and should not be used.

Positioning Methods

Stand Alone

This is the main method used for GPS positioning and makes use of code-generated pseudo-ranges. This is usually used for a non-carrier phase GPS receiver. This way of positioning can only get a chance to an average accuracy of 5-10 meters for civilian use, but more accurately can be obtained by achieving a military code.

Recipient of a series of satellites to listen to downloaded Almanac basis of the information, since each satellite in view is to see the satellites are identified by the various C / A code and the camps are produced using identical code with the same amount of seeds used for the satellite signal. These two signals can then be compared to provide a delay between the signals and receiving signals generated in the. The resulting values known as the Pseudo-range.

Pseudo-range, and can be considered as a range of values that does not take hours offset between the receiver and satellite, and as a result can never be completely accurate. This is because at that time typically use inexpensive GPS receiver clock crystal average only GPS synchronized time. This took several steps such as (at four under, to complete the four unknowns, the unknown for the third place and one not known for a while) that the time errors can be eliminated. This is easily done, as the receiver clock offset is the same for every satellite as a signal from each satellite will be received simultaneously.

It is this kind of position that we target in this document.

DGPS

Initially the GPS position signal is available to civilian users deliberately degraded. This capability is known as selective availability (SA), and is intended to prevent enemy forces using an accurate GPS position. SA is active with the best available position for stand-alone solutions recipients around 100m. SA finally shut down in 2000, he had to do is to encourage the development of powerful tools to improve GPS position solution.

DGPS is based on the use of some recipients (two or more). One of the recipients remained at a known point, and act as a reference, while others may move around. If this opportunity known reference point position correction solutions that produce quantifiable. This correction can be transmitted by radio stations around the links so that they can improve the solution. This technique is accurate only about 500 km between the centers of a different and mobile reference station, because more than that and weather satellites are used in the more likely to change.

Carrier Phase

Carrier phase (some times known as a relative) positioning is another way to use GPS signals to the solution. It doesn't depend on pseudo-random number code generated by GPS satellites to produce a solution is dependent on the stage of the incoming carrier signal in two or more recipients. Positioning systems are used by many plots and accurate application to several mm.

The phase angle measured incoming waves, perhaps this information with knowledge of the wavelength of the carrier wave to produce a solution space. The distance between the satellite receiver and a number of waves (known as the ambiguity integer) plus part of a wave that is received at the end of the recipient (sometimes known as carrier phase).

In this way the store is higher than it is difficult to use Pseudo code range, not only in terms of data processing, but also as a key to sustainable treatment and that the satellites used for positioning the various solutions. Loss of keys (known as round-left) means that the process of finding an integer ambiguities must start again from scratch if the intent and purpose of all there is a new integer ambiguity values.

Now possible to do this kind of position in real time, which has led to the development of real-time kinematic positioning techniques. Improving the system is possible because the development of real-time technology to enable data transfer over short distances is important for some of these systems, increasing computing power available to recipients of success in microprocessor design, and improving methods used in mathematics for the position solution.

Augmentation Systems

Also known as overlay their services come in two types, satellite-based (SBAS) and ground-based (GBAS). They are required for satellite navigation systems are designed citizen requests in mind.

Civilian applications that require more effective are available today, the system ensures that the operating system functions and parameters of this system is not broken in some way and they also provide the blame if something goes wrong people (roles).

To provide solutions to these needs correction DGPS augmentation systems usually provide information about the reliability of position solutions. Position system is also based augmentation to have additional sources of there own to GPS satellites broadcast a signal, and thus increase the availability of the position signal.

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