GPS (Global Position System)

There are 24 satelites which 3 three of them are spare satelites, in 6 orbital planes in GPS system. Four satelites are equally spaced in each plane.They are at the distance of 11.000 miles from the earth. The angle between each satellite orbit is 60 degrees. In this way, for each position in the earth there are at least 4 satellites that can see that position.The satellites transmit accurately time codes along with a message that includes satellites position, precise time correction signals to the receiver. GPS receiver compares the his code and code received from the satellite and measures how much time took the electromagnetic code to arrive to him. It allows the receiver to calculate the distance between satellite and receiver. As I said above a receiver can see 4 satellites at the same time. It allows the receiver to have four range datas from the satelites. Because, the posiitions of the satellites are known the position of the receiver can be found by intersecting all four ranges.

GPS system consist of three main parts. These parts are satellites, Land

Earth Stations, and receivers. I mentioned above about the satellites.

The Main Parts of the GPS;

The Federal Radionavigation Plan has designated the Navigation System using Timing and Ranging (NAVSTAR) Global Positioning System (GPS) as the primary navigation system of the U.S. government. GPS is a spaced-based radio positioning system, which pro-vides suitably, equipped users with highly accurate position, velocity, and time data. It consists of three major segments: a space segment, a control segment, and a user segment. The space segment contains 24 satellites. Precise spacing of the satellites in orbit is arranged such that minimums of four satellites are in view to a user at any time on a worldwide basis. Each satellite transmits signals on two radio frequencies, superimposed on which are navigation and system data. Included in this data is predicted satellite ephemeris, atmospheric propagation correction data, satellite clock error information, and satellite health data. This segment consists of 21 operational satellites with three satellites orbiting as active spares. The satellites orbit in six separate orbital planes. The orbital planes have an inclination relative to the equator of 55° and an orbital height of 20,200 km. The satellites complete an orbit approximately once every 12 hours.

The control segment includes a master control station (MCS), a number of monitor stations, and ground antennas located throughout the world. The master control station, located in Colorado Springs, Colorado, consists of equipment and facilities required for satellite monitoring, telemetry, tracking, commanding, control, uploading, and navigation message generation. The monitor stations located in Hawaii, Colorado Springs, Kwajalein, Diego Garcia, and Ascension Island, passively track the satellites, accumulating ranging data from the satellites’ signals and relaying them to the MCS. The MCS processes this information to determine satellite position and signal data accuracy, updates the navigation message of each satellite and relays this information to the ground antennas. The ground antennas then transmit this information to the satellites. The ground antennas, located at Ascension Island, Diego Garcia, and Kwajalein, are also used for transmitting and receiving satellite control information. The user segment is designed for different requirements of various users. These receivers can be used in high, medium, and low dynamic applications. An example of a low dynamic application would be a fixed antenna or slowly drifting marine craft. An example of a medium dynamic application would be a marine or land vehicle traveling at a constant controlled speed. Finally, an example of a high dynamic application would be a high performance aircraft or a spacecraft. The user equipment is designed to receive and process signals from four or more orbiting satellites either simultaneously or sequentially. The processor in the receiver then converts these signals to three-dimensional navigation information based on the World Geodetic System 1984 reference ellipsoid. The user segment can consist of stand-alone receivers or equipment that is integrated into another navigation system. Since GPS is used in a wide variety of applications, from marine navigation to land surveying, these receivers can vary greatly in function and design.

Frequencies used for GPS Coding;

GPS satellites transmit pseudorandom noise (PRN) sequence-modulated radio frequencies, designated L1 (1575.42 MHz) and L2 (1227.60 MHz). The satellite transmits both a Coarse Acquisition Code (C/A code) and a Precision Code (P code). Both the P and C/A codes are transmitted on the L1 carrier; only the P code is transmitted on the L2 carrier. Superimposed on both the C/A and P codes is the Navigation message. This message contains satellite ephemeris data, atmospheric propagation correction data, and satellite clock bias.

The Principles of Obtaining a Fix in GPS;

A GPS position fix is obtained by measuring the ranges from a series of selected satellites to a receiver. Ranges are determined by measuring the propagation time of the satellite data transmissions. However, it is not possible to precisely synchronize the satellite and receiver clocks, the ranges measured are not true ranges, but are termed ‘pseudoranges’ since they contain a receiver clock offset error. In order to achieve a two-dimensional (2-D) fix on the Earth’s surface at least three ‘pseudoranges’ must be obtained; the receiver microprocessor can then resolve the three range equations to remove the effects of receiver clock offset error. Similarly four ‘pseudoranges’ would be required to obtain a 3-D fix.

Types of GPS receivers onboard;

We have two normal type (not dgps) on board.

Ecdis;

We have no ecdis on board so I have no sufficient information about ecdis.