|Friday, 27 October 2006|
A GPS receiver is composed of an antenna unit for receiving high frequency satellite signals and a processing unit for processing the signals to compute positional coordinates of the GPS receiver. A differential GPS receiver includes a GPS antenna to receive the GPS signals transmitted from one or more GPS satellites, a GPS processor to calculate the GPS antenna's position mid time of observation of that position from the GPS signals, a display processor to convert the GPS position and observation time into information that is useful for an application, and a display to present the information to the user. A DGPS antenna and receiver are provided in a separate unit or units connected to the GPS processor. The processing unit includes an analog circuit which is responsible for amplifying the high frequency satellite signal, reducing the high frequency signal to an intermediate frequency signal, and converting the signal into a digital signal. Also included in the processing unit is a digital circuit which processes the digital signal to compute the positional coordinates of the GPS receiver. Since the digital circuit handles the digital signal, it is a source of developing a noise which interferes with the antenna and impedes the antenna gain. In order to avoid the interference, the digital circuit has to be surrounded by an EMI (electromagnetic interference) shield. Typically, a GPS antenna element is utilized by GPS receivers to receive the signals transmitted. The global positioning system can be used to determine the position of a GPS antenna to within a few centimeters by using well developed carrier phase differential interferometric techniques between an aircraft mounted GPS antenna and a nearby surveyed GPS reference receiver. In order for the GPS receiver to compute its geographic location, the antenna element of the receiver must be oriented to receive an acceptable level of the signals. Optimally, the flattened surface of the GPS antenna element is righted against the force of gravity such that a maximum surface area of the antenna faces the satellites. A GPS antenna is flat, and small in size, and generally this antenna and a high-frequency circuit board (for effecting processings for the received radio waves) are combined together to form a unit (GPS antenna unit). Antennas for GPS receivers are divided into a ceramic patch antenna and a helix antenna. The ceramic patch antenna is formed by casing a ceramic patch of a rectangular parallelepiped shape with a plastic injection. The helix antenna is formed by twisting a feeder line path around a Teflon-based cylindrical member and then casing the cylindrical member with a plastic injection. In the GPS, active antenna electronics adjust the gain and the phase of a multi-element antenna array in order to steer the antenna pattern away from an interference source which may serve to jam the satellite ranging signals.
A variety of portable electronic devices have bee incorporated with GPS receivers, and are becoming increasingly popular due to the availability of low cost GPS receivers and a wide variety of GPS applications. A GPS receiver may be incorporated into a cellular phone or handheld GPS navigation system to provide various functionalities such as to determine its location, to provide navigation information or to remotely provide others with the information about the user's current location. Cellular telephones are commonly used together with GPS receivers for communicating the location of the receiver. Cell phones receive GPS signals so that operators in a public safety answering center are able to determine the location of the cell phone by receiving a GPS signal via the cell phone. This feature assists in locating cell phones and their users during emergency situations. There is a variety of global positioning system (GPS) and global system for mobile communications (GSM) based communication equipment developed to fulfill such needs. Recently, GPS receivers and cellular telephones have been packaged together for reducing their combined cost and size by sharing the same housing and power supply. Generally, a mobile telephone apparatus containing a GPS unit is not always used in a place where the GPS information from the GPS satellite can be received in a good condition but is often used in an unreceivable place, typically, an underground mall. In order to effectively utilize the GPS information in such cases, the underground mall or a tunnel is provided with a GPS communication mobile telephone base station which covers a local region as a communication area. Conventionally, antenna of mobile phone and antenna of GPS based communication equipment are separated. That is, antenna of mobile phone can only receive communication signals, while GPS based communication equipment can only receive coordinate signals. However, due to functional as well as aesthetic considerations, it is desirable to be able to eliminate the dual mounting of two separate antennas for GPS and cellular communication by having a single module for accomplishing both functions.