Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
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Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors. Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all.
GPS units typically will not work indoors, underwater or underground. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping. GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is using the capacity or is in an idle state.
GPRS is a best effort packet switched service, as opposed to circuit switching, where a certain Quality of Service QoS is guaranteed during the connection for non-mobile users. The last has been typically used for applications like wireless payment terminals, although it has been removed from the standard. UE User Equipment. In practice, the mobile built-in browser uses IPv4. But if the mobile is used as a modem to the connected computer, PPP is used to tunnel IP to the phone. Such devices are known to be available today. Must be switched manually between one or the other service.
A true Class A device may be required to transmit on two different frequencies at the same time, and thus will need two radios. A DTM-capable mobile may use simultaneous voice and packet data, with the network coordinating to ensure that it is not required to transmit on two different frequencies at the same time. Such mobiles are considered pseudo-Class A, sometimes referred to as "simple class A". Some networks are expected to support DTM in GPRS is new technology in which speed is a direct function of the number of TDMA time slots assigned, which is the lesser of a what the particular cell supports and b the maximum capability of the mobile device expressed as a GPRS Multislot Class.
There are two protocols for data transmission over GPRS. They are explained below:. Using UDP, programs on networked computers can send short messages sometimes known as datagrams using Datagram Sockets to one another. The protocol was designed by David P. Reed in and formally defined in RFC Datagrams may arrive out of order, appear duplicated, or go missing without notice. Avoiding the overhead of checking whether every packet actually arrived makes UDP faster and more efficient, for applications that do not need guaranteed delivery.
Time-sensitive applications often use UDP because dropped packets are preferable to delayed packets. UDP's stateless nature is also useful for servers that answer small queries from huge numbers of clients. Unlike TCP, UDP is compatible with packet broadcast sending to all on local network and multicasting send to all subscribers. In the Internet protocol suite, UDP provides a very simple interface between a network layer below e. UDP adds only application multiplexing and checksumming of the header and payload.
If any kind of reliability for the information transmitted is needed, it must be implemented in upper layers. In particular, TCP provides reliable, in-order delivery of a stream of bytes from one program on one computer to another program on another computer. Besides the Web, other common applications of TCP include e-mail and file transfer. Among its management tasks, TCP controls message size, the rate at which messages are exchanged, and network traffic congestion.
TCP provides a communication service at an intermediate level between an application program and the Internet Protocol IP. That is, when an application programmer desires to send a large chunk of data across the Internet using IP, instead of breaking the data into IP-sized pieces and issuing a series of IP requests, the programmer can issue a single request to TCP and let TCP handle the IP details. IP works by exchanging pieces of information called packets. A packet is a sequence of bytes and consists of a header followed by a body.
The header describes the packet's destination, which routers on the Internet use to pass the packet along—generally in the right direction—until it arrives at its final destination. The body contains the data which IP is transmitting. Due to network congestion, traffic load balancing, or other unpredictable network behavior, IP packets can be lost or delivered out of order. TCP detects these problems, requests retransmission of lost packets, rearranges out-of-order packets, and even helps minimize network congestion to reduce the occurrence of the other problems.
Once TCP at the receiving end has finally reassembled a perfect copy of the large data chunk originally transmitted, it passes that single chunk up to the application program at the receiving end. Thus, TCP greatly simplifies the application programmer's network communication task.
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However, because TCP is optimized for accurate delivery rather than timely delivery, TCP sometimes incurs relatively long delays in the order of seconds while waiting for out-of-order messages or retransmissions of lost messages, and it is not particularly suitable for real-time applications such as Voice over IP. TCP is a reliable stream delivery service that guarantees delivery of a data stream sent from one host to another without duplication or losing data.
Since packet transfer is not reliable, a technique known as positive acknowledgment with retransmission is used to guarantee reliability of packet transfers. This fundamental technique requires the receiver to respond with an acknowledgment message as it receives the data. The sender keeps a record of each packet it sends, and waits for acknowledgment before sending the next packet.
The sender also keeps a timer from when the packet was sent, and retransmits a packet if the timer expires. The timer is needed in case a packet becomes lost or corrupt. Google Maps for a time named Google Local is a free web mapping service application and technology provided by Google that powers many map-based services including the Google Maps website, Google Ride Finder and embedded maps on third-party websites via the Google Maps API.
It offers street maps, a route planner for bicycles, pedestrians routes less than 6. Google Maps also covers many cities including Moscow, Istanbul, and most of India. Various governments have complained about the potential for terrorists to use the satellite images in planning attacks.
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Google has blurred some areas for security mostly in the United States , including the U. Naval Observatory area where the official residence of the Vice President is located , and until recently, the United States Capitol and the White House which formerly featured erased housetop. Other well-known government installations are visible including Areas 51 in the Nevada desert. With the introduction of an easily pannable and searchable mapping and satellite imagery tool, Google's mapping engine prompted a surge of interest in satellite imagery.
Sites were established which feature satellite images of interesting natural and man-made landmarks, including such novelties as "large type" writing visible in the imagery, as well as famous stadia and unique earth formations. Although Google uses the word "satellite", most of the high-resolution imagery is aerial photography taken from airplanes rather than from satellites. As the user drags the map, the grid squares are downloaded from the server and inserted into the page.
The script-insertion tool Greasemonkey provides a large number of client-side scripts to customize Google Maps data. Combined with photo sharing websites such as Flickr, a phenomenon called "memory maps" emerged. Using copies of the Keyhole satellite photos of their home towns or other favorite places, the users take advantage of image annotation features to provide personal histories and information regarding particular points of the area.
This has since been rectified.
Both were released to coincide with the O'Reilly Web 2. Maps lacks international support, but included a geocoder in the first release. As of October , Google Gadgets' Google maps implementation is much easier to use with just the need of one line of script. The drawback is that it is not as customizable as the full API. In late , Yahoo began a campaign to upgrade their maps, to compete better with Google Local and other online map companies.
Several of the maps used in a survey were similar to Google maps. Google Maps actively promotes the commercial use of their API. One of its earliest adopters at large scale are real estate mashup sites. Google's case study is about Nestoria, a property search engine in the UK and Spain. In late , Google introduced a Java application called Googel Maps for Mobile, which is intended to run on any Java based phone or mobile device.
Most, if not all, web based features are available from within the application. On November 28th, , Google Maps for Mobile 2. The "my location" feature works by utilizing the GPS location of the mobile device, if it is available. This information is supplemented by the software determining the nearest cell site. The software then looks up the location of the cell site using a database of known cell sites.
The software plots a blue icon with a blue circle around the estimated range of the cell site based on the transmitter's rated power among other variables. The estimate is refined using the strength of the cell phone signal to estimate how close to the cell site the mobile device is.