Hyper Text Transfer Protocol - HTTP

HTTP
HTTP stands for Hypertext Transfer Protocol. It is an TCP/IP based communication protocol which is used to deliver virtually all files and other data, collectively called resources, on the World Wide Web. These resources could be HTML files, image files, query results, or anything else. 
An HTTP conversation


HTTP is the set of rules governing the format and content of the conversation between a Web client and server.

HTTP Message
•  HTTP uses the client-server model: An HTTP client opens a connection and sends a request message to an HTTP server; the server then returns a response message, usually containing the resource that was requested. After delivering the response, the server closes the connection.

HTTP transaction

      HTTP Message
    The format of the request and response messages are similar and will have following structure
    - An initial line
    - Zero or more header lines
    - A blank line
    - An optional message body like file, query data or query output. 
HTTP Methods
  •      GET
  •      POST
  •      HEAD
HTTP Request Types
•GET (default) and POST do basically the same thing: Send data from the client to the server. 
However, they have some differences:
•GET
–Appends form data directly to the end of the UR— visible to users 
(not suitable for sending passwords)
–Limited to 2,048 characters for the entire URL
–Result page can be bookmarked and cached

•POST
–Sends form data in the HTTP request—invisible to users
–Virtually no limit (but check your specific configuration) 
–Results are not cacheable or bookmarkable

The HEAD Method
 A HEAD request is just like a GET request, except it asks the server to return the response headers only, and not the actual resource (i.e. no message body). This is useful to check characteristics of a resource without actually downloading it, thus saving bandwidth. Use HEAD when you don't actually need a file's contents.
     The response to a HEAD request must never contain a message body, just the status line and headers.

HTTP Header Fields
Allow
Authorization
Content-Encoding
Content-Length
Content-Type
Date
User-Agent
Referer
Server
HTTP Status Codes


Example of an HTTP Request from a Web browser





HTTP version 1.1 specifies a persistent connection by default.


  scenario in which an electronic store can benefit from the use of cookies. Assume a shopper wants to buy a toy from an electronic store named BestToys. The shopper browser (client) sends a request to the BestToys server. 
























Global Position System -GPS


         What is GPS?
GPS, which stands for Global Positioning System, is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere. 
The three parts of GPS are:    
  • Satellites
  • Receivers
  • Software

         
     Satellites
There are quite a number of satellites out there in space.  They are used for a wide range of purposes: satellite TV, cellular phones, military purposes and etc.  Satellites can also be used by GPS receivers.

GPS Satellites
The GPS Operational Constellation consists of 24 satellites that orbit the Earth in very precise orbits twice a day.  GPS satellites emit continuous navigation signals.


Receivers and Satellites
GPS units are made to communicate with GPS satellites (which have a much better view of the Earth) to find out exactly where they are on the global scale of things. 





GPS Signals
Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant.
Physically the signal is just a complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.


Time Difference
The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.

     Calculating Distance
Velocity x Time = Distance
Radio waves travel at the speed of light, roughly 186,000 miles per second (mps)
If it took 0.06 seconds to receive a signal transmitted by a satellite floating directly overhead, use this formula to find your distance from the satellite
186,000 mps x 0.06 seconds = 11,160 miles

    Triangulation
Geometric Principle: 
You can find one location if you know its distance from other, already-known locations.











3-D Trilateration


Atomic Clocks
 

GPS satellites use Atomic Clocks for accuracy, but because of the expense, most GPS receivers do not.  

Line of SightTransmissions
Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way.  GPS transmission are line-of-sight transmissions.


Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.

Light Refraction
Sometimes the GPS signal from the satellite doesn’t follow a straight line. 


Refraction is the bending of light as it travels through one media to another.








Signal Refraction
Signals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.











Signal Interference
Sometimes the signals bounce off things before they hit the receivers.













Satellite Distribution
When the satellites are all in the same part of the sky, readings will be less accurate.



PDOP

PDOP = Positional Dilution of Precision

All of this combines to make the signal less accurate, and gives it what we call a high “PDOP.” 
         A PDOP of <4 is excellent


         A PDOP of 4-8 is good
         A PDOP of >8 is poor

Differential Correction
Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location - the "base station“- and comparing that data with GPS positions collected from unknown locations with "roving receivers."


















Postprocessing / Real-time


In a Nutshell



Latitude and Longitude
Latitude and Longitude are spherical coordinates on the surface of the earth. Latitude is measured North or South of the Equator. Longitude is measured East or West of Greenwich.  GPS uses Latitudes and Longitudes to reference locations.


Waypoints
Waypoints are locations or landmarks that can be stored in your GPS. Waypoints may be defined and stored in the unit manually by inputting latitude and longitude from a map or other reference.
Or more usually, waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point.


Data Dictionary
GPS units collect data in:
       Points
       Lines
       Areas
These are called features.
A data dictionary is a means by which we collect specific information about a data feature.

Roving File
A roving file is like a drawer of a filing cabinet containing many feature files.
Collect all the features collected in one interval in one roving file.

GPS Uses
         Police and Emergency Medical Services
         Firefighters  
         Map makers
         Science 

References