Location-based services

14 04 2013

A location-based system is a system of satellites and earth-based stations that provide geo-spatial positioning with global coverage (GNSS=Global Navigation Satellite System). Small electronic devices with appropriate receivers may determine their location (longitude, latitude, and altitude) to within a few meters using time signals transmitted as electromagnetic waves from satellites and pseudolites.

Pseudolite is an abbreviation of ‘pseudo-satellite’, used to refer to an earth-based station which operates like a real satellite. Pseudolites support satellite-based positioning in case of blocked, jammed or simply missing GPS signals.

A simple 2D method …

To understand how the location of objects can be achieved look at this method which was developed about 500 years ago, the triangulation method.

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The math is relatively easy but the prerequisites definitely do not meet the requirements when walking through the streets with an iPhone. You will not find reference points like lighthouses when climbing up the Everest, moving through the Everglades or driving through Australia’s outback.
And moreover it’s a 2-dimensional static method which doesn’t help to fix the location of an object in 3 dimensions, that means to fix the longitude, latitude, and altitude.

A complex 3D method …

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Using many satellites fix all these problems as 3 or 4 of them can be ‘seen’ from every location on the earth. The math to localize objects with satellites is complicated so just look at this image and possibly read the more detailed notes or look on this website and I’m sure you will be back at iNotes4You within a part of a second.

GNSS Positioning Calculation

The details …

The satellite broadcasts a signal that contains orbital data (from which the position of the satellite can be calculated) and the precise time the signal was transmitted. The orbital data is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation.

The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time using an adapted version of trilateration.

Each distance measurement places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated.

Notes

However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver’s angle to the satellite, because that changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.

Location services on iOS devices …

Location Services allows location-based apps and websites (including Maps, Camera, Safari, and other Apple and third-party apps) to use information from cellular, Wi-Fi, and Global Positioning System (GPS) networks to determine your approximate location.

For example, an app might use your location data and location search query to help you find nearby coffee shops or theaters, or your device may set its time zone automatically based on your current location. To use features such as these, you must enable Location Services on your device and give your permission to each app or website before it can use your location data.

For safety purposes, your iPhone’s location information may be used for emergency calls to aid response efforts regardless of whether you enable Location Services.

Apps using location services …

  • Camera
    assigning the actual location if you make a photo
  • Find my iPhone
    locating your devices on a map
  • GPS Toolbox
    locating the actual position and saving them in a database

GPS Toolbox …

GPS Toolbox by Audama Software Inc. is a location-based tool for Apple’s iOS devices.

This app provides a variety of GPS and map related tools which enable you to save locations to exportable lists, convert between coordinate formats, and view different map formats (road, satellite, hybrid, or terrain maps) from both Google maps and Bing maps. These features allow you to do things such as:

  • Find GPS coordinates for a location without ever going there.
  • Find what is located at a given GPS coordinate.
  • Map out a list of locations (such as a hiking route) before heading out.
  • Compare map and satellite data from multiple sources (Google and Bing).
  • Create location lists and load them into other programs such as Google Earth.
  • Create location lists and load them into other third-party GPS devices.
  • Locate, view, and convert between GPS coordinates in any of the following formats:
    Decimal degrees, Degrees/minutes, Degrees/minutes/seconds, UTM, MGRS.
  • Manage a collection of GPS points within data files. These data logs can be easily viewed, edited and exported. The following export formats are currently supported:
    Tab delimited, Comma separated values (CSV), GPX, KML, KMZ, HTML tables.
  • Additional features include viewing and saving the current GPS data as well as converting between GPS coordinates and physical addresses.

This gallery shows you some features of the app and the locations I have been. Some of these locations have been recorded with the app others have been entered manually because at the time I visited these places Apple was in the middle of its life crisis and far far away from purchasing smartphones or tablets.

Apple’s privacy policy …

If you use location services on you iOS device look here for informations about your privacy.

Apple Inc. , May 21, 2012
Location-Based Services
To provide location-based services on Apple products, Apple and our partners and licensees may collect, use, and share precise location data, including the real-time geographic location of your Apple computer or device. This location data is collected anonymously in a form that does not personally identify you and is used by Apple and our partners and licensees to provide and improve location-based products and services. For example, we may share geographic location with application providers when you opt in to their location services.
Some location-based services offered by Apple, such as the “Find My iPhone” feature, require your personal information for the feature to work.

Links …


iPhones dining habits

LG: Location based Services for Mobile


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