19 01 2014

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.

GPS satellites are basically clocks broadcasting time. So, finding where a receiver is, is a triangulation problem. The scope of satellite’s time precision is immensely complicated, continuously recalibrated for relativistic effects of Earth’s gravity, and electromagnetic wave propagation delay through the atmosphere. Maths and physics make it work.

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.


The math is relatively simple 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 …

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 on iNotes4You within a part of a second.

GNSS Positioning Calculation


It takes a minimum of 3 satellites to reduce the number of possible positions to 2, which means an x + y fix. It takes a minimum of 4 satellites to get an x, y + z fix.

A partial fix with 2 satellites would be a circle, and a single satellite would be an uncertain sphere. Locating a device with one or two satellites is not possible. You better ask someone near to you where you are and where the next bakery is.

The ideal constellation is many satellites in view, far apart as possible near the horizon with one directly over head.
Accuracy in the horizonal (x,y plane) unassisted (without A-GPS) can get to 1 m on industrial receivers. Smartphone receivers rarely have access to or show constellation status or accuracy. With kinematic/differential/A-GPS horizontal (x,y) accuracy approaches 5-10 mm (read that again: it might be less than a centimeter!).

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.

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.

Built-In apps using location services …

  • Maps
    showing your actual position in a map
  • Weather
    to show the weather forecast for your actual location
  • Compass
    to additionally show you actual location
  • Find my iPhone
    locating your devices on a map
  • Find Friends
    to show the location of your friends using an iOS device
  • Reminders
    reminds you if you are near a defined location
  • Camera
    assigning the actual location if you make a photo
  • Apple Store
    to show the nearest store
  • iWork
    it’s still a secret for me
  • Safari
    when visiting geolocation-enabled websites for sharing your location with the website to get location-based information, such as showing you the nearest post office or bank ATM.
  • Siri
    to answer location based questions like the weather forecast

The utilities …

Here is a brief overview about the needed components which allow to locate you iOS device. Information about sensors is taken from the iPhone 5S, first released in September 2013.


Feel free to download this map from my Box account.

The alternative file formats have been created with iThoughts HD for iPad (.ITMZ file format). Compatibility to other tools is limited.

Application File format
Adobe Reader PDF
iThoughts ITMZ
MindManager MMAP

Isn’t it a giant effort to find the nearest bakery?

The battery …

GPS uses more energy of your battery than any other antenna or sensor.

There is no time division like in other communication mechanisms, necessitating powering the antenna for the duration of any communication. Worse, while the GPS is on, the system cannot enter a sleep state.
Mobile devices such as the iPhone and Android devices achieve their battery life largely because they can aggressively and quickly enter into and exit from sleep states.

GPS’s battery draining behavior is most noticeable during the initial acquisition of the satellite’s navigation message: the satellite’s state, ephemeris (position of astronomical objects at a given time), and almanac (stats about astronomical data like time of rising sun). Acquiring satellites can take up from 10 sec to 10 min. During all of this, your device is unable to enter a deep sleep.

A-GPS (Assisted GPS) partially solves this, by sending the navigational message to your mobile device over your cellular data network or even WiFi. As the bandwidth of either of these greatly dwarves the 50bps of the GPS satellites, the time spent powering the GPS antenna or avoiding deep sleep is greatly reduced.

Most mapping software is therefore processor-intense. A well-designed app can make a significant difference here.
For mobiles, it really depends on the device and the app. Apps can be allowed to select the accuracy (how close is that pin to reality) and update frequency (continuously in the background or just major changes).

Summary …

GPS combined with earth-bounded, so-called pseudolites, allows astonishing precision in locating devices.

Retail GPS sensors have more fancier algorithms due to cost, intellectual property and power consumption. Precisely-mapped base-stations are able to send corrections along with constellation state and so increase the speed of location acquisition (when the position and time are known to be good).

The battery drains but the companies sending you location-based ad reached their target and acts like the big brother watching you.

So turn off location services for all apps except for Maps and navigation apps.

Related links …

Apple about Location Services

Sensitiveness, about iPhone sensors

GNSS Positioning Calculation

Thanks for visiting iNotes4You.



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