Localization Services

 Some localization services are GPS, Galileo, BeiDou, and GLONASS. The United States Airforce operates GPS. Galileo is operated by the EU system. BeiDou is operated by China, and GLONass is operated by the Soviet Union. 

GLONASS and GPS were made to track missiles. GPS was encrypted in 2012 for civilian use. 

Localization:

To localize someone, we need a point of reference, and constraints to these points of reference.

An example is someone is in their office. The office is the point of reference and the person is "in office", thus constrained in the office area. 

The space segment:

24 satellites in 12 hour orbits.

At any point in time, 4 satellites are above an area and we use time signals to perform trilateration. 

We can figure out R1, R2, and R3 through GPS travel time. 

Find 3 distances between the satellites and solve the 3 distances and therefore solve for the location. 

However, this is a problem. This only works if the clocks are SUPER accurate.

Three circles joined gives a location.

If the GPS receiver clocks are inaccurate, it causes a distance error. 

There is going to be a drift, instead of measuring Td, you are measuring Td + drift. For a drift, we can add a fourth equation, thus satellites. 3 satellites for points and 4th for drift. 

Big building can cut off signals. This is why GPS and Google maps are so hard to use. This phenomena is called urban canyons. 

To solve this use wireless localizations, which locates wireless devices to help make the location even more precise. 

The typical algorithms are as follows:

Trilateration

Triangulation

Fingerprinting

And distance estimation:

Received Signal Strength, etc. 

The following equation is bluetooth to derive distribution:

These equations are not very accurate, and there are thousands of papers trying to correct this model. HP and Cisco are developing solutions for this.  

The lmitation is the exact time synchronization needed. The uses of location distances are useless.

The time of arrival approach uses time of transmission, speed, and time of arrival to compute distance. The time-difference-of-arrival approach uses two different signals with different propagation speeds. The limitation is that this requires extremely expensive hardware.