GPS vs U-TDOA | Difference between GPS and U-TDOA

GPS vs U-TDOA | Difference between GPS and U-TDOA(Uplink OTDOA)

This page compares GPS vs U-TDOA and mentions difference between GPS and U-TDOA. GPS stands for Global Positioning System and U-TDOA stands for Unlink Time Difference of Arrival. U-TDOA is uplink method similar to downlink OTDOA.

We know that emergency calls made from mobile phones are increasing day by day. Victims who have made calls can be anywhere on the earth such as indoors, on the tall buildings, away from the city areas etc. The positions of these peoples should be located very accurately in order to provide them emergency services from various hospitals etc.

There are various methods to determine the location of mobile which include A-GNSS, GPS, GLONASS, OTDOA (Downlink Observed Time Difference of Arrival), U-TDOA (Uplink Time Difference of Arrival), etc.

GPS | Global Positioning System

It uses GPS chips inside the mobile phone (i.e. UE) which receives signals from orbiting satellites. It consecutively determines location of mobile phone based on distance of satellites.

Total 4 satellites are needed to determine 3D position of mobile phone as shown in the figure-1. The GPS chip inside the phone determines T1, T2, T3 and T4 times of the pulse transmitted by the orbiting satellites with respect to internal time reference. Based on this and with the following formula GPS receiver chip determines its position on the earth.
Distance = transit time x speed of light
Speed of light is known constant with value 3 x 10⁸ m/sec.

Refer GPS basics >> and GPS tutorial >>.

U-TDOA | Uplink Time Difference of Arrival

Sensitive receivers in the cell towers (i.e. eNBs/Base Stations) receives the signal from mobile phone (or UE). The difference in time the signal reaches different cell towers is used to calculate position of the UE. Like downlink OTDOA; the method used to determine position of UE using uplink transmissions is known as U-TDOA. Refer OTDOA basics and working >>.

Following table mentions difference between GPS and OTDOA methods used to determine position of UE.

GPS	U-TDOA
The full form of GPS is Global Positioning System.	The full form of U-TDOA is Uplink Time Difference of Arrival.
Uses GPS satellites to determine UE (i.e. Mobile phone) position.	Uses time difference of arrival of uplink signals by multiple eNBs to determine UE position. Position of eNBs are already fixed which helps in the UE position estimation.
For GPS to work effectively mobile phone should be in unobstructed LOS (line of sight) with the GPS satellites. This is because, if the mobile phones are inside the buildings or are in locations where surrondings are populated with tall buildings, satellite signals will become weak or sometimes do not reach the mobile phones.	In U-TDOA, eNBs use highly sensitive receivers which receives mobile phone signals even through the buildings and hence mobile phones (i.e. UEs) can be accurately located.
Trees will block the satellite signals going towards the UE and hence GPS do not perform well in densely populated forest areas. Hence people in such areas can not be located even in emergency situations easily as satellites will not have clear LOS path to the UE.	With U-TDOA, trees do not block the signals and hence UE can be easily located.
Not effective for emergency situations.	Very effective for emergency situations, public safety and national security applications.