The Global Positioning System (GPS) is a worldwide radio-navigation system built and managed by the U.S. government that encompasses a minimum of 24 satellites orbiting the earth at a distance of 10,900 miles, twice a day in 6 orbital planes at a 55° inclination angle with respect to the equator.

The orbiting satellites are connected to a network of ground stations and navigation systems and use triangulation to locate the user, as each satellite transmits signals.

Satellite communication is basically achieved in three steps: The satellite transmits its own position, its time and a pseudo random noise code (PRN) used by receivers to calculate the range. The GPS receiver converts the received signals into position, velocity and time estimates and calculates the exact position of the transmitting satellite and the distance (from the transmission time delay) between it and the receiver. Coordinating the signal from satellites determines position. Each of the other Global Navigation Satellite Systems (GNSS) work on a similar principle.

Why Wi-Fi positioning?

However, GPS or GNSS is not always the answer, particularly in indoor environments or in the dense ‘Urban Canyon’ where the low-level satellite-based signals are critically compromised by obscuration and environmental degradation. In these situations, other technologies already in the consumer device are being utilised to bridge the gap, such as Cell-ID and particularly Wi-Fi. Positioning techniques based around the pattern of observations associated with multiple Wi-Fi ‘hotspots’ include ‘fingerprinting’ where observations are compared to previously mapped locations, and “trilateration”, where received power is used as an indication of distance from the transmitter and a geometric calculation against known transmitter locations is used to locate the device.

Many observers believe that the urban canyon and indoor problem will be progressed, maybe even fixed, over the next few years. The candidate technologies are already in place and significant progress is already being made. This revolution has already started with assisted-GPS, where the almanac and ephemeris (precise satellite location) data is uploaded to reduce the search window, resulting in quicker Time To First Fix (TTFF) and the ability to acquire lower power signals, including limited ability to navigate indoors. At the user level, we are seeing new applications, falling prices and better performance. GNSS is appearing everywhere.

Wi-Fi positioning is, however, rapidly gaining acceptance as a complement and supplement to GNSS positioning for indoor environments. Wi-Fi hotspots are prevalent in the very areas where GNSS starts to struggle and many ‘smart’ devices are already equipped with Wi-Fi technology that can support positioning applications. Numerous service providers are offering the essential databases that enable the overall positioning capability.

Why use a Wi-Fi positioning simulator?

A-GPS/A-GNSS, Wi-Fi and Cell ID positioning are emerging as the key positioning technologies that comprise hybrid positioning capabilities for any location-aware mobile device (although there are many other radio-based candidates).

Spirent’s unique GSS5700 multiple Access Point Wi-Fi positioning simulator provides a controlled and repeatable environment of observable Wi-Fi hotspots to support Wi-Fi positioning test. This product may be integrated with Spirent’s GNSS solutions to provide a hybrid stimulation package for device R&D.