[YS Learn] LBS building blocks: Global Positioning System (GPS)

27th Mar 2013
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Decoding location based services. Image courtesy: enormlabs
Decoding location based services. Image courtesy: enormlabs

This is part of an educational series on the location based services(LBS) domain. Through this series, we will explore features of Maps and LBS, potential areas and opportunities and companies that operate in the domain. The first few articles will cover the building blocks of LBS.

As people working in the technology industry, we all have heard of GPS. But if you could peek at a normal sales process for a phone in India, you’d be amazed how many lay consumers are confused between GPS and GPRS.

 The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more of the 24 GPS satellites. It is maintained by the US government and is freely accessible to anyone with a GPS receiver.

Standalone GPS receivers work when there is clear line of sight to GPS satellites.

The constellation of GPS satellites is maintained by the US government.
The constellation of GPS satellites is maintained by the US government.

A GPS receiver scans the satellites for signals of satellite position and time taken for the signal to reach the receiver. By locking into the signals of four satellites, it then calculates its own position in three-dimensions (latitude, longitude and altitude) and precise time.

A key parameter in measuring GPS receivers is Time-to-first-fix (TTFF), or the time it takes before the receiver can acquire enough information to calculate its own position. Typically, GPS systems take a longer time when they first connect (cold start) and a shorter time subsequently (warm start).

When Standalone GPS receivers are switched on the first time, they must scan signals afresh in order to get a position fix. This is referred to as a cold start, and may take up to 12 minutes for TTFF. On subsequent uses, the GPS receiver retains some of the information previously used to speed up the fix times. This results in lower TTFF and is called a warm start, typically a minute or less.

Standalone GPS receivers do not work well in cases where there were obstruction to line-of-sight to satellites e.g.: when the user is indoors, skies are overcast, or the user is on the road between tall buildings obstructing signals. This is why you often see GPS struggling to get a fix when you are indoors.

AGPS

A key improvement to GPS is the AGPS or assisted GPS system. In this case, the GPS receiver uses cellular network information to get a quicker TTFF.

AGPS relies on network information to provide a faster location fix
AGPS relies on network information to provide a faster location fix. Image courtesy: xda developers

AGPS requires a working data connection on the phone. In case this is not available, AGPS systems fallback into standard GPS mode. AGPS takes just a few kB of data during the initial GPS fix, and can help obtain GPS position within a few seconds.

Nowadays, GPS chipsets also offer means to synthetically calculate information so as to enable faster TTFF.

Yes, we know that was a lot of technical jargon! However, some of this forms the base of building GPS systems, and there is a lot of confusion about the basics.

In the next post, we will explore some alternatives to GPS positioning systems.

Let us know your comments in the section below.

The author can be reached at shrinath@yourstory.in and at @shrinathv.

 

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