Location Based Services: Feedback

Written by: Jonathan Briggs

February 18, 2006 [2198 views]

I was impressed by the number of people who attempted this work; over 120. Many of you had clearly spent more than the minimum time on the task and there were some good answers. Some of you are clearly very confused however and need to do some more work to separate the technologies in your head.

A few of you simply copied and pasted paragraphs from the Internet and although this is satisfactory, I hope that you tried to understand it and could put it into your own words.

How does a GSM mobile phone ‘know’ where it is?

GSM phones maintain contact with a specific cell base station (tower) although as the user moves they will be handed over from one tower to another. A base station is therefore able to report that it is or has been in contact with a specific phone. This is the minimum level of location and allows local information (at the cell level) to be delivered.

More accurate positioning is possible where the phone can communicate with 3 or more cell towers. By measuring the distance between the phone and each tower (radio waves travel at a fixed speed over distance) it is possible to ‘triangulate’ a position. This can be accurate to about 50m. Of course in some locations 3 towers cannot be reached and in this case “cell level” location will be all that is possible.

How accurate is this location?

As explained above the accuracy depends on the available cells. Some of you got this completely wrong and started talking about satellites. There are no satellites involved in cell positioning. It is really important that you separate in your mind the different technologies.

The reason for the confusion is because you mix up GPS and GPRS, they are not connected. GPRS is an upgrade to GSM that allows data to be sent over the cell network at faster than modem speeds.

GSM = GPRS = no satellites!

What about GPS?

GPS, Global Positioning by Satellite, is a technology that has been around as an idea since the 60’s and is not directly linked to mobile phones. The system is based on 24 satellites put into orbit around the earth by the American Department of Defense.

These satellites transmit streams of synchronised timing data that can be received on land, at sea and in moving vehicles. By receiving data from 3-6 satellites the location of the receiver can be calculated (using triangulation). No data is transmitted from the handset to the satellite (ever).

What signals are sent from the handset to the satellite?

About 50% of you imagine that your in-car GPS system is sending messages to satellites. Just imagine that this was possible. There are millions of GPS receivers in use and each one of these would have to have a separate conversation with the satellite. No satellite could be built to make this possible. The good news is that it is not necessary. The GPS receiver can work out where it is without sending out a single message; indeed there is no transmitter inside a GPS.

What about GPS enabled phones?

We are just beginning to see the launch of GPS enabled phones; GSM or 3G phones that have a GPS module built into them. You can, for example, buy a GPS module for a smartphone such as the SP2. These phones still make no contact with the satellites. Instead they can report back the position of the handset (calculated by the GPS) to a server, using GPRS or 3G data services.

Using these two separate technologies we could imagine a service that would measure the location, send the location (as data) and retrieve a map or data from a server. I asked you “how could a GPS enabled mobile report its position back to an application?”

Many of you did not answer this question at all. Take great care to research and answer the questions you are asked rather than what you think you might have been asked.

Oystercards contain RFID technology

The next question was about RFID. This is an identification technology that consists of a small antenna inside each Oystercard. This antenna can be “programmed” to hold an ID number and when you purchase a card this is done to activate the card. Season tickets and pre-pay budgets are stored against this ID number. Your personal details are also stored if you have chosen to register your card fully.

When you pass through the gate, the Oystercard reader reads the ID of the card and is able to look up the validity of the card (value) and allow you to pass through the gate. In fact there is sufficient memory in the card to allow the value to be stored on the card too, allowing the card to operate even if there is no connection to the network. This allows Oystercard to be used on buses as well as tubes.

The barrier could therefore “know” who has passed through the gate by reading the ID numbers of all registered users who pass through the gate.
I was surprised that some of you appear not to have used the Oystercard system or even to know that it exists.

RFID identification is also used for product security

In most music retailers RFID tags are used to protect the products. A tag can be incorporated into a label and stuck to the packaging. This contains a product ID, store ID and the status (paid/unpaid) of the product. When you buy the item it is passed over a reader (just like the Oystercard) which sets the status of the product to paid (and updates the store inventory).

If you try and steal the item you need to pass through an electronic gate which contains a reader. This detects that the item status is unpaid and activates an alarm. Note that the reader can be a 1-2 metres from the product and can read the RFID tag even while the customer is moving.

The Oystercard system could read your Oystercard at a greater distance but this has the drawback that you could be charged for a journey on a passing bus. Different RFID systems have different sensitivities and each system has to be configured for a specific environment.