Android Sensors and Location based services

Here is the talk i presented about
Sensors and location based services on Android 

at B.A.(U).G / BLR-DROID. 

Felt awesome talking to people, answering Qs about nGPS.

Download Sensors and location based services on Android

nGPS : Location fix without GPS

NOTE: Skip this post if you do NOT live on planet earth.

This is one of the ideas that i hit upon when preparing for a talk Sensors on Android @DroidCon2011. It is an unusual application of the on-board sensors present most Android devices. Due to lack of time i was unable to present it in much detail during my talk. So here goes...

nGPS (NO GPS) is a way of obtaining a location fix without using any GPS, AGPS, Wi-Fi Positioning and cell-site triangulation technologies.

Why would anyone want to use nGPS

- Pure GPS based systems take upto 10mins for 1st fix.

- AGPS, Wi-Fi positioning require an active data-connection.

- Cell-site triangulation requires network coverage.

So without any of these technologies at our disposal, how do we obtain a "location-fix" i.e. a latitude-longitude pair representing our current position. The answer lies in the magnetic-field sensor.

The Earth's magnetic field, as measured by a magnetic sensor on the Earth's surface, is combination of of several magnetic fields generated by various sources. These fields interact with each other and the net resultant what the magnetic sensor measures. 

World Magnetic Model (WMM)

Major contributors to a magnetic-field: 

+ Conducting, fluid outer core.
+ Earth's crust and upper mantle.
+ Electrical currents in the atmosphere. 

+ Local magnetic interference.

By filtering the local magnetic interference due to other electronic/electrical devices, we have a unique magnetic-field signature present at each place on earth. The WMM aims to provide an accurate estimate of this field. A device (having a magnetic sensor) can measure the components of this field. Then comparing it with the WMM values of the earth's field, one can identify the latitude/longitude of the present location.

Android contains built-in support for the WMM using the GeomagneticField class. The GeomagneticField class utilises the WMM internally to provide an estimated magnetic field at any given point on Earth at a given time. The important thing to note is that this class accepts the location (alongwith altitude and time) and provides the expected magnetic-field at that position (at that particular altitude and instant of time).

To determine the location using the GeomagneticField class, requires some reverse-lookup trickery on our part. More on it in another post.

UPDATE : A recent talk on Sensors and Location based services on Android at blr-droid meetp#12 featuring nGPS among other things.