Mapping Hacks

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Chapter 5, Mapping with Gadgets

#61 Build a Map of Local GSM Cells

HACK

If you want to see the cell ID data go by as you collect it, you can use the

very handy

tee shell command:

$ ./cellstumbler.pl -p /dev/rfcomm0 | tee cell_data.txt

The standard output of cellstumbler.pl contains one sample per line, with

seven comma-separated fields. In order, they are:

timestamp (in iCal format)

GPS status

latitude

longitude

CID (in hexadecimal)

LAC (in hexadecimal)

signal strength (measured in dBm)

The value in the GPS status field is returned directly from gpsd, which will

be

0 if the GPS has no satellite lock, 1 if it has a satellite lock, and 2 if the

location reading is differentially corrected (e,g,, via a WAAS signal).

Evaluating the Data

Given a set of readings of the ID of some radio frequency beacon and its sig-

nal strength, and the latitude and longitude and time at which the measure-

ment was taken, how can we figure out where the radio antennas are? We

could start with a naive interpolation technique that doesn’t even make use

of the signal strength data.

First, we convert the latitude and longitude from the GPS readings into

UTM, to make Cartesian calculations easier. (See “Work with Different

Coordinate Systems”

[Hack #26] for more details on this process.) We take the

numerical average of all the GPS points collected for the same cell-ID read-

ing, and call this the “center” of the cell. We take the ...