## A First Look At GPS Data

May 21, 2013

In previous installments, we looked at how to read data from a serial-over-USB GPS, then how to parse the data. However, getting data isn’t everything, we must also ascertain it’s valid

The first thing that comes to mind to test the device’s precision (while I don’t really know how to check for the accuracy) by plug it in, leave it stationary, and collect the readings for a good while

## Average node depth in a Full Tree

May 14, 2013

While doing something else I stumbled upon the interesting problem of computing the average depth of nodes in a tree. The depth of a node is the distance that separates that node from the root. You can either decide that the root is at depth 1, or you can decide that it is at depth zero, but let’s decide on depth 1. So an immediate child of the root is at depth two, and its children at depth 3, and so on until you reach leaves, nodes with no children.

So the calculation of the average node depth (including leaves) in a tree comes interesting when we want to know how far a constructed tree is from the ideal full tree, as a measure of (application-specific) performance. After searching a bit on the web, I found only incomplete or incorrect formulas, or stated with proof. This week, let us see how we can derive the result without (too much) pain.

## Parsing GPS data with Bash

May 7, 2013

Last time we looked at how to get the data to the GPS and now we will have a look at how to parse the data. Turns out that except for the check-sum, everything is pretty straight forward, even in Bash.

So, why bash in the first place? Well, there’s not real reason except that for the something else I’m working on, it’s the ideal glue-code language, allowing me to invoke simply other programs that I do not want to re-code (or take parts of) to do what I want. I must say that I even have a C# version of the GPS data grabber, but while fancier, it does not bring much more than the Bash version.

## Reading GPS data with Bash

April 30, 2013

I am presently working on something that requires geolocation. Not knowing much about GPSes and related topics, I decided to get a USB GPS. This week, let’s have a look at how we can extract information from the USB GPS using Bash.

The first step is to locate your USB GPS as a device. If it’s NMEA compliant, it should mount automagically as a USB serial port. You would think that lsusb -v would show you the device, but it does not always. Sometimes it shows as “Brand X GPS”, sometimes it only shows as a generic device, say “MediaTek Inc.”, or even as a modem. It will typically show up as /dev/ttyUSB0 or /dev/ttyACM0.

## Breaking Caesar’s Cipher (part III)

April 23, 2013

In the last installment of this series, we looked at Markov chains as a mean of estimating the likelihood of a given piece of text of actually being a message, written in English, rather than mere gibberish.

This week, we finally piece everything together to obtain a program to crack Caesar’s cipher without (much) human intervention.

April 21, 2013

Caroline Paul, Wendy MacNaughton — Lost Cat: A True Story of Love, Desperation, and GPS Technology — Bloomsburt, 2013, 176 pp. ISBN 1608199770

In the last installment of this series, we had a look at Caesar’s cipher, an absurdly simple encryption technique where the symmetric encryption only consists in shifting symbols $k$ places.