Category Archives: Tutorial

LESSON 24: Understanding GPS NMEA Sentences

GPS units spit out data in a dizzying array of formats. It can become a challenge to take the output from a GPS unit, like the Adafruit Ultimate GPS breakout board, and get that data to play nicely with mapping programs like Google earth. It is as if no one is speaking the same language. In this lesson we will look at what the data coming off the GPS means, and how you can work with it, and get it to display properly in programs like Google Earth.

Arduino GPS
We are using the Adafruit Ultimate GPS

When you connect a GPS board up, and look at the data coming off of it, you are likely to see something like this:


This is what the data looks like when your GPS does not have a fix. When it does have a fix, there will be all types of numbers between the commas in the lines above. So, how do we make sense of all this? The first thing is to learn some of the lingo. Each line above is referred to as a NMEA sentence. There are different NMEA sentence type, and the type is indicated by the first characters before the comma. So, $GPGSA is a certain type of NMEA sentence, and $GPRMC is a different type of NMEA sentence. To make this whole thing a manageable problem, the first thing we must realize is that the data that we want will be in the $GPRMC sentence  and the $GPGGA sentence. If your GPS unit has options to turn the other sentences off, then turn them off to simplify the data getting thrown at you. If you can not turn the other sentences off, then just ignore them, and focus on the two you care about,  $GPRMC, $GPGGA.

If your GPS has a fix, then your GPS sentences should look something like this:

Lets start by breaking down the $GPRMC sentence.

The $GPRMC is simply telling what sentence type is on the line. The next number,  represents the Coordinated Universal Time (UTC). It works like this, 194530.000 would be 19:45 and 30.0 seconds in UTC. Since converting from UTC to your local time is simply a matter of knowing what time zone you are in, you can see that one thing you get from a GPS is a very accurate clock. The next letter just lets you know if your signal is Active (A) or Void (V).  An ‘A’ indicates you are getting a signal and things are working properly. A ‘V’ means you do not have a signal.

Now, for the good stuff, the next number and following letter gives you your lattitude.  3051.8007,N should be interpreted as follows:

Your Latitude is: 30 degrees, 51.007 minutes, in the Northern Hemisphere.

Similarly, the next number, 10035.9989,W,  is Longitude. This is interpretted as:

Your Longitude is:  100 degrees 35.9989 minutes, in the Western Hemisphere.

Degrees could be a one, two or three digit number. Hence you have to be careful parsing this data. What is always the case is that the first two numerals to the left of the decimal place, and the numbers to the right of the decimal represent the minutes. So, always take the number starting two to the left of the decimal, and those following all the way to the comma, and that is your minutes. Unfortunately, if you simply try to put 3051.8007,N,10035.9989,W, into Google Earth, it will not recognize it. The simplest thing you can do to get Google Earth to recognize and properly display the coordinate would be to enter the following:

30 51.8007N, 100 35.9989W

Notice the space inserted between degrees and minutes, and no comma before hemisphere designation. This would properly display that single point via the Google Earth search bar. The challenge however is that if you want to log data from your GPS and then show a track of where you have been, you will have to create a KML or KMZ file. These files are even more particular about the format of the coordinate data. These files want both latitude and longitude as decimal numbers, so we must convert degrees, minutes to decimal degrees. You can do this simply by realizing that there are sixty minutes in a degree. So, for the case above, 51.8007 Minutes = 51.8007/60 degrees = .86335 degrees. So, to make a KML file that Google Earth will like, 30 51.8007N should be converted to 30.86335. We still have to deal with the N and W. KML files do not know what to do with the N, S, E, W. So, you must do the conversion. On Latitude, if you have a N, leave Latitude positive, if you have a S make your Latitude negative. On Longitude, if you have an E leave your number positive. If you have a W make your Longitude negative. Following these rules:

Should become:

30.8633, -100.5999

Those numbers will not only display properly in a KML file, they will also work if directly typed into the search bar on Google Earth.

We will talk more about KML files in the next lesson, but we will state here that in the KML file, you should put Longitude first, followed by Latitude. Opposite of how you put the numbers in Google earth, which wants latitude followed by longitude.

OK, that is the hard part, now for the rest of the characters in the sentence. The next number, 1.49 in the example above is the speed in knots. I was walking, so you can see this is a small number in the above example. The next number represents track angle, not something I find useful or use.  Then after that, you see the number  310714. This is the date, which would be DD/MM/YY so this date is July 31, 2014. You can see on my NMEA sentence, I just get commas after that. Some units report magnetic variation here, but mine doesn’t. Then finally, the final characters represent a checksum, something we do not need to worry with.

So there, our first NMEA sentence parsed. You can see that you get just about everything you need in this one sentence.

So why do we need the $GPGGA sentence you ask. Well, it has a couple of nuggets of info we need, particularly if we are doing High Altitude Ballooning/Edge of Space work. Lets look at the $GPGGA sentence:

Again, the first number represents the time.

194530.000 would be 19:45 and 30.000 seconds, UTC

The next numbers represent Latitude and Longitude, just as in the $GPRMC sentence above, so I will not go through that again.

Now the next number, 1 above, is the fix quality. a 1 means you have a fix and a 0 means you do not have a fix. If there is a different number it is more details on the type of fix you have. Next number is how many satellites you are tracking. For us, 4. The larger number here the better. Next up, 2.18 is the horizontal dilution of position. Something I don’t worry with. Next one is an important one . . . 746.4 is my altitude in meters above mean seal level. This is hugely important for projects like high altitude ballooning. It is a direct indication of your altitude. Next number is the altitude of mean sea level where you are. This is something you do not need to worry about in most applications but is related to a geometric approximation of the earth as an ellipsoid. Look up WGS84 Ellipsoid if you really want to understand this number.

OK, lets bring this all home now. Using the  $GPRMC and $GPGGA NMEA sentences, we can get our latitude and longitude coordinates, and convert them to numbers that can be recognized by Google Earth, or put into a KML file. We also have our speed in knots, and our altitude. These are all the things we would want for projects like unmanned aerial drones, or high altitude balloons.

LESSON 22: Build an Arduino GPS Tracker

OK, it is time for us to take our projects up to that next level. We are going to build a GPS tracker from scratch. This is going to take several lessons to complete, but it will build on what you already know, and is really not going to be that difficult of a project. We will be using the most excellent Adafruit Ultimate GPS module. This is an excellent GPS. I like it because it is affordable, easy to use, and is one of the few that will work at extreme elevations, making it ideal for our Edge of Space/High Altitude Balloon work.

Arduino GPS
We are using the Adafruit Ultimate GPS

This unit is pretty easy to hook up, as you can see in the Table below:

Connecting the Adafruit Ultimate GPS Unit to Arduino
GPS PinArduino Pin
RXPin 2
TXPin 3

Our goal in this lesson is to get the GPS connected, and get it reading NMEA sentences. NMEA is a data format used by GPS and mapping devices and software to keep track of position coordinates. There are lots of different NMEA sentences, but the two that contain the most useful information are the $GPRMC and $GPGGA sentences.

Our interest is in creating a location tracker for our High Altitude Balloon work, and the $GPRMC and $GPGGA sentences contain all the information and data we would need for that work. These sentences contain the lattitude, longitude, time, altitude, and velocity.

The GPS modules are pretty easy to work with. When you apply power to the GPS, it immediately starts spitting out NMEA sentences to its serial port. Our job on the arduino side is to simply read these data strings, and then parse them into useful data. The thing that is a challenge is that they constantly spit out data, whether you want it, or whether you are ready for it or not. In developing the software, we have to be mindful the the data is always spewing out of the GPS. Typically, we will have other components on our package, like temperature, pressure and inertial sensors. While our arduino is out making measurements on these other sensors, data continues to stream in from the GPS, likely overflowing our serial buffer. When we return to make a GPS measurement, it is very likely that the serial buffer will have old or corrupt GPS data in it. Hence,  we must be mindful to deal with this issue in developing our software.

The video above takes you step-by-step through connecting and reading the NMEA sentences step-by-step. Then in the next lesson we will parse and log the data to create a portable GPS tracker. The code below is explained in the video. You need to watch the video to understand this code, and so you will be able to begin to work with this code to create a portable GPS tracker.

In order for this software to work, you need to download and install the Adafruit GPS Library.


Arduino LESSON 21: Log Sensor Data to an SD Card

In most of our work so far, we have just watched our data go by on the Serial Monitor. In most cases, you will want to have some means to store your data. The easiest way to do this is to use a simple SD card reader. For this example, we use the Virtuabotix SD Card Reader.

SD Card Reader
Arduino connected to a BMP180 pressure sensor and an SD Card Reader

In this tutorial, we will need to have some sensor hooked up so we will have some data to store. We will be using the BMP 180 Pressuer and Temperature sensor from adafruit.  We have a complete tutorial on this sensor  HERE.  You will need to go to that lesson and get the sensor hooked up, the library installed, and the software done. All this is explained step-by-step in the LESSON.

The BMP180 is connected to the arduino as follows:

Connecting Up the BMP180 Pressure and Temperature Sensor
BMP180 PinArduino Pin

Once you have the BMP180 connected, test and make sure your code is working, and you are getting good pressure and temperature readings. Once that is working, you are ready to connect your SD card Reader/Writer.

The SD card reader should be connected as follows:

Connecting the SD Card Reader
Sd Card Reader PinArduino PinDetails
GNDGNDCommon Ground
3.3 V – (NOT USED)
CS4Chip Select
GNDGNDCommon Ground

In the video we will show step-by-step how to develop the software. You should follow along in the video, and not copy and paste the code below. You will never learn to program if you do not write your own code. The code below is to help you in case you get stuck.

If you have the BMP180 and the SD card connected correctly,  this should create a file called PTData.txt on the card, and write comma delimited data to the file. Note that if the file does not exist on the card, the command:

will create the file. If the file already exists, this command will append data to the existing file. If you want to start with a clean new data set, erase the old PTData file.

When you run the program, you end up with a PTData.txt file on the SD card. When you have finished logging your data, you can pop the card out, put it into your PC, and then import the data into excel. You should now be able to plot, graph or analyze the data using all the powerful features of Excel.

Python with Arduino LESSON 13: Calculating Height from Pressure measurements from BMP180 Pressure Sensor.

It is time to bring together a lot of things we have learned in our earlier lessons to create a Height-O-Meter, which will plot how high our BMP180 pressure sensor is above the floor. For this lesson we make simplifying assumption of constant temperature. When we use the sensor for our space probe or other high altitude experiments we will need to derive the equation again to take into account changing temperature. We went through the math of calculating height from changing pressure in LESSON 12.

In this lesson, we start with the software we developed in LESSON 11 for measuring, streaming, and plotting pressure and temperature data from the BMP180 sensor.

Remember, we connect the sensor to the Arduino as follows:

Connecting Up the BMP180 Pressure and Temperature Sensor
BMP180 PinArduino Pin


The software we are using on the arduino side is shown below, from LESSON 11.

We modify the Python code from LESSON 11 as explained in the video above to get this code for the Python side.

 Please go through video for complete description of this software. Remember this is only valid for small changes in height over which temperature is constant.

Python with Arduino LESSON 11: Plotting and Graphing Live Data from Arduino with Matplotlib

We now have all the pieces put together to allow us to plot live data from the Arduino. If you have kept up with the earlier lessons, you will now have everything you need. If you have not done the earlier lessons, make sure you have python 2.7, vPython and pySerial installed from Python with Arduino LESSON 2.  Make sure you have installed matplotlib (Python with Arduino LESSON 7), and install drawnow (Python with Arduino LESSON 10). Also, you need to build the BMP180 circuit and get the arduino programmed up as explained in Python with Arduino LESSON 9. With this business taken care of, you are now ready to start plotting live data.

Pressure Data
This chart shows live pressure and temperature data being plotted in real time

We are using the Adafruit BMP180 pressure sensor.  We showed how to hook it up and program it in LESSON 9. As a reminder, we are using this code for the arduino. LESSON 9 explained in detail how the code works.

The video in this lesson above explains step-by-step how to develop the code on the Python side, and how matplotlib and drawnow work together to make live graphs and plots of data streaming from the arduino in real time. The code below is what we developed in the video. Do not simply cut and paste this code, but make sure that you understand it so you are able to create your own live graphing programs from scratch. If you are in my class, you will be required to be able to develop live graphing code like this from scratch, so don’t take a shortcut and copy and paste.

You should be seeing data like the graph on the top of this lesson. You will probably need to adjust your y-axis scale parameters in Python to ensure the scale is suitable for the data you are taking. If your chart is blank, likely your y-scales are not right for your data measurements.