GPS ReceiverBuild and program your own GPS receiver.

Contents

• Overview• Challenge 1: Understanding GPS Data

o Collecting Your Componentso Connecting the Receiver to Your Computero Setting up a Terminal Emulatoro Understanding GPS Data

• Challenge 2: Controlling the Receivero Collecting Your Componentso Connecting the Receiver to the Chipo Displaying Raw GPS Data on the LCDo Parsing the Raw GPS Datao Selecting GPS Output Streamso Displaying Specific GPS Data Elementso Formatting GPS Data on Two Lines

Overview

In this project, you will build an interface to the GlobalSat EM-406A GPS Receiver. The EM-406A is a 20-channel GPS receiver based on the SiRF StarIII chipset, and it features a built-in antenna. After building the interface circuit, you will learn how to customize it to display more than 30 standard GPS data types, including latitude, longitude, altitude, velocity, heading, time, and date.

Figure 1. GlobalSat EM-406A GPS Receiver.

Like most GPS receivers, the EM-406A conforms to the standard NEMA SiRF interface protocol, so the code you write in this project will work with most other GPS receivers.

Challenge 1: Understanding GPS Data

In order to better understand the GPS data format, you will first connect your GPS receiver directly to your computer through the Machine Science programming cable. GPS receivers are capable of transmitting a large volume of data, which can be difficult to see on a small LCD screen. By viewing the data on a computer monitor, you will be able to learn about the different GPS data streams and data types.

Collecting Your Components

In order to connect your GPS receiver to your computer, you will need the following components:

Part Quantity DescriptionA 1 GlobalSat EM-406A GPS Engine B 1 Connector harness (6-pin) C 1 Bent header (6-pin) D 1 Bent header (4-pin)

Connecting the Receiver to Your Computer

Using Figure 2 as a guide, connect the GPS receiver to the programming board, which will link it to your computer. To avoid intefering with the microcontroller, build this circuit in an area of the board away from the chip. Figure 3 shows one way to lay out this circuit. To orient the GPS unit properly, note the position of the grey wire in Figure 3.

Figure 2. Connecting the GPS receiver to the computer (schematic).

Figure 3. Connecting the GPS receiver to the computer (photo).

Setting up a Terminal EmulatorConnected as shown in Figure 2, the GPS receiver will send satellite data to your computer’s COM port. To view the data, you will need to run a terminal emulator--a program that emulates an older-style text-only computer terminal. Depending on your operating system, your computer may already have a terminal emulator, or you may need to download one from the Internet:

• Windows (Vista): TeraTerm Pro Web is available here: http://www.ayera.com/teraterm/

• Windows (pre-Vista): HyperTerminal is available in the Start menu under Programs > Accessories > Communication.

• Macintosh: ZTerm comes pre-installed with OSX and above. • Linux: CuteCom can be found at: http://cutecom.sourceforge.net/

The instructions in this section are for installing TeraTerm Pro Web, but should give you enough information to help you set up any of the other programs listed above.

1. Determine which COM port your Machine Science programming board is using by viewing the Options menu in the Programming Window.

2. Download TeraTerm Pro Web from http://www.ayera.com/teraterm/ 3. Unzip the .zip file to a convenient location on your hard drive. 4. Open the folder and double click on the file ttermpro.exe. You will see a dialog

box like the one shown in Figure 4.

Figure 4. New connection dialog box.

5. Select Serial and the COM port being used by your Machine Science programming board. You should now see unintelligible characters scrolling across and down your screen as shown in Figure 5.

Figure 5. TeraTerm Window.

6. The reason you see a jumble of characters is that you need to set the baud rate in baud rate in your terminal program to match the baud rate of the GPS receiver. Select “Serial port...” from the Setup menu. You should now see a dialog box like the one shown in Figure 6.

Figure 6. Serial port setup.

7. Select 4800 for the baud rate. You should now see lines of GPS data being updated every second on your screen, as shown in Figure 7.

Figure 7. Terminal emulator.

Understanding GPS DataGPS receivers are capable of transmitting eight types of GPS data streams, shown below:

Option DescriptionGGA Time and position dataGLL Latitude, longitude, time and statusGSA GPS receiver operating mode, satellites used in the position solution,

and DOP valuesGSV The number of GPS satellites in view, satellite ID numbers, elevation,

and azimuthMSS Signal-to-noise ratio, signal strength, frequency, and bit rate from a

radio-beacon, receiverRMC Time, date, position, course and speed dataVTG Course and speed information relative to the groundZDA Pulse Per Second (PPS) timing message

Referring to Figure 7 (or your terminal emulator, if it is still running on your computer), you should see four types of GPS data streams: GGA, RMC, GSV, and GSA. These are the four default data streams that are transmitted by the GPS receiver. Each stream is transmitted on a single line. An example of a GGA data stream is shown below.

$GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M, , , ,0000*18

Each GPS data stream can have up to sixteen GPS data fields. The data fields are separated by commas, as shown in the sample above. Table 3 below details the data fields contained in the GGA data stream.

Name Example DescriptionMessage ID $GPGGA GGA protocol headerUTC Time 161229.487 hhmmss.sss (h=hours,

m=minutes, s=seconds)Latitude 3723.2474 ddm.mmmm (d=degrees,

m=minutes)N/S Indicator N N=north or S=south

Longitude 12158.3416 dddmm.mmmm (d=degrees, m=minutes)

E/W Indicator W E=east or W=westPosition Fix Indicator 1 0=fix not valid, 1,2, or 3= fix

validSatellites Used 07 Range 0 to 12

HDOP 1.0 Horizontal dilution of precision

MSL Altitude 9.0 Meters above sea levelUnits M Meters

Geoid Separation MetersUnits Meters

Age of Diff. Corr. SecondsDiff. Ref. Station ID 0000

Checksum *18 Check for valid data<CR><LF> Carriage return and line feed

indicating end of message

Don't worry about understanding all of the GPS data types shown in this table. The ones of greatest interest to you will probably be time, latitude, longitude, and altitude. In the next challenge, you will learn how to display this information in a meaningful way on your LCD.

Challenge 2: Controlling the Receiver

In this challenge, you will construct a circuit connecting the EM-406A to the Atmega Board and program the chip to display readable GPS data on the LCD.

Collecting Your ComponentsIn order to build the circuit, you will need the following components:

Part Quantity DescriptionA 1 GlobalSat EM-406AB 1 Connector harness (6-pin)C 1 Bent header (6-pin)D 1 Atmega Board

Connecting the Receiver to the ChipUsing the schematic in Figure 8 as a guide, add your components to the breadboard. Figure 9 shows an example of a completed board for this circuit. Since the leads on the GPS receiver are not labeled, you may need to examine the picture to make the correct connections.

Figure 8. Connecting GPS receiver to the Atmega168 (schematic).

Figure 9. Connecting GPS receiver to the Atmega168 (photo).

Displaying Raw GPS Data on the LCDThe following code will allow you to begin receiving data from your GPS receiver and displaying the information on your LCD. In this first step, you will not be parsing the data from your receiver. As you learned in the previous challenge, GPS receivers can produce a lot of data, so your tiny LCD will quickly fill up with a lot of numbers, letters, and symbols. While you will not be able to read the data, it will indicate that your circuit is working properly. (It also looks kind of cool!)

IMPORTANT NOTE: The GPS receiver and the Machine Science programming board both use the same pin for transmitting data to the Atmega168. You must disconnect the wire linking the GPS pin 4 to the Port D0 of the Atmega168 before downloading your code. You can immediately reconnect this wire once the code has been downloaded.

1. #include "mxapi.h"2. #include "usart.h"3. #include "lcd.h"4. 5. int main (void)6. { 7. char gps_characters; //Declare a variable for

storing GPS characters

8. usart_init(4800); //Initialize the USART to 4800 baud

9. lcd_init(); //Initialize your LCD10. while(1==1) //Start an infinite loop11. {12. gps_characters=usart_read(); //Read each

character from the GPS13. lcd_character(gps_characters); //Display

each character on the LCD 14. }15. }

Parsing the Raw GPS DataIn order to better view the data on your LCD, you will need to parse the raw data stream from the GPS receiver. In Figure 7, you can see that the terminal emulator on your computer displays each data stream on a separate line. It does this by checking for a carriage return and line feed, the ASCII characters 13 and 10, at the end of each data stream (as shown in the last rows of Table 3). When the terminal emulator finds these two characters, it starts the next data set on a new line, making the data much easier to read than if it were simply a continuous stream.

The microcontroller has no built-in ability to parse data. As you may have seen in the previous exercise, the carriage return and line feeds at the end of each data string are displayed on the LCD as more data. As a result, using the previous code, the GPS data was displayed on the LCD in long stream without easily discernible breaks.

The next code sample introduces breaks after each line of data, just like the terminal emulator did. It checks continuously for a line feed character (ASCII 10), and when it gets one, it repositions LCD's cursor at the start of the first line before displaying the next character. The data on the display remains somewhat difficult to read, since all of the different GPS data streams are displayed, but it is easier to pick out patterns in the data.

1. #include "mxapi.h"2. #include "usart.h"3. #include "lcd.h"4. 5. int main (void)6. {7. char gps_characters; //Declare a variable to

store GPS characters8. usart_init(4800); //Initialize the USART to

4800 baud9. lcd_init(); //Initialize the LCD10. while(1==1) //Start an infinite loop

11. {12. gps_characters = usart_read(); //Read

each character from the GPS13. lcd_character(gps_characters); //Display

each character on the LCD14. if (gps_characters == 10) //Check for the

line feed (ASCII 10) character15. {16. lcd_instruction( FIRST_LINE ); //Move

the cursor to the first line17. }18. }19. }

Programming ChallengeModify your code so that each line new line scrolls up in the same way that it displays on your computer monitor. This is difficult!

Selecting GPS Output StreamsThe following code turns on and off specific GPS data streams. Remember that the receiver is capable of transmitting any or all of the eight different GPS streams listed in Table 2 at a rate of up to 100 Hertz each. In order to display the data on the LCD, it is best to turn on only one stream and turn off the rest, using a function called gps_setdata. This function takes two arguments: the first argument selects the GPS stream that you want to turn on or off; the second argument determines how frequently (in Hertz) the GPS receiver transmits the stream. A value of 0 turns off the stream entirely, while a value of 1 to 99 specifies a transmission frequency of 1 to 99 times a second.

1. #include "mxapi.h"2. #include "usart.h"3. #include "lcd.h"4. #include "gps.h"5. 6. int main (void)7. {8. char gps_characters; //Declare a variable to

store GPS character9. usart_init(4800); //Initialize the USART to

4800 baud10. lcd_init(); //Initialize the LCD 11. delay_ms(2000); //Delay while GPS receiver

warms up 12.

13. /* Turn off all NMEA formats except the one you want to see. */

14. gps_setdata(NMEA_GGA, 1); //Turn on GGA format at 1 Hertz

15. gps_setdata(NMEA_GSA, 0); //Turn off GSA format (0 Hertz)

16. gps_setdata(NMEA_GSV, 0); //Turn off GSV format (0 Hertz)

17. gps_setdata(NMEA_RMC, 0); //Turn off RMC format (0 Hertz)

18. while(1==1) //Run the following code in an infinite loop

19. {20. gps_characters = usart_read(); //Read

each character from the GPS21. lcd_character(gps_characters); //Display

each character on the LCD22. if (gps_characters==10) //Check for the

line feed (ASCII 10) character23. {24. lcd_instruction(FIRST_LINE); //Move the

cursor to the first line25. }26. }27. }

You do not need to select a GPS data stream every time you program the device. Once a stream is selected, the receiver displays only that stream until the gps_setdata function is called again.

Programming ChallengeTry changing the GPS stream displayed by your GPS receiver. Try changing how often the GPS stream is updated.

Displaying Specific GPS Data ElementsWith the following code, you can display specific GPS data elements, such as time, latitude, longitude, and altitude, from the selected data stream. To simplify matters, every GPS data stream is parsed in the background by a function within the “gps.h” header file. The parsed data is stored in an array called gps_data[ ], with each value in the array representing a different GPS data element. You can reference any element in the gps_data[ ] array in your code.

For example, with the GGA data stream, selected a line of data might look like this:

$GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M, , , ,0000*18

In this instance, gps_data[0] would equal $GPGGA, gps_data[1] would equal 161229.487, gps_data[2] would equal N, and so on.

The following table shows the gps_data[] values for some of the more useful data elements in the GGA, RMC, and VTC data streams.

GPS Data Example Units Format GGA RBC VTGMessage ID $GPGGA 0 0 0

Latitude 3723.2475 Degrees and

minutes

ddmm.mmmm 2 3

North/South N 3 4Longitude 12158.3416 Degrees

and minutes

dddmm.mmmm 4 5

East/West W 5 6Altitude 9.0 Meters 9Speed 0.2 Kilometers

/ hour7

Course 309.62 Degrees 8 1Time 161229.487 hhmmss.ss 1 1Date 120598 ddmmyy 9

Satellites 07 0 to 12 7

This program uses the chip's internal interrupts, which allow the microcontroller to execute multiple tasks at the same time. A few new statements are added to enable and initialize the interrupts.

1. #include "mxapi.h"2. #include "usart.h"3. #include "lcd.h"4. #include "gps.h"5. 6. int main (void)7. {8. usart_init(4800); //Initialize the USART to

4800 baud

9. usart_interrupt_rx(ENABLE); //Enable interrupts for the USART

10. sei(); //Turn on interrupts11. lcd_init(); //Initialize the LCD 12. 13. while(1==1) //Start an infinite loop14. {15. lcd_instruction(FIRST_LINE); //Move the

cursor to the first line16. lcd_instruction(CLEAR); //Clear the LCD 17. lcd_text(gps_data[2]); //Display the 3rd

value in the selected data stream 18. }19. }

Formatting GPS Data on Two LinesThe last code example in this project will display and format multiple GPS data fields on both lines of the LCD. The added functions in this example should be familiar to you from other Machine Science projects.

1. #include "mxapi.h"2. #include "usart.h"3. #include "lcd.h"4. #include "gps.h"5. 6. int main (void)7. {8. usart_init(4800); //Initialize the

Atmega168s USART to 4800 baud9. usart_interrupt_rx(ENABLE); //Enable

interrupts for the USART10. sei(); //Turn on interrupts11. lcd_init(); //Initialize your LCD 12. 13. while(1) //Run the following code in an

infinite loop14. {15. lcd_instruction(FIRST_LINE); //Move the

cursor to the first line16. lcd_text(gps_data[2]); //Display value

of third data field 17. lcd_character(':'); //Display ':'

character

18. lcd_text(gps_data[3]); //Display value of fourth data field

19. 20. lcd_instruction(SECOND_LINE); //Move the

curse to the first line21. lcd_text(gps_data[4]); //Display value

of fifth data field 22. lcd_character(':'); //Display ':'

character 23. lcd_text(gps_data[5]); //Display value of

sixth data field 24. lcdBlankLine(); //Send blanks to

clear the lines25. }26. }