Setting GPS Altitude Reference Point to compare Altitude Above Ground

Projects Programming
1

I am working on a project to display my drone's altitude above ground. After researching how GPS receivers determine and present their altitude readings (above sea level), it appears that I need to do some tweeking.

I am wondering how I might set my initial GPS altitude reading as a 0 reference point to then be able to compare all following altitude readings to this initial reference point (ground level).

Ultimately, I want to create an If statement in which if my drone goes at least 400 feet above the established reference point, I receive a warning on my remote LCD.

I am using a Arduino Uno R3, a SparkFun GPS Shield Kit (includes a SparkFun GPS Shield (SparkFun GPS Logger Shield - GPS-13750 - SparkFun Electronics) and a GPS Receiver - EM-506 (48 Channel) (https://www.sparkfun.com/products/12751) attached.

I already have the GPS on the drone comparing latitude and longitude coordinates and sending messages to an LCD screen attached to another Uno on my remote control if coordinates match a pre-set range. The two are communicating via two XBee Pro 60mW Wire Antenna - Series 1 (802.15.4) (XBee Pro 60mW Wire Antenna - Series 1 (802.15.4) - WRL-08742 - SparkFun Electronics).

Here is the "transmitter" code (Uno and GPS on drone) that I have working so far. The only altitude portion that I have so far is at the bottom (just displays current altitude):

#include <SoftwareSerial.h>
#include <TinyGPS.h>

//In this case, the GPS module's TX pin will connect to the Arduino's RXPIN which is pin 3.
#define RXPIN 2
#define TXPIN 3

// This is the serial rate for your terminal program.
// Set this value equal to the baud rate of your terminal program
#define TERMBAUD  9600

// Set this value equal to the baud rate of your GPS
#define GPSBAUD  4800

int flag=0;

// Create an instance of the TinyGPS object
TinyGPS gps;

// Initialize the SoftwareSerial library to the pins you defined above
SoftwareSerial uart_gps(RXPIN, TXPIN);

// This is where you declare prototypes for the functions that will be 
// using the TinyGPS library.
void getgps(TinyGPS &gps);

// In the setup function, need to initialize two serial ports; the 
// standard hardware serial port (Serial()) to communicate with the 
// terminal program an another serial port (SoftwareSerial()) for the 
// GPS.
void setup()
{
 
  // Sets baud rate of your terminal program
  Serial.begin(TERMBAUD);
  // Sets baud rate of your GPS
  uart_gps.begin(GPSBAUD);
  
  //Serial.println("");
  //Serial.println("GPS Shield QuickStart Example Sketch v12");
  //Serial.println("       ...waiting for lock...           ");
  //Serial.println("");
}

// This is the main loop of the code. All it does is check for data on 
// the RX pin of the Ardiuno, makes sure the data is valid NMEA sentences, 
// then jumps to the getgps() function.
void loop()
{
  while(uart_gps.available())     // While there is data on the RX pin...
  {
      int c = uart_gps.read();    // load the data into a variable...
      if(gps.encode(c))      // if there is a new valid sentence...
      {
        getgps(gps);        // then grab the data.
        delay(4000);        // delay in grabbing GPS data until to length of time required to display message
      }
  }
}

// The getgps function will get and print the values we want.
void getgps(TinyGPS &gps)
{  
  // Define the variables that will be used
  float latitude, longitude;
  // Then call this function
  gps.f_get_position(&latitude, &longitude);
  
  if (latitude >= 41.479133 && latitude <= 41.493510 && longitude >= -71.541359 && longitude <= -71.522058)
  {
    flag=1;
    Serial.write(flag);
  }
  else
  {
    flag=0;
    Serial.write(flag);
  }
   
  // Can print the altitude and course values directly since there is only one value for the function
  // Serial.print("Altitude (meters): "); Serial.println(gps.f_altitude());
}

Thank you very much! Your input is highly appreciated!

2

Ultimately, I want to create an If statement in which if my drone goes at least 400 feet above the established reference point, I receive a warning on my remote LCD.

How are you going to establish the reference point ? Will it be the altitude at startup ? If so, read the altitude at startup, save it in a variable then compare all subsequent altitudes to the initial one. If some other event it to trigger the establishment of the reference point then the same principle applies,

3

Grab it in setup() only.

4

Steve_P71:
I am wondering how I might set my initial GPS altitude reading as a 0 reference point to then be able to compare all following altitude readings to this initial reference point (ground level).

You would probably set up a loop in setup() to wait until you get a stable altitude fix and save that altitude in a global variable before returning from setup(). Then in loop() you can subtract that global variable from the new GPS altitude to get altitude relative to your start point.

5

Hi Steve

Couple of things to try:

Add in some Serial.print() statements to see what value gps.f_altitude() returns at different points in your program - in setup when you assign the value to "a" and then also when you call it in getgps().

In loop(), you only call getgps() after gps.encode() returns true to indicate that a valid sentence has been received ...

  while(uart_gps.available())     // While there is data on the RX pin...
  {
      int c = uart_gps.read();    // load the data into a variable...
      if(gps.encode(c))      // if there is a new valid sentence...
      {
        getgps(gps);        // then grab the data.

But you don't do the same in setup() before calling getgps(), so I'm not sure that gps.f_altitude() will return a valid value.

Regards

Ray

6

@Steve_P71 gets a one day ban for excessive cross-posting. When he returns he will apologize for wasting the time of so many people. Or be permanently banned.