Skip to main content

GAGA-1: Flight Computer radio's first transmission

Work is progressing on the flight computer with the first RTTY transmission. In the picture you can see the computer (the Arduino) in the foreground, connected with test leads to a small breadboard containing a voltage divider made from three resistors which then connects to the radio itself. The radio is connected to the ground plane antenna in the capsule.

The radio module is a Radiometrix NTX2 which transmits using FSK on 434.650Mhz. Its TX pin takes a voltage between 0 and 3V and converts that into a frequency shift of 0 to 5kHz. For the RTTY transmission I'll be using a shift of 425Hz between the mark and space frequencies.

That shift corresponds to a difference in voltage on the TX pin of 0.255V. The configuration I'm using is based on picoatlas where a pair of pins from the Arduino are used to power a voltage divider. The set up is as follows:
Arduino                         Radio
7 ---27K--------------\
                       ---------- TX
8 ---22K----2K2-------/
The pins 7, 8 and are switched so that one gives 5V and the other 0V. Thus the TX will be fed 2.36V or 2.64V depending on which is high and which is low. That's a difference of 0.27V. The TX pin has an input impedance of 100K and in the real measured circuit the voltages at the TX pin were 2.01V and 2.25V for a 0.24V shift. That should correspond to a shift of 400Hz. I'll measure that later.

The test code is as follows:
// These are the digital pins used to control the Radiometrix NTX2
// module.  TX0/TX1 are for the two tones used for RTTY and EN is
// connected to the Enable pin to enable or disable the module

#define EN  6
#define TX0 8
#define TX1 7

// These are the pins used to read/write two the internal and
// external temperature sensors

#define TEMP_INT
#define TEMP_EXT

void setup()
{
  // Set up the pins used to control the radio module and switch
  // it on
  
  pinMode( EN,  OUTPUT );
  pinMode( TX0, OUTPUT );
  pinMode( TX1, OUTPUT );
  digitalWrite( EN, HIGH );
  
}

void loop()
{
    char test[255];
    sprintf( test, "GAGA-1 High Altitude Balloon Test Transmission on 434.650 @ 10mW" );
    rtty_send(test);
    
    delay(5000);
}

// ---------------------------------------------------------------------------------
// RTTY Code
//
// Code to send strings via RTTY.  The RTTY parameters are defined by constants
// below.
// ---------------------------------------------------------------------------------

// The number of bits per character (7), number of start bits (1), number of stop bits (2)
// and the baud rate.

#define ASCII 7 
#define START 1
#define STOP 2
#define BAUD 50
#define INTER_BIT_DELAY (1000/BAUD)

// rtty_send: sends a null-terminated string via radio to the ground trackers
void rtty_send( char * s ) // The null-terminated string to transmit
{
  char c;
  while ( c = *s++ ) {
    int i;
    for ( i = 0; i < START; ++i ) {
      rtty_bit(0);
    }
    
    int b;   
    for ( i = 0, b = 1; i < ASCII; ++i, b *= 2 ) {
      rtty_bit(c&b);
    }
    
    for ( i = 0; i < STOP; ++i ) {
      rtty_bit(1);
    }
  }
 
  // Note that when idling RTTY specifies that it be in the 'mark' state (or 1).  This
  // is achieved by the stop bits that were sent at the end of the last character. 
}

// rtty_bit: sends a single bit via RTTY
void rtty_bit(int b) // Send 0 if b is 0, 1 if otherwise
{
  digitalWrite(TX0,(b>0)?HIGH:LOW);
  digitalWrite(TX1,(b>0)?LOW:HIGH);
  delay(INTER_BIT_DELAY);
}
Thanks to everyone on #highaltitude for tips and encouragement.

Comments

Popular posts from this blog

Your last name contains invalid characters

My last name is "Graham-Cumming". But here's a typical form response when I enter it: Does the web site have any idea how rude it is to claim that my last name contains invalid characters? Clearly not. What they actually meant is: our web site will not accept that hyphen in your last name. But do they say that? No, of course not. They decide to shove in my face the claim that there's something wrong with my name. There's nothing wrong with my name, just as there's nothing wrong with someone whose first name is Jean-Marie, or someone whose last name is O'Reilly. What is wrong is that way this is being handled. If the system can't cope with non-letters and spaces it needs to say that. How about the following error message: Our system is unable to process last names that contain non-letters, please replace them with spaces. Don't blame me for having a last name that your system doesn't like, whose fault is that? Saying "Your

All the symmetrical watch faces (and code to generate them)

If you ever look at pictures of clocks and watches in advertising they are set to roughly 10:10 which is meant to be the most attractive (smiling!) position for the hands . They are actually set to 10:09.14 if the hands are truly symmetrical. CC BY 2.0 image by Shinji I wanted to know what all the possible symmetrical watch faces are and so I wrote some code using Processing. Here's the output (there's one watch face missing, 00:00 or 12:00, because it's very boring): The key to writing this is to figure out the relationship between the hour and minute hands when the watch face is symmetrical. In an hour the minute hand moves through 360° and the hour hand moves through 30° (12 hours are shown on the watch face and 360/12 = 30). The core loop inside the program is this:   for (int h = 0; h <= 12; h++) {     float m = (360-30*float(h))*2/13;     int s = round(60*(m-floor(m)));     int col = h%6;     int row = floor(h/6);     draw_clock((r+f)*(2*col+1), (r+f)*(row*2+1),

The Elevator Button Problem

User interface design is hard. It's hard because people perceive apparently simple things very differently. For example, take a look at this interface to an elevator: From flickr Now imagine the following situation. You are on the third floor of this building and you wish to go to the tenth. The elevator is on the fifth floor and there's an indicator that tells you where it is. Which button do you press? Most people probably say: "press up" since they want to go up. Not long ago I watched someone do the opposite and questioned them about their behavior. They said: "well the elevator is on the fifth floor and I am on the third, so I want it to come down to me". Much can be learnt about the design of user interfaces by considering this, apparently, simple interface. If you think about the elevator button problem you'll find that something so simple has hidden depths. How do people learn about elevator calling? What's the right amount of