Introduction to the near space imaging project

Hi

Welcome to wider image. This site will serve as a portal for my near space imaging project.

I am aiming to gain photos and video footage of our earth from near space. To achieve this I will be using a helium meteorological balloon which will ascend to somewhere in the region of 25-40km above the surface of the earth. Coupled to the balloon will be a payload consisting of a stills camera, a video camera and a flight computer.

The Flight Computer and tracking basics

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The flight computer will serve as a multi functional computer which will be responsible for tracking the flightpath of the balloon and broadcasting its position over ground via radio transmissions. The radio is a very low power device which broadcasts at less than 10mW in line with UK radio transmission laws. A GPS chip will monitor the position using the network of satellites positioned in space, the chip will then output a data string in a standardised format known as NMEA.

The string will look similar to the following code:

$GPGGA,092750.000,5321.6802,N,00630.3372,W,1,8,1.03,61.7,M,55.2,M,,*76
Where:
     GGA          Global Positioning System Fix Data
     123519       Fix taken at 12:35:19 UTC
     4807.038,N   Latitude 48 deg 07.038' N
     01131.000,E  Longitude 11 deg 31.000' E
     1            Fix quality: 0 = invalid
                               1 = GPS fix (SPS)
                               2 = DGPS fix
                               3 = PPS fix
			       4 = Real Time Kinematic
			       5 = Float RTK
                               6 = estimated (dead reckoning)
			       7 = Manual input mode
			       8 = Simulation mode
     08           Number of satellites being tracked
     0.9          Horizontal dilution of position
     545.4,M      Altitude, Meters, above mean sea level
     46.9,M       Height of geoid (mean sea level) above WGS84
                      ellipsoid
     (empty field) time in seconds since last DGPS update
     (empty field) DGPS station ID number
     *47          the checksum data, always begins with *

The flight computer will then process this data into what is known as a string. The string will then be passed to the radio module which will broadcast it as RTTY. This is basically serial data, which is broadcast at 50 bits of information per second.

The signal is then broadcast via an antenna, which should allow it to be picked up on the ground. From the ground, one of two things can be done with the data.

Firstly and most simply, the signal can be tracked using what is known as a directional antenna. (Think of wildlife programs where bears and other wild animals are fitted with radio collars. The people who are concerned with tracking the animal then use the same sort of antenna to give them an idea as to the direction the animal is in. The strength of the signal acts a guide to the range. Here is a clip of conservationists using just that technique) This is a very rough method for the purpose of tracking a balloon at the sorts of altitude that is will hopefully reach.

The second method, and it is more complex, though much more useful for the purposes of tracking, is to use what is known as software defined radio.

This is a pretty nifty thing. Basically the radio transmits on a certain frequency and the receiver is tuned to a slightly lower frequency. The result is simply the broadcast frequency minus the receiver frequency. This is easily within the audio range of the speaker and the human ear. (here is a sample of RTTY over radio) The sound is then passed onto a computer via the soundcard. A piece of software then decodes this RTTY string and processes it to give the data that the flight computer was passing to the radio. From this it is possible to ascertain the location of the balloon, its height and whether is is ascending or descending.

I’ll update the site with more information, documenting the project and showing how all the systems will be utilised, as I go through the project. Either subscribe to site updates using the RSS feed or follow on Twitter @nearspaceimages

 

 

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