Stratosphere 

Troposphere 

Atmospheric 

Measurement 

Glider 

 

Outline of Research Motivation:

Rawinsonde observations are generally considered to be the most important input to Numerical Weather prediction models. These rawinsondes are generally expendable packages which are attached to a helium or hydrogen-filled weather balloon (see Figure 1). The number of rawinsondes launched each year by meteorological agencies numbers in the millions worldwide and only a small number of these sensors are retrieved. The main aim of the current project is to examine the possibility of producing a recoverable rawinsonde sounding system. In addition, several recent experiments, which incorporated high-value instruments, have been launched into the stratosphere via balloon. Several of these projects would have significantly benefited from the retrieval of the instrument and future research projects within the atmospheric group would benefit from this ability. The data retrieved by these instruments generally includes pressure, temperature, relative humidity and many now have some form of GPS-based wind speed and direction measurement ability. The benefits of a recoverable system are –

 

•         more accurate calibrated sensors can be utilised which could not be used in an expendable platform because of cost and time constraints.

•         measurements can be made on the up- and the down-leg of the mission.

•         the use of a recoverable rawinsonde package could significantly reduce the cost of routine observations by meteorological services and thus may be important in increasing the amount of atmospheric information measured. The cost-effectiveness of such a system may be of significant use in increasing the number of launches made in third world countries.

 

Another benefit of such a system which is under consideration is the ability of the system to loiter over a specified area, defined by a set of waypoints, this may be of significant use in intensive measurement campaigns and in mesoscale model studies. A prototype powered version of the current STAG design is indicated in Figure 2.

The aim of this project is to design and produce an efficient instrument retrieval system that could be used with balloon-borne systems. In addition, this research programme aims to make detailed measurements of atmospheric dynamics (namely horizontal wind speed and direction) using GPS receivers. On-board sensors will also measure atmospheric pressure, temperature and humidity. Later versions of STAG will also carry Electrochemical Concentration or Chemiluminescent Ozonesondes.

Figure 1: Standard Balloon launched instrument (rawinsonde)

 

The retrieval system envisaged is based on a design which returns the high value components of the system to the ground via a glider. This system will allow the possibility of high resolution measurements of temperature, humidity, pressure to be made on the up and down legs of the systems flight. This has the added benefit of sampling the atmosphere at the same altitude with very fine spatial and temporal scales. Only previously available by launching multiple balloon-borne instruments (more commonly called rawinsondes). This work will also provide the Atmospheric Physics group with a significant ability to measure the atmosphere in conjunction with the ST radar system being developed. 

 

Figure 2: The current powered prototype of the STAG instrument.

The system envisaged (called STAG) is a small glider (»1.5m wingspan) equipped with a set of meteorological measurement sensors which, after prototyping, will be launched from a meteorological balloon at altitude. STAG will make measurements of pressure, temperature, relative humidity, wind speed and direction, and ozone concentration which will be telemetered back to a ground station in a similar fashion to a standard rawinsonde. However, the system will be able to autonomously guide itself back to the ground station using information from a digital compass, GPS data and a set of accelerometers processed onboard by a network of task-specific microcontrollers. This ability will allow measurements of atmospheric parameters to be made inexpensively and on the up- and down-legs of the flight. The autonomous guidance ability of a powered version of the glider is currently under test at low altitudes. An initial description of the prototype and discussion about the guidance performance to date is detailed. The stages of prototyping to be performed will also be discussed. The possibility of using the powered version of STAG for low–level pollution studies and air-quality monitoring is to be considered.

Mission

The final STAG system should allow the following operations to be performed–

•         Record pre-launch information, which includes pressure, temperature, relative humidity and a launch position based upon GPS data.

•         Launch by a helium or hydrogen-filled meteorological balloon

•         Ascend to the balloon burst altitude (or to a user-specified altitude at which point a release servo will be activated) while transmitting data to the ground station via an on-board UHF transmitter. Onboard memory will also allow the measurements to be downloaded at a later date if the data is not received at the ground station.

•         Travel under autonomous control to an area above the launch point. An algorithm examines the current GPS position and the launch point position and calculates whether this is possible based on the gliders aerodynamic parameters (the glide slope being the most important parameter). If the launch point can not be attained (which is likely in high wind conditions) a secondary landing site should be targeted.

•         Once the glider is above the launch position (or the secondary landing site) the glider should either circle the launch point or define a path between a set of pre-defined waypoints until a lower altitude is reached. At this point a servo is triggered which releases a parachute. A homing signal is also activated at this point which allows the STAG instrument to be recovered.

 

Autonomous Control and the Environmental Measurement System

A block diagram which indicates the major components of the STAG electronic systems is shown in Figure 3. The system can be separated into five sections, these are –

•         Enviromental measurement unit

•         Navigation microcontroller

•         Aircraft control micro-controller

•         Navigation and  aircraft attitude sensors

•         STAG Servos

It should be noted that because of the difficulty and expense of frequent balloon launches’ a powered design has been used to test the autonomous control system. The use of a powered design requires the glider to be controlled from the ground at take-off and landing and a standard radio-control receiver has been integrated into the onboard electronics to allow this function to be performed. The heart of the environmental measurement unit is a microcontroller, the readings from the atmospheric sensors (pressure, temperature and relative humidity) are passed via an analog to digitial converter to this mircocontroller. Digital data from the GPS unit is then integrated into a data stream in the microcontroller. The microcontroller codes this data stream for input to the on-board UHF transmitter which transmits the data to the ground, this data stream also being passed to onboard memory which will allow the measurements to be downloaded at a later date if the data is not received at the ground station. Navigation commands are derived in the Navigation microcontroller from the current compass heading and current GPS location relative to those required to reach the launch site. The command, consisting of a compass heading, is passed to the aircraft microcontroller which interprets the navigation command and produces the modulated signal used to control the aileron, rudder and elevator servos. It should be noted that in the powered prototype these servos can also be controlled via a standard model aircraft radio-controller. Navigation commands defining a set of predefined waypoints can also be downloaded to the navigation microcontroller pre-launch, this ability has been of particular use during the prototyping and testing phases of the autonomous control system where the glider can be set to autonomous control then commanded to perform a loiter manoeuvre. Currently, a signal from the radio-control transmitter is used to tell the aircraft microcontroller to relinquinsh control of the glider servos so that the powered aircraft can be landed under human control. It should be noted that the range of the radio-control system is relatively limited (less than 2km) and this has limited test flights to relatively short test periods for the autonomous control system in order to ensure that the glider can be placed under human control at any time.

Figure 3: The current powered prototype of the STAG instrument. 

 

A PDF version of  a poster describing some initials results from the STAG system which was presented at the 2002 Conference of The Meteorological Society of New Zealand can be found here (229kBytes).

A brief article in the Christchurch Press on the glider from 13th May 2003 can be found here (590kBytes).

A copy of an invited talk given to NIWA at the Greta point office in Wellington from 31st October is available here (3.7MBytes).

A PDF version of a poster describing initial results of what was thought to be a sea breeze (but after discussion may be associated with a lee trough) presented at the 2003 Conference of The Meteorological Society of New Zealand can be found here (252kBytes). Note this work was primarily carried out by Andreas Baumgartner an Honours student.

 

Acknowledgements

Dr. McDonald would like to acknowledge grant U6331 awarded by the University of Canterbury .

 

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