GPS World, September 2011

Algorithms Methods INNOVATION T The Right Attitude Experimenting with GPS on Board High Altitude Balloons Peter J Buist Sandra Verhagen Tatsuaki Hashimoto Shujiro Sawai Shin Ichiro Sakai Nobutaka Bando and Shigehito Shimizu IT IS NOT WIDELY RECOGNIZED that relative or differential positioning using GNSS carrier phase measurements is an interferometric technique In interferometry the difference in the phase of an electromagnetic wave at two locations is precisely measured as a function of time The phase differences depend amongst other factors on the length and orientation of the baseline connecting the two locations The classic demonstration of interferometry showing that light could be interpreted as a wave phenomenon was the 1803 double slit experiment of the English polymath Thomas Young Many of us recreated the experiment in high school or university physics classes A collimated beam of light is shone through two small holes or narrow slits in a barrier placed between the light source and a screen Alternating light and dark bands are seen on the screen The bands are called interference fringes and result from the waves emanating from the two slits constructively and destructively interfering with each other The colors seen on the surface of an audio CD the colors of soap film and those of peacock feathers and the wings of the Morpho butterfly are all examples of interference Interference fringes also reveal information about the source of the waves In 1920 the American Nobel prize winning physicist INNOVATION INSIGHTS with Richard Langley Phase differences depend on the length and orientation of the baseline Albert Michelson used an interferometer attached to a large telescope to measure the diameter of the star Betelgeuse Radio astronomers extended the concept to radio wavelengths using two antennas connected to a receiver by cables or a microwave link Such radio interferometers were used to study the structure of various radio sources including the sun Using atomic frequency standards and magnetic tape recording astronomers were able to sever the real time links between the antennas giving birth to very long baseline interferometry VLBI in 1967 The astronomers used VLBI to study extremely compact radio sources such as the enigmatic quasars But geodesists realized that high resolution VLBI could also be used to determine very precisely the components of the baseline connecting the antennas even if they were on separate continents That early work in geodetic VLBI led to the concept developed by Charles Counselman III and others at the Massachusetts Institute of Technology in the late 1970s of recording the carrier phase of GPS signals with two separate receivers and then differencing the phases to create an observable from which the components of the baseline connecting the receivers antennas could be determined This has become the standard high precision GPS surveying technique Later others took the concept and applied it to short baselines on a moving platform allowing the attitude of the platform to be determined In this months column we look at how a team of Dutch and Japanese researchers is using GPS to determine the attitude of a payload launched from a highaltitude balloon www gpsworld com September 2011 GPS World 57