Tuesday, 8 October 2013

Space Gazing & Firing Lasers by Graham Appleby

Herstmonceux Geodetic Observatory
In April 2013 the NERC Space Geodesy Facility (SGF aka HGO - Herstmonceux Geodetic Observatory in East Sussex) transferred management to BGS. HGO and its staff are now part of the Earth Hazards and Observatories programme which covers Space Weather to Earthquakes. So what is SGF/HGO, what does it do, and why is its work of interest to BGS, you and to science in general? Over to Head of HGO Service, Dr Graham Appleby.....

SGF/HGO makes a strong observational and computational contribution to defining the International Terrestrial Reference Frame (ITRF) at millimetre-levels of precision and accuracy, represented by the geo-centred coordinates of a variety of geodetic instrumentation at a number of observatories worldwide, including Herstmonceux. Four complementary space techniques are capable of reaching the required levels of precision and together they define the ITRF and the link to the celestial reference frame.

The first technique is Satellite laser ranging (SLR) and in 2012 Herstmonceux was awarded the status of a 'New Technology SLR Site' by the Global Geodetic Observing System (GGOS).  Short pulses of visible laser light are directed at selected orbiting spacecraft carrying reflectors, and by detecting the reflections the satellite range is found from precise time-of-flight measurements. To reach millimetre precision, the times-of-flight of the laser photons have to be measured to a millionth of a millionth of a second. Using data from the global network of accurately located satellite tracking systems, precise orbits are computed for Earth observation satellites, such as CryoSat-2 and Jason-2. Critically, laser ranging to these satellites ensures that the mathematical models of their orbits are centred on the geocentre so that the satellites’ radar observations of sea level and ice-sheet altitude and thickness may be referred to a truly global reference frame.

The second, universally recognized geodetic technique is Global Navigational Satellite Systems (GNSS), including GPS (USA), GLONASS (Russian), and Galileo (European). These navigational systems, at the heart of sat navs, smart phone location services, and surveying and road building engineering techniques, need a global distribution of receivers at known locations to geo-reference the GNSS orbits and monitor the satellite clocks. One of the GNSS receivers at Herstmonceux is driven by an extremely stable Hydrogen maser clock (it loses 1 second in 30 million years) and, this, combined with its location close to the SLR, makes it a important contributor to these global services.

We have ambitions to add the third and fourth techniques to the site; a very accurate Doppler satellite-tracking beacon for the French DORIS system and, much more ambitiously, a Very Long Baseline Interferometry (VLBI) 12m radio telescope. Having a full suite of measurement techniques would place SGF/HGO on the world stage as a core station of the emerging Global Geodetic Observing System, the position service of the International Association of Geodesy.

We are also operating and researching the use of an absolute gravimeter to study local and regional site motion in conjunction with the results from GNSS and laser ranging, and have a PhD thesis in progress in collaboration with UCL and NOC Liverpool. Research into atmospheric aerosol layers is carried out in collaboration with Cambridge University, using a LiDAR system developed in-house.

Starlette - the first satellite tracked at Herstmonceux
This year we’re celebrating 30 years of SLR observations. The laser ranging instrument was originally developed to continue work carried out by the Royal Greenwich Observatory using observations of star positions to infer small changes in the length of the day and in polar motion; the laser ranging technique improved determination the Earth’s rotation vector by an order of magnitude. This has practical use in targeting radio-communications with spacecraft travelling through the solar system for instance. The geophysical drivers of the impulses driving polar motion and length-of-day variations include seasonal variations in atmospheric angular motion determined from weather modelling and core-mantle interactions that can be studied only by inference from geodetic observations. In a very real sense, the core matters to us too!

Graham Appleby

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