Jodrell Bank Observatory
Jodrell Bank Observatory:
Simon Garrington, Director Jodrell Bank Observatory, University of Manchester, [an extract] comments on Cheshire East Development Strategy ‘Shaping our Future’. 25 February 2013
Radio astronomy provides a unique view of the heavens and over the last 60 years it has transformed our understanding of the Universe. Jodrell Bank Observatory (JBO) embodies much of the huge contribution made by the UK to the development of radio astronomy. Cheshire East can be rightly proud of having such an iconic, world-leading centre of scientific research in its midst. The new Discovery Centre at Jodrell Bank is an award winning Cheshire tourist attraction, with over 100,000 visitors per year – it plays a major role in inspiring future generations of scientists. The BBC Stargazing Live programmes, broadcast each year from Jodrell Bank attract more than 10M viewers each year. JBO is operated by the University of Manchester and through a series of continuing major investments funded by the University, funding agencies and the EC its facilities represent the state-of-the-art, keeping it at the forefront of astrophysics research. These facilities are in great demand from hundreds of UK and international astrophysicists for world-leading research on topics from the formation of planets and stars to the evolution of galaxies and the universe. The Lovell Telescope is the third largest radio telescope in the world and is used by astronomers in the UK and as part of international collaborations for world-leading research programmes – it remains a powerful, highly visible and uniquely accessible icon of the UK’s scientific and engineering capabilities and ambitions.
The signals that radio telescopes aim to detect are usually extremely weak. Even with dishes with collecting areas of several thousand square metres focussing signals to state-of-the-art receivers cooled to -250 degrees, radio astronomy generally requires long integration times and/or wide bandwidths together with sophisticated signal processing techniques. Radio astronomy relies on the national and international protection of the radio spectrum but even in those parts of the spectrum where all transmissions are prohibited, interference arises from a wide range of devices which emit radio noise unintentionally. Such devices are increasingly used in our homes, and it is not uncommon for a typical home to contain many computing devices (PCs, laptops, tablet computers, hi-fi devices, games consoles, computer peripherals etc ) which have a significantly higher threshold for unintentional emission allowed by EC directives (eg those set by CISPR) compared to other domestic devices.
Radio frequency interference (RFI) can cause spurious artefacts in the images and spectra produced by radio telescopes, it limits the precision of certain measurements such as the timing of pulsars, which is now arguably the most promising way to test theories of gravity and detect gravitational waves, it can limit the dynamic range of spectra and images (ie the ability to detect weak features in the presence of stronger ones) and it can require that significantly longer observations are needed to reach a given sensitivity. Since many scientific programs are aiming to detect the very faintest objects possible and since it costs thousands of pounds per day to operate the telescope, this is additional cost is significant.
At a distance of several kilometres individual domestic IT devices can exceed the internationally agreed and widely adopted threshold for radio emission deemed detrimental for radio astronomy (ITU-R 769). The aggregated radio emission from hundreds of homes each containing several devices is therefore a significant threat.
It is for this reason that Jodrell Bank is in general concerned about the increase in residential developments near the Observatory and has in some cases opposed some of the larger developments closest to the Observatory. This is part of the University’s efforts to safeguard the scientific future of the Observatory given the huge investments that it has made in the past and is continuing to make over the next 10 years to protect and enhance the operation and scientific productivity of the Lovell Telescope and the other radio telescopes operated from Jodrell Bank.
JBO is pleased to see that the Policy Principles document continues to recognise the importance of protecting the scientific operation of Jodrell Bank in its policy SE14 , and that the Council are willing to develop the detail of this policy further.
JBO appreciates that all of the proposed sites for potential major developments in the strategic plan are either outside the Jodrell Bank Consultation zone, or at least near its outer edge. However it remains the case that over 6000 new homes, more that 25% of the total plan for Cheshire East are envisaged within or on the fringe of the JBO Consultation zone and this represents a significant potential impact on the successful scientific operation of Jodrell Bank Observatory.
In order to provide a quantitative view on the relative impact from proposed sites which are just within or just outside the consultation zone, an analysis of the path loss from each site to the Lovell Telescope has been performed. This ‘path loss’ is the extent to which radio interference from the site is decreased by virtue of the distance and diffraction by intervening terrain. The path loss is measured in dB, and is a logarithmic unit, so that every additional 10dB of path loss means that the interfering signal is 10 times weaker.
The path loss has been calculated using the methodology recommended by the international Telecommunications Union (ITU) when considering the potential interference between one radio service and another (ITU-R P.452 (2009) ‘Prediction procedure for the evaluation of interference between stations on the surface of the Earth at frequencies above about 0.1 GHz’). This procedure takes several factors into account, including diffraction over a specified actual terrain profile. The loss was calculated for a frequency of 1.42GHz, the ‘prime frequency’ for the Lovell Telescope, and a height of 63m was used for the height of the telescope (the minimum and maximum heights for the telescope focus are 50m and 76m).
A more detailed analysis would also need to take into account the difference in local ‘clutter’ due to buildings and other obstructions near each site, between the site and the Observatory. For the Middlewich sites, there may be some additional shielding for Middlewich 1 by other buildings. The Macclesfield sites considered here are all on the S or SW side of the town – there may be some shielding of Macclesfield 2, but Macclesfield 3 is essentially in a ribbon almost perpendicular to the direction from the Observatory. The Congleton sites are all on the N of the town, so there is little benefit there. Similarly the Sandbach sites are on the N of the town, so there would be little benefit from buildings in the town.
JBO is also generally more concerned about potential interference from southerly directions, since the telescope is often pointed close to the horizon looking South to observe pulsars towards the centre of our Galaxy. Hence the sites in Congleton and Middlewich should be flagged for additional concern.
Given all the above considerations and taking into account the potential numbers of houses on each site, then the those which give the greatest concerns are probably Middlewich 2 (although this is just outside the consultation zone), and Congleton 1 and 3, together with Congleton 4 (also just outside the consultation zone). The Macclesfield 3 site is of a significant concern, especially given the very large number of houses involved, and a more detailed analysis of the local terrain is required for this site.
A detailed analysis has not been performed for the various alternative sites listed. However, we note that for Macclesfield, Congleton and Sandbach, some of the alternative sites could be preferable in terms of reducing potential interference since they lie at greater distances from JBO, and may be more shielded by buildings in the towns themselves or by local terrain features.
It is unfortunate that in these three towns the preferred sites are all on the side of the town closer to JBO. For the alternative new settlements, JBO would be very concerned about the Siddington site which is only 5km from JBO and well within the consultation zone and the Chelford site which is 4km from JBO. We also note that that the Wardle site is 4km from the e-MERLIN telescope operated by JBO at Darnhall.
JBO and the University of Manchester would welcome the opportunity to continue to provide input to this consultation process and where necessary carry out further more detailed analysis. Although the Lovell Telescope stands above the Cheshire Plain it is possible to use even modest features in the local terrain to provide some protection and we would welcome the opportunity to use the tools now available to investigate this. Higher resolution terrain models are certainly available to facilitate more detailed investigations.