TROPOSPHERIC CHARACTERISATION OF SKA SITES Document number... WP3 040.020.000 R 001 Revision... 2 Author... R.P. Millenaar Date... 2011 02 09 Status... Approved for release Name Designation Affiliation Date Signature Submitted by: R.P. Millenaar Site Engineer SPDO 2011 02 09 Approved for release as part of SKA System dcodr documents: P. Dewdney Project Engineer SPDO 2011 02 09
DOCUMENT HISTORY Revision Date Of Issue Engineering Change Number Comments 1 2010 01 29 Release for system Co0DR 2 2011 02 09 Updated to reflect progress and status during past year. Released for system scodr DOCUMENT SOFTWARE Package Version Filename Wordprocessor MsOffice Word 2003 WP3 040.020.000 R 001 2_TropoCharac ORGANISATION DETAILS Name Physical/Postal Address SKA Program Development Office Jodrell Bank Centre for Astrophysics Alan Turing Building The University of Manchester Oxford Road Manchester, UK M13 9PL Fax. +44 (0)161 275 4049 Website www.skatelescope.org 2011 02 09 Page 2 of 7
TABLE OF CONTENTS 1 INTRODUCTION... 4 2 EQUIPMENT... 4 2.1 Equipment options... 4 2.2 Principles of operation... 5 3 CAMPAIGN... 5 4 STATUS... 6 4.1 System production... 6 4.2 Satellite choice... 6 4.3 Installation... 6 LIST OF FIGURES Figure 1: Interferometer being tested on roof of JPL building.... 6 Figure 2: Inside antenna electronics box... 7 Figure 3: Central electronics rack... 7 LIST OF ABREVIATIONS JPL... Jet Propulsion Laboratory SKA... Square Kilometre Array SMA... Sub-Millimetre Array SPDO... SKA Program Development Office WP... Work Package 2011 02 09 Page 3 of 7
1 Introduction WP3 040.020.000 R 001 The wavefront that the SKA receptors are sampling runs the risk of undergoing significant modification in the last hundreds of kilometres of the signal s journey from source to detector. The two media that affect phase and amplitude of the signals are the ionosphere and troposphere. The degree of modification of the wavefront by these media is a selection criterion for the choice of the SKA site. For that reason the ionospheric and tropospheric properties over the two shortlisted sites will have to be investigated in order to make well informed decisions. PrepSKA Work Package 3 requires us to do: WP3.4: Carry out studies of the effects of tropospheric turbulence on high frequency observations. Study the high frequency limits of phase referencing and self calibration, and determine the implications for the SKA design. The ionosphere over the two sites is studied in Work Package 3.3. Where the ionosphere affects the lower frequencies most, the tropospheric phenomena increasingly cause phase and amplitude variations as frequencies increase. Because of the fact that SKA mid will be operating at low land locations up to at least 10 GHz, tropospheric characterisation is deemed to be very relevant. For interferometric observations a prime factor to be investigated are the phase fluctuations introduced by variations in precipitable water vapour. These phase fluctuations can be measured by means of an interferometer observing a reference source, and fluctuations in amplitude are usually investigated with a water vapour radiometer. It has been decided that WP3.4 should concentrate on characterising the tropospheric phase fluctuations. For that reason a process has been started up to investigate the possibilities for obtaining instrumentation and plan for a measurement campaign of at least one year, taking place at the two candidate sites at the same time. This document reports on the state of this project, January 2011. 2 Equipment 2.1 Equipment options Several options for getting equipment for measuring the tropospheric phase have been investigated in 2009 and 2010. Among these were new self build, loan and contracting options. It has been decided to join the effort at JPL to duplicate a number of complete interferometer systems. As it happens JPL was planning to obtain, install and operate a number of systems at their DSN sites. The system selected for duplication is the newly developed equipment for the Sub Millimeter Array (SMA) on Mauna Kea. In fact this is a 5 station interferometer, giving a maximum of 10 baselines. This system is being incrementally installed during 2009 ad 2010, and is showing excellent performance. 2011 02 09 Page 4 of 7
JPL has agreed to build two of these systems for the SPDO. It was decided that the systems should consist of two antennas each (single baseline) for budgetary reasons. It was deemed sufficient for site characterisation to measure just one tropospheric drift vector. 2.2 Principles of operation For the SKA site investigations two stations are deployed in the field. The stations are built up from commercial ~1m class dishes that are pointed towards a geostationary broadcasting satellite. The signals from the stations are transported by fibre to a central correlator system that derives the phase fluctuations and takes care of storing the measured phases. The system therefore measures the path length differences from source to surface on a baseline that has been specified to be 200 metres. This is the effective length, perpendicular to the azimuth of the geostationary satellite. The system uses low cost mass produced dishes and front end electronics. The downconverters have been modified to accept an external local oscillator signal, which is distributed to each element via fibre from a central source. The system observes geostationary digital television broadcast transmissions in the 11.7 to 12.2 or 12.2 to 12.7 GHZ bands. The electronics measure a 200 MHz wide video broadcast. For each baseline an analog complex correlator generates phase and amplitude information at a rate of 4 khz. This data stream is Walsh demodulated and integrated to improve signal to noise. The final data stream per baseline is then generated at 10 Hz rate, with 0.02 degrees phase resolution. Postprocessing then performs phase unwrapping and the diurnal phase shift caused by satellite motion is corrected for. Data products are stored in files that will be transmitted to a central server at regular intervals. The sites require an in door area for placement of an electronics rack, a computer with a medium bandwidth Ethernet connection. The fibres need to be buried and the dishes will be mounted on a sturdy pad that minimises wind buffeting. The central electronics rack will be shielded to prevent RFI from impinging on the instruments, such as the precursor telescopes at the site. The two electronic boxes, containing antenna electronics will be placed directly at the antennas, and require environmental proofing. 3 Campaign The measurement campaign will last one full year at least, in order to yield meaningful, statistically representative data. It can be expected that the tropospheric turbulence is strongest during the local summer months. The two sites will be investigated at the same time, using the two produced systems. The proposed core locations will be used for setting up these systems. After the one year period is over a report on the results will be written and submitted to the site selection team. An interim report will be written after about four months into the campaign. The campaign is likely going to be continued after the initial first campaign year to further improve the statistical quality. Data collection and processing will be done by the SPDO remotely, and the site proponents will provide day to day maintenance and overseeing. 2011 02 09 Page 5 of 7
4 Status 4.1 System production The two systems were produced at JPL, under the project leadership of L. D Addario, during 2010. The systems were completed and tested by the end of November. The SPDO Chief Site Engineer inspected the systems and familiarised himself with them during a visit to JPL. Results that were taken from the roof of the building showed that the systems were functional and were providing the required data products. 4.2 Satellite choice During 2010 the choice of suitable geostationary satellites has been a topic of discussion amongst SPDO, site proponents and JPL. For the Australian case the choice was easily made, but for the South African case the availability of suitable satellites was not so obvious. The South African site proponent was requested to provide help in coming to a suitable selection. At the time of writing the final selection will likely not be taken before initial field tests at the site. 4.3 Installation The systems were delivered to the site proponents in the first week of 2011. The Australian team first subjected the system to EMI tests before shipping the systems to the site. Both site proponents need more time than anticipated to get the systems to the sites and install and connect them. It is anticipated that both systems will be up and running in March 2011. A visit of the Chief Site Engineer to both locations will be required to validate the systems and organise delivery of data to SPDO. Figure 1: Interferometer being tested on roof of JPL building. 2011 02 09 Page 6 of 7
Figure 2: Inside antenna electronics box Figure 3: Central electronics rack 2011 02 09 Page 7 of 7