End of an Era

After some 23 years, the University's well-known landmark 'dish' antenna on the roof-tower of BB has been lifted down by crane and unceremoniously hauled away to the knackers' yard.

In 1975, a young PhD student (Martin Sweeting) first persuaded the UK Admiralty to donate a 4-ton Bofors anti-aircraft gun mount from the destroyer HMS Wakeful to Surrey for use as a basis for a satellite tracking mount - and then persuaded the Head of the Electronic and Electrical Engineering Department that it should be balanced on the roof. During a freezing and snowy February, EEE workshop staff and (a not very popular) MNS slowly dismantled the gunmount in Portsmouth and transported it to Surrey.

After initial tests on the ground powered by a stack of car batteries, the gun-mount was de-rusted and re-painted over an Easter holiday and hoisted by a complaining crane onto the BB tower. Most of the University turned out to watch - although their interest was primarily in witnessing the collapse of the building.

The room-sized 'amplidyne' rotating machine amplifiers used to drive the 1kW DC azimuth/elevation motors for the gun-mount (capable of slewing at up to 60 degrees per second to chase aircraft!) were replaced by 'latest technology' solid-state linear amplifiers designed by academic and workshop staff in EEE. 'Improving' the motor control circuitry driving the 4-ton mount proved a fruitful source of E4 and MSc projects for a number of years but, on more than one occasion, experimental control loop instabilities and limit cycles caused the whole building to shudder and shake - much to the consternation of the inhabitants of BB.

The first satellites to be tracked at Surrey were the OSCAR-6 amateur radio communications and NOAA weather satellites in 1976 - using reels of punched-paper-tape generated weekly by special arrangement with the University's Computing Unit and their ICL1900 mainframe.

Surrey's first microsatellite, UoSAT-1, was controlled via the dish - initially commanded in orbit by setting a bank of 8 data switches and a 'send' push-button. Over the years, this was gradually replaced by dedicated computer lines, VDUs and software - and, by the time of launching of UoSAT-2, a hand-built 8080 microcomputer, a Commodore PET, banks of BBC microcomputers and finally PCs through three generations of Satellite Control Room.

Eventually, after tracking more than 40,000 satellite passes, the difficulty of servicing (and lack of spares for) the 'dish' mount and its ageing electronics proved too great a maintenance burden. Communications with the growing number of Surrey-built satellites in orbit necessitated moving operational antennas to the two lattice towers on the adjacent BB roof - these now track 9 satellites and over 100 satellite transits each and every day. However, a brand new (lightweight) tracking mount and 2-metre dish antenna will soon be mounted back on the BB roof tower to attempt to restore the landmark and to provide high-speed L/S-band communications for the upcoming UoSAT-12 minisatellite mission.

-- Martin Sweeting, 9 April 1997

End of an Era II

In preparation for the launch of the DMC spacecraft, Thursday saw the replacement of the SSTL S-band antenna on the BB tower with a new 3.7-metre diameter dish and X/Y tracking mount in place of the old 2.4m dish which was installed in 1997 and which in turn had replaced the even older 2m 'AA Bofors gun-mount' tracking system installed in 1975! The new antenna system has been built by a local company to SSTL's specification and is identical to that which will be installed shortly in both Algeria and Turkey.

The pedestal is an X/Y tracking mount, rather than the traditional Az/El mount - this solves the "keyhole" problem which is apparent in Az/El rotators which makes it difficult to track a spacecraft when it is passing close to overhead. The X/Y mount can also give the illusion that it is able to continuously rotate in azimuth without wrapping up the cables!

The spine road was closed whilst the antenna system was assembled and, whilst we were lucky with the weather, the wind was extremely marginal during the final lift of the completed system which weighs in at over 1200kg. With a large surface area, the dish was being blown around as it was lowered into its final position making alignment to the mountings extremely difficult.

The most commonly asked question during the installation: "Why didn't we get an SSTL logo put on the dish..." there are two reasons for this:

1. this requires extra planning permission as it constitutes an advertisement, and
2. due to the operation of the X/Y mount, anything written on the surface of the dish will rotate and will therefore be upside down (or at least at an angle) for most of the time!

Thanks to all those involved, the installation went extremely smoothly, and by Friday afternoon we were using the new system to receive data from SNAP.

-- Chris Jackson, January 2002.