Author: "H. Fagard" <herve.fagard@ign.fr> 

Title: Antenna support change at Chatham (New Zealand)

Dear Colleagues,
The current support of the Starec antenna at Chatham - a 2 m guyed tower - has been replaced with a more rigid tower, 32 cm
sided, bolted on the same concrete block.

The beacon was turned to standby on November 27 at 3.00 UTC, and transmissions resumed on November 29 around 19 UTC
after the support change.

Before removal of the old support, the centring of the antenna was checked. The base of the antenna was found to be centred
within 1 mm above the ground mark, but corrosion of the aluminium supporting plate had caused a small tilt of the antenna to
the East. The measured excentricities for the antenna phase centres are:
- the 400 MHz phase centre is 2.9 mm E, 0.5 mm N from the ground mark,
- the 2 GHz phase centre is   5.6 mm E, 0.5 mm N from the ground mark.

After replacement of the supporting tower and antenna supporting plate (which is now made of a high quality stainless steel),
the antenna could be pretty well centred above the mark, and the supporting plate's height could be adjusted so that the height of
the antenna above the ground mark remain unchanged.
The excentricities of the new antenna position, with respect to the ground mark (and hence to the initial position upon
installation in Feb '99) are :
- the 400 MHz phase centre is 2.5 mm E, 0.3 mm N from the ground mark,
- the 2 GHz phase centre is   2.2 mm E, 0.3 mm N from the ground mark.

Because these values are very small, and the antenna height has not changed, we have decided to keep the current acronym
(CHAB) and DOMES number (50207S001) for the antenna reference point. This will allow this station to be used by the
on-board navigator (DIODE) of the new DORIS instrument on Jason-1.

Many thanks to John Beavan (IGNS) for kindly carrying out the antenna survey and support change.

Best regards,

Herve Fagard