Carnarvon Location Project


GEOS-1 - Image: CIRA of Colorado State University
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GEOS-1 - Image: CIRA of Colorado State University

Contents

The evolution of satellites and the various NASA tracking networks provided the opportunity to refine the geodesy of the Earth and, in turn, improve the accuracy of navigation at lunar range. Geodetic Earth Orbiting Satellite, GEOS-B or GEOS-2, was designed to meet such geodetic objectives.

Leading up to the Apollo missions, a Tracking Camera and a Mobile Laser were located at the SPAN site to assist with the more precise locatation of the USB antenna. The project led to more accurate tracking data for Apollo’s journey to the Moon.

Geos-B

SPAN systems

Back to Station Equipment

GEOS-B was launched on 11 January ’68 into a high retrograde orbit to maximize the occurrence of shared-visibility periods for several stations at a time. Its aim was to fix the precise location of participating observation stations to within 10m.

But it also provided Carnarvon with many local opportunities to compare the tracking results of its several tracking systems through the simultaneous access to a wide variety of GEOS-B facilities: an optical beacon controlled by an accurate onboard clock, R&RR transponders, a C-band radar transponder (used by FPQ-6 radar), a passive radar reflector, and Laser corner reflectors. Regular Carnarvon access was facilitated by STADAN network GEOS-B command control through the local R&RR station.

Tracking Camera

A Satellite Tracking Camera arrived at the SPAN site early in 1968. On command from R&RR, the satellite could transmit a series of 5 or 7 precisely-timed flashes which the camera photographed against the star-filled background of the night sky. The photographs were returned to NASA for analysis.

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Mobile Laser

A Ruby Laser Tracking System, affectionately known as ‘Ruby Baby’, arrived at SPAN early in 1969 to contribute to the location project. Aided by pointing data provided by the FPQ-6 computer, the Laser was tested in February ‘69 by reflecting pulses of light from a small array of corner reflectors carried on Geos-B. To trigger the laser required the discharge of a van full of large fully-charged capacitors; Jim Gregg remembers this producing a “thump like an artillery gun” accompanied by the popping of exploding capacitors which then had to be replaced for the next shot.

Navigating accurately to the Moon also required a better scientific understand the Earth-Moon relationship. A Laser such as ‘Ruby Baby’ could measure the distance to the Moon to an accuracy of 3 cm, so on each Apollo lunar landing a laser ranging retro-reflector (LRRR - a square array of 100 corner cubes) pointing back at Earth, was set up by the astronauts near their Lunar Module to reflect light back to the point of origin where ever that was.

The Laser’s very-narrow intense pulse-of-light spread to a diameter of about 6.5 Km at the Moon’s surface. A beam this wide still posed a technical challenge to point the laser accurately enough to hit a specific reflector array. Back on Earth the ‘echo’ was received by a 40cm reflector telescope mounted alongside the transmitting Laser. It was a minuscule reflection even during the best of Earth’s atmospheric conditions - only about one photon every few seconds. Extremely sensitive filtering and amplification equipment was needed to detect such a small reflection.

Over many years the Apollo reflectors revealed some surprising information. Perturbations of the Earth and Moon show that the Moon may have a liquid core and that the length of the Earth day has “distinct small-scale variations of about one thousandth of a second over the course of a year caused by the atmosphere, tides, and Earth’s molten core.” [1]

Ref [1] ‘Apollo Laser Ranging Experiments Yields Results’, LPI Bulletin, No. 72, August 1994

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Project results

Simultaneous three-way Geos-B ranging experiments using the laser, the optical camera, and R&RR produced a more exact R&RR location: -24deg 54min 11.4sec; 113deg 42min 58.9sec. [2]

The FPQ-6 radar also participated in a world-wide refinement of the C-band radar network using Geos-B. Calculations from both sets of results improved the accuracy of the USB antenna location.

Ref [2] NAA: PP538/2 Box 37; X-552-71-52, Goddard Range and Range Rate and LASER station coordinates from GEOS-II Data, January 1971

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