Carnarvon Location Project
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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. PAssive GEOdetic Satellite , PAGEOS-1, and Geodetic Earth Orbiting Satellite, GEOS-B or GEOS-2, were designed to meet such geodetic objectives.
Leading up to the Apollo missions, a Tracking Camera was located at SPAN and a Mobile Laser at GRARR. Each assisted with the development of a more precise location of Carnarvon and, in turn, more accurate tracking data for Apollo journeys to the Moon.
PAGEOS-1
In June 1966, PAGEOS-1 - a passive 30.48m diameter highly-reflective balloon - was launched into a 4000-Km polar orbit. Its high polar orbit avoided the Earth’s shadow so it could be observed as a slow-moving ‘star’ at any time of the night.
Tracking the balloon positioned more than 40 stations around the world on all continents to an accuracy of 3 to 5 metres – approximately 20 times more accurate than terrestrial triangulation at the time.
GEOS-B
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; GRARR 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 GRARR station.
GEOS-B also carried a satellite tracking system which was a precursor to the Tracking and Data Relay Satellite System (TDRSs)which would lead to the closure of all earth tracking stations except the Deep Space Network (DSN)
Tracking Camera
A Satellite Tracking Camera arrived at the SPAN site early in 1968 . It had two different shutter systems: an iris shutter used for PAGEOS and a focal-plane shutter for GEOS. Jim Gregg recalls that the focal-plane shutter was shattered upon arrival; he replaced it with a thin sheet of balsa wood painted matt black.
Tracks were recorded on 190 x 215mm glass photographic plates. For PAGEOS the iris shutter was open and shut synchronously with the USB time standard producing a dotted balloon-track against the star-track background. For GEOS, the focal-plane shutter was opened for 30 seconds while GRARR commanded the satellite to transmit a series of 5 or 7 precisely-timed flashes which appeared as dots against the star-track background. The photographic plates were returned to NASA for analysis.
Mobile Laser
A Ruby Laser Tracking System, affectionately known as ‘Ruby Baby’, arrived at GRARR 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]
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: -24° 54′ 11.4″, 113° 42′ 58.9″. [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.
References
[1] ‘Apollo Laser Ranging Experiments Yields Results’, LPI Bulletin, No. 72, August 1994
[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
