WA wheat town home to site where we measure the earth

“The main part of our job still involves shooting a very bright green laser at things flying around in space. Which (grown up) kid wouldn’t love doing that! I still do after 35 years.”

Those are the excited words of Randall Carman, station manager of the Geoscience Australia (GA) facility at Yarragadee in outback WA. Carman is tasked with, among other things, measuring Australia.

Known as Satellite Laser Ranging (SLR), this activity measures the Earth’s centre of mass. SLR is one of 4 space geodesy activities that take place at the Yarragadee Geodetic Observatory (YGO), about 300km north of Perth – a “geodetic supersite.”

“Geodesy,” says National Geodesy Director Nick Brown, is “the science of the measuring the Earth. There are huge forces interacting with Earth as it flies through the universe.”

“Geoscience Australia works with a range of international partners to track the many constant changes occurring within and around the rock we call home,” Brown says. “It is hurtling through space in orbit around the Sun and being pulled in various directions by the Moon and other planets. This complex relationship causes the surface of the oceans and Earth to rise and fall over time.”

Yarragadee’s laser, the NASA MOBLAS-5, hits its satellite targets with the speed and precision necessary to measure where Earth is in time and space.

MOBLAS-5 – ironically meaning “Mobile Laser 5” – has sat at Yarragadee since October 1979.

There are currently 5 stations in the second generation MOBLAS network, with two in the continental United States, one in South Africa, one in Tahiti and Australia’s MOBLAS-5, which consists of three vans, a support building and an antenna on a hectare of bitumen.

These stations sit under clear skies and “near perfect weather” to allow them to make measurements to orbiting satellites.

Yarragadee, wa
Yarragadee, WA (Image GA)

The equipment is maintained to NASA’s standards. Computers that, despite operating for 25 years, still enable Yarragadee “to be the best performing SLR station in the world” according to GA.

Yarragadee staff, like Carman, live in the nearby coastal towns of Dongara and Geraldton.

“Our facility is operated around the clock, so we get to experience all times of the day and night at the semi-remote location,” says Carman, who’s been at the station since its inception.

“Being perched on the edge of a low plateau, the views over the cropping land are really impressive, especially at sunrise and sunset. Of course, the summer heat and associated dust storms can be tough, and I do sometimes wonder why we are stuck in the middle of a wheat paddock. But, as long as the air-conditioning is running and we can keep the equipment cool, it doesn’t really bother us.

“I love both history and technology and working with the NASA equipment, a lot of which dates back to the 1970s, is really rewarding as well as challenging.”

A Space Shuttle Air to Ground VHF terminal operated from 1982-1989.

There are four main space geodetic techniques used at Yarragadee.

Satellite Laser Ranging (SLR), which fires green ultra-short, high-powered laser pulses towards orbiting satellites. The pulses reflect back from specially-designed retroreflectors on the satellite and the site measures exactly how long the laser pulse takes for the round trip.

Precise Global Navigation Satellite System (GNSS). Precision antennas and receivers use the carrier signals from the GPS like constellations to precisely determine positions on Earth.

Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) is a beacon which acts like GPS in reverse. GA sends GPS-like signals up to orbiting satellites equipped with DORIS receivers. This provides the satellite operators with very precise orbital positioning information but also, over time, can provide precise station positioning.

Very Long Baseline Interferometry (VLBI) which uses traditional-looking antenna dishes (12m  diameter) to point at very distant radio sources and time the arrival of noise variations. If dishes on different continents look at the same quasars (the incredibly powerful but exceedingly remote targets usually employed) at the same time, minute differences in arrival time of the signals can give rise to all sorts of incredible information.

“When I first started working at Yarragadee,” Carman says, “we only used one technique – the SLR system. In the late 70s, NASA and other scientists, wanted to verify and measure plate tectonic motion, so they placed MOBile LASer systems around the globe.

“Australia already had one SLR system at Orroral Valley in the ACT, so we agreed to host the mobile system on the west coast so together they could measure the movement of the Australian Plate.

It was moving about 7cm north-north-east a year!

“The laser system has continually improved over the years – the laser pulses are much shorter and the timing equipment much more accurate. There are also many more satellites to track – from 5 when I started to more than 120 now. All of this has meant our accuracy and precision have reduced from metres in the 70s to about a millimetre now.”

“The SLR was also much more labour intensive in the early days. It took 3-4 people on each shift to operate the laser and control the telescope. Now, one person can control the system and we can afford to operate 24/7.”

By any measure: Another rural town for science

Some of the science products derived from the global space geodesy effort include Earth orientation, polar motion or wobble, length of day, tectonic motion, Earth centre of mass or measuring where the centre of the Earth is at any time (it moves about!) and realisation of the all-important International Terrestrial Reference Frame.

“YGO also now hosts the other three techniques mentioned above. Each method of measuring the Earth has strengths but also inherent biases or errors. However, when you locate the four techniques together, it provides a very robust and accurate measurement. Yarragadee is currently one of only two observatories in the southern hemisphere that hosts all four techniques.

“This has opened up a lot of possibilities. Humanity has become incredibly reliant on precise positioning and timing – think driverless or technology-assisted cars and GPS-steered agriculture, as well as precise timing for banking and stock exchange transactions.

“To enable all of these things we need an International Terrestrial Reference Frame (ITRF). All GPS enabled devices rely utterly on an accurate and up-to-date ITRF. The only way to determine and update the ITRF is by using space geodesy.”

In recent months YGO equipment has been sustained with significant enhancements.

“The VLBI system has had a major upgrade,” says Carman.

“It has gone from operating a receiver in two narrow radio bands – S&X at ambient temperature, to a cryogenically cooled broadband system that can detect signals from 2GHz to 15GHz. It has just come on-line since the upgrade, and we can’t wait to see some of the science that will come from that.

“We have also joined the huge AARNet optical fibre running down from the SKA site in the Murchison. Now instead of having to post our VLBI data to correlation centres around the world, we just send it down the “pipe”. It also means soon we will be able to perform things like real-time VLBI and remote operations of the site.”

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