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by Kaye Dee

I love watching satellites — and it seems like every week now there are new stars in the sky as more satellites are launched to help us learn more about outer space and the Earth itself. Just in the past two weeks, we’ve seen three new satellites dedicated to discovering more about the Earth’s atmosphere and the way it works.

Explorer-XX: Topside Down

The first of the recent launches was Explorer-XX, finally orbited on 25 August from Vandenberg Air Force Base in California after problems with its Scout X-4 booster that took many months to resolve. Explorer-XX has a string of aliases: it’s also known as Ionosphere Explorer IE-A, Ionosphere 2, Science S-48, Topside-sounder, TOPSI and Beacon Explorer BE-A! Underneath all those monikers, it’s the latest in the series of scientific research satellites that began with America’s first satellite, Explorer-I, back in 1958.

Explorer-XX under construction

Explorer-XX’s main purpose is to act as a topside sounder, which means that it takes measurements of the ion concentration within the ionosphere from orbit above it. This data can then be compared with measurements taken from the ground. Since the ionosphere is what makes global radio communications possible, understanding its composition and characteristics is important to scientific and defence research, as well as international radio telecommunications operators.

Unlike some satellites, Explorer-XX doesn’t have an onboard tape recorder, so it can only transmit data when it’s in range of a ground station. One of those ground stations happens to be just outside the Woomera Rocket Range, at NASA’s Deep Space Instrumentation Facility at Island Lagoon. Island Lagoon is actually a dry salt-lake (and not a bad picnic spot for a nice Sunday outing from Woomera Village), and its shores proved to be an ideal location for NASA’s first deep space tracking station outside America. Last year, the Minitrack radio-interferometry tracking system that was originally installed on Woomera’s Range G to support satellite tracking during the International Geophysical Year, was moved to the Island Lagoon site. Minitrack is part of NASA’s Satellite Tracking and Data Acquisition Network and it can receive the Explorer-XX data. Some of the sounding rocket work out at Woomera also involves taking ionospheric soundings for defence and civilian scientific research, so I’m sure my colleagues at WRE will soon be incorporating the data from Explorer-XX into their research as well.

NASA's Minitrack station at Island Lagoon, near Woomera – one of the data receiving stations for Explorer-XX

Following in Canada's Footsteps

Explorer-XX is only the second topside sounder ever launched. The first was Alouette-1, Canada’s first satellite, which went into orbit almost exactly two years ago and is still in operation. Alouette-1, by the way, was part of a program in which the United States generously offered to launch satellites for other countries. Great Britain and Canada have already had their first satellites launched this way, and Italy will soon have a satellite launched by NASA as well. Australia had an invitation to take part in this project, too, but while I was working for the WRE, I heard that our government had rejected the offer on the basis that the country couldn’t afford it — which is pretty short-sighted thinking, if you ask me!

Canadian scientists celebrating the launch of their first satellite-Alouette-1. Wish there was a picture of Australian scientists doing the same.

Nimbus-1: Second-Generation Weather Satellite

Even if the Australian Government lacked the vision to take up America’s offer of a satellite launch, it is interested in taking advantage of the practical ways in which satellite can benefit the country. Last month, I mentioned Australia’s intention to be part of the INTELSAT communications satellite consortium, and our Bureau of Meteorology is fast becoming a major user of weather satellites. Its ground station was one of 47 outside the United States to receive live weather images broadcast directly from space from the TIROS-8 weather satellite launched last December. Some test transmissions were received from TIROS-8 on Christmas Day, just a few days after its launch, and images have been regularly received since January 7 this year.

Now, the first of a new weather type of weather satellite is in orbit, from which Australia is also receiving data. Nimbus-1 (aka Nimbus-A) was launched from Vandenberg just a few days after Explorer-XX, on August 28. It’s now in polar orbit, more eccentric than desired because of a short second-stage burn, but all its instruments are functioning and ground stations are receiving regular data.

Some people think Nimbus-1 looks like a butterfly, though it reminds me of an ocean buoy with solar panels attached either side!

Like TIROS-8, Nimbus-1 can transmit live cloud images from orbit using the Automatic Picture Transmission instrument. This television system is designed to photograph an area of 800 miles square, which is the largest field of view to date. The pictures are transmitted using a slow-scan system of four lines per second, similar to the way radio photographs are sent. Each ground station is designed to receive three pictures per orbit. Nimbus can also store data on board and retransmit it later if it is not in range of a ground station. But what makes Nimbus-1 different from TIROS-8 is that its High-Resolution Infra-red Radiometer enables it to take images at night and measure the night-time radiative temperature of cloud tops and the Earth’s surface, so that data is being acquired all day, every day.

Here's a diagram of Nimbus-1 showing its main components and instruments.

On its first day in orbit, Nimbus took a picture of Hurricane Cleo as it travelled north along the US east coast after devastating parts of the Caribbean and Florida. This really demonstrates that with the data and images from the TIROS and Nimbus satellites, the Bureau of Meteorology will now be able to reliably track the development of conditions over the Pacific, Southern and Indian Oceans that determine the weather across different parts of Australia. The poet Dorothea Mackellar didn’t call Australia the “land of droughts and flooding rains” for nothing, but weather satellites will undoubtedly improve the forecasters’ abilities to see when these weather conditions are coming!

Hurricane Cleo imaged by Nimbus-1. Its strike on Florida delayed the launch of the Gemini-2 unmanned test flight.

Orbiting Geophysical Observatory-1: A New Design Paradigm

Just two days ago, 5 September (Australia time), NASA’s third recent satellite was launched. This time it was the Orbiting Geophysical Observatory, or OGO-1, the first of a series of satellites that is intended to study the atmosphere, magnetosphere and the space environment between the Earth and the Moon, making sure that it will be safe for the Apollo astronauts to traverse this region of space.

This philatelic cover marking the launch of OGO-1 highlights its role in manned spaceflight safety.

OGO-1 is the largest and most complex scientific satellite that NASA has launched to date. With the OGO series, NASA is taking a new approach to satellite design. Until now, each satellite has been designed to accommodate the instruments and experiments that it would carry. However, with OGO, the satellite design is fixed and the experiments are tailored to fit the satellite. Each satellite will carry about 20 experiments.

Diagram of the universal OGO bus that will be used for all the satellites in the series.

OGO-1 has been placed into a highly elliptical orbit with an apogee of almost 93,000 miles, and the plan is for future OGO missions to alternate between this type of orbit and low polar obits. At 31° inclination (its angle with respect to the equator), the OGO series needs additional tracking stations to supplement NASA’s STADAN network. One of these support stations will be established next year in Darwin, in the Northern Territory, as an outstation of the STADAN station at Carnarvon. This facility is part of the NASA Carnarvon tracking station that I mentioned in my last article, which is a prime tracking station for the upcoming Gemini missions.

Unfortunately, one of OGO-1's long booms and one of its short booms did not properly deploy. As a result the satellite used up most of its stablisation-thruster fuel attempting to lock the satellite into its Earth-stabilised orbit. For the moment, scientists have decided not to turn on any of OGO-1's instruments while they work out ways to operate it as a spin-stablised satellite. Let's hope they succeed as this satellite and its successors promise a wealth of new data on the near-space environment.

OGO-1's deployment from its folded launch configuration to its operational configuration is rather complex. I guess it's not surprising that this new satellite has had some problems in properly unfolding!

It’s exciting to see so many new space missions occurring and knowing that, through the tracking stations around the country (managed by the WRE on NASA’s behalf and operated by local engineers and technicians) Australia is playing its part in the exploration and peaceful use of outer space. I can scarcely wait to see what goes up next month!