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Aviation History
1984
1984 - 0098.PDF
SPACEFLIGHT Water, water everywhere, and Topex sees it all PASADENA Nasa is now examining indus try proposals for ocean topog raphy experiment, Topex, a satellite designed to map the sea surface with a radio- altimeter. In April up to three companies will be selected for detailed work, and one of these should win Topex contractorship by October 1985. Topex is a more accurate successor to Nasa's Seasat, but without synthetic aper ture radar. Seasat's obser vations have won widespread acclaim, and its failure after only about 100 days was a great disappointment. Nasa expects Topex to measure wave-height with an accuracy of about 0-14 m —about twice the precision achieved by Seasat. Topex's orbit is such that it will be able to see most of the world, but the polar regions are beyond its sight. What we can expect from Topex, then, is a world-wide mapping of ocean behaviour—including currents, wave-heights, and troughs and peaks in the seabed. These results will mean safer navigation in coastal areas, through more precise knowledge of tides and currents. Supertankers cross ing hazardous shoals, such as the approach to the English Channel, will be among those to benefit. Today, the draught of Supertankers approaching the Channel is only just less than the depth of water avail able, and captains must wait for the extra clearance provided by a high tide. Fore casts of such tides are based on sparse shore-based data, however, and are often incom plete. The offshore oil industry should also reap the fruits of Topex labour, thanks to accu rate measurements of wave heights over several years, in areas such as the North Sea. This sort of statistical infor mation is vital in designing oil rigs, which have to be able to withstand the largest wave likely in 100 years on site. The larger the anticipated wave, the stronger and taller the rig must be. With each metre of height adding something like $1 million to rig cost, the savings could be considerble. Dumping of toxic waste is a na iswif5 It uVt, itiiUi i-iitlij iiliwi Pi. - "- "^ •» ^^ •. 4 am "' sit. lllllllP^HiSs [[<Wf- 'f '',2- X-- :2. ••'•%: :...-•.::....:•••.•• Nasa's ocean topography experiment (Topex) will map the sea-surface, beginning in the late Eighties controversial issue today, with pressure groups being particularly vocal about the risk of contamination. Here Topex can help by producing detailed maps of the ocean floor. This "mapping" is done indirectly from the radio- altimeter readout, since experience has shown that seabed peaks are typically mirrored by an upswelling of water on the surface. Seabed details could also prove useful in undersea drilling for oil and gas. A tight budget has led Nasa to adopt a new approach in defining Topex—that of modifying an existing satellite bus. Project manager Charles Yamarone explains that the Topex specification calls for "either a bus that has flown, or an existing design". Flight understands that there are at least three main contenders: Hughes Aircraft's HS376 bus, RCA Astro Electronics' Block 5D, and TRW's Fleetsatcom design. The winning company will act as Topex prime contractor, but the radio- altimeter is being supplied by the Applied Physics Labora tory at Johns Hopkins University. Only one other sensor will be aboard Topex, and that is a microwave radiometer. The task of this instrument is to measure water vapour in the atmos phere, necessary for corrections to the radio altim eter. Nasa does plan to fly a complementary sensor aboard another satellite, however. Called a wind radar scatter- ometer, the device will record wind speeds and directions within a day or so of each Topex pass. The scatter- ometer will be supplied by JPL and industry, and will probably fly on the Navy research oceanographic satel lite system (Nross) in late 1988. The orbit chosen for Topex is 1,300km high, circular, and inclined at 65°. At this height the influence of atmospheric drag is minimal, which helps to achieve the accuracy expected of Topex (Seasat's orbit was 800km high) That Nasa is serious about Topex is clear from the investment already made in studies—some $18 million in FY 1983-84. A further $6 million is due to be expended on Topex in FY 1984-85. It is planned to initiate the project in Nasa's FY 1985-86 budget, according to Yamarone, who adds that the venture is top of the list for new starts in that year. This would mean the start of serious Topex work in October 1985. Total cost of Topex is esti mated at $270 million in real- year prices. This includes the satellite, three years of oper ation, and contingency margins, but excludes launch. The satellite will account for about $50 million of this total (or $85 million including its contingency margin). Topex will be designed to last for at least five years, and will Weigh around 1,500kg at lift-off. The earliest that Topex could fly is February 1989. If the project remains purely American, then it will ride on Shuttle. There is a chance that the venture will be shared with French space agency CNES, however, which would mean Topex flying on Ariane (it would be the first Nasa craft to do so). In essence, France would provide the Ariane ride in return for a share in the results plus carriage of two sensors, an experimental altimeter and an orbit tracker. Construction and operation of Topex would remain Nasa's responsibility, and each coun try would finance its own experiments. A decision on whether to co-operate is likely to be taken before August- September this year. As so often happens with co-operative projects, it is likely to be political goodwill that defines whether France and America will co-operate. Nasa is going ahead either way, however, so Topex's future is assured. Topex's data-rate is rela tively low, and the plan is to record observations on board, and relay them to ground when convenient, via Nasa's tracking and data-relay satellites. The craft's orbit is such that the same parts of the ocean will be covered every ten days. This "rep etition" allows Topex to sepa rate currents from changes in sea-surface contour caused by gravity (these are relatively static). 146 FLIGHT International, 21 January 1984
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