Decametre in the context of "Geopotential height"

The hectare (/ˈhɛktɛər, -tɑːr/; SI symbol: ha) is a non-SI metric unit of area equal to a square with 100-metre sides (1 hm), that is, 10,000 square metres (10,000 m), and is primarily used in the measurement of land. There are 100 hectares in one square kilometre. An acre is about 0.405 hectares and thus one hectare is about 2.47 acres.

In 1795, when the metric system was introduced, the are was defined as 100 square metres, or one square decametre, and the hectare ("hecto-" + "are") was thus 100 ares or ⁠1/100⁠ km (10000 square metres). When the metric system was further rationalised in 1960, resulting in the International System of Units (SI), the are was not included as a recognised unit. The hectare, however, remains as a non-SI unit accepted for use with the SI and whose use is "expected to continue indefinitely". Though the dekare/decare daa (1000 m) and are (100 m) are not officially "accepted for use", they are still used in some contexts.

The Ukrainian T-shaped Radio telescope, second modification (official abbreviation UTR-2) is the world's largest low-frequency radio telescope at decametre wavelengths. It was completed in 1972 near the village of Hrakovo (49°38′N 36°56′E / 49.633°N 36.933°E / 49.633; 36.933), 15 km west-south-west from Shevchenkove, Ukraine. The telescope is operated by the Institute of Radio Astronomy of the Ukrainian Academy of Sciences.

The UTR-2 consists of an array of 2040 dipole elements in two arms each containing 6 rows of elements, oriented in a T shape: a north–south arm consisting of 1440 elements covering an area of 1800×60 meters, and an east–west arm consisting of 600 elements covering an area of 900×60 meters. The basic element is a broadband cage dipole 1.8 m in diameter and 8 m long made of galvanized steel wire, mounted 3.5 m above the ground, with a balun to connect it to the transmission line. The dipoles are all oriented along the east–west axis, with the spacing between rows of 7.5 m in east–west direction and 9 m in north–south. It has a total area of 150,000 square metres (1,600,000 sq ft), and a resolution of about 40 arcminutes at the middle frequency 16.7 MHz. The operating frequency range is 8–33 MHz. The sensitivity is about 10 mJy.

High frequency (HF) is the ITU designation for the band of radio waves with frequency between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters (ten to one hundred meters). Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies can be used for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (3.95–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.