A coordinate positioning tool, using a combination of satellites
that can rapidly and accurately determine the → latitude,
→ longitude, and the → altitude
of a point on or above the Earth's surface. The GPS
is based on a constellation of 24 Earth-orbiting satellites at an
altitude of about 26,000 km. The system is a direct application of the thories of
→ special relativity and
→ general relativity.

Fr.: Système stéréoscopique de haute énergie (H.E.S.S.)

An array of → IACT telescopes for studying
cosmic → gamma rays in the 100 GeV to 100 TeV energy range.
The HESS observatory is located in Namibia, southern Africa, at an altitude of
1800 m, and the project is an international collaboration of more than 100 scientists
from nine countries. In its Phase I, HESS used four telescopes
each consisting of a light collector with a diameter of 13 m and a focal length of 15 m
placed at the corners of a square 120 m apart. Each telescope
is segmented into 380 round mirror facets of 60 cm diameter and uses
a camera consisting of 960 closely packed → photomultiplier
tubes. The first of the telescopes went into operation
in Summer 2002. Phase II includes a fifth
telescope, called Large Cherenkov Telescope (LCT), of 27 m diameter, located
in the centre of the initial array. This upgrade lowers the triggering
threshold of the HESS array to about 20 GeV, thus broadening the
energy window in which gamma-ray astronomy can be done, opening up more
opportunities in astrophysical research
(see, e.g., Bernlöhr et al. 2003, Astroparticle Physics 20, 111).

H.E.S.S., short for High Energy Stereoscopic System, is also
intended to pay homage to Victor F. Hess (1883-1964), an Austrian-American physicist
who received the Nobel Prize in Physics in 1936 for his discovery of
→ cosmic rays.

The conversion of a → number system
with a given → base to another system with a
different base; such as the conversion of a → decimal number
(base 10) to a → binary number system
(base 2).
In order to convert a number into its representation in a different
number base, we have to express the number in terms of powers of the other base.
For example, to convert the decimal number 100 to base 3, we must figure out how to
express 100 as the sum of powers of 3. We proceed as follows:
1: Divide the decimal number to be converted (100) by the value of the new base
(3).
2: Get the remainder from Step 1 (that is 1) as the rightmost digit (least
significant digit) of new base number.
3: Divide the quotient of the previous divide (33) by the new base.
4: Record the remainder from Step 3 (0) as the next digit (to the left) of the new base number.
Repeat Steps 3 and 4, getting remainders from right to left, until the
quotient becomes zero in Step 3 (2 and 0).
The last remainder thus obtained (1) will be the most significant digit of the new base number.
Therefore, 100_{10} = 10201_{3}.
Conversely, to convert from another base to decimal we must:
1: Determine the column (positional) value of each digit.
2: Multiply the obtained column values (in Step 1) by the digits in the corresponding columns.
3: Sum the products calculated in Step 2. The total is the equivalent value in decimal.
For example, the binary number 1100100 is determined by computing the place
value of each of the digits of the number:
(1 × 2^{6}) + (1 × 2^{5}) + (0 × 2^{4}) +
(0 × 2^{3}) + (1 × 2^{2}) + (0 × 2^{1}) +
(0 × 2^{0}) = 64 + 32 + 0 + 0 + 4 + 0 + 0 = 100.

A class of gravitationally loose stellar conglomerate with a
notable apparent shape making it
different from typical → bound
→ star clusters. The UYSS class
includes a large range of objects,
which extend at various size scales and at various degrees of
self-binding; from small (semi-)compact
→ unbound systems named
→ stellar associations,
to huge extended superstructures of → massive stars
that make up whole parts of Galactic → spiral arms,
known as → stellar complexes
(Gouliermis, D. A., 2018, PASP 130:072001; arXiv:1806.11541).