SETI being the search for extraterrestrial intelligence, looking for some kind of signal from the stars that intelligent life exists. Do we humans broadcast anything in particular to other possible intelligence, or are EM broadcasts like in Carl Sagan’s “Contact” all that we’re offering?
IIRC, we did send out one message. But AFAIK we don’t intentionally broadcast messages for the purpose of letting aliens know we’re here.
IANA expert though.
As far as I know, we’re not broadcasting any specific messages right now. projects to do this have been done in the past though; here’s one example. There was another similar one that runs to many pages, but I can’t find the link. For fun, I tried deciphering it once without reading any of the accompanying blurb; it was surprisingly robust and pretty much unpacked itself.
Here is the message I had a crack at decoding; much of it is quite simple and should be understandable to any being with a reasonable grasp of mathematics and science.
A buddy told me about Project OZMA back in the mid-60’s but I never heard more about it (or that sort of thing) until Sagan and SETI.
Looks fun!
Page 1: Digits 0 - 9 and the equals sign, decimal notation, prime numbers 2 - 89, Mersenne prime 2^3021377 - 1 (notation for exponentiation and the minus sign)
Page 2:Basic arithmetic: 1 + 1 = 2, 1 - 1 = 0, etc. Addition, multiplication, division signs, decimal point, representation of negative numbers, infinity, sign for recurring decimals.
Page 3:Exponentiation. Negative and fractional exponents. sqrt(2) = 1.4142135… Note that “recurring decimal” sign from Page 2 is used for irrational decimals.
Page 4:Basic algebra: x + 3 = 4 : x = 1, etc. Three symbols for algebraic variables. Cartesian graph of y = x^3. Note that similarity betwen the symbols for the variables suggests they’re related alphabetically in the same way that our “x”, “y”, and “z” are.
Page 5:Logogram for “Radius”, or perhaps just another letter. Symbol for pi. Value of pi. Note that the “recurring decimal” sign from Page 2 is not used. Pythagoras’ theorem. Another variable letter in addition to the “y” and “z” from Page 4.
Page 6, I can’t read at sight, will have to transliterate first. 
Page 6: Hydrogen and helium atoms. Logograms for proton, neutron, electron, mass, and charge. Ratio of proton/electron and proton/neutron masses. Logograms for hydrogen, helium, carbon, nitrogen, oxygen, aluminium, silicon, iron, sodium, and chlorine.
Page 7:More elements. Relationship between mass of carbon and masses of proton and neutron. PROBLEM - do they mean the carbon atom or the carbon nucleus? Definition of practical unit of mass by reference to carbon. Masses of proton, neutron, and electron in these units.
Page 8:Hydrogen spectrum - wavelengths and frequencies in terms of units of length and time. Relationship between frequency, time, wavelength, and speed of light. Speed of light in units of length and time.
Page 9:Basic mechanics. Definition and units of power, energy, pressure and force. A couple of physical constants.
Page 10:Temperature. Melting and boiling points of various elements and water at standard pressure. PROBLEM. What if this standard pressure is way outside the range the alien engineers and scientists are used to working with?
Page 11.The solar system. Mass and size of Jupiter - mass, size, and temperature of the sun.
Page 12.Mass and size of the moon. Mass and size of the Earth. Distance between Earth and moon and Earth and sun.
Page 13.Year, month, and day. Age of Earth and Sun.
Page 14.The Earth’s surface. Chemical composition of land, sea and air. Depth of oceans and height of atmosphere. Acceleration due to gravity.
Page 15. TEH PR0N!!!
Page 16.Human population, age, mass and temperature. Range of human hearing and vision. The primary colours.
Page 17. (Too obvious for spoilers from now on). DNA bases.
Page 18. The cell, and the structure of DNA.
Page 19/20. Map of the Earth.
Page 21. Length and frequency of the transmission. Details of the radio transmitter that sent it.
Page 22. Properties of vacuum. µ[sub]0[/sub] and other constants.
Page 23. Questions for the aliens.