The Tibetan Government in Exil gives an very good official introduction in Tibetan Astrology on the site: http://www.tibet.com/Med_Astro/astro.html
with the title: An Introduction to Tibetan Astro. Science
by Professor Jampa G. Dagthon, translated by Jhampa Kalsang

“Tibetan Astro. Science is the ancient art of calculation and interpretation of celestial phenomena. We classify Astro. Science into two categories: Elemental Astrology (Byung-rtsis) and Astronomy (Kar-rtsis), and have drawn from the teachings of neighboring countries such as India, China, Persia and Greece in synthesis with the tenets of Buddhism to create a uniquely Tibetan system.”

and further more on the site: http://www.tibet.com/Med_Astro/astro2.html

with the title: Outline of the Tibetan horoscope
By Mrs. Tsering Choezom, Astrologer

“There is nobody in this universe who doesn’t have some kind of problem, whether big or small. On a large scale, this applies to countries, communities and organisations. On a small scale, it applies to the family and the individual. Every country has its own traditional ways to solve such problems, and we Tibetans have a unique system of our own. In important matters, we seek advice from oracles or request a divination from a high llama. We also rely on astrological calculations.”

(read more ibn there)