You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
BeiDou_Navigation_Satellite_System

Context:
On July 25, 2015, the 18th and 19th satellites were successfully launched from the Xichang Satellite Launch Center, marking the first time for China to launch two satellites at once on top of a Long March 3B/Expedition-1 carrier rocket. The Expedition-1 is an independent upper stage capable of delivering one or more spacecraft into different orbits.

Question:
What were the 18th and 19th satellites for the BeiDou system launched with?

Long March 3B/Expedition-1 carrier rocket