In an unusual move China has launched two Beidou navigation satellites on the same Long March 3B, one of China’s largest launch vehicles.
The two spacecraft were placed into an initial highly elliptical transfer orbit and will use their own engines to move into about a 13,400 mi. operational orbit. They will also use their own propulsion systems to separate into the proper spacing within the Beidou constellation.
China has never before launched two large prime mission payloads simultaneously let alone into such high orbits. According to China, the country will launch three more Beidou satellites this year.
By the end of 2012 the Beidou system will involve more than 10 satellites covering the Asia-Pacific region. The complete global navigation system, which consists of 35 satellites, is planned to be in place by 2020.
China’s Beidou satellite navigation system (also known as COMPASS) has been fast-tracked as part of China’s 12th Five-Year Plan that extends through 2015.
But in an unusual move, China’s “Global Times” website recently aired shortcomings about the Beidou system. The frank assessment occurred in mid 2011, but went unnoticed at the time. The sour notes raised are notable because the site is essentially a communist government mouthpiece. The same article also praised the nation’s decision to undertake such an ambitious program.
Unlike GPS, which uses a large constellation of satellites, the Beidou system adopted a dual-satellite positioning method, whereby the ground station sends inquiry signals to users via two satellites, and the user terminal sends a response that is then transmitted in return via the same two spacecraft.
The user’s two-dimensional position is then derived using geometrical calculations based on the travel time of the signals and the positioning of the satellites, mapped against stored three-dimensional terrain data, and the information sent back to the user.
The service became available to civilians in April 2004, making China the third country, after the US and Russia, to deploy an operational satellite navigation system.
Beidou is classified as an “active system,” as users have to broadcast data rather than simply receive their position.
According to the Global Times “An active system has disadvantages, because electronic support measures, such as radio location systems, can pick up the signal and derive the direction and position of users.”
This was a veiled reference by the site that Chinese users, including the military, can make themselves a target by using their own Beidou system.
“In a passive system such as the American Global Positioning System GPS, the terminal receives signals from at least three to four satellites at any given place and derives its own 3D position by computing the distance to each satellite,” the website noted.
The Global Times source “also confirmed that the active Beidou system is not very capable at tracking velocity, explaining the signal travels for about one second during the whole positioning process.”
“Aircraft can travel for over several hundred meters during one second, resulting in relatively high errors in measurement,” the Chinese source explained to the government website.
In contrast, GPS terminals have been integrated in most modern military aircraft and the guidance kits of many precision guided bombs and missiles.
Beidou is advancing toward several hundred thousand users, but GPS has several hundred million users.
Remarkably, at least 95 percent of the Chinese satellite navigation market –even the People’s Liberation Army–is already “occupied by GPS,” according to internal Chinese studies.
“Beidou’s market is limited,” another Chinese insider told the Global Times website, “but I guess there will be ‘some policy support’ (aka major long term Chinese government funding ) to ensure the “profitability” of the future Beidou system,” the source told the Chinese Global Times website. Another graphic view of the Beidou network is shown below.Missions » GPS »