The BBC's How China's GPS 'rival' Beidou is plotting to go global mentions a scenario where end-users can send signals to other Earth locations using BeiDou and later says:

But despite its technological sophistication, BeiDou has a supposed flaw - a two-way transmission process that involves satellites sending signals to earth and devices transmitting signals back. This can compromise accuracy and takes up more spectrum bandwidth.

In contrast, GPS devices do not have to transmit signals back to the satellites.

That link in turn points out some of potential disadvantages of BeiDou requiring two-way signaling, which include a larger and more complex BeiDou transceiver compared to a simple receiver for GPS (and presumably for GLONASS and Galileo and perhaps whatever a post-Brexit UK decides to build), as well as potentially revealing your location by having to transmit a signal to obtain your location.

I'm trying to understand the following. In order to get your position to even the nominal ~10 meter accuracy with BeiDou:

  1. Do you absolutely have to transmit a signal?
  2. Are you really transmitting all the way to MEO or GEO? If not, is it to ground stations?
  3. Do you potentially make available both your unit's individual identity and its location when you use it?

Or perhaps is this two-way exchange only required for improved accuracy?

  • $\begingroup$ @TonyEErocketscientist any thoughts? $\endgroup$
    – uhoh
    Commented Sep 21, 2018 at 13:53

1 Answer 1


It appears BeiDou 1 location service was bi-directional, involving a central ground station calculating the position and sending it back "the system capacity is 540,000 users per hour, and serve up to 150 users simultaneously"

The above source goes on to indicate BeiDou2 uses the more conventional GNSS method.

The Asia times story doesn't give sources for the "Japanese and Taiwanese military commentators" so it's entirely possible they were referring to the original system.

Galileo also provides a two way communication service, but that isn't required for the location service.

  • $\begingroup$ Thanks, that certainly makes a lot of sense. Maybe my linked articles are not referring to the newest available information. $\endgroup$
    – uhoh
    Commented Sep 21, 2018 at 14:42
  • 2
    $\begingroup$ given the date of the article, @uhoh, it should be talking about the new system, but... journalists.... $\endgroup$
    – user20636
    Commented Sep 21, 2018 at 19:30

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