According to Wikipedia's article on GPS,
Each GPS satellite continuously broadcasts a navigation message... Each complete message takes 750 seconds (12 1/2 minutes) to complete.
My understanding of how GPS works is that the receiver receives the broadcast message and calculates location based on that. So how can a GPS system start delivering reasonably accurate locations within a few seconds?
Wikipedia mentions that the message is divided into frames and sub-frames:
The message structure has a basic format of a 1500-bit-long frame made up of five subframes, each subframe being 300 bits (6 seconds) long... [A] complete data message requires the transmission of 25 full frames.... [T]his gives 750 seconds to transmit an entire almanac message (GPS). Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite.
But the iPhone's GPS, for example, delivers about a location point a second. Even if a frame or a subframe is enough, that's 6-30 seconds. How can the hardware update its position with reasonable (reported) accuracy every single second? Is the iPhone lying to me?
No comments:
Post a Comment