I've been reading up on the GPS signal structure, and it's true that it takes 12 1/2 minutes for the full sequence, but... and this is a big but... the GPS time and GPS/UTC offset are sent with every sub frame, and they come by every six seconds. So you can forget about waiting 12 1/2 minutes. before being certain of the time! It's synchronized every 6 seconds. Its all well explained in Wikipedia http://en.wikipedia.org/wiki/GPS_signals
On Monday, April 23, 2012 5:47:21 PM UTC+10, andrewg_oz wrote: > > Some of the time confusion arises because GPS units can report a variety > of "GPS Time", "UTC" and semi-corrected GPS Time. I discovered this when > building a precision timer for an industrial corrosion monitoring device. > > On startup the GPS would report UTC plus one second. Within 12.5 minutes > it would report exactly UTC, i.e. after reception of the GPS/UTC correction > factor. I could only assume that the GPS developers had pre-programmed in > the UTC/GPS clock offset that was current when the device was manufactured. > Since then a UTC leap second had been added. > > Importantly, there is no way to tell which of those times the GPS is > reporting. No doubt the precise behavior varies from chipset to chipset. > For my project I was using a SiRF III receiver and could switch on the raw > 50bps data steam and watch for the GPS/UTC correction to be sure when I had > UTC time. > > AFAIK, there is no way of doing that on Android, so for truly accurate > times your best bet is to leave the GPS on for 12.5 minutes before reading > the time, and hope that it has successfully received the clock correction. > > -- > Andrew > > On Monday, April 23, 2012 5:47:21 PM UTC+10, andrewg_oz wrote: > > Some of the time confusion arises because GPS units can report a variety > of "GPS Time", "UTC" and semi-corrected GPS Time. I discovered this when > building a precision timer for an industrial corrosion monitoring device. > > On startup the GPS would report UTC plus one second. Within 12.5 minutes > it would report exactly UTC, i.e. after reception of the GPS/UTC correction > factor. I could only assume that the GPS developers had pre-programmed in > the UTC/GPS clock offset that was current when the device was manufactured. > Since then a UTC leap second had been added. > > Importantly, there is no way to tell which of those times the GPS is > reporting. No doubt the precise behavior varies from chipset to chipset. > For my project I was using a SiRF III receiver and could switch on the raw > 50bps data steam and watch for the GPS/UTC correction to be sure when I had > UTC time. > > AFAIK, there is no way of doing that on Android, so for truly accurate > times your best bet is to leave the GPS on for 12.5 minutes before reading > the time, and hope that it has successfully received the clock correction. > > -- > Andrew > > -- You received this message because you are subscribed to the Google Groups "Android Developers" group. To post to this group, send email to android-developers@googlegroups.com To unsubscribe from this group, send email to android-developers+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/android-developers?hl=en
