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How can a GPS device give exact timing even when the number of available satellites are zero?

I'm monitoring GPS data using VisualGPS.

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If a GPS receiver doesn't see satellites, then what you see in the software is not the GPS time. Probably, your GPS unit has some backup timekeeping (quartz crystal).

Some GPS units have a low power crystal with a backup battery. It provides an approximate time, which allows the receiver to reduce the search space, which reduces the time to first fix.

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  • \$\begingroup\$ um, never thought that way. probably you are right. The module i use is SKM53. Just looked at the datasheet and it says there's an internal back-up battery. Thanks! \$\endgroup\$ – Anubis Jan 7 '13 at 4:43
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    \$\begingroup\$ Well, it says "Internal back-up battery and 1PPS output". I don't know what exactly is meant by "1PPS output" but can i get an idea like for how long the module is capable of keeping the backup? \$\endgroup\$ – Anubis Jan 7 '13 at 4:47
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    \$\begingroup\$ @Anubis, 1pps output means 1 pulse per second output. Many gps modules provide this output line. \$\endgroup\$ – Chetan Bhargava Jan 7 '13 at 4:57
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    \$\begingroup\$ @Anubis 1PPS is "one pulse per second". It's a square wave output, which can be used for precise synchronization. Imagine interrupt service routines (ISR) triggered off 1PPS, which are synchronous... everywhere under the skies. \$\endgroup\$ – Nick Alexeev Jan 7 '13 at 4:58
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GPS has two (maybe more - newer ones) startup modes. Cold start is when GPS doesn't know any satellites and tries to acquire the satellites. Once the number of required satellites are acquired, GPS gets current coordinates and time information from them. After it receives that information, it updates its local caches and RTC.

The other mode is warm start where GPS maintains in its RAM or NVRAM the recently acquired satellites and recent time. The recent time is maintained in GPS's local RTC. When the time and satellite information gets updated after a warm start, GPS updates it's local satellite and time information.

The reason why you get a correct time on a GPS start is that GPS may already have information from previous satellite sync. The GPS's local RTC provides you the time information.

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  • \$\begingroup\$ Note on cold start the GPS must wait for certain data fields which are only infrequently sent. Assisted GPS helps out by filling in that data via a network connection. \$\endgroup\$ – Bryce Mar 27 '14 at 8:15

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