# Reference time(GPS) in Settings\>GPS\>Orbits

**URL:** <https://learn.safran-navigation-timing.com/t/reference-time-gps-in-settings-gps-orbits/346>\
**Category:** Skydel Discussion\
**Created:** [January 18, 2022, 3:43pm UTC](https://learn.safran-navigation-timing.com/t/reference-time-gps-in-settings-gps-orbits/346 "2022-01-18T15:43:46Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![hongzhaozheng](https://yyz2.discourse-cdn.com/flex030/user_avatar/learn.safran-navigation-timing.com/hongzhaozheng/32/115_2.png) [@hongzhaozheng](https://learn.safran-navigation-timing.com/u/hongzhaozheng)\
**Post date:** [January 18, 2022, 3:43pm UTC](https://learn.safran-navigation-timing.com/t/reference-time-gps-in-settings-gps-orbits/346/1 "2022-01-18T15:43:46Z")

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Hi,

Could anyone tell me what role the reference time (GPS) plays here? Will this have impact on the positioning solution?

Kindly,  
Hongzhao Zheng

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**Author:** ![M.Tamazin](https://yyz2.discourse-cdn.com/flex030/user_avatar/learn.safran-navigation-timing.com/m.tamazin/32/126_2.png) [@M.Tamazin](https://learn.safran-navigation-timing.com/u/M.Tamazin)\
**Post date:** [January 19, 2022, 9:21pm UTC](https://learn.safran-navigation-timing.com/t/reference-time-gps-in-settings-gps-orbits/346/2 "2022-01-19T21:21:34Z")

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Hi @hongzhaozheng,  
In general, Orbits settings in Skydel can be used to control/change the satellites’ locations within the constellation. By changing the Reference time (GPS) of one or more satellites, you are changing the satellite(s) location in the navigation message, which will not impact the PVT solution in the receiver.

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**Author:** ![hongzhaozheng](https://yyz2.discourse-cdn.com/flex030/user_avatar/learn.safran-navigation-timing.com/hongzhaozheng/32/115_2.png) [@hongzhaozheng](https://learn.safran-navigation-timing.com/u/hongzhaozheng)\
**Post date:** [January 19, 2022, 9:35pm UTC](https://learn.safran-navigation-timing.com/t/reference-time-gps-in-settings-gps-orbits/346/3 "2022-01-19T21:35:47Z")

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> [@M.Tamazin](#):
>
> e satellite(s) location

Should I change reference time if I set the start time to be my current computer time? The reference time now is back in 2021/6/24.

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**Author:** ![stephane.hamel](https://yyz2.discourse-cdn.com/flex030/user_avatar/learn.safran-navigation-timing.com/stephane.hamel/32/26_2.png) [@stephane.hamel](https://learn.safran-navigation-timing.com/u/stephane.hamel)\
**Post date:** [January 19, 2022, 10:26pm UTC](https://learn.safran-navigation-timing.com/t/reference-time-gps-in-settings-gps-orbits/346/4 "2022-01-19T22:26:17Z")

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You rarely need to change the reference time. Skydel will automatically extrapolate the orbital parameters to create a GPS navigation message where the ephemeris/almanac are valid for the scenario time (past of future). The purpose of the reference time is to define the time when the orbital parameters were defined. If you change the reference time, you actually move the satellite. However, all this has no effect on the receiver position because Skydel will simulate the appropriate propagation delay regardless of where the GPS satellite is to match the desired receiver position as defined in your scenario.

In fact, with any GPS simulator, the simulated GPS satellites position don’t have to match the real position and the receiver won’t see the difference. In general this is not an issue, but if you desire to replicate exactly the GPS satellite trajectory, you need observation file for the time matching your scenario time.

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**Author:** ![hongzhaozheng](https://yyz2.discourse-cdn.com/flex030/user_avatar/learn.safran-navigation-timing.com/hongzhaozheng/32/115_2.png) [@hongzhaozheng](https://learn.safran-navigation-timing.com/u/hongzhaozheng)\
**Post date:** [January 20, 2022, 8:44pm UTC](https://learn.safran-navigation-timing.com/t/reference-time-gps-in-settings-gps-orbits/346/5 "2022-01-20T20:44:38Z")

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I see. Thank you Stephane.