[ntp:questions] Shared PPS source/Multiple PPS sources

A C agcarver+ntp at acarver.net
Sat Feb 7 18:17:14 UTC 2015


On 2015-02-07 02:48, David Woolley wrote:

> What the satellites do do is to to transmit some additional information
> about ionospheric propagation, which allows for receiver to compensate
> for the signals travelling slower than the speed of light in vacuo.

None of the primary satellites in the constellation of the Navstar
system (those with PRNs less than 100) transmit ionospheric information
over L1.  Ionospheric delay is a function of path length through the
ionoshphere so it varies depending on the location of the satellite and
the receiver.  The WAAS satellites (PRN > 100) can provide some
corrective data on a side-channel that are folded into the calculations
on the receiver but the receiver must be able to use the WAAS signal to
do this.  WAAS provides a more detailed ionospheric correction using a
series of ground reference stations whose positions are known via ground
surveys and transmitted by two geostationary satellites.  For Navstar it
is not available globally, only over North America.

Military Navstar receivers use a minimum of two different frequencies
(L1 1575.42 MHz, L2 1227.60 MHz) and can utilize that to compute
ionospheric delays from each satellite in near real-time at the
receiver's location (in addition to using WAAS).

The newest satellites are capable of transmitting a civilian code on L2
(L2C).  Receivers that can use L2C in theory could also perform the
ionospheric compensation calculations but it will take some time before
the entire constellation is capable of transmitting L2C.


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