|
Time synchronisation for WAAS over Indian airspace using GPS
Two Station Common View Time Transfer Technique The principle of this technique is illustrated in Fig. 1. Two GPS receivers are kept at stations A and B, separated by a distance of 1000 kms or more. Both the stations receive the signals of the same satellite at the same time, and synchronize their clocks to the satellite clock. At both the stations, the 1 PPS of the local clock of the GPS Receiver is compared with the 1 PPS of an atomic time standard, by measuring the time delay between the two 1 PPS, using a time interval counter. If these time interval measurements at stations A and B are respectively, DT A and DT B, , then (DT A -DT B ) will be the offset of one atomic standard with respect to the other.  In this approach, the satellite clock error contributes nothing. Some of the other errors like satellite ephemeris error, ionospheric effect, tropospheric effect, receiver delay are reduced. Because of this, there is an improvement in time synchronization accuracy between two station clocks. In summary, the common view concept is as follows. If a transmitted event (like 1 PPS synchronized to the Navigation data stream) from a GPS satellite is viewed simultaneously from two stations A and B, each maintaining an independent clock, then the coordinate times of the event can be computed at sites A and B as t’GA and t’GB respectively. If the true coordinate times of the transmitted events are tGA and tGB, respectively, then one can write tGA= t’GA + tDA ———————— (2) tGB= t’GB + tDB ———————— (3) where the error terms tDA and tDB include errors in both measurement and calculation. These errors arise principally from errors in knowledge of the satellite position, and of the time delays in the propagation paths and the receivers. Taking the difference between (2) and (3) yields an estimate of the true coordinate time difference between the A and B site clocks, where tGA–tGB) = t’GA-t’GB+DtD ..................... (4) DtD=tDA-tDB .................... (5) Experimentally it has been found that the size of the errors on the left hand side of (5) are about an order of magnitude smaller than those in either term on the right hand side, due to common mode cancellation of errors. Thus one sees the significant advantage of the simultaneous common-view approach. In Table 2 below, typical error budgets of GPS time comparisons in Common View (CV), between two stations at a distance d, using C/A code receivers is given Table. 2: Typical error budget for GPS time comparison in Common View (CV) Time Transfer Technique, between two stations using C/A code receivers
It is assumed in this table 2, that the noise of the laboratory clocks and the rise time of reference pulse bring unintelligible contributions, which are not considered here. The uncertainty in the antenna coordinates are assumed to be of the order of 3 m. In practice, errors of coordinates can sometimes reach 30-40 m. Two cases are considered : a single common view of about 13 minute duration and a daily average of 10 common views. This budget is established for normal operating conditions. Much large errors may occur in the case of a defective receiver, lack of delay calibration, poor environment of the antenna, adoption of wrong antenna coordinates etc. |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||