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Time synchronisation for WAAS over Indian airspace using GPS
 Sangeeta Nagrare & M. R. Sivaraman Satcom & IT Applications Area Space Applications Centre Ahmedabad –380015, India nsangita@yahoo.com Abstract Indian Space Research Organisation (ISRO) in collaboration with Airport Authority of India(AAI), is planning to implement Wide Area Augmentation System(WAAS) over Indian Airspace, to meet the requirements of Category I precision approach of aircrafts. Indian WAAS will consist of a network of ground stations called ‘Range & Integrity Monitoring Stations(RIMS), One or two Master Control Centre (MCC), One or two Navigation Land Earth –Stations (NLES), CXL & CXC Navigation Transponders on one or two Geostationary Communication satellites. RIMS,MCC and NLES will have atomic clocks that should be kept synchronised to a network time within 1-3 ns and the clocks at NLES and MCC should be kept synchronised to, 20ns with GPS time. We have already tested Two-Way Satellite Time & Frequency Transfer (TWSTT) Technique and achieved a time synchronisation accuracy of 1ns between two remotely kept atomic clocks. RIMS are basically GPS dual frequency tracking stations, located at well surveyed points, whose coordinates are well known. Hence at RIMS, in addition to the atomic clocks , a dual channel P code GPS Receiver will be also installed for GPS tracking. They will essentially collect pseudorange and carrier phase measurements, from all the visible GPS satellites. At Space Applications Centre, Ahmedabad, we propose to try out GPS Common View time transfer technique, basically using the simultaneous pseudorange and carrier phase measurements, to synchronise the atomic clocks at two stations separated by thousands of kms. and estimate its accuracy by comparing the stations. We will present in this paper the details of the GPS common view method, the errors involved in the method and some preliminary results obtained in our experiment. Introduction Global Positioning System (GPS) satellites are capable of providing global time & frequency dissemination 24 hours a day. GPS is slowly evolving into a primary system for the distribution of Precise Time and Time Interval (PTTI), both at national and international level. It has been shown that GPS can be used to synchronize clocks to tens of nanoseconds over large distances. All GPS satellites carry an atomic clock (either a Cesium beam standard or a Rubidium vapour standard) on board. The GPS Control Segment located in US, monitors the satellite clocks and determines their offset from a GPS System time maintained by US Naval Observatory. The clock offsets are then uploaded to the satellites, which are stored and transmitted as part of the transmitted GPS Navigation message. GPS time is a continuous time usually measured in weeks and seconds, from the GPS time zero point of midnight, January 5, 1980. Controlled by Universal Time Coordinated (UTC), GPS time is not corrected for leap seconds and so it is currently ahead of UTC by a few seconds. With the exception of the integer number of leap seconds, GPS time is steered to within one microsecond of UTC with a difference reported in the GPS Navigation message to a precision of 90 nanoseconds. In a simple GPS Timing Receiver, a replica of the C/A code transmitted by one of the visible GPS satelllites, synchronised to a local clock 1 PPS, is generated by the receiver and crosscorrelated with the received signal. Since the local clock 1 PPS will be offset from the satellite clock 1 PPS, the local clock 1 PPS will have to be shifted by a time Dt, so that the cross correlation is maximum. By doing this, the GPS Receiver stripes off the C/A code and the carrier has only BPSK modulated Navigation data. In addition , Dt gives pseudorange from the receiver to the satellite (PR). The receiver resets the local clock time to the time given by in HOW (time of week) and the local 1 PPS is synchronised to the beginning of that subframe. Because of the path delay and many other reasons, still the local clock will be running behind the satellite clock, by a few milliseconds. The receiver estimates the time difference between the local clock and satellite clock (GPS time), as given by Dt’ = PR – (R/C+dS+dI+dT)-dD+dC .........(1) Where PR – Measured pseudorange, determined by measuring the time difference between two the transmitted and received identical codes. R – Geometric range between the satellite and the receiver, computed from broadcast ephemeris and the known coordinates of the GPS receiver antenna. C – Speed of light dS – Correction due to Sagnac Effect dI – Propagation delay due to Ionosphere dT – propagation delay due to troposphere dD – Receiver delay dC – Difference between the onboard clock and GPS time. Basically GPS can be used for time synchronisation, in two different ways. They are (1) One-way mode (2) Two-way Common-mode ,Common-view Time Transfer. Single Station Time Transfer Technique A single GPS receiver can deduce GPS time, from measurements according to (1). A typical GPS receiver performs the above process, during 13 minute tracking session as follows.
Table. 1: Typical error budget for Single Station GPS technique.
The error due to receiver coordinates and satellite coordinates, causes error in calculation of geometric range (R) in eq. 1 above. In addition they are responsible for the error due to Sagnac effect. The accuracy to which an atomic standard can be synchronized to GPS time depends on local conditions of observations, mainly on the accuracy of the receiver antenna coordinates and on the amount of acquired data. If the antenna coordinates have an uncertainty of 10 m, the accuracy ranges on an average from 40-65 ns, for 13 minute track, to a few tens of nanoseconds for averaging times of one day or more. The accuracy of time synchronisation, using P code receiver, in single station mode is better than C/A code receivers, because (1) P code receivers uses dual frequency measurements, to remove ionospheric effects and (2) the receiver noise in P code receivers is 1/10 times less than C/A code receivers.
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