Time synchronisation for WAAS over Indian airspace using GPS


where
FiAM is the single difference between RIMS,A and MCC,M.

(DtuA-DtuM) is the clock offsets between RIMS, A and MCC, M, that can be determined.

A third approach is based on using Kalman Filter Technique. Each measurement from a RIM, given by eq. (8) has 5 unknowns viz. three components of satellite ephemeris error (i.e. along track error, cross tack error and radial error), satellite clock error (Dtis) and RIMS clock error (DtuA). If there are n RIMS, we have n equations for a single satellite but (n+4) unknowns (viz. 3 components of satellite ephemeris error, satellite clock error & RIMS clock error in n RIMS.). Kalman Filter approach can be used to solve n equations for (n+4) unknowns.

Experimental Plan
The experimental setup to demonstrate GPS Common View Time Transfer Technique is shown in Fig. 2. Two GPS P code receivers, whose specifications are given in Table 4 below will be the main equipment required.

The two GPS receivers would be installed at Ahmedabad (AES) and Delhi Earth Stations (DES), along with modems used in TWSTFT Experiment. The built in Rubidium Clocks in the modems are used as Station clocks. The time interval between the 1 PPS derived from Rubidium clocks and the 1 PPS derived from GPS receivers are measured at AES and DES (DT A and DT B respectively) and recorded as time tagged data in a PC. A GPS C/A code receiver is used at AES and DES, for initial coarse synchronisation of the Rubidium clocks. This will make sure that the two atomic clocks are synchronised to

Table. 4: Specifications of GPS Receivers Required
1. Receive Frequencies: 1575.42 & 1227.6 MHz (L1 & L2)
2.Codes used: C/A code on 1575.42 MHz, P code on 1227.6 & 1575.42 MHz,(BW = 20.46 MHz)
3.Operation Mode: The receiver should automatically acquire GPS satellites and synchronise its clock with GPS time, transmitted from the satellite.
4.Timing Pulse  
 (a) Accuracy of timing pulse: The receiver should output 1 PPS pulse accurate to less than 40 ns to GPS time, transmitted from the satellite.
 (b) Programmable Rate: 0.1 sec. to 20 sec.
 (c) Specifications of Pulse: + 12 V DC, 25 msec rise time
5.Antenna: Choke Ring
6.Data Logging: Internal data logging of Satellite Navigation data received as well as pseudorange & carrier phase measurements on L1 and L2.
7.Additional Features: (1) A built in software to remove ambiguity in carrier phase measurements in L1 & L2 and compute ionospheric propagation delay
(2) A built in software for relative precise antenna coordinate determination (of the order of few cms.) from carrier phase measurements.

each other within about 60-80 ns. It is made sure that the Rubidium clocks are allowed to run freely, after initial synchronisation. It is also made sure during the experiment that the GPS P code receiver are locked to a common GPS satellite and its 1 PPS locked to the received 1 PPS from that satellite. Along with time tagged data, the GPS Navigation Data is also recorded. The data collection for GPS Time transfer Experiment is carried out round the clock for atleast about 3 months. .The TWSTFT experiment & data collection is done for about 1-2 hours daily during the period of GPS data collection. During the experiment, meteorological data (viz. Pressure, Temperature & Relative Humidity) are also recorded atleast at intervals of 15 minutes, using automatic meteorological sensors.

The following are the major equipments required for conducting the experiment.

Two Dual channel P code GPS Receivers (According to specifications laid down in Table 4).

Two Time Interval counters (With a precision of time interval measurement better than 1 nsec).

Two modems with built in Rubidium Clock, a GPS Single Channel C/A code receiver for Coarse time synchronisation of atomic clocks & Time interval counter.

Four PCs with printers.
Two Earth Stations and a satellite transponder.
A software for data analysis (To be developed).
Automatic Meteorological data recorder
The specifications of Time Interval Counter, required for the experiment are given in Table 5.

The specifications of the meteorological data recorder is given in Table 6.

Table. 5: Specifications of Time Interval Counter Required
1.Internal Oscillator Frequency:5 MHz (Should be able to phase lock to an external atomic frequency standard)
2.Oscillator Stability: Better Than 5X10-11
3.No. of Channels: Min. 2
4.Time Interval Measurement accuracy : 25 to 100 ps
5.Range: 100 ps to 10 s
6.Sample size: 1 to 100 per second
7.Interface: RS 232 or IEEE 488
8. Power Supply: 230 V AC, 50 Hz.
9. Temperature : 0 to 550C (Operational)
  -40 to 700C (Storage)
10.Humidity: 5% RH @ 400C


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