|SATRE – Two-Way Satellite Time and Frequency Transfer Modem [back]|
TimeTech GmbH is the sole European manufacturer of Two-Way
Satellite Time and Frequency Transfer
modems (SATRE TWSTFT). Most of the main Standard Institutes worldwide
use this method to compare high accuracy clocks.Time-Broadcast
and Satellite Ranging Modem, SATRE (Satellite Time and Ranging
Equipment), which is based on PN-Technology, is a development of our
company. The system is being used for two-way ranging and multi-hop
ranging applications for geo-stationary satellites.
The SATRE modem is being extended to Two-Way Carrier Phase capabilities, including theoretical analysis and field trials.
Applications and Configuration
Applications and systems:
External Reference Interfaces:
Ground Station Interface:
Major Functions of SATRE Modem
Satellite round-trip ranging:
The purpose is to determine the slant range of a satellite derived from “round - trip” signal delay measurements. Conversion from round-trip time-delay to one-way range is performed by speed-of-light under the consideration of instrumental delays (G/S and transponder, which have to be either calibrated out or solved for in the orbit-determination post-processing). Signal flow: one station transmits and receives its own signals (‘round-trip’).
It is important to note, that one can perform satellite ranging without
knowing the exact time, when this is executed, especially, when
satellite motion is slow. In geo-synchronous orbits, satellite motion
is at the m/s (meter/s) level, hence timing accuracy of some ms
The purpose is to determine the slant range of a satellite derived from monitoring a ranging signal. This signal is transmitted by a different station. To calculate pseudo-range, additional data is required. This means:
An accurate clock (GPS or provided via two-way time transfer) and Ranging signal transmission is synchronised to an accurate clock at transmitting side (GPS or provided via two-way time transfer). Signal flow: One station transmits and receives, another station receives. The receiving station gets the measurement results of the ranging station over RF, which allows calculation of the pseudo-range data in real time.
Two-Way Time Transfer:
This is a standardised method recognised, amongst others, by the BIPM, Paris, for highly accurate synchronisation of clocks. It is important to note, that one can perform 2-way time transfer without knowing the precise orbit of the satellite.
Signal flow: two stations transmit simultaneously and each one receives the transmissions of the other one (‘two-way’). Modems exchange measurement data for remote station monitoring and for the calculation of the real-time two-way solution at both sites. This provides both the clock time and frequency offsets with a latency of 3 seconds.
SATRE 3-Channel Receiver:This receiver is designed for use as master receiver in a trilateration system. Such a system is fully operation. All stations perform standard ranging (round-trip) at their individual location. The 3-channel receiver can replace three individual modems at the master station, which are used to listen to the transmissions of the remote stations during their ranging activity.
Data transmission is performed via the ranging signal with an appropriate modulation. This data is internal measured range. Supplemental local user input data, e. g. meteoro-logical data can be transferred to the master from the remote sites.
As additional benefit, the 3-channel receiver performs pseudo-ranging with respect to the transmissions received from the remote sites.
Combined ranging and pseudo-ranging:One master performs round trip measurements and the slave stations are operated in receive-only mode. All units use their built-in GPS receiver and the master transmits a set of corrective data (GPS, clock, orbit) to the slaves.Receive-only modems perform pseudo-ranging taking into account the corrective GPS data (somewhat similar to real-time common view). The GPS receiver manufacturer states an accuracy of 50ns or approx. 15 m (1 sigma) for the GPS time.Data is relayed via landline to the master. Ranging data history is kept in the modems (24 hrs minimum) and this history data can be read out by an external computer or via a TCP/IP connection. (Example read outs: ‘provide range at 12:00’, ‘provide velocity at 12:00’ etc.).An internal data base holds raw measurement data and pre-processed data. Single shot measurements are stored for 1200 s (20 min), after this, a sophisticated data compression algorithm is used, which preserves the accuracy of the measurement data.
SATRE: Two-Way Satellite Time and Frequency Transfer Modem SATRE TWSTFT modems provide high accuracy clock comparison, where results are available in real time. The real time capability allows integrity monitoring in critical systems; time to alarm is a few seconds only. TWSTFT is recognised by BIPM as a second method – independent from GPS - to compare clocks of standard laboratories in order to form TAI (international atomic timescale). An extension of the TWSTFT is the remote time synchronisation, transferring the stability of high accurate clocks to a SATRE modem anywhere in the field.
Customer List for SATRE TWSTFT Modems: