Evaluation of mitrex modem transmit and receiver delay instability

摘要

The IEN time and frequency laboratory has developed an automated TWSTFT station to join the experimetns network on INTELSAT satellite already operative. The hardware and software that were first checked in April 1999 when the IEN station participated in the international measuring sessions using the OCA code and time slot are still under test. The chance of having three MITREX modems available at the laboratory, allowed us to measure transmitting and receiving delays by suitable cross-connections between the modems. In the measurement setup a common reference frequency has been applied to the modems and all the possible combinations of (Tx+Rx) time interval measurements have been performed. Unfortunately, the measurement scheme doesn't allow the estimation fo the values of the separate transmitting and receiving delays but, under suitable assumptions, it may be shown that the variances and covariances of the indiviual delays may be estiamted from the variances and covariances of the measured quantities. This estimation requires a noise de-coupling technique similar to the known N-cornered-hat method used to evaluate the noises of the individual clocks in comparison measures. This can givean insight of the instabilities of the modem channels both in the short term and in the long term as one of the instability contribution in the calculation of the TWSTT uncertainty budget.