## 6 Discussion  
This study investigated how uncertainties in the orbit of the TDRS satellites propagate into the DOWD-based orbit of the user spacecraft when the TDRS orbit is determined solely based on TT&C measurements. The following TDRS orbit determination scenarios were considered:  
* Reconstruction of the TDRS orbit using current operational values of the a priori conditions. * Monte-Carlo analysis to assess uncertainties in the TDRS orbit reconstruction in the case of inadequate knowledge of the a priori conditions. * Analysis of the relationship between the quality of the TDRS orbit reconstruction and arc length dependency. Each reconstructed TDRS orbit was used to reconstruct the orbit of the user spacecraft. The error between the reconstructed orbits and operational solutions was evaluated. The FDF's operational TDRS solution, derived from BRTS and TT&C measurements, achieves an accuracy better than 75 m, whereas the DOWD-based operational solution for user spacecraft reaches an accuracy of up to 300 m.  
The study found that the TDRS orbit determined solely from TT&C measurements is highly sensitive to measurement bias and its covariance. If these values are not adequately accounted for during orbit determination, it can result in a significant orbit error of about 17 km. In such cases, extending the arc length of the TT&C measurements by five or more days can improve the estimation of the TT&C bias and result in better orbit determination accuracy. However, if there is insufficient knowledge of the measurement bias and its covariance, it is recommended to avoid estimating the TT&C bias to prevent substantial orbit errors. In this case, the maximum error in the orbit is directly affected by the underlying error present in the TT&C bias. Consequently, an increase in the magnitude of measurement error will result in a higher level of orbital error. Therefore, it is important to assess the accuracy of TT&C measurements and consider their associated uncertainties. If the precise values of these quantities are known, the resulting TDRS orbit can be reliable, and the orbit error would be within the current TDRS orbit accuracy requirement. Likewise, the study showed that uncertainties in the TDRS orbit can propagate into the orbit of the user spacecraft and significantly impact the accuracy of the user spacecraft orbit reconstruction. A linear correlation was discovered between errors in the TDRS and user spacecraft orbits. This correlation's slope is influenced by several factors, including the number of DOWD passes acquired, the relative position of the TDRS concerning the user spacecraft during the acquisition of DOWD measurements, and dynamic errors in the user spacecraft's orbit. However, because these factors are highly interrelated, the analysis does not isolate their individual contributions to the correlation slope. Insufficient number DOWD passes (typically less than four) can lead to significant errors in the user spacecraft's orbit, with the potential for errors of up to 33 km. However, with sufficient DOWD passes, the accuracy of the TDRS orbit plays a vital role in determining the precision of the user spacecraft orbit. Depending on the TDRS orbit error, the user spacecraft orbit error can range from as little as 16 m to as much as about 5 km. TDRS orbit accuracy degradation can have severe consequences, particularly for spacecraft with stringent orbit precision requirements. Therefore, research indicated that prematurely discontinuuing the Bilateration Ranging Transponder System, BRTS, without a thorough understanding of the TT&C bias and its associated uncertainty might obstruct the attainment of the necessary orbital accuracy for both TDRS satellites and user spacecraft. However, despite the reduced precision of the TDRS and user spacecraft orbit, the estimation of Local Oscillator Frequency drift remains accurate without any noticeable impact. Furthermore, a TDRS orbit based solely on range-only TT&C could have repercussions that extend beyond a DOWD-based user orbit.