## 3 Tracking Measurement Overview  
### DOWD Measurements  
DOWD employs an open-loop methodology for tracking data to mitigate any impact from the user spacecraft transmitted frequency offset. This technique involves OWD measurements, where a radio signal is transmitted from the spacecraft, received by two TDRSs, frequency translated, and then independently relayed to the ground stations.  
Figure 1 illustrates the OWD tracking configuration utilized by user spacecraft in conjunction with two TDRS satellites. The signal transmission process involves two distinct paths, starting with the user spacecraft at Node #1. The first path entails transmitting a signal from the user spacecraft to two TDRS satellites located at Node #2. The second path involves transmitting a signal from a ground station positioned at Node #3 to the TDRS satellite at Node #2. The received signal at the TDRS satellite is then combined with the signal from the first path before being transmitted back to the ground station at Node #4. This configuration is also known as three-legged Doppler measurements.  
In general, the accuracy of OWD measurements is influenced by a variety of factors. These include atmospheric refraction, the offset of the user spacecraft antenna, the reference clocks, delays in the transponder, and the bias in the oscillator frequency. Consequently, these measurements are not ideal for determining orbits with high precision. The FDF has developed a technique that involves subtracting two distinct OWD measurements obtained from the pair of TDRS satellites that share the same time tag. This process results in a Differenced One-Way Doppler, DOWD, measurement [8].  
Assuming a spherically symmetric atmosphere, the signals received from different TDRSs are subject to essentially the same atmospheric corrections, and taking the difference between pairs of OWD measurements can effectively cancel out these corrections. As a result, the resulting DOWD tracking measurements can be as accurate as two-way Doppler measurements [5; 9]. However, errors arising from the differences in propagation delays between the user spacecraft and the two receiving TDRSs, as well as those resulting from the drift rate of the user spacecraft's transmission frequency, persist in the signal [8].  
DOWD Benefits and Limitations  
The DOWD measurement is critical during the launch and early orbit phases of a spacecraft, particularly when onboard navigation systems are not yet fully operational, and the spacecraft lacks coherent signal capabilities. By addressing these challenges, DOWD eliminates the need for an ultra-stable oscillator (USO) during these initial stages. For example, while many spacecraft are equipped with GPS receivers, these systems are often activated only after the spacecraft reaches orbit. The DOWD technique not only provides an interim OD solution during this period but also aids in verifying the accuracy of GPSreceivers. Additionally, integrating DOWD measurements with other tracking data, such as radar range and angle data, improves OD precision while simultaneously reducing the number of radar passes required, which could lead to cost savings for the mission.  
Despite its advantages, the DOWD technique has notable limitations. At least four DOWD passes are generally needed to obtain a reliable OD solution. Furthermore, the technique requires two TDRS spacecraft to simultaneously receive an S-band signal from the user spacecraft for DOWD measurements to be performed. Since these measurements rely on the precise positioning of TDRSs in space, the accuracy of DOWD processing is heavily dependent on the precision of TDRS orbital data. As a result, higher uncertainties in TDRS orbits can restrict the overall accuracy of the user spacecraft's orbit determination.