Analytical Determination of Kepler Uplink Lock Frequency

In the third year of its mission, the Kepler spacecraft began to experience issues locking onto the X-band uplink carrier frequency transmitted by the Deep Space Network (DSN). The spacecraft uses a Small Deep Space Transponder (SDST) that ‘rests’ at a predetermined or “best lock” frequency based on the temperature of an internal oscillator until an uplink sweep catches the rest frequency and the receiver locks onto the carrier signal. The problems with uplink carrier lock prompted an investigation into the true rest frequency of the SDST. A short-term solution was implemented by doubling the sweep range until an investigation could show a long-term solution. While this change helped to achieve uplink, it subtracted from commanding time and added complications to station handovers. The analysis detailed in this report used both spacecraft and ground telemetry to indicate that the best lock frequency of the receiver was at least 7.2 kHz lower than the listed frequency at 250C previously used. This offset is comparable to the width of the frequency sweep performed by the DSN for Kepler. After the DSN decreased its operational frequency by 8 kHz, no further issues were observed in which the carrier signal sweep failed to achieve lock with the spacecraft. This paper drafts a new method of determining best lock frequency without the need of the internal oscillator temperature telemetry.