Carrier acquisition and tracking for Europa relay communications

In deep space communications, carrier acquisition is pivotal for demodulation of weak signals. Traditionally, NASA's Deep Space Network will sweep the uplink carrier to ensure that the receiver has acquired carrier lock before sending data. In a deep space relay link, the carrier acquisition process will be performed by one of the transponders. Using the Johns Hopkins Applied Physics Laboratory (JHU/APL) Frontier Radio Software Defined Radio architecture, the swept behavior required for unaided carrier acquisition was migrated to the radio's receiver through updates to the algorithms that perform carrier acquisition and tracking. A notional relay link between the NASA Europa Lander spacecraft and the Europa Clipper and Carrier Relay Spacecraft (CRS) is used as a case study for the technique. The nominal center frequency of the carrier acquisition phase-locked loop (PLL) is able to sweep at a user-defined rate within user-defined frequency bounds to ensure that the receiver can lock onto a carrier without the transmitted carrier sweeping through the receivers quiescent frequency. Carrier loop sweeping continues until lock is detected and the loop is switched to a tracking design. The technique that enabled receiver loop sweeping also allows the receiver to accept a Doppler prediction frequency to offset the quiescent frequency of the loop. The turn-around ratio is maintained through the acquisition process, as well as with Doppler prediction offset and tracking. As a result of implementing self-sweeping in the radio receiver, an offset can accumulate in the PLL filter state as the tracking design is nearly Type-II. Furthermore, this offset may compound with repeated acquisitions. The resulting filter state can cause an unwanted offset in the acquisition frequency of the loop. Since the internal state of the loop filter is not directly accessible in the Frontier Radio architecture, this state must be bled-off by temporarily altering the design of the second-order PLL to one with very short time constants. This bleed-off loop is applied at the beginning of each acquisition sequence for a short period of time, then the nominal loop parameters are reapplied before sweeping commences. Algorithms and analysis of the techniques are described below.