Of the three signal parameters amplitude, frequency and phase, frequency measurements are most widely used for the entry probes, since they provide direct data on probe velocity. Typically, in the receiver, the signal bandwidth is shifted down to an intermediate frequency (IF) with a heterodyne signal formed by mixing the received signal with a reference oscillator.
In the closed-loop system the signal is then acquired and filtered with a phase-locked loop (PLL), and the frequency of the voltage-controlled oscillator (VCO) of the PLL is the resultant parameter that yields the probe Doppler velocity.
As a result of the highly dynamic and often poorly predictable behaviour of a probe, and consequently the signal, during the entry phase, the real-time PLL might not acquire the signal or may lose tracking. An open-loop system will improve reliability and flexibility in signal detection, filtering and frequency measurements, as well as data acquisition. In the open-loop system the IF signal is usually digitized with the sampling frequency exceeding 21F in the one-channel scheme, or exceeding 1F in the two-channel conversion. The digitized signal is recorded for further digital filtering and processing resulting in frequency measurements. Multiple runs with different moving filters allow frequency measurements to be made in extreme situations. Critical data from two Soviet probes - Venera 7 after landing (Avduevsky et al., 1971) and Mars 6 during the entry phase (Kerzhanovich, 1977), as well as radio science data on many US probes, have been retrieved with an open-loop system.
The root mean square (RMS) error of frequency measurements in a one-way system caused by system noise af can be estimated as where pL = Pc/PN0, BL is the equivalent bandwidth of the PLL or digital filter, T the integration time, and index C means carrier. This equation can also be used to estimate the frequency error in a two-way link provided that the SNR in the up-link is significantly greater than the SNR in the down-link. The corresponding velocity error can be estimated using Equation 10.1. The same equations are applicable to relay links.
A generic diagram of probe data transmission is shown in Figure 10.5. Data collected on the probe can be coded to improve link performance. Coded data either modulate a subcarrier, which in turn modulates the carrier frequency, or modulate the carrier directly. On the down-end of the link, the acquired signal is demodulated and decoded. In cases where an orbiter or flyby spacecraft relays the data, it is received and decoded onboard, and then retransmitted to ground stations on Earth.
Of the different types of modulation (frequency modulation, amplitude modulation, etc.), BPSK (binary phase-shift keying) is the most widely used for entry probes. This modulation can be implemented with or without a subcarrier, and with carrier or without carrier. The highly dynamic behaviour of the signal
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