For deep space communication systems, the decision of whether or not to suppress the transmitted carrier has always been an issue. For certain missions that use high data rates, the available bandwidth might be a limiting factor. In such cases, it is preferred to use a completely suppressed carrier system that is more bandwidth efficient.
Radio science (RS) experiments currently rely on an unmodulated CW RF signal carrier for spectral purity and maximized signal-to-noise ratio. This requires missions to carefully schedule them away from periods of high-rate telemetry. In the era of optical communications, currently designed systems experience the same problem.
RS measurements are performed by using only a residual carrier scheme. When a carrier signal is passing through the atmosphere of a planet, RS information is extracted from the amplitude and phase variations of the received carrier. For certain critical missions, pure carrier transmission without data might be used for some duration of time to enhance the quality of received RS observations.
This work proposes using optical links for RS measurements. For RS, the optical signal can be unmodulated (known data) or modulated (with unknown data). In the proposed optical receiver, the modulation is considered to be a binary phase shift keying (BPSK) modulated laser, or an intensity-modulated optical signal such as pulse position modulation (PPM).
A data processing architecture was developed that will yield high-accuracy RS, or link science type of information on the ground from readily transmitted optical communication signals coming from space assets. This technique is intended to save power, bandwidth, and scheduling demands on the spacecraft. The approach is applicable to a broad suite of modulations (phase and/or intensity) and receiver types (coherent and/or non-coherent), thus providing an architectural improvement to present state-of-the-art communication systems utilized by NASA, as well as to future systems.
This method is an optical module to the existing optical communication receiver architecture. For optical links with intensity-modulated laser transmission or phase-modulated CW laser communications, the proposed optical receiver provides both data detection and signals required to extract RS data such as amplitude, phase, and frequency due to planetary atmospheric changes. The same information required for RS data can be extracted using differential methods of encoding. At the optical receiver, a local laser, a phase shifter, and an array of photon detectors are used.
This work was done by Dariush Divsalar, Bruce E. Moision, and Samuel J. Dolinar Jr. of Caltech for NASA’s Jet Propulsion Laboratory.
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Free Space Optical Receiver for Data Detection and Radio Science Measurements
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Overview
The document presents a technical report on a Free Space Optical Receiver designed for data detection and Radio Science measurements, developed by researchers at NASA's Jet Propulsion Laboratory. The report outlines the challenges faced in deep space communications, particularly the need for reliable information data transmission alongside accurate Radio Science observations, which are crucial for understanding planetary atmospheres and other celestial phenomena.
Traditionally, Radio Science experiments have relied on un-modulated continuous wave (CW) radio frequency (RF) signals to ensure spectral purity and maximize signal-to-noise ratios. However, this approach necessitates careful scheduling to avoid conflicts with high-rate telemetry periods. The proposed optical receiver aims to address these limitations by utilizing optical communication signals, which can be more bandwidth-efficient and power-saving.
The report details the architecture of the proposed optical receiver, which is capable of processing various modulation schemes, including Binary Phase Shift Keying (BPSK) and pulse position modulation (PPM). The system is designed to extract essential Radio Science data, such as amplitude, phase, and frequency variations caused by planetary atmospheric changes. This is achieved through differential encoding methods and the use of a local laser, phase shifter, and photon detectors at the optical receiver.
One of the key innovations discussed is the method for removing data from the received signal to isolate the information necessary for Radio Science measurements. This involves techniques such as squaring the received samples or applying hyperbolic tangent functions, although the report notes potential phase ambiguities that may arise from these methods.
The document emphasizes the versatility of the proposed optical receiver, which can be adapted for various types of coherent and non-coherent receivers, making it applicable to a wide range of future space missions. By improving the efficiency of data transmission and enhancing the quality of Radio Science observations, this technology could significantly advance our understanding of planetary atmospheres, surfaces, and other astrophysical phenomena.
In summary, the report outlines a promising approach to integrating optical communication technologies into deep space missions, potentially revolutionizing how data is collected and analyzed in the field of Radio Science.
