In a scheme to increase the overall data return from a phase-modulation, suppressed-carrier telemetry transmitter, the telemetry playback symbol rate is adjusted essentially continuously. More precisely, the playback symbol rate is adjusted frequently (as often as once per symbol period) in small increments. The adjustment of the rate is made in accordance with the principle that the supportable data rate at any given instant is a function of the instantaneous total signal-power-to-noise spectral-density ratio (PT/N0) at the receiver. The scheme was devised for transmission of telemetric data from deep-space missions, but could also be applied to satellites in orbit around the Earth.
PT/N0 is a known function of the receiving-station gain-to-noise-temperature ratio (G/T), which, in turn, is a known function of the position of the transmitter relative to that of the receiver. Thus, once the trajectory of the transmitter relative to the receiver position has been predicted, then G/T or PT/N0 as a function of time can be estimated (see figure); this estimate can be computed aboard the spacecraft or uploaded to the spacecraft prior to a portion of a trajectory (a tracking pass) and the corresponding interval of time during which the telemetry signal is expected to be received. Then during the tracking pass, the playback symbol rate at the transmitter is adjusted in accordance with the predicted G/T.
Traditionally, the telemetry symbol rate (equivalently, the telemetry data rate) for a deep-space mission is either fixed or is changed only a few times during a pass. A change in the data rate often entails a large, instantaneous jump, which can sometimes cause the ground receiving system to lose symbol lock. Upon losing lock, the receiver begins to lose some or all of the incoming telemetry data and must go through a signal reacquisition process in order to regain lock. The time lost in reacquisition could be large enough to nullify any advantage gained by changing the data rate. Although there are ways to minimize the probability of losing receiver lock during data-rate changes, they are operationally complicated and often require very precise predictions of the times when the rate changes will take place.
In the present scheme, a precise prediction of the instantaneous data rate is not needed. [However, coarse data-rate predictions are needed for initial acquisition and for reacquisition after mode changes (not to be confused with rate changes).] In this scheme, the symbol clock in the transmitter has a continuous phase; in other words, even when the clock frequency changes abruptly, the clock phase remains continuous. Because of the continuity of phase and the smallness of the clock-frequency increments, the symbol-tracking loop in the receiver is subjected to only small transient phase errors that do not cause it to lose lock. Experiments and computational simulations for some typical cases have shown that a receiver can, indeed, track the small frequent rate changes and that the telemetry returns achievable by use of this scheme exceed, by 1 to 2 dB, those achievable by use of the best-single-rate scheme.
This work was done by Miles K. Sue, Jeff B. Berner, Selahattin Kayalar, and Henry Hotz of Caltech; Peter Kinman of Case Western Reserve University; and Harry Tan of Q-Plus for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
NPO-21024
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Variable Telemetry Playback Rate for Increased Data Return
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Overview
The document discusses an innovative telemetry playback scheme designed to enhance data transmission for deep-space missions, particularly focusing on the use of phase-modulation and suppressed-carrier telemetry. The core idea is to adjust the playback symbol rate continuously and frequently, allowing for small incremental changes in the bit rate. This method is advantageous because it enables the receiver to track the downlink signal without needing precise predictions of bit-rate changes, which can be challenging in traditional systems that rely on larger, infrequent adjustments.
The document outlines the theoretical and experimental foundations of this approach, demonstrating that a Block-V receiver can effectively manage these small bit-rate changes without losing lock on the symbol loop. The ability to maintain lock is crucial, as losing it can complicate reacquisition during tracking passes, although such events are expected to be rare.
A significant aspect of the scheme is its reliance on the instantaneous total signal-power-to-noise spectral-density ratio (PT/N0) at the receiver. The achievable static bit rate is directly related to this ratio, allowing for optimal data transmission based on current signal conditions. The document includes various figures illustrating the gains relative to single-rate strategies and the performance of the system under different conditions, such as the zenith total power-to-noise spectral density ratio.
Additionally, the document emphasizes the importance of this telemetry scheme for NASA's deep-space missions while also noting its potential applicability to other government agencies and satellite companies operating in Earth orbit. The authors advocate for the dissemination of this work through platforms like NASA Tech Briefs, highlighting its relevance to a broader audience interested in advanced telemetry solutions.
In summary, the document presents a novel approach to telemetry playback that enhances data return by allowing for continuous adjustments in bit rate based on real-time signal conditions. This method not only improves the reliability of data transmission during deep-space missions but also offers a framework that could benefit various sectors involved in satellite communications.
