Browsing International Telemetering Conference Proceedings, Volume 04 (1968) by Issue Date
Now showing items 1-20 of 51
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Single RF Carrier Time-Sharing by Remote LocationsIt is of vital national interest to know the essential real-time factors involved in the evaluation of an air attack versus a ground defense. This need led military planners to request the development of a computerized system to determine the victors and the vanquished in a war game on a par with an actual combat situation. From an engineering point of view, the evaluation system would permit all "combatants" full scope of operation and would not introduce, of itself, any "artificialities" into a complexity of split-second duels taking place over a wide geographical area. This paper discusses a unique time-division telemetry technique that was designed to resolve the data and control flow to and from remote locations, in this case, tactical aircraft. The actual system that evolved from this approach transfers all "aim and fire" events, coming from a group of aircraft engaged on a "mission", to a central communications and data processing facility. The control in the form of timing synchronization is sent from the facility to all aircraft. It should be noted that this time-sharing method could not utilize classical time-division multiplexing, e.g., PAM or PDM, since the test elements were all physically separate from one another (up to 120 miles). Preliminary test data is presented herein as an indication of the validity of this new technique. The paper concludes with a brief description of this method as applied to air and water pollution control and other posited applications.
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The Effect of Coding on Rate Equalization of Digital ChannelsThe pulse stuffing technique for rate equalization of digital channels is generalized in this article to the stuffing of a sequence of pulses (a word), which can be coded. The extra capacity needed for signaling the stuffed word decreases exponentially with the number of pulses in it, and may, in fact, be eliminated at a negligible increase in the error rate of the channel.
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Performance of a Bandwidth Limited PCM/PSK/PM Telemetry SystemThis report investigates the use of coherent and non-coherent FM/PM detectors as applied to recovery of PCM telemetry data. Given a PCM Manchester II encoded FM carrier, a theoretically perfect bit detector was derived. A laboratory prototype was built and evaluated under simulated threshold conditions. Agreement with theory was obtained within 1.50 db, using a coherent demodulator without a limiter stage. Amplitude and phase characteristics are shown in addition to the filter circuit and component values. Several commercial demodulators are compared against the theoretical model. The results of the comparisons are discussed, and recommendations concerning deficient areas are submitted.
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A PCM-Telemetry System for Sounding Rock PayloadsComplex sounding rocket payloads require on-board data processing and channel capacity which frequently exceed the capability of the standard FM-FM-telemetry system. To obtain the full benefit of the accuracy and information density of sounding rocket experiments a PCM-telemetry system has been developed which provides sufficient flexibility in the choice of channel number, bit rate, time resolution, and accuracy. A first version with Bo channels for scientific data, and 62 channels for technical data will be flown on board of five Black Brants from the Esrange in Kiruna.
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Output SNR of an FM Discriminator with Non-Ideal LimitingThe effect of abrupt-limiting on the output of a frequency discriminator has been treated thoroughly by Middleton. This paper considers the case of smooth band-pass limiting both for the simple differentiator and for the balanced discriminator. The error function is used as a model for the smooth limiter. The idealness of the limiter is related to the quantity "μ" - the limiting hardness. The analysis reveals that for a balanced discriminator, the output signal-to-noise ratio can be made largely immune to changes in "μ". However, for the unbalanced discriminator, the signal-to-noise is not only appreciably "μ" dependent, but also a function of carrier frequency.
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Performance of Block-Coding Systems When Transmitting Through a Ary Discrete Channel in the Presence of White Gaussian NoiseThe basic problem of transmitting digital (PCM) data with the least signal to noise ratio per bit of information is considered if the data is encoded into code words consisting of a finite number of symbols where each symbol belongs to an alphabet of N elements. For quantitative results the error probability of the decoded data has been taken as equal to or below one out of 10⁵ bits of information in the case where the transmitted symbols are corrupted by additive white Gaussian noise and are detected "symbol per symbol" by correlation methods. Mainly coherent detection is considered. After the derivation by a geometrical method of the upper bounds (based upon random coding) of the minimum signal to noise ratio per bit, the performance of several constructive code methods are compared with these bounds. The amount of hardware and the number of operations required respectively for encoding and for decoding the most promising class of codes (binary and N-ary Bose-Chaudhuri codes) are indicated. Considerations are given to synchronous demodulation requirements using a "Costas" phase lock loop type of demodulator.
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International Telemetering Conference Proceedings, Volume 04 (1968)International Foundation for Telemetering, 1968-10
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Tranmission of Cardiovascular Data from DogsProlonged acquisition of dynamic blood pressure data from animal subjects in various experimental conditions has become a special research area in numerous institutions. To properly conduct many of these experiments, the subject must be instrumented with blood pressure sensors and a means of conveying the indicated pressure level to a remote station. Quite often data must be obtained over several weeks in which recalibration can not be conducted. Telemetry techniques are quite adaptable to these problems and in many instances are the only solution available. To illustrate the special applications of telemetry, several experiments are described along with the hardware required to conduct these experiments.
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The Signal-to-Noise Ratio Estimation Techniques for PCM SignalsReliable estimation of signal-to-noise (S/N) ratio in a demodulated PCM telemetry signal can be useful in evaluating the performance of the complete telemetry link, including its signal detection and data processing portions. This paper describes three potentially practical methods developed at Goddard Space Flight Center for estimating the S/N ratio in a PCM signal. One method referred to as "spectral null" method uses spectral characteristics of PCM' signals to estimate the S/N ratio, the other two use statistical properties of the signal, i.e., its mean value and variance. These two methods are known as "variance estimations and "null zone." The implementation of each method is discussed. The spectral null method takes the least amount of equipment, but is more difficult to calibrate and operate over a wide range of bit rates, than the other two systems. All three approaches, however, are uncomplicated enough to be included into almost any existing PCM data handling system. An analysis of the performance characteristics of each system is made. It is shown that the variance estimation method is the most versatile. It can reliably estimate the S/N ratio to within 1.5 db over a range of S/N ratios from 0 db to +10 db. (The S/N ratio is defined as the ratio of signal energy per bit/noise power density.) Under certain conditions all three methods can provide estimates to within 1 db, especially over a S/N ratio range from +3 db to +10 db.
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Flight and Laboratory Testing of a Double Sideband FM Telemetry SystemThis paper discusses the NASA Flight Research Center's laboratory and preliminary flight evaluation of a double sideband suppressed carrier constant-bandwidth telemetry system that will be used as an airborne high-frequency data recorder. Some practical limitations are illustrated, and laboratory and flight-test results are compared. No attempt is made to compare this system with systems using other forms of modulation. Results obtained using an RF link are compared with magnetic tape recording of data. Calibration requirements are included for each system.
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Performance Characteristics and Specification of PCM Bit Synchronizer/Signal ConditionersThe PCM BR Synchronizer/Signal Conditioner, hereafter called "synchronizer," plays a vital role in telemetry data recovery, and is perhaps the most important and complex component of telemetry data processing systems (DPS). The synchronizer, being the "front end" of the system, makes an irrevocable decision as to the binary value of each data bit, and provides the fundamental timing signal (clock) for the entire DPS. Thus, the performance characteristics of the synchronizer substantially determine the system's capabilities, and it may be said that the system is as good (or bad) as the synchronizer. This paper presents and discusses test data obtained on synchronizers available to date, and used at Goddard Space Flight Center (GSFC) and its satellite tracking and data acquisition network (STADAN) stations. Performance characteristics such as bit synchronization (bit sync), bit sync acquisition, tracking, bit error rate, and intersymbol interference have been measured with respect to split-phase (SP) and NRZ-L input signals between 500 bps and 300 Kbps, perturbed by "white" Gaussian noise plus jitter. The effect of tape recording and band limiting of these signals on synchronizer performance is also discussed. It is shown that bit error rate alone does not "tell the whole story" about synchronizers, particularly when operating with low (less than 7 dB) SNR's plus jitter. The test data indicate that there is no single synchronizer excelling in all respects. For example, a synchronizer which operates well down to SNR of -3 dB has inferior acquisition, and slippage characteristics when jitter is added to noise. Generally, the performance threshold for random jitter (defined later) is at SNR greater than 10 dB. Some synchronizers seem to perform better with SP than NRZ-L signals, and vice versa. Finally, discussed and suggested are definitions of performance parameters which would uniformly and unambiguously describe and specify synchronizers. A lack of precisely defined and measurable performance parameters and characteristics has caused misinterpretation and misunderstanding of specifications presented by both vendor and customer.
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Laser Applications to Biology and MedicineThe unique physical characteristics of coherence, intensity, and monochromatically offered by laser instrumentation has placed renewed importance on studies in photochemistry and photobiology. Sufficient research experience has now been accumulated to demonstrate the potential usefulness of laser energy to fundamental cell biology, and to the diagnosis and treatment of certain pathological conditions. Current progress in laser applications to ophthalmology, oncology, and dentistry is briefly summarized.
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Cause and Effect of Time Base errors in Coherent Demoudlation of a Suppressed Carrier AM MultiplexTwo types of time base error, TBE, are discussed. One type results from variations in tape speed (flutter) and the other type is the result of additive noise. Measured data on TBE from a typical tape machine are included. Quantitative effects of TBE on coherent demodulation of DSB, SSB and quadrature DSB are discussed.
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Bit Error Rates in the Presence of Untracked Time Base FluctuationThis paper presents a simple four-step procedure for estimating the error probability of an NRZ PCM Synchronizer and Detector operating on an NRZ Bit Stream in the presence of a fluctuating data frequency source. The four steps are as follows. First, the bit error probability is calculated for Gaussian time base fluctuation as a function of the energy per bit to noise power density ratio. The second step is to model the synchronizer as an ordinary linear servo for small phase errors and a closed loop bandwidth, small compared to the bit rate, so that effect of the randomness of the data is averaged out. With the linear model, the time base error in tracking the input signal is calculated also utilizing this approximation as if there were no additive noise. The third step is to calculate the mean squared time base error due to the additive Gaussian noise alone. The fourth step is to combine the errors found in steps two and three as if they were independent and use the graphs found in Step 1 to determine the error rates. It is assumed that the total untracked time base fluctuation is Gaussian. The calculated error probabilities are compared with measured data. There appears to be good correspondence between the calculated and measured error probability.
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Performance of Binary PSK Communication SystemsThe degree of RF coherence which can be established between transmitter and receiver greatly influences the performance of binary communication systems. Practical systems are partially coherent; the main classes are transmitted reference (TR) and single channel (SC). Although SC systems are potentially superior, they are difficult to analyze and have an inherent mark-space ambiguity problem. In this paper, four SC PSK systems have been studied using Monte Carlo simulation on an IBM 360/50 digital computer. Differential data encoding was used. The systems investigated include Decision Feedback (DF), Squaring (SQ), and a variation of SQ called Absolute Value (AB). In addition, a new Maximum Likelihood (ML) SC system, which is optimum in a restricted sense, is derived and simulated. The simulation results show that all of these systems yield comparable average probability of error. This is in contrast with results which have been published previously. Furthermore, the systems can all be shown to reduce to Differential PSK when the number of reference bauds is one. Finally, a method is introduced for studying the effects of various methods of data encoding on SC system operation.
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A Two-Channel Monopulse Telemetry and Tracking Antenna FeedThe two-channel monopulse telemetry and tracking antenna feed operates over the frequency range from 1435 to 2300 MHz. The feed was designed and developed for a 7-foot parabolic reflector located in the nose cone of an EC-135N aircraft. This airborne system maintains in-flight voice and telemetry communications with Apollo spacecraft during the injection and reentry phases of the Apollo missions. The feed consists of a planar multimode dual-polarized cavity-backed spiral radiator and two printed circuit comparators. The spiral radiator is excited in the sum and difference modes at both its inner and outer filament terminals. This allows the simultaneous reception of right-hand and lefthand circularly polarized signals. The sum and difference modes of excitation are precisely controlled to provide the proper reflector illumination functions for improved sidelobes and efficiency. Sidelobe levels in excess of 22 db, null depths greater than 40 db, boresight shifts of less than 0.25 degree, and system efficiencies of greater than 40 percent have been achieved.
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Multifunciton Receiver System for Integrated Tracking, Telemetry and Ranging Data AcquisitionThe state-of-the-art for Tracking, Telemetry and Ranging data acquisition has reached a point where simultaneous performance of each of these functions is possible with one receiving system. In addition to simultaneous reception of data with one receiver, this Multifunction Receiver System was developed to be compatible with the other DOD and NASA Tracking and Data Acquisition systems besides the specific system for which it was designed, the Goddard Range and Range Rate System. NASA/Goddard Space Flight Center initiated development of this integrated receiver system in September 1967 and will have the first system operational in December 1968 at Rosma4, North Carolina. Three more systems will be installed: one each at NASA STADAN stations in Alaska, Tananarive, and Carnarvon, Australia. The receiver system was designed to cover all the currently known NASA and DOD frequency bands from VHF to 10 GHz. The data handling capability of the system is optimized for both narrowband and wideband data. AM, FM and PM data is accommodated in varying bandwidths from 10 kHz to 10 MHz. The primary objectives for developing such a system were to achieve improved mission effectiveness of NASA STADAN operations and reduce life-cycle costs in carrying out NASA Tracking and Data Acquisition responsibilities.
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Optimum and Sub-optimum Detection of Digital Sequences Corrupted by White NoiseThis paper presents an optimum and a sub-optimum, but easily recognizable, method for the detection of K binary symbols corrupted by white Gaussian noise with unknown mean. Basically the optimum procedure requires picking the sequence from a set of 2ᴷ sequences which minimizes a certain functional. However this procedure requires a great deal of computation. This computational problem is considerably reduced by the use of an efficient searching procedure developed in this paper. However a sub-optimum procedure exists and is very simple to instrument with the advantage that decisions are made in a bit-by-bit fashion. This procedure is analyzed and the average error probability is obtained.
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Digital-Data Transition Tracking LoopsThis paper is devoted to the problem of tracking data-transitions in digital communication systems by means of decision-directed phase-tracking loops. Such techniques are of interest because these methods provide the receiver with a knowledge of the time instants when the modulation may change states without using the additional transmitter power. The results presented are useful in designing synchronizing circuitry for a wide variety of digital systems. One particular system mechanization plan is presented.
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A Maximum Likelihood Bit SynchronizerA method of implementing a maximum likelihood synchronizer for baseband signals in Gaussian noise is presented along with analysis and measurements of its noise performance. Results are given showing the noise jitter of the synchronizer as a function of the energy per bit to noise power density ratio for various parameters of the synchronizer system. The Cramer Rao inequality is used to give a qualitative description of the system noise performance in terms of the signal structure. Finally the noise performance of this technique is compared with several other techniques which are currently used to synchronize baseband PCM signals.