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An integrated computer program for TDMA data link synchronization and position determination from Loran-C and data link signal times-of-arrival was written for an IBM 4pi computer and flight tested. The primary intent was to verify on a small scale in the 'real world' a simulation being used in development of the technique. Results highlighted the importance of measurement geometry and synchronization problems when using overland Loran-C.

This paper defines the software design of the position location and synchronization algorithm in use during the final flight tests of the Electronic Grid Integration Program.

The problem of efficiently packing many mobile user signals into the available UHF uplink bandwidth of an advanced communications satellite is considered. The basic approach is FDMA with QPSK modulated data streams individually modified by spectral shaping using data windows. From extensive computer simulations it is concluded that with relatively little degradation 20 unsynchronized 16 kbps satellite users can be packed into a 500 kHz uplink bandwidth that is shared with terrestrial LOS allocations. Alternatively, 7 unsynchronized 2400 bps users can occupy a 25 kHz bandwidth. Signal strengths may vary by as much as 10 to 15 dB at the satellite without the need for transmitter power control.

The results of the preliminary calibration tests of the testbed system and Loran-C equipment to be used in the flight tests of the Electronic Grid Integration Program are reported. These ground and airborne tests were conducted to obtain data on basic system characteristics of Loran-C and the TDMA data link ground system needed for the construction of a realistic simulation to be used in testing the Kalman filter navigation algorithm under development for a combination data link/navigation program and for use in performance analysis of the live flight tests of that program., Electronic Systems Div., Hanscom AFB, Mass.

This paper describes a research project to engineer a security kernel for Multics, a general-purpose, remotely accessed, multiuser computer system. The goals are to identidy the minimum mechanism that must be correct to guarantee computer enforcement of desired constraints on information access, to simplify the structure of that minimum mechanism to make verification of correctness by auditing possible, and to demonstrate by test implementation that the security kernel so developed is capable of supporting the functionality of Multics completely and efficiently. The paper presents the overall viewpoint and plan for the project and discusses initial strategies being employed to define and structure the security kernel. This paper also appears in the Proceedings of the Association of Computing Machinery SIGOPS Fifth Symposium on Operating Systems Principles which was held at the University of Texas in November 1975., Prepared in cooperation with Massachusetts Inst. of Tech., Cambridge. Project MAC. Presented at the Symposium on Operating Systems (5th) Nov 1975, Texas Univ.

A time division multiplexing scheme is described for use with FLEETSAT. Available communications circuits would be shared among a much larger pool of users by demand assignment, with a central controller allocating satellite resources. The system achieves rapid circuit assignments and preemptions with relatively high efficiency.

In this paper we discuss the lessons learned in the design and implementation of the ALOHANET, a packet broadcasting radio network in operation at the University of Hawaii since 1970. The major part of the paper consists of a detailed discussion of the communications protocol choices that have evolved since the initial stages of the design of ALOHANET. Choices concerning the design of the radio communication subsystem are then examined, followed by an evolutionary view of the important impact that microcomputer technology has had on the user interface design and resulting system capabilities. The concluding section summarizes our present views with respect to the basic system configuration and properties of packet broadcasting networks., Report on the ALOHA SYSTEM.

In this paper we analyze the thruput of an ALOHA channel when compared to that of a conventional point-to-point channel of the same average power. This analysis is of interest in the case of a satellite information system employing thousands of small earth stations. For a satellite system the fundamental limitation in the downlink is the average power available in the satellite transponder rather than the peak power. Our results show that in the limit of large numbers of small earth stations the ALOHA thruput approaches 100% of the point-to-point capacity. This the multiple access capability and the complete connectivity (in the topological sense) of an ALOHA channel can be obtained at no price in average thruput. Furthermore, since our results suggest the use of higher peak power in the satellite transponder (while the average power is kept constant) the small earth stations may use smaller antennas and simpler receivers and moderms than would be necessary in a conventional system., Report on the ALOHA SYSTEM.

In this paper we present some of the important design issues for packet switching networks with both satellite and terrestial components -- which we call mixed media packet switching networks. Satellite packet switching has considerable promise for low cost, high bandwidth data communications. However there is inherent high delay in satellite links which do not appear in ground links. Therefore a mix of the two communications media offers the advantages of low cost/high bandwidth together with low delay communications where required. In this paper we examine a number of tradeoffs which offer guidelines for the design and optimum utilization of mixed media networks. (Author), Report on the ALOHA SYSTEM.

A unique multi-user computer system for data acquisition and experimental. control is now in use at the US Army Research Institute of Environmental Medicine at Natick, MA. Based on a 32K PDP-11/40 which is running the multi-task executive RSX-11M, the system provides real-time data acquisition and control for several independent biomedical experiments simultaneously. The system handles analog I/O (64 channels in, 10 out) through the LPS 11, switch closure input, relay closure output, parallel binary or BCD I/O (DR 11 interface) and TTY I/O (DL 11 interface). Analog and digital signals are carried to and from approximately 20 laboratories in the building via cables of 11-pair individually shielded wire. Instrumentation amplifier modules (Analog Devices AD610J) preceded by passive filter networks are used at the computer end to isolate and filter each analog input channel at a cost of approximately $50/channel. Applications which have been developed include: analysis of myocardial constractility, breath-by-breath respiratory parameter evaluation, temperature (skin and rectal) measurement and digitizing of miscellaneous graphical data. No automatic control is presently being done, but on-line manual control is being done based on data fed back to the labs in the real time from the computer.

Sponsored by the FAA, the Electromagnetic Compatibility Analysis Center conducted tests from April through June, 1975 at the National Aviation Facilities Center to investigate the electromagnetic compatibility between existing systems in the 960-1215 MHz radionavigation band and four candidate signal formats of the Joint Tactical Information Distribution System (JTIDS). This report documents the test procedures, and contains graphical representations of the data along with an interpretation of the test results.

The report covers the activities on a research and development program for a Troposcatter Convolutional Encoder-Viterbi Decoder (codec). The report contains physical and functional descriptions of the codec, a description of work accomplished during the reporting, conclusions, and recommendations., See also report dated Jun 74, AD-781 573.

We study in this paper a problem concerning the scheduling of a set of jobs on a single processor computer system. In our model, a job consists of a periodic stream of identical requests. That is, a job J(i) demands periodically C(i) units of computation time in every T(i) units of time. We shall use J(1), J(2), ..., J(n) to denote the jobs, T(1), T(2), ...T(n) to denote the request periods, and C(1), C(2),..., C(n) to denote the computation times. (Throughout our discussion, we shall assume T(1) < or = T(2) < or = ...< or = T(n) for convenience in notations). That is, a job J(i) is completely characterized by an ordered pair of numbers (C(i), T(i)). By scheduling a set of jobs we mean to determine, at any time instant, the particular request to which the processor should be devoted. In our model, the criterion of scheduling is to satisfy each request prior to its deadline, which is the arrival time of the next request of the same job. A set of jobs is said to be schedulable by a certain scheduling algorithm if such a criterion can be met. Throughout our discussion, we assume that the preemptive scheduling discipline is employed. That is, the execution of a job can be interrupted by the execution of another job.

This report contains a quantitative and qualitative analysis and discussion of several currently known military and commercial satellite multiple-access systems. The report has two main objectives. A tutorial exposition of multiple-access techniques is provided. In addition, the multiple-access realizations are presented in such a manner that given partial information of a multiple-access system the most plausible deduction can be made concerning the missing information of a system. Four multiple-access techniques will be discussed: frequency division multiple-access (FDMA), time division multiple-access (TDMA), code division multiple-access (CDMA) (Sometimes known as spread spectrum multiple-access (SSMA)) and pulse address multiple-access (PAMA). The primary applications of each technique and their major advantages and disadvantages will be given. The basic operational characteristics of each system are described and references to currently implemented systems are included. The time-domain and frequency-domain characteristics of each system are given. Also, the ground station EIRP (effective isotropic radiated power) and G/T (antenna gain-to-noise temperature ratio) requirements as a function of the satellite received system saturation flux density, G/T, and transmitted EIRP are given. The efficiency of each system will be compared., See also Volume 4, AD-A027 601.

Concepts and techniques needed to instrument time division multiple access (TDMA) satellite communication systems containing a large number of small, highly-mobile terminals, e.g. aircraft, are being developed under a three year contract with the United States Air Force. Progress made during the second year of that contract is described in the report. The work has involved the development of prototype TDMA satellite communication modems and the evolution of techniques and associated technological bases needed to incorporate adaptive null-steering arrays in TDMA systems. An overview of the results obtained to date is given and the tasks to be performed during the remainder of the contract are described., See also rept. no. 1, AD-778 437.

Contents: Introduction; Orbital Mechanics; Satellite Tracking; System Description; Multiple Access of Satellites; Jamming of Satellites; Satellite Communications Link Parameters; Earth Terminal Components, Devices, and Systems; Solid State Techniques; Digital Techniques; Testing Philosophy; Communication System Measurements., Includes Change 1 dated 10 Nov 75 and Change 2 dated 22 Jul 76. Supersedes report dated 15 Feb 68.

We study the behavior of a population of data terminals which use packet-switching to communicate with a central station over a multiple-access radio channel. In order to reduce the interference among conflicting packets, we introduce Carrier Sense Multiple Access (CSMA) as a method for multiplexing the terminal traffic in the packet radio environment. In this access mode, a terminal will inhibit its transmission when it senses the carrier of any other transmission. This method is effective when the propagation delay is small compared to the packet transmission time. However, the performance of CSMA is highly dependent on the sensing ability of each terminal; unfortunately, many situations exist in which some terminals are hidden from others (either because they are out-of-sight or out-of-range). The existence of hidden terminals significantly degrades the performance of CSMA. To eliminate the hidden terminal problem, we consider a natural extension of CSMA, namely, the Busy Tone Multiple Access (BTMA). BTMA with hidden terminals performs almost as well as CSMA without hidden terminals. A second alternative for multiplexing the terminals on a packet radio channel is to use reservation techniques. Here we study the Split-channel Reservation Multiple Access mode as one implementation of such reservation techniques. These many access modes are compared with regard to throughput and delay. (Author)

Three major areas of activities in support of packet radio communications are reported in this document. These are: (1) Analysis of the pseudorandom code spread spectrum technique, (2) evaluation and selection of the pseudorandom codes for the experimental repeater, and (3) development of a signal capture model. The analysis of the code spread spectrum includes considerations of synchronization and multiple access capabilities. The code selection is based on the computer-derived side lobe properties and crosscorrelation of high data rate codes with low data rate codes. The developed capture model is based on the repeater design currently being developed and will be used to model capture in the existing packet radio network simulation models, which presently have no capture. Also, a brief status report of the repeater equipment development is included in the introduction., See also report dated 17 Apr 1974, AD0778345, and AD0778344.

A technique is discussed for the design of multiple access frequency hopping patterns which will allow a number of users to communicate reliability over a limited bandwidth in the presence of doppler and timing asynchronies. The patterns are based on a subset of the Reed-Solomon codewords. They possess nearly flat ambiguity and cross-ambiguity functions and thus could be useful as discrete radar signals. In addition the structure of the patterns permits a straightforward implementation. (Author)

In a time-division multiple-access (TDMA) system, many transmitting stations time share the same RF spectrum. Individual transmissions must be identifiable to distant receiving stations and must be synchronized so that no interference overlap occurs. A unique word sequence can serve both purposes. The basic concepts of a satellite communications TDMA system are briefly reviewed, with emphasis on functional receiver requirements. A correlation detection scheme is hypothesized, from which false and miss-detection probability expressions are derived. Criteria are then established for the selection of sequences suitable for use as unique words. Representative unique words of length 15 through 20 are included, along with an explanation of the difficulties encountered in devising an efficient computer search routine. (Author)

The report presents a summary of a study conducted for the Transportation Systems Center of promising access control techniques which are applicable to an aeronautical satellite system. Several frequency division multiple access (FDMA) and time division multiple access (TDMA) configurations are analyzed and compared which are capable of providing voice, data and independent surveillance services. One of the FDMA concepts and a burst TDMA system are rated highest and are presented in greatest detail. Procedures are outlined for different types of entry, beam switching and handling various types of interconnections. Included are preliminary designs of the avionics instrumentation. (Author)

The objective of the program is the development of a device to lower the bit error rate of a troposcatter channel by correcting errors which occur between the transmission and reception of the digital signal. The decoder is capable of accepting; (1) an analog signal from which a three-bit parallel soft decision (eight-level quantization) can be made or, (2) a one-bit hard decision type signal. The interleaver/deinterleaver concept is to be employed to enhance system performance during periods of signal fading while in the hard decision mode of operation.

The report documents a study of modifications required to allow for the use of the experimental Distress Alerting and Locating System (DALS) as a preoperational SAR system. In particular, the following areas are addressed: Multiple access and false alarms in an attempt to prevent system saturation; Automatic deployment techniques for the user device (hand-held unit); Situation coding to describe emergency status of distressed vessel/person; The feasibility of a frequency modification for operation on an allocated safety and distress frequency; Repackaging techniques incorporating a non-destruct antenna, long shelf life batteries and a sealed (environmentally protected) case; and The desirability and feasibility of adding to the distress device an 'on-air' indicator and/or a 'response to your call' indication. (Author)

Determination of the intermodulation power levels in the case of soft-limiting multiple-access repeaters is often critical to system design. An approximate solution to the biased nu th law problem is derived that simplifies the determination of the critical intermodulation power resulting from N carriers. Experimental measurements of a diode simulator are compared with the corresponding approximate analytical results. Finally, measurements with an amplifier operating in the VHF/UHF spectrum are compared with calculations from a biased nu th law model. (Modified author abstract)

Contents: Multiple Access Modulation Techniques; Time-Division Multiple Access; Frequency-Division Multiple Access; Spread-Spectrum Multiple Access; Pulse-Address Multiple Access With Narrow-Band Pulses; Pulse-Address Multiple Access with Spread Spectrum Pulses; Network Timing; Signal Waveform Analysis; Probability Distributions.