Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1998-01-09
2001-01-23
Vo, Don N. (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
C375S316000, C342S450000
Reexamination Certificate
active
06178207
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to processing of radio-frequency transmitted data and, more particularly, to processing of real-time data transmitted between fixed ground stations and as mobile aircraft as part of an aircraft combat training system.
2. Description of the Related Art
Military training of aircraft combat crews involves maneuvering aircraft across specified terrain and airspace in simulated battle operations. Instrumentation mounted in the aircraft provides position information and performance data to ground-based stations, which collect the aircraft data, process it, and generate reports on likely combat effectiveness of the crews and equipment. A wide variety of aircraft operations can be simulated, including air combat and ground attack. The data processing can determine the likely effect of weapons delivery, keep track of objects being fired upon, and assess likely damage to targets and to attacking aircraft.
The instrumentation is generally carried by aircraft in pods mounted at aircraft weapons stations, and is referred to as Air Combat Maneuvering Instrumentation (ACMI). One well-known instrumentation specification and data protocol, used by the armed forces of the U.S.A., is referred to as Tactical Air Combat Training System (TACTS). Data in TACTS is transmitted at the rate of either approximately 62 kilobits per second (KBPS) or 198.4 KBPS. The TACTS data includes aircraft identification and operational data such as weapons load and remaining fuel. The digital data is modulated and mixed with a carrier frequency for transmission over the radio frequency (RF) band.
The TACTS data transmitted by an aircraft permits aircraft position to be derived by a process known as multilateral triangulation. In multilateral triangulation, an aircraft receives data from a ground station and transmits data to multiple ground stations. The range (straight-line distance) from the aircraft to any one of the ground stations is determined by measuring the phase of a sinusoidal signal modulated onto a carrier frequency received and re-transmitted by the aircraft. Range data from any three ground stations will determine the aircraft position in terms of latitude, longitude, and altitude.
The speed of the aircraft involved in ACMI systems can vary greatly, from zero (hovering speeds) in the case of helicopters to hundreds of miles per hour in the case of supersonic aircraft. The ACMI ranges cover many square miles of land and the altitudes involved can vary from ground level to tens of thousands of feet. All of the data must be transmitted, received, collected, and processed in real time. This presents a very demanding signal processing task to ensure accuracy and reliability. The TACTS specification has been in use since approximately the 1970s.
A system having greater data transmission capability has been proposed for use at the military range located at Nellis Air Force Base, Nevada, U.S.A. The new system is called Nellis Air Combat Training System (NACTS) and specifies data transmission at the rate of 1.44 megabits per second (MBPS), or 1440 KBPS. Because of the advent of systems such as the Global Positioning Satellite (GPS) system, the NACTS protocol does not rely on multilateral triangulation for determining aircraft position. As with the TACTS implementation, the NACTS data is transmitted from pods attached to aircraft and relayed to ground stations, where the data is processed. The increased data rate of NACTS can support, for example, an increased number of aircraft participating in any training exercise or an increased amount of data transmitted for each aircraft.
As noted above, the TACTS specification has been in use for many years. Many training facilities have used, and will continue to use, the TACTS specification. Thus, it would be advantageous if an ACMI pod for use with training systems could support both the TACTS and NACTS specifications. A characteristic of continuing importance is the accurate detection of received data and, in particular, the identification of data pulses. Efficient construction and operation, in the form of low weight and low power requirements, also is important so as to minimally impact aircraft operation.
From the discussion above, it should be apparent that there is a need for a processing system that can operate with multiple data protocol specifications, waveforms, and data rates, while ensuring accurate and reliable detection of data streams in an aircraft operational environment. The present invention fulfills this need.
SUMMARY OF THE INVENTION
The present invention provides an application specific integrated circuit (ASIC) chip that can interface with multiple data protocol specifications. The ASIC includes a demodulator and modulation generator for data messages in a protocol at a first data rate, such as TACTS, and includes a demodulator and modulation generator for data messages in a protocol at a second data rate, such as NACTS. The ASIC chip is installed in a system for the transmission and receipt of data messages in the two protocols, and operates under control of a microprocessor, which selects between the available message protocols. Additional data rates and protocols beyond the first two can be accommodated. The different demodulators share processing structures that reduce the circuit components otherwise necessary for operation. This reduces the weight and power requirements of the chip. In this way, the ASIC chip provides a processing system that can operate with multiple data protocol specifications and data rates, while ensuring accurate and reliable detection of data streams in an aircraft operational environment.
In one aspect of the invention, the ASIC chip includes a demodulator for the first protocol (TACTS) with a data clock recovery circuit that permits precise phase alignment of the demodulator clock with the clock of the data generating device, such as the aircraft. This demodulator permits quicker response, better immunity to noise, and better signal updating as new sample data is received.
In another aspect of the invention, the ASIC chip includes a demodulator for the second protocol (HDR) with a digital correlator that acts as a matched filter to quickly and accurately detect a message preamble, and a carrier phase tracking circuit to more precisely track the carrier phase for signals under consideration. In particular, a predetermined number of prior signal samples is summed to form a carrier reference signal. The dot product of the reference with the next incoming data sample of the signal is checked to determine if the sample should be characterized as in phase or out of phase with the reference.
In yet another aspect of the invention, the ASIC chip includes a dual function transmit path digital interpolator (DI) that is used as an interpolator for range measurement functions and then also functions as a parametric waveform generator when transmitting pulse waveform data. The interpolating filter is implemented as a Hogenauer filter that is less complicated than conventional finite impulse response (FIR) filters typically used for interpolation filtering, and uniquely permits dual use as a waveform generator. The dual use feature provides improved performance with simplified structure and reduced weight.
In another aspect of the invention, the ASIC chip includes a digital correlator that more accurately estimates the time of arrival of a pulse signal. This is particularly useful when the ASIC chip operates in a pulse position modulation mode, in which data messages comprise only predetermined pulse waveforms such that a delay interval between consecutive pulse waveforms comprises pulse position modulation data. In the pulse position modulation mode, a vector summer determines when a peak sample value of a data message sample occurred, and a counter register stores a count representing the local time of arrival of each peak sample value, such that the host processor can then calculate the elapsed time between sample peak sample values and the
Landen Lloyd E.
Richards Wayne E.
Schafer James W.
Cubic Defense Systems Inc.
Jester Michael H.
Phu Phuong
Vo Don N.
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