Abstract: A protocol and mechanism which provide for centralized asset tracking communications from a central station are based on a control/polling forward channel; i.e., a narrow band channel from the central station to tracking units affixed to the assets. A narrow band service channel is used by the asset tracking units to transmit to the central station to aid the central station in efficient scheduling of asset reporting, and a plurality of narrow band channels that are appropriately multiplexed are used for conveyance of data from the asset tracking units to the central station.

Abstract: A mode of communication for asset tracking units involves communication between a central station and the individual tracking units, usually through a satellite link. This mode is the primary communication link for tracking the assets. A second mode is the local area network (LAN) or "mutter" mode, in which a subset of tracking units communicate with each other in a mobile LAN. This mode is used as the secondary communication mechanism to conserve power, since mutter mode communication requires much less energy for local communication between the tracking units as opposed to direct satellite communication with the central station. In addition, mutter mode leads to increased reliability of the asset tracking system by enabling tracking of units which have batteries too weak to support communication with the central station, but strong enough to support communication in mutter mode.

Abstract: A digital radio communications system employs a digital information source for providing digital information such as message bits, a transmitter for transmitting encoded digital information into data symbols in a radio-frequency (RF) signal to a plurality of antennae which sense the transmitted RF signal. A post detection measure of signal quality, the signal-to-impairment ratio, (SIR), is utilized by the receiver to perform post detection combining of signals received by a plurality of antennae. The antennae are coupled to a receiver which for each received signal: digitizes the signal, determines phase angles of the digitized signal, converts the signal to unit vectors, determines a signal-to-impairment ratio (SIR) estimates .gamma..sup.j of the digitized signals. The SIR estimate .gamma..sup.j is weighted by combining weight computation element and multiplied by each unit vector to provide an in-phase is component and a quadrature component for each signal.

Abstract: Directional antennae on aircraft and on cellular telephone base stations, having a polarity opposite that of potentially interfering ground system signals, minimize signal strength of air cellular signals received by ground cellular stations. Aircraft directional antennae comprise patch antennae or vertical arrays of loop elements or vertical arrays of virtual loop elements. Additionally, air cellular signals are switched to channels not currently in use by ground cellular systems.

Abstract: A method of estimating the quality of a communication channel from a differential phase angle between a received signal and the corresponding transmitted phase angle employs determining a signal to impairment ratio (SIR) as an indicator of channel state information (CSI). A maximum likelihood estimation procedure is employed to calculate this CSI metric as a function of the differential phase angle between be received signal and transmitted signal An alternate embodiment employs an estimation that incorporates average SIR information for a Rayleigh fading channel. Since CSI is derived from the phase angle of the received signal, and does not require signal amplitude information, it is attractive for use with differential detectors, phase-locked loops (PLLs) and hard-limited signals. The CSI provided can be used for implementing post detection selection diversity, by selecting the signal from a plurality of antennae which has the best SIR.

Abstract: A symbol synchronizer derives a symbol clock reference from a sampled baseband signal which may be of a variety of suitable digital signalling formats, including non-return-to-zero (NRZ) and Manchester formats. A one-bit quantizer receives the sampled signal and provides a binary representation thereof to a level-transition detector which generates a sequence of pulses at the symbol rate of the binary-valued data sequence and at a predetermined timing offset from the symbol timing epochs of the data sequence. The timing offset depends on the digital signalling format and is on the interval ##EQU1## where T is the symbol period. The level-transition detector employs delay circuitry, logic circuitry, and a monostable multivibrator to mask transitions in the quantized data sequence that do not occur at the predetermined timing offset from the timing epochs and to generate the symbol clock pulse sequence.

Abstract: High efficiency air filters are fabricated by forming two end assemblies from rectangular wall members with side plates attached to extend upwardly from opposite sides thereof. A removable wall of expendable material such as ordinary masking tape is applied to the other two sides, extending between the ends of the side plates to form a shallow pan which is filled with a liquid adhesive. The pleated filter media is attached along its two side edges to side wall members having notched ends for mating with the side plates of the end assemblies. One edge of the filter media between the two side walls is immersed in the pool of adhesive and the side walls are attached to the side plates. After the adhesive hardens, the masking tape is removed and the other end assembly is attached in the same manner.

Abstract: An air filter unit of the pleated media, high efficiency type wherein the media pleat edges are sealed to the supporting frame to prevent bypass of air with a ceramic adhesive and fibrous ceramic mat which allows the unit to be exposed to high temperatures (e.g., up to 2000.degree. F.) without danger of seal breakdown. While in the form of a slurry the adhesive is applied, for example, with a trowel to the zig-zag pleated edges of the media which, together with corrugated spacers, forms the filter core. The latter is then surrounded on four sides by the compressible mat of fibrous ceramic material and inserted in a box-like support frame with the slurry filling the space between the pleated edges of the media and the fibrous mat.

Abstract: Location of an object to be tracked is determined by measuring, at a receiver situated at the object, the propagation time differences between the signals from a plurality of GPS satellites, each of which is received by the receiver situated at the object. The measured propagation time difference values are transmitted to a central station where the location of the object to be tracked is calculated based upon the propagation time differences of the signals transmitted from the satellites and data derived from a receiver apart from the object but also receiving signals from the satellites.