An observation satellite is used for obtaining information about electromagnetic energy emitted from the earth. The observation satellite orbits the earth in an orbit having an inclination larger than 90 and smaller than 270. Further, the observation satellite comprises at least one receiving antenna, the at least one receiving antenna having a receiving pattern directed towards the earth, and suitable for receiving electromagnetic energy in the radio frequency range as the observation satellite is orbiting relative to the surface of the earth. The observation satellite also comprises a transmitter configured for at least one of: (i) retransmitting, to a relay spacecraft, the received electromagnetic energy, (ii) transmitting, to the relay spacecraft, information representing the received electromagnetic energy, and (iii) transmitting, to the relay spacecraft, information derived from the received electromagnetic energy. The invention also relates to systems and methods therefor.

An illustrative embodiment disclosed herein is a method, by a satellite, including sending a downlink signal to a first endpoint and a second endpoint at a first time and receiving a first uplink signal from the first endpoint at a second time. The second time is based on a first delay value calculated by the first endpoint. The method further includes receiving a second uplink signal from the second endpoint at a third time. The third time is based on a second delay value calculated by the second endpoint. The method further includes calculating the first delay value and the second delay value based on one or more identifiers of the first endpoint and the second endpoint, respectively, and an algorithm and determining a first distance between the satellite and the first endpoint and a second distance between the satellite and the second endpoint.

An illustrative embodiment disclosed herein is a method including estimating, by an endpoint, a first rate of change of a Doppler frequency offset during a downlink reception from a satellite associated with the Doppler frequency offset and applying, by the endpoint, a second rate of change of the Doppler frequency offset to an uplink transmission to the satellite. The second rate of change of the Doppler frequency offset compensates the first rate of change of the Doppler frequency offset.

Systems, methods, and apparatus are provided for automated identification of open space in a wireless communications spectrum, by identifying sources of signal emission in the spectrum by automatically detecting signals, analyzing signals, comparing signal data to historical and reference data, creating corresponding signal profiles, and determining information about the open space based upon the measured and analyzed data in near real-time.

Based on a ratio between Doppler frequencies calculated from a plurality of reception signals outputted by a receiver, a position detector calculates a relative position of a plurality of antennas in a mobile wireless apparatus to a stationary wireless apparatus. A train position detector calculates a position of a train from the calculated relative position, a placement position of the stationary wireless apparatus and placement positions in the train of the plurality of antennas.

Geolocating an emitter of a low probability of detection (LPD) signal being transmitted from the emitter in an environment with a noise floor, where the LPD signal is below the noise floor. At a sensor node, a version of the LPD signal is received from the emitter. For the version of the LPD signal, cyclostationary feature detection or energy detection of the version of the LPD signal is performed. A low probability of detection descriptor word, including at least one of a frequency feature of the version of the LPD signal or an energy feature of the version of the LPD signal is created. The low probability of detection descriptor word is provided to a data processor, where the data processor is configured to use a plurality of low probability of detection descriptor words from different sensor nodes for different versions of the LPD signal to geolocate the emitter.

First information corresponding to a radio signal received at a first sensing device from a candidate location is obtained. Second information corresponding to a radio signal received at a second sensing device from the candidate location is obtained. A first relationship between the first sensing device and the candidate location and a second relationship between the second sensing device and the candidate location are determined. A first inverse and a second inverse of respectively the first and second relationships are obtained. A first estimate of the radio signal at the first sensing device is determined from the first information and the first inverse. A second estimate of the radio signal at the second sensing device is determined from the second information and the second inverse. Energy emitted from the candidate location is measured based on the first estimate and the second estimate.

First information is obtained from a sensing device at a first time. The first information corresponds to a radio signal received at the device from a candidate location. The device is at a first location at the first time. Second information is obtained from the device at a second time. The second information corresponds to a radio signal received at the device from the candidate location. The device is at a second location at the second time. A system determines that a pattern is in each of the first and second information and determines relationships between the candidate location and the device at each first and second location. The system obtains inverses of the relationships and determines estimates of the received radio signals based on the information and inverses. The system measures or estimates energy emitted from the candidate location based on the estimates.

An illustrative embodiment disclosed herein is a method including estimating, by an endpoint, a first rate of change of a Doppler frequency offset during a downlink reception from a satellite associated with the Doppler frequency offset and applying, by the endpoint, a second rate of change of the Doppler frequency offset to an uplink transmission to the satellite. The second rate of change of the Doppler frequency offset compensates the first rate of change of the Doppler frequency offset.

An illustrative embodiment disclosed herein is a satellite including a transceiver, a first antenna, a second antenna, and a switch to enable only the first antenna by electrically coupling the first antenna to the transceiver responsive to a satellite constellation having less than a threshold number of satellites and enable only the second antenna by electrically coupling the second antenna to the transceiver responsive to the satellite constellation having greater than the threshold number of satellites.