NorthWest Research Associates

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Coherent Radio Receiving Systems

ITS Receivers are now manufactured and marketed by Northwest Research Engineering, LLC.

For further information please contact Mr. Frank Smith:

For assessing the transionospheric communication channel, NWRA has developed two radio receiving systems capable of measuring the intensity of and relative phase between VHF and UHF signals broadcast from the "Transit" satellites of the U.S. Navy Navigation Satellite System. They are the NWRA ITS10 and the NWRA ITS10S.

The ITS10 and ITS10S employ simple dipole antennas fixed above a ground screen, which deliver between -132 and -109 dBm of power at each of the Transit radio frequencies (RFs), depending upon the class of "Transit" satellite being observed and its elevation angle. The receiver noise figure for both models is 4.2 dB, and they employ analogue phase-locked loops to track the received UHF signal as its Doppler shift changes during a satellite pass (typically over a pass duration of about 15 minutes). An internal frequency synthesizer is used to translate the RF signals to an intermediate frequency (IF) of 50 kHz.

Both receivers sample the IF version of the VHF signal (employing the UHF as a phase reference) at a sample rate of 200,000 sample per sec (sps). An NWRA-proprietary digital signal processor (DSP) converts the IF samples to the desired outputs. The DSP circumvents imbalances and drifts often encountered in (analogue) coherent receivers, rendering the ITS10 and 10S suitable for remote operation.

The ITS10 receiver outputs the in-phase (I) and quadrature (Q) components of the VHF signal at a rate of 10 sps. An on-line personal computer (PC) converts I and Q to VHF intensity and dispersive phase. With cycle crossings accounted for before smoothing, the latter is proportional to the integral of electron density along the radio path, relative to an additive constant. Final outputs, computed either by means of standard software developed by NWRA and provided as part of the ITS10 or in user-specific software, are stored on the PC's disk for retrieval via modem or other means. The NWRA standard outputs consist of dispersive phase at 1 sps and both the relative intensity (power) of the received VHF signal and its square, both averaged over 20 sec. The mean and mean-squared intensity are provided as indicators of scintillation activity (and/or VHF interference or equipment problems); the ITS10 provides no UHF intensity information.

The ITS10S (with the S standing for "scintillation") does measure relative UHF intensity, and its output data rate is substantially higher than that of the ITS10. Specifically, the DSP of the ITS10S passes the UHF I and the VHF I and Q to its online PC at 50 sps each. Again, the PC computes dispersive phase and VHF intensity. It then stores dispersive phase and both VHF and UHF intensity (the latter being equal to UHF I, since UHF Q = 0 by definition of the receiver's phase reference) at 50 sps. Thus, the user has available both dispersive phase and VHF and UHF intensity for assessment of complex-signal scintillation, including spectral analysis thereof. The standard software provides averages and stores the intensities and their squares over 20 sec for ready computation of the intensity scintillation index at both frequencies (either online or after modem, or other, data retrieval).