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).
