HyperTrack™
Software Instruction
110
Quasonix, Inc.
same concept to the gain upstream of the receiver. This dramatically improves the performance of the HyperTrack™
Ground Station in environments with nearby interfering signals.
For many years, the telemetry frequency bands were largely free of interfering signals. This led to a design
philosophy of using very high gain amplifiers early in the RF path, most often in the feed itself. With high gain early
in the RF path, the system noise temperature was very low, and the system sensitivity was very high. If there were
no interfering signals, the system performance was good.
Today, however, interfering signals are ubiquitous and operate at extremely high power. Filters can decrease the
level of these interferers, but high gain amplifiers in the antenna front end can still be driven into compression.
Amplifiers in compression will create intermodulation products that no filtering can ever remove. For this reason,
the HTAC employs Intelligent Gain Control (IGC) in the SCM feed.
The SCM feed has three gain settings (high, medium, and low). The IGC ensures that the feed uses no more gain
than needed to achieve error-free performance. This works as follows:
At installation, before each mission, or whenever
commanded by the operator, the antenna is first pointed to “cold
sky” to determine the absolute lowest noise level achievable. The noise level is recorded for all three gain settings.
Then, the antenna is rotated through 360° of azimuth at zero degrees elevation (where interference is always worst).
Using the Ground Station Analyzer (refer to section 8.4.7) as a simulated signal source during this sweep, the HTAC
rec
ords the “noise floor” (including interference) at the frequency (or frequencies) of interest, and in the full 360° of
azimuth. Some azimuths are likely to be marked as “interference
-free.
” The use of the GSA as a signal source
ensures that any intermodulation products caused by the mixing of the interferers with the desired signal will be
detected.
This azimuth sweep is automatically repeated for each of the three gain settings, and then the three sets of noise
floor results are stored in the HTAC. This baseline data (measured and stored in half-beamwidth increments) allows
the HTAC to compute the true signal-to-noise ratio (SNR) at that frequency (or frequencies) while tracking the test
article.
While in track mode, the HTAC knows its azimuth and the current feed gain setting (and therefore the noise floor).
It also knows the received RF signal level (embedded in the HyperTrack™ control stream from the receiver). The
HTAC then continuously calculates the SNR. If the current DQM (also embedded in the HyperTr
ack™ control
stream) is high and the computed SNR is above a pre-defined threshold (typically about 30 dB), then the feed is
using more gain than is necessary to yield error-free telemetry data and the HTAC will lower the gain in the feed.
This dramatically decreases the risk of compression and intermodulation in the feed.
The IGC is not fooled by the high-level interference because, although the interferers can create very high SNR,
they will also create a very low DQM. Using the pre-mission survey of noise floor together with the real-time signal
strength and DQM, the IGC can adjust the front-end gain to maintain error-free performance while reducing
susceptibility to interference.
IGC cannot entirely eliminate telemetry dropouts due to interfering signal, but it does significantly reduce their
likelihood. Only a HyperTrack™ Ground Station offers this feature.
8.4.3
Incidental AM Rejection
Because every target vehicle has an irregular antenna pattern, target motion imposes amplitude modulation on the
RF signal, and the frequency range of this AM generally increases as the target velocity increases. In certain
instances (spinning missiles, for example), the frequency of the AM induced by the target can overlap with the AM
induced by the antenna scanning action. Conventional ACUs can be severely degraded by this incidental AM.
Through the HyperTrack™ protocol, however, the receiver and the HT
AC share common knowledge of the
scanning signal, which allows the HTAC to perform the equivalent of matched filter detection of the AM signal.
This vastly improves the rejection of incidental AM from target motion and improves tracking performance on
targets that induce significant AM on the RF signal.
