High-fidelity model development for navigation warfare simulation studies (2002)

A software project is under development that models
propagation of electromagnetic (EM) signals over terrain
from Global Positioning System (GPS) jamming
equipment and the capability and accuracy of simulated
GPS and Inertial Navigation System (INS) equipment to
form a navigation solution under the modelled conditions.
The purpose of the software tool is to predict, in a static or
dynamic environment, the likely navigation accuracy over
a period of simulated time that would be possible given
different combinations of navigation equipment and
jammers.
The EM propagation model performs diffraction
calculations using modelled terrain profiles to determine
the path loss that occurs when GPS jamming radiation
propagates from a transmission point to a reception point.
The propagation model takes into account radiation
frequency, bandwidth, polarisation and waveform.
Environmental factors such as refractivity profiles and
clutter are included in the diffraction calculations to
provide an accurate estimate of path loss.
Obscuration of EM signals by the host platform body is
modelled to a fidelity controlled by the user. Obscuration
modelling applies to both GPS signal reception (if
platform mounted) and to jamming signal transmission (if
the jammer is platform mounted). Platform mounted
receivers and jammers can undertake simulated, pre-
determined flight trajectories where platform attitude,
signal obscuration and antenna gain orientation are taken
into account at discrete points in simulated time to
calculate interference levels at the GPS receiver.
Antenna gain patterns can be associated with GPS
jammers and with the GPS reception system. The GPS
reception system can be modelled as a simple Fixed
Reception Pattern Antenna (FRPA) or as a Controlled
Reception Pattern Antenna (CRPA) array with realistic
geometry (eg array elements flush to curved surfaces) and
with an associated modelled antenna control unit. For
CRPA array modelling, a key aspect in predicting
navigation performance under jamming is modelling the
practical limitations on cancellation performance. The
CRPA model employed is based on the digital
architectures that will be used in the future and takes into
account the limitations of the antenna array, the radio
frequency (RF) conversion electronics, the analogue-to-
digital (A/D) converters and the digital signal processing.
The GPS constellation and propagation model takes a base
set of orbital parameters for any number of satellites
(ingested directly from System Effectiveness Model
(SEM) or Yuma almanacs if desired) and performs
calculations to predict satellite positions at any required
point in time. Ionospheric propagation, tropospheric
propagation, ephemeris and residual satellite clock errors
are all modelled.
The GPS receiver model comprises code and carrier
correlator-discriminator models, code and carrier tracking
control functions for up to 12 satellite signals and a
receiver clock error model. The correlator-discriminator
models account for the effects of interference and receiver
noise. The tracking control function includes the
capability to adapt the tracking loop bandwidths to the
interference level. By comparing the pseudo-ranges and
pseudo-range rates output by the receiver model with their
truth counterparts, it is determined whether the receiver is
correctly tracking code and carrier.
An INS model comprising a low order navigation
equations function and accelerometer and gyro error
models has also been implemented. This is used to aid
code tracking when carrier tracking is not available and to
provide a guide to navigation accuracy when GPS
tracking is lost.