Definition
According to the
dictionary guidance is the 'process of guiding the path of an object
towards a given point, which in general may be moving'. The process of
guidance is based on the position and velocity if the target relative to
the guided object. The present day ballistic missiles are all guided
using the global positioning system or GPS.GPS uses satellites as
instruments for sending signals to the missile during flight and to
guide it to the target.
SATRACK is a system that was developed
to provide an evaluation methodology for the guidance system of the
ballistic missiles. This was developed as a comprehensive test and
evaluation program to validate the integrated weapons system design for
nuclear powered submarines launched ballistic missiles.this is based on
the tracking signals received at the missile from the GPS satellites.
SATRACK has the ability to receive record, rebroadcast and track the
satellite signals.
SATRACK facility also has the great
advantage that the whole data obtained from the test flights can be used
to obtain a guidance error model. The recorded data along with the
simulation data from the models can produce a comprehensive guidance
error model. This will result in the solution that is the best flight
path for the missile.
The signals for the GPS satellite
navigation are two L-band frequency signals. They can be called L1 and
L2.L1 is at 1575.42 MHz and L2 at 1227.60 MHz.The modulations used for
these GPS signals are
1. Narrow band clear/acquisition code with 2MHz bandwidth.
2. Wide band encrypted P code with 20MHz bandwidth.
L1 is modulated using the narrow band C/A code only. This signal will
give an accuracy of close to a 100m only. L2 is modulated using the P
code. This code gives a higher accuracy close to 10m that is why they
are encrypted. The parameters that a GPS signal carries are latitude,
longitude, altitude and time.
The modulations applied to each
frequency provide the basis for epoch measurements used to determine the
distances to each satellite. Tracking of the dual frequency GPS signals
provides a way to correct measurements from the effect of refraction
through the ionosphere. An alternate frequency L3 at 1381.05MHz was also
used to compensate for the ionospheric effects.
