Page 208 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes Meteor Trails

Meteors enter the earth’s atmosphere at speeds between 10 and 80 Kilometres per second and
burn up due to friction at altitudes between 80 and 120km, dependent on their size, speed and
angle of entry. As the meteors burn up their atoms vaporise leaving a trial of positive and negative
electrons. The trial formed is typically only a few metres wide but can be over 20km long.

Meteor trial can be divided in to 2 types depending on the density of the electrons. These are
known as “over dense” and “under dense”. Over dense trials produce relatively strong signal
reflections. With a high electron density signals do not totally enter these trails and are reflected.
The duration can be several seconds and are the result of larger sized meteors.
Under dense trails are formed by very small meteors, typically the size of a grain of sand. With a
lower electron density these act in different way to over dense trails. A signal penetrates the trail
and is scattered rather than being reflected. Only a small amount of signal is returned to earth,
the signal typically rising in strength for a few hundred milliseconds and then decays but can
last for a few seconds.

!
Caution Signals are subject to Doppler shift but are less affected at lower vhf frequencies, 4
and 6M bit can be a s much as 2kHz on high frequencies.
12.2.8 Future Developments

The functionality of the MBC network will make it suitable for a large number of other applications
where tracking and/or messaging on an occasional basis is required.

Example: are the tracking of cargo containers or complete trailers, alarms in remote and
stationary objects, and control of (environmental) monitoring equipment in remote places.
The advantage of a fixed application is the use of a directive antenna increasing the quality of the
communication link and thus reducing the waiting time. Due to its low cost and high reliability,
the system is ideal for communication from remote areas where other communication systems
are not available or expensive.

The evolving proliferation of less expensive and improved microprocessor technology applied to
adaptive signal processing techniques, electronic system controllers, cryptographic devices and
user terminal microprocessors offers great promise to improve significantly the performance of
MBC systems. A careful assessment of the advantages offered by technologically advanced MBC
systems likely will confirm that such systems can improve the diversity of information transport
services available to our national security operations of the 21st Century.

Self-Assessment

Fill in the blanks:
9. Meteor burst communications is a specialized form of propagation that can be successfully
used for ………………....… over paths that extend up to 1500 or 2000 km.
10. Frequencies above 30 MHz but below 50 MHz in the lower very high frequency (VHF)
spectrum propagate most efficiently using ………………....…
11. Meteor Burst communications has been a viable communications medium since the
………………....…
12. ………………....… is a two-way messaging system for the transport sector, providing
communication between mobiles stations and a central office for vehicle dispatch and
tracking in support of fleet management systems.

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