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LF/MF range, radio signal reflections are of little consequence. (Periodically conditions may exist which cause some
atmospheric refraction.)
High frequencies (HF or short wave) range from about 3 MHz up to about 30 MHz and propagate by combinations of line of
sight, refraction, and reflection. Propagation characteristics tend to vary from day to night, summer to winter. This is because
the dominant propagation of short waves is dependent upon the earth’s ionosphere, and as the ionosphere’s characteristics
change through day, night, summer, and winter, so do its propagation characteristics. Because of susceptibility to changing
conditions, HF radio navigation and communications reliability can be highly variable. Propagation ranges thus can vary from
line of sight (50 km) during poor conditions or up to thousands of kilometres during optimal conditions. These long distance
propagations are possible because of ionized particles in the ionosphere causing the RF signals to refract or reflect back down
to the earth’s surface. And when the earth’s surface is highly reflective (such as over water bodies) multiple reflections
between the earth and ionosphere occur allowing long distance propagation (multi-hop).
Very high frequencies (VHF) range from about 30 MHz up to about 300 MHz and propagate primarily by line of sight. For
most applications this limits reliable propagations to about 30 - 50 km. However, these distances can be increased by raising
the transmitter and receiver antennas as high as possible above the terrain. Obviously, aircraft may have line of sight
capabilities of 200 km or more. VHF occasionally experiences special atmospheric conditions known as tropospheric ducting
where temperature inversions in the atmospheric layers cause signal refraction due to sudden changes in the air mass dielectric
constants, thus causing the signal to follow the inversion duct. Ducting can extend the propagation distance to hundreds of
kilometres. At VHF frequencies, many natural and man-made objects become fairly good RF signal reflectors.
For ultra high frequencies (UHF) ranging from 300 MHz up to about 3000 MHz (3 GHz) where the signal wavelengths are
submetre, line of sight is the primary mode of propagation. Thus, signal propagation distances on the earth are generally
limited to 50 km. Aircraft can achieve hundreds of kilometres depending on their altitude above the earth’s surface. Natural
terrain as well as man-made objects can have great influence on the signal as it travels along the propagation route. Because
of the small wavelengths in this frequency range, reflections occur off almost any metal structure larger than a few
centimetres in size. This frequency range is the primary segment used for satellite communications and navigation systems
including TRANSIT and GPS.
The GPS RF Signal
The GPS satellite constellation maintains precise orbit patterns at approximately 10,898 nautical miles above the earth’s
surface. Each of the 24 orbiting satellites (exact number may vary) broadcast continuous PRN code as well as precise orbit
data on the GPS L1 channel (1575.42 MHz carrier) and L2 channel (1227.60 MHz carrier). Each satellite transmits these
signals using a right hand circular polarized (RHCP) antenna.
As the GPS signal must travel approximately 10,898 nmi, some refraction and delay of the signal does occur as it travels
through the changing propagation mediums (ionospheric layers and troposphere). Some GPS receivers, such as the NovAtel
GPSCard, model out much of the ionospheric and tropospheric delays. However, the most successful method for cancelling
the effects of atmospheric refraction and delays is to use a dual frequency GPS receiver (L1/L2). These two frequencies have
a special ralationship which, when combined with special down-conversion and mixing techniques, allows them to be
combined in such a way as the effects of atmospheric delay are almost completely cancelled.
Because GPS is a radio ranging and positioning system, it is imperative that ground station signal reception from each satellite
be of direct line of sight. This is critical to the accuracy of the ranging measurements. Obviously anything other than direct
line of sight will skew and bias the range measurements and thus the positioning triangulation (or more correctly,
trilateration).
The Role of Receiving Antennas
The role of a receiving antenna is to intercept electromagnetic waves as they propagate through space (or the earth’s
atmosphere). In order for an antenna to efficiently intercept a desired frequency or band of frequencies, it must be “tuned” to
the frequency band where optimal reception is desired. Tuning helps reject out-of-band signals as well. Once intercepted, the
electromagnetic wave induces alternating currents in the receiving antenna that accurately duplicate the RF signal as
transmitted from the transmitting antenna. Once intercepted, the signal can be further amplified, filtered, downconverted, and
demodulated or decorrelated by the receiver circuitry.
Radio waves exhibit polarity characteristics based on the transmitting antenna design. For example, vertical (or whip)
antennae are considered as vertically polarized, whereas dipole and other horizontally oriented antennae are considered
horizontally polarized. Space radio systems such as GPS tend to use antennae that are circular polarized. For optimum signal
transfer, radio links require that the receiving antennae polarization match that of the transmitting antenna. If cross
polarization occurs between the two systems, the receiving system may lose anywhere from 10 to 30 dB of the available