Viavi 87339, Руководство по эксплуатации

Страница: 166 / 198

B - 5
Ionosphere, Ionospheric
Delay
The Ionosphere is that band of atmosphere extending from about 50 to
1000 kilometres above the earth's surface in which the sun's ultraviolet
radiation ionizes gas molecules which then lose an electron. These free
electrons influence the propagation of microwave signals (speed,
direction and polarization) as they pass through the layer. The
Ionospheric Delay on GPS signals is frequency-dependent and hence
impacts on the L1 and L2 signals by a different amount (unlike that
within the Troposphere). A linear combination of pseudo-range or carrier
phase observations on the L1 and L2 carrier waves can be created to
almost entirely eliminate the Ionospheric Delay. The resulting
observable is known as the Ionosphere-Free carrier phase (or
pseudo-range). For single-frequency receivers it is not possible to
account for this signal bias in this way. A broadcast model is contained
within the transmitted Navigation Message, however, it is a relatively
poor model (unlikely to account for more than 50% of the effect) as the
Delay is very difficult to predict. The magnitude of the Ionospheric Delay
is a function of the latitude of the receiver, the season, the time of day,
and the level of solar activity. The Delay in the Zenith direction can be
several tens of metres, increasing as the elevation angle of the satellite
signal reduces (being 3-5 times greater than in the Zenith direction).
The Delay is largely eliminated in Relative or Differential Positioning,
however, the residual Ionospheric Delay increases as the baseline
length increases and may be a significant source of error (especially in
the height component) for very high precision GPS Geodesy. Even when
using dual-frequency instrumentation, the Ionospheric Delay can still
cause problems during the process of rapid Ambiguity Resolution when
phase and range combinations other than the Ionosphere-Free one are
used.
L1 Frequency 1575.42MHz GPS carrier frequency which contains the C/A-Code, the encrypted
P-Code (or Y-Code) and the Navigation Message. Commercial GPS
navigation receivers can track only the L1 carrier to make pseudo-range
(and sometime carrier phase and Doppler frequency) measurements.
Rice Factor
The ratio between the power of the direct component and the mean
power of the Rayleigh faded component in Rician fading, expressed in
dB. A Rice factor of +∞ dB implies no fading, while -∞ dB implies
Rayleigh fading.
Satellite (or User)
Location (or Trajectory)
The location (or trajectory) of the center of gravity of the satellite (or
user).
Standard Keplerian
Elements (almanac)
Those elements which describe the position of a satellite in a perfectly
elliptical orbit and which comprise semi-major axis, eccentricity,
inclination angle, longitude (or right acention) of the ascending node (or
at epoch), argument of perigee, and one of mean, true, and eccentric
anomaly.
Sub-carrier
One of the separate modulated streams on a given frequency (either I
and Q, or A, B and C).
Term Description