14.1 Pseudo-Ranging Position Finding Method of an Object and Design …
247
The correlation receiver allows to obtain the best value of the signal-to-noise ratio
at the output and thereby ensure reliable detection of the NSV signal.
The use of optimal methods of NSV signals processing permits to use antenna of
a small size, and in general, the user equipment has a relatively small size, weight
and relatively low cost.
One of the most important reasons for using a pseudo-random code in an SRNS—
is the using possibility by all satellites the same carrier frequency in their transmitters.
But since each NSV transmits a code inherent only to it, the receiver can easily
distinguish the signals of a particular satellite. In addition, the use of a pseudo-random
code in the SRNS allows to control the mode of access to it.
14.2 Factors Affecting the Accuracy of Satellite Radio
Navigation Systems
The user’s spatial location in the SRNS is determined relatively to the NSV. It is
clear that the more precisely the position of the satellite in orbits is known, the more
accurately the coordinates of the object will be determined. The current coordinates
of the satellite are calculated by the onboard receivers using known orbital elements
(ephemeris). Orbital elements of the NSV are calculated on the ground and periodically transmitted to the NSV. In addition to the orbital elements themselves, their
derivatives (rates of change caused by disturbing factors) are also transmitted. All
these data as part of the navigation message are sent to the onboard receiver, which
calculates the current coordinates of the satellite. The errors in determining the AV
coordinates increase over time, because the calculated data are outdated. The error
in calculating the coordinates of the NSV is the greater the more time has passed
since the time, at which the orbit parameters were determined. At the same time,
these parameters are updated quite often, so a significant error, as a rule, does not
have time to accumulate.
The error in determining the ephemeris (coordinates) of the NSV is closely
related to the error in time measuring. The SRNS operates in its own system time,
determined by a high-precision ground central synchronizer (instability 10
–13 –10
–15 ).
However, this system time deviates from UTC and the value of the deviation is
periodically transmitted to each satellite. Each NSV uses its own frequency standard
(i.e., clock), its own time scale with the worst stability. Therefore, in fact, each
satellite uses its own time. It is periodically corrected from the Earth by entering
corrections, but does not completely coincide with the system time, for which the
orbital elements are calculated. It turns out that the onboard receiver determines the
coordinates of the satellite not quite for the moment in time for which it is required,
which is equivalent to an additional error. Along the way, we note that the error of the
onboard clock of the receiver (on AV) does not make any contribution to the error,
since it is continuously considered in the process of measuring the pseudo-range
(see Formula 14.1). As a result of the inaccuracy of ephemeris information and time,
247
The correlation receiver allows to obtain the best value of the signal-to-noise ratio
at the output and thereby ensure reliable detection of the NSV signal.
The use of optimal methods of NSV signals processing permits to use antenna of
a small size, and in general, the user equipment has a relatively small size, weight
and relatively low cost.
One of the most important reasons for using a pseudo-random code in an SRNS—
is the using possibility by all satellites the same carrier frequency in their transmitters.
But since each NSV transmits a code inherent only to it, the receiver can easily
distinguish the signals of a particular satellite. In addition, the use of a pseudo-random
code in the SRNS allows to control the mode of access to it.
14.2 Factors Affecting the Accuracy of Satellite Radio
Navigation Systems
The user’s spatial location in the SRNS is determined relatively to the NSV. It is
clear that the more precisely the position of the satellite in orbits is known, the more
accurately the coordinates of the object will be determined. The current coordinates
of the satellite are calculated by the onboard receivers using known orbital elements
(ephemeris). Orbital elements of the NSV are calculated on the ground and periodically transmitted to the NSV. In addition to the orbital elements themselves, their
derivatives (rates of change caused by disturbing factors) are also transmitted. All
these data as part of the navigation message are sent to the onboard receiver, which
calculates the current coordinates of the satellite. The errors in determining the AV
coordinates increase over time, because the calculated data are outdated. The error
in calculating the coordinates of the NSV is the greater the more time has passed
since the time, at which the orbit parameters were determined. At the same time,
these parameters are updated quite often, so a significant error, as a rule, does not
have time to accumulate.
The error in determining the ephemeris (coordinates) of the NSV is closely
related to the error in time measuring. The SRNS operates in its own system time,
determined by a high-precision ground central synchronizer (instability 10
–13 –10
–15 ).
However, this system time deviates from UTC and the value of the deviation is
periodically transmitted to each satellite. Each NSV uses its own frequency standard
(i.e., clock), its own time scale with the worst stability. Therefore, in fact, each
satellite uses its own time. It is periodically corrected from the Earth by entering
corrections, but does not completely coincide with the system time, for which the
orbital elements are calculated. It turns out that the onboard receiver determines the
coordinates of the satellite not quite for the moment in time for which it is required,
which is equivalent to an additional error. Along the way, we note that the error of the
onboard clock of the receiver (on AV) does not make any contribution to the error,
since it is continuously considered in the process of measuring the pseudo-range
(see Formula 14.1). As a result of the inaccuracy of ephemeris information and time,