Geoscience Reference
In-Depth Information
3. When the data on the receiver's panel display is normal, observers can begin the
self-test and input control information for observation stations and intervals of
observation time.
4. When the receiver begins to record data, the observer should use function keys
and selection menus to check information on observation stations, number of
received satellites, satellite catalog number, channel SNR (signal-to-noise ratio),
phase measurement residuals, results and changes in real-time positioning,
records of storage media, etc.
5. During the period of observation, the receiver should not be turned off and
restarted. The antenna height and limits of the elevation angle of satellites should
not be changed. Observers are supposed to prevent vibration and especially
displacement of the receiving devices. The antenna or signals should not be
touched or obstructed.
6. When all the operation projects are confirmed to have been completed as
required, the station can be moved.
7. In long-distance GPS measurements at higher levels, meteorological elements
should be measured as required.
Observational Records
Observational records are automatically formed by GPS receivers onto storage
media, which include carrier phase observations, pseudo-range observations,
corresponding GPS time, parameters of GPS satellite ephemeris, clock offset
parameters, and initial information of observation stations such as name, catalogue
number, time intervals, approximate coordinates, antenna height, and so on. The
information on observation stations is generally first input by observers into the
receivers or recorded manually in measurement handbooks.
3.4 The Gravity Datum and Gravity Control Networks
To obtain the value of gravity quickly, we usually adopt relative gravity measure-
ment. As a consequence, there must be some reference points with known gravity
values that belong to a unified system. The point with known absolute gravity value
is referred to as the gravity base station (gravity reference station), and the gravity
value at the base station is known as the gravity reference value. The gravity datum
is derived from a network formed by a series of gravity base stations. Just like the
three-dimensional coordinate control network and horizontal and vertical control
networks, the establishment of the gravity control network is also a basic project of
constructing geodetic datums. Establishing high-precision gravity networks plays a
prominent role in determining and refining the gravity field of the Earth and the
geoid. This section is dedicated to the introduction of gravity datum, gravity basic
networks, and establishment of gravity networks in China.
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