Guidelines for the use of GNSS in surveying and mapping

Appendix A: GNSS verification, testing and
maintenance
GNSS receivers should be checked and maintained
on a regular basis. The checks should be included
in any quality assurance documentation. Testing
and maintenance can be split into internal company
checks and external manufacturer checks.
A1 GNSS verification (internal company
checks)
See ISO standard 17123: Field procedures for
testing geodetic and surveying instruments – GPS
field measurement systems in real-time kinematic
(RTK).
There is little that the average user can do to check
the internal components of a GNSS system.
However, baselines of a known length can be
surveyed. These should be previously established
test baselines, suitably adjacent to a company’s
offices. Most land survey companies should have
access to a test baseline for EDM instruments.
These will typically be a few points in a straight line
over a total distance of 200m to 700m with differing
internal distances.
Where these EDM baselines are suitable (see
section 5), it is recommended that testing of GNSS
receivers and ancillaries used for dynamic surveys
is carried out. A base receiver may be established
at one end of the test base. Roving receivers
should then be set over each of the other baseline
points. The GNSS-derived distance should then be
compared with the internal distances measured
using EDM. The manufacturer’s tolerances and
precision values for both the EDM and GNSS
equipment should be taken into account during the
comparison. Generally, results should compare to
within the precision stated in Tables D1 to D3 (p.
11), or to within the specific manufacturer’s
precision given for the GNSS system under test.
Land survey and equipment hire companies, who
purchase or own static GNSS systems, are
recommended to establish a GNSS test network.
This should be a simple 5–10km baseline circuit of
three or four interconnected baselines, with
appropriately monumented stable control stations.
This should be established when the receivers are
first purchased. A static survey of several
independent sessions, using different satellite
constellations and different times of day should be
carried out to fix the baselines. At least two
stations in the circuit should have control coordinates
to better than 1m absolute in ITRS89.
The high-precision static survey guidelines should
be followed for GNSS receiver testing, using the
network at intervals of typically six months.
Receivers should be tested after firmware
upgrades, or on return from a manufacturer’s
upgrade or repair. Post-processing software should
be tested on archived data after a software
upgrade or new release. Any differences exceeding
the stated precision of the instruments or system
should be reported to the manufacturer.
Ancillary survey equipment should be tested
regularly – in particular tribrachs, adaptors and
other centring equipment, as well as tapes or
height rods, compasses and clinometers. If weather
observations are made when using GNSS
equipment, instruments such as barometric
pressure recorders, wet/dry thermometers and
humidity meters should be tested.
A2 Equipment testing (external
manufacturer checks)
The manufacturer’s advice regarding service
intervals and firmware upgrades should always be
followed when purchasing GNSS equipment.
Receivers should always be upgraded to the latest
applicable firmware/software versions. Receivers
and antennas should be returned to the
manufacturer when repairs are necessary.
It is good practice to register GNSS equipment with
the manufacturer to ensure that software and
hardware changes and revisions are kept up to
date.
GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING |59