Humboldt 5000 BLU Light Weight Deflectometer - Manual (Page 49)

7.5 Menu GPS / TIME
7.5.1 GPS Reception
The device is delivered with the status set to ‘GPS IS ON’.
The GPS reception is automatically activated when turning on the device,
unless it has been switched off previously. After turning on the device,
wait until the GPS reception has logged in. While searching for satellites
a ‘ ? ‘ will be shown on the GPS icon. Once the GPS reception is
established, the display will show an ‘OK’ on the GPS icon during
‘STATUS REQUEST’. This process may take a few minutes.
If a test is started without waiting for GPS reception during ‘STATUS
REQUEST’, the GPS will not log in and all tests will be performed
without the GPS function.
If the GPS reception is turned on, the GPS coordinates of the testing point will automatically appear on
the test protocol. They will also be saved automatically to both the internal memory and the external
media (if in use) for later analysis on a PC and connection to the Google
® Maps interface.
7.5.2 Accuracy of GPS Reception
The GPS satellite positioning system was developed by the U.S. Department of Defense at the beginning
of the 1970s. 24 satellites were placed in space. Due to the orbits of the satellites and the deployed
transmission technology, the accuracy of the determined position is limited.
We would like to briefly explain this further: A GPS receiver calculates its position by analysing the
signal of several GPS satellites. The more satellite signals it receives, the more accurate the determined
position. If the view of the sky is clear, the signals of up to 12 satellites may be received at the same time.
To determine a valid position, the data of at least three satellites is necessary. This works very well in open
field. In this case, the accuracy is below 30 metres.
On a moving vehicle, accuracies of one metre and below can be achieved. Unfortunately, construction
work is not always conducted in open space. In cities or in a built-up areas, various disturbances occur,
shade for example. In this case, one or more satellites are likely to be blocked by high buildings. It is
possible that the signals of only western, northern and southern satellites may be received, while all
eastern satellites are blocked completely. This may be the case when standing very close to a building, for
example.
Reflections are another cause of inaccuracy. The signals sent out by the GPS satellites are radio signals,
which spread in wave form and can be reflected, by large metal surfaces (such as aeroplanes) for
example. Due to these reflections the GPS module may not receive the signals directly, but after being
reflected by another object or surface. The signal propagation delay may change, which may result in a
greater divergence than normal (outlier). In a densely built area with high buildings, this may mean errors
of tens of metres.
A car navigation system tries to offset the effects of these problems by assuming that the car is moving,
and by checking that the position determined by the GPS module makes sense on the current road (a signal
which indicates a sharp turn into a nearby field 100 metres before a junction is most likely an error, for
example).
49
D I 3 0 / 0 6 / 1 3 1 5 : 5 0 : 2 2
OK
OK OK
GPS
6 , 3 V
1 0 0
GPS AVAILABLE
D I 3 0 / 0 6 / 1 3 1 5 : 5 0 : 2 2
?
? ?
GPS
6 , 3 V
1 0 0
GPS NOT AVAILABLE