Flytec 6040 * - Manual (Page 49)

Operation Manual FLYTEC 6040 GPS
47
11 Additional Information
11.1 Altimeter
An altimeter is really a barograph because it doesn’t directly measure altitude, but air pressure. Altitude
is then calculated from air pressure. The pressure at sea level is used as zero point height for the
calculation of absolute altitude (according to the international height formula).
Why does pressure change with height? Pressure at any given point on the earth is created by the
weight of air in the atmosphere above it. Therefore, pressure reduces with height – there is less air
above your head! A change in pressure of 1 mbar (mb) at 500 metres above sea level is a height
difference of about 8 metres.
In practice, however, it is not as simple as that because of the many other factors that influence air
pressure. Therefore, air pressure is also depending on temperature and of course, on weather
conditions. On a stable day, temperature induced differences of 1mbar mean a height difference of ± 10
metres. Depending on the weather conditions, air pressure at sea level (QNH) may vary from 950 mbar
to 1050 mbar. In order to eliminate this influence of weather, the altimeter has to be calibrated at
certain intervals. This means the altimeter has to be set to a known altitude and
it must then show this value.
During rapid weather changes (e.g. passage of a cold front), the air pressure can change by
5 mbar during one day. This means a height difference of 40 metres!
Another way to calibrate an altimeter is to set it to the actual QNH.
What is the QNH? General air traffic needs a common zero point. This means that at a certain altitude
all aircraft show the same altitude on the altimeter. The common basis for this is the QNH. The QNH is
the actual pressure in hPa (1 hPa=1mbar) calculated back to sea level. It is calculated several times a
day and can be taken from the weather forecast for aviation or it may be requested by radio from
airfields.
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11.2 Navigation
11.2.1 Reception quality of GPS
The GPS-Receiver of the FLYTEC 6040 GPS can follow up to 16 satellites simultaneously. After turning
on the unit it is necessary to receive at least 4 satellites to fix position for the first time. Once logged on,
3 satellites are sufficient (for 2D positioning) to determine the location. However, if one wants
to record altitude too (3D positioning), then definitely four satellites are required. There is a table in the
receiver, The Satellite Almanac, in which the path, place, and time data of all satellites are kept with
reference to the receiver. The Almanac is continuously updated during signal reception. However, if
the signal to the Almanac memory is disrupted completely or the unit is taken 200 km or more from the
last reception point, then the Almanac has to be re-established. Power is still supplied to the almanac’s
memory even when the instrument is turned off.
Normally the instrument recognizes its position under unobstructed view conditions after
a few minutes. If the receiver is switched-off for a short time (less than 2 hrs.), the time for new position
finding is less than a minute. Buildings, mountains or thick forest affect reception quality
of the receiver. Therefore, you should always look for the best possible visibility around you and the
antenna in the casing should point upwards if possible. In particular when mounted on the steering bar
of the hang glider, we recommend not to have the instrument fixed under the pilot’s head on the middle
of the basis, but indeed sideways. In this position the FLYTEC 6030 GPS should not have more than
45° deviation from horizontal position so that the antenna points upwards.
Due to the fact that receiving strength of the satellite signals is only approx. 1/1000 of mobile radios,
these radio sets and other disruptive factors
The FLYTEC 6040 GPS is fitted with a 16-channel GPS-Receiver which is featured with lesser power
consumption and also a significantly shorter satellite detection time. Accuracy is between
7 to 40m. As an average one may assume approx. 15 m.