Tektronix NetTek YBT250, User Manual

Page: 96 / 338

GPS
2- 56
YBT250 Field Transmitter & Interference Tester User Manual
being taken may not be accurate and will become invalid if the
GPS timing or position information does not improve.
H
Red: The GPS TDOP/HDOP timing or position information is
poor.
When the useable satellites are widely positioned in the sky (larger
vertical angles between the receiver and each satellite), the DOP
values are close to 1 and timing and position information is more
accurate. When the useable satellites are more closely positioned
together, the DOP values are higher and accuracy is degraded
because of the small angles between the satellites. In general, the
errors due to other sources are multiplied by the DOP value.
The four common DOP values are PDOP, HDOP, VDOP, and TDOP
(3--D position, horizontal position, vertical position, and time).
Timing is the major function of the Tektronix YBGPS1, so TDOP is
the important DOP value for this product.
The TDOP (shown when using the Tektronix YBGPS1 GPS Timing
Reference) indicates the quality of the timing reference signal. A
lower TDOP value means more accurate timing information.
HDOP (shown when using GPS receivers other than the Tektronix
YBGPS1 GPS Timing Reference) indicates the quality of the
receiver latitude and longitude position data. A lower HDOP value
means more accurate position information.
For reasonable timing accuracy, there must be at least 4 useable
satellites, the TDOP value should be less than 4, the signal strength
of the satellites used in the solution should be above 43 (SNR), and
you should wait 30 seconds after locking onto the fourth satellite to
allow the satellite time to transmit its precise position information.
For more information, search the Internet using the keywords GPS,
Theory, Satellite, Geometry, and DOP.
GPS Information Field
The GPS Information field shows the GPS receiver geophysical
location (latitude and longitude), altitude, time, and date (UTC --
Coordinated Universal Time). Altitude information is labeled MSL
(Mean Sea Level) or HAE (Height Above Ellipsoid), depending on
the GPS receiver. See Figure 2--20.