6-2
Other information that is displayed on this Screen includes:
DOP (Dilution of Precision), This is a number between 00.0 and 99.0 that represents the dilution of
quality of the calculated fix due to satellite geometry. 00.0 is best, 99.9 is worst. If this figure is
greater than 5.0, performance of the system is likely to be degraded because some of the visible
satellites appear too close to each other. DOP is calculated from the angular separation between the
various visible satellites. Greater separation results in better fix geometry and a lower DOP. (This
DOP figure is not measured in any specific units.)
UTC/DATE, which is Universal or Greenwich Mean Time and Date.
LOCAL OFFSET, which is the difference between UTC and local time.
LOCAL TIME, which is calculated by adding Local Offset to UTC.
LAST FIX. If the unit does not have a current valid fix, this is the position at which it last had a fix. If
the unit has a fix, this is the present position.
SATS (eight satellite numbers), SIGS (signal strength for each satellite) and STAT (status of each
satellite).
Each satellite has a PRN (Pseudo Random Noise) or identification number. The satellite PRN
numbers are displayed in a line beside the word SATS. The two digit number under each PRN
number is an indication of the signal strength being received from that satellite expressed in terms of a
percentage. Best is 99, worst is 00. These numbers can be used for finding and eliminating electrical
interference. Readings of 60 and above indicate a good installation.
Under each signal strength number is a two character code. These indicate the status of each
satellite. These codes are: CS, CA, AS, FA, BD, MD, TA, EA, and AP. These relate to the eight
possible receiver modes and are detailed below.
CS : Code Search. This is the receiver's initial stage in acquiring a satellite. It means that the receiver
is trying to match its internal code to the satellite signal.
CA: Code Acquire. This means the satellite code has been received and matched to the receiver-
generated code.
AS: AGC Set. This means the satellite strength has been assessed and the Automatic Gain Control
has been set.
FA: Frequency Acquire. This means the receiver has correctly locked onto the satellite data
frequency.
BD: Bit Sync Detect. This means the receiver is synchronised with the satellite's data bit stream.
MD: Message Detect. This means the receiver is synchronised with the satellite's message stream.
TA: Time Available. This means the satellite is fully locked in and has sent down UTC time and date
information to the receiver.
EA: Ephemeris Acquire. This means the receiver is reading the constellation health status message
from the satellite. This usually takes around two minutes and is a function that is performed in the
background even if the receiver has a fix.
AP: Available for Position. This means the satellite is fully locked in and tested and can be used for
calculating position. A minimum of three satellites are necessary for a 2D fix.
Key 1 returns to Main Menu. Date, Time, Local Time Offset and Present Position can be adjusted by
selecting one of Keys 2, 3 or 4. Direct access to the MAP Mode is available by pressing Key 5.
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