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4.2 GPS / Satellite Tracking Status
Display screens fig 5: [b,c,d] all show a three-character status field at the bottom right-hand side
of the display. (The examples shows values 8 7 A.) The three-character field provides visual
feedback on the most important parameters affecting the operation of the internal GPS receiver.
The meaning of each character is as follows:
1
st
character: (Hexadecimal
1
number: range of values is 0-D, or decimal value range 0-13)
This shows the total number of satellites that are possibly able to be “seen” at this physical
location, based on the internal GPS receiver’s knowledge of the orbit parameters of each GPS
satellite (the GPS “Almanac”). A value of zero means that NTS01’s GPS receiver has no stored
almanac data. This condition can occur if the unit has been in storage for an extended period, or
as a result of the embedded GPS receiver being reset. Under these circumstances, the NTS01
starts searching the sky randomly to find satellites, and the time required to obtain
synchronisation can be much longer than usual – up to several hours in the worst case. As soon
as a satellite is detected, the NTS01 begins updating its almanac record from data down-loaded
from the satellite. This task typically takes about 20 minutes, after which the status digit changes
to a non-zero value to show that the stored almanac is now valid. The display example in fig5 b
indicates that 8 satellites are present in the hemisphere of sky overhead.
2
nd
character: (Hexadecimal
1
number: range of values is 0-C, or decimal range 0-12)
This shows the number of satellites currently being used to compute the time solution. “0”
indicates that no updated time solution is available – the “out of lock” condition. If this condition
persists for the “Sync Hold” time, (See section 5.3 p15) the clock will go to “Warning State 2”
described below. The display examples in fig 5 show that the signals from 7 satellites are
currently in use to form the time solution.
3
rd
character:
Possible values are:
A NTS01 in Acquisition mode - attempting to get satellite fixes
G Poor satellite geometry – can occur when multiple satellites are tracked, but are positioned
in almost a straight line, degrading the ability of the unit to calculate accurate time. If the
unit has achieved a 3-dimensional (see “3” below) fix at some point in the past,
synchronisation will be maintained under this condition
2 A 2-dimensional position solution is in use. (No height calculation). If the unit has achieved
a 3-dimensional (see “3” below) fix at some point in the past, synchronisation will be
maintained under this condition.
3 A 3-dimensional position solution is in use, which includes a height calculation. 4 or more
satellites are being tracked. This condition is
required before the NTS01 first enters a
synchronised state after switch on.
*
The NTS01 unit is configured to operate in Slave mode, where a precision time signal is
expected on the IRIG-B input P4. (RS422 signal IRIG-B with IEEE1344 extensions).
1
A Number system with 16 characters instead of 10, with “A”-“F” having decimal values 10-15.
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