Users manual
N4430-O-G
Revision #:
B
Date:
8-23-18
Page
14
of
74
To measure the range from the satellite to the receiver, two criteria are required:
signal transmission time and signal reception time. All GNSS satellites have several
atomic clocks that keep precise time and are used to time-tag the message (i.e.
code the transmission time onto the signal) and to control the transmission
sequence of the coded signal. The receiver has an internal clock to precisely
identify the arrival time of the signal. Transit speed of the signal is a known constant
(the speed of light), therefore: time x speed of light = distance.
Once the receiver calculates the range to a satellite, it knows that it lies somewhere
on an imaginary sphere whose radius is equal to this range. If a second satellite is
then found, a second sphere can again be calculated from this range information.
The receiver will now know that it lies somewhere on the circle of points produced
where these two spheres intersect.
When a third satellite is detected and a range determined, a third sphere intersects
the area formed by the other two. This intersection occurs at just two points. A
fourth satellite is then used to synchronize the receiver clock to the satellite clocks.
In practice, just four satellite measurements are sufficient for the receiver to
determine a position, as one of the two points will be totally unreasonable (possibly
many kilometers out into space). This assumes the satellite and receiver timing to
be identical. In reality, when the receiver compares the incoming signal with its own
internal copy of the code and clock, the two will no longer be synchronized. Timing
error in the satellite clocks, the receiver, and other anomalies, mean that the
measurement of the signal transit time is in error. This, effectively, is a constant for
all satellites since each measurement is made simultaneously on parallel tracking
channels. Because of this, the resulting
ranges calculated are known as “pseudo-
ranges”.
To overcome these errors, the receiver then matches or “skews” its own code to
become synchronous with the satellite signal. This is repeated for all satellites in
turn, thus measuring the relative transit times of individual signals. By accurately
knowing all satellite positions, and measuring the signal transit times, the user’s
position can be accurately determined.
This LED illuminates green when the unit is locked to the GNSS system. If the LED
is flashing green, the unit is operating on the OCXO and is attempting to gain GNSS
Lock.
.
The GNSS lock status is also available from monitoring serial NMEA/status on the
power/serial 10-pin connector.
If the GNSS indicator remains flashing green for an extended period of time, it could
be an indication of an antenna, cabling or unit malfunction. Confirm the antenna is
still connected and has not become obstructed from a clear view of the sky. To
