SXBlue II GPS Series Technical Reference Manual
47
3.4 Differential Correction Operations
This section provides basic configuration steps for various modes of differential corrections.
3.4.1 DGPS RadioBeacon Operation (SXBlue II-B GPS)
The SXBlue II-B GPS may be operated in Automatic mode, Manual Tune or Database mode. In
Automatic mode
, the receiver will identify and tune to the station providing the strongest DGPS
signal using two receiver channels. In
Manual mode
, you specify the frequency to which the
receiver will tune. In Manual mode, only the primary receiver channel is used. In
Database
mode
, the receiver will search for the closest station based on its current location and distance
to the internal list of station locations.
Refer to Section 4 for commands relating to changing the operating mode and monitoring
receiver performance.
3.4.1.1 Automatic Mode
•
Automatic Beacon Search (ABS) Mode
The SXBlue II-B GPS operates in ABS mode by default, selecting and tuning to the most
appropriate beacon without operator intervention. The SXBlue II-B GPS uses its two
independent channels to identify and lock to the best DGPS beacon.
ABS mode is ideal for navigation applications over considerable areas with various
beacon stations available, eliminating the need for operator intervention when traveling
from one beacon coverage zone to another.
•
ABS Global Search
When powered for the first time in ABS mode, the SXBlue II-B GPS initiates a Global
Search (GS), examining each available DGPS beacon frequency, and recording Signal
Strength (SS) measurements in units of decibel micro volts to the Global Search Table.
The receiver uses these measured values to compute an average SS, noise floor and to
sort the frequencies in descending order of SS.
This initial phase of the GS takes approximately 3 to 4 seconds to scan all 84 beacon
channels in the frequency band using both channels. Once scanned, and with SS
numbers identified for each beacon channel, both receiver channels cooperatively
examine the frequencies with the highest SS measurements, above the computed noise
floor, to determine the station providing the strongest RTCM signal. The receiver's
primary channel locks to the first identified DGPS broadcast, while the second channel
continues searching in the background for superior beacon signals. If no signal is
available, the SXBlue II-B GPS will initiate a fresh GS, continuing this cycle until it finds a
valid beacon.
The secondary acquisition phase of the GS composes the remainder of the time
required to acquire the beacon signal. The time required to acquire a beacon in ABS
mode is dependent upon the signal quality of DGPS beacons in your area, and their
relative strength to other non-DGPS beacon signals in the 283.5 to 325.0 kHz band. As
this frequency range is a navigation band, it is shared with other navigation aides such
as non-directional beacons used for aviation and marine navigation. Depending on the
signals available, initial acquisition may take less than 15 seconds for a strong beacon
