SBX-4 Integrator Guide
Appendix A-Troubleshooting
Page A2
Operating Modes
The SBX-4 may be operated in Automatic Beacon Search (ABS) mode, Manual Tune or Database mode.
In ABS 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 Hemisphere GNSS’ Technical Reference Manual Manual for commands relating to changing the
operating mode and monitoring receiver performance.
Automatic Beacon Search (ABS) Mode
The SBX-4 beacon receiver operates in ABS mode by default, selecting and tuning to the most
appropriate beacon without operator intervention. The SBX-4 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 SBX-4 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 SBX-4
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 make take less than 15 seconds for a strong beacon station, from a cold start or up to a
couple minutes if a beacon site is weak relative to other signals. During the acquisition phase, if bit errors
occur due to a weak DGPS beacon signal or environmental noise occurs, the time to acquire may be
longer.
