Product specifications

21
S IGNALS
This section provides more detail on the signals available via the 34-pin header
connector.
RF I NPUT
The Crescent Vector module is designed to work with active GPS antennas
with an LNA gain of 10 to 40 dB. The purpose of this LNA gain above the
minimum requirement of 10 dB is to accommodate for losses in the cable
system. Essentially, there is a maximum cable loss budget of 30 dB for a 40
dB gain antenna. Depending on the antenna you choose, your loss budget
will likely be lower (a 24 dB gain antenna would have a 14 dB loss budget).
When designing your internal and external cable assemblies, and choosing
your RF connectors, please be sure not to exceed your loss budget, otherwise
the tracking performance of the Crescent Vector module will be compromised.
C OMMUNICATION P ORTS
The Crescent Vector module has four communications ports designated Port
A, Port B, Port C and Port D. The three main ports are Port A, B and C. The
fourth port, Port D is used exclusively for interface to the SBX beacon module
or a source of external corrections. This port will not output normal GPS-
related NMEA messages.
Communicating into either Port A, B or C, you may establish a virtual
connection to the device on Port D using the $JCONN command. Further, you
may connect the device on Port A directly to the device on Port B using a
derivative of this same command.
As the Crescent Vector serial ports are a 3.3 V CMOS level, you may have to
translate this level in order to interface to other devices based upon your
product requirements. For example, if you route the Port A / B / C serial
ports directly to the outside world, you will likely want to translate to an RS-
232-compatible level for communication with PC computers. However,
translation of the signal levels is entirely at your discretion based upon your
product requirements.
C OMMUNICATION P ORT D
The exclusive function of Port D is for external correction input to the
Crescent Vector. The source of corrections may depend on the geographical
use of your final product, market, customer, and positioning performance
requirements. Appendix C provides a variety of information on SBAS and as
you will see, there is finite coverage of these services. If you wish to market
products outside of SBAS coverage, you may want to allow your product to be
used with external correction input, or integrate a second source of
corrections along with the Crescent Vector, such as the Hemisphere GPS SBX
beacon module.