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Chapter 8: GPS and a Real- T ime Situational Displa y 199

Features/Specifications Description
Sensitivity Acquisition: −148 dBm
Tracking: −159 dBm
(These are very sensitive levels.)
Chipset SiRFstar III
Channels 20, simultaneous tracking
Data protocol NMEA 0183 v2.2 GGA, GSV, GSA, RMC (optional VTG, GLL)
Power Typical 65 mA @12 V (Chip uses 3.3 V to 5 V)
Antenna Internal patch with provision for external connection
Storage Rechargeable battery stores real-time clock (RTC) data and
receiver configuration settings
Connections Premade cable with connector for power and data interconnections
LED functions Power on/off and navigation
Start time 30 seconds

Table 8.1 PMB-688 Features and Specifications

The −159 dBm tracking sensitivity means that the signal, once acquired, can lose up to 90%
of its original strength, yet remain locked in by the receiver.
Having an NMEA 0183 output operating at 9600 baud means that the receiver generates
standard GPS messages at a rate twice as fast as comparable receivers. The 30-second start-
up time is excellent and due in part to the receiver’s extreme sensitivity.

GPS Receiver UART Communications
Universal Asynchronous Receiver Transmitter (UART) is the serial data protocol used between
the GPS receiver and the Propeller Mini processor module (which is discussed in a later
section). Three data pins are the minimal amount necessary to establish a communications
link between the receiver and the processor. They are identified on the GPS as TTLTX
(transmit), TTLRX (receive), and GND (ground or common), as shown in Figure 8.8.

Figure 8.8 UART pins.