Global Positioning Sytem (Gps) - Motorola APX 5000 Service Manual

Theory of Operation
3-60

: Global Positioning Sytem (GPS)

3.3 Global Positioning Sytem (GPS)
The APX 6000/APX 6000XE/SRX 2200 GPS architecture employs the Texas Instruments NL5500
GPS IC (U2401) which decodes GPS signals at 1575.42 MHz (L1 band). It is capable of producing a
final position solution including full tracking and data decode capability. The GPS signal is received
by the main RF/GPS combination antenna. The GPS signal is then diplexed at the antenna port via a
series resonant network, C1309 and L1305 which provides a very low capacitive load to the
transceiver. The signal is routed through a low noise amplifier (LNA, U1304) on the RF board (and a
SAW filter, FL1301 for UHF1 and UHF2 radios) and its output is applied to the RF-Vocon interface
connectors (P101 to J1001) where it is eventually routed to the expansion board via the J4001 and
P2001 connectors for processing by the GPS IC. Additional GPS diplexing components include
C1150, L1114, C1122, and L1103 which provide proper termination at the Peregrine switch (U1102)
output to minimize GPS signal leakage at the antenna port tap point at C1301. When the GPS signal
reaches the expansion board, it goes through a SAW filter (FL2401), LNA (U2404), and a second
SAW filter (FL2403), which then connects to the NL5500's GPS RF input (U2401 pin L2). The
NL5500 IC is connected to the main OMAP processor via UART2. It is a two wire UART interface
(TX and RX with no handshaking).
NOTE: The enable signal for the LNA U1304 on the RF board is generated by the GPS IC, and DC
coupled onto the GPS RF signal which goes through the RF-Vocon-Expansion Interconnect.
The GPS receiver is setup in an autonomous continuous navigation mode where the current position
is updated once per second. The GPS receiver continuously tracks satellites for as long as the radio
is powered on to ensure the best possible accuracy. In the event the radio loses visibility of the
satellites due to terrain or environmental factors such as driving through a tunnel or entering a
building, the GPS will temporarily lose its position fix. If the signal outage is long enough, a power
savings algorithm will then cycle the GPS in and out of a sleep mode to save battery life until the
radio has moved back into an environment where the GPS signal is present.
The following table lists the power, clock, and I/O connections from the GPS IC to various
peripherals.