Figure 3-9. Gps Diagram - ASTRO APX 7000 Basic Service Manual

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3.2.1.1 GPS
The GPS architecture employs a single chip GPS receiver which decodes GPS signals at
1575.42 MHz. It is capable of producing a final position solution including full tracking and data
decode capability. The GPS receiver will operate in the autonomous mode only.
The GPS signal is tapped at the antenna port via a series resonant network which provides a very
low capacitive load to the transceiver. The signal is routed though a GPS LNA and its output is
applied to the RF-Controller interface connector where it is eventually routed to the expansion board
for processing by the GPS IC.
The GPS receiver is setup in an autonomous one track always (OTA) mode, also known as
continuous navigation. This means the GPS will continuously track satellites for as long as the radio
is powered 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. A power savings algorithm will then cycle the GPS in and
out of a sleep mode at approximately 90 second intervals until the radio has moved back into an
environment where GPS signals are present.
The user will be able to view the current latitude, longitude, and time/date stamp on the radio's
display. The radio can also be configured to send its location to the system at predetermined
intervals (LRRP). Depending on system options, the user may be able to enable/disable the GPS
receiver.
Antenna
3.2.1.2 VHF Front-End
From the first band select switch, a VHF signal is routed to the first pre-selector filter followed by an
LNA and a second pre-selector filter. Both filters are discrete and fixed designs and are used to band
limit the incoming energy and suppress known spurious responses such as Image and the ½ IF spur.
The output of the second pre-selector filter is applied to the RF port of the Mixer IC via a second
VHF/700,800 or VHF/UHF1, or VHF/UHF2 band select switch. The Mixer IC is also excited by a
Local Oscillator (LO) signal at the LO port to down-convert the RF signal to a 109.65 MHz
intermediate frequency (IF). The down converted IF signal is passed through a crystal filter and IF
amplifier which drives the input of the Abacus 3 Analog to Digital Converter IC (AD9864).
Basic Theory of Operation: Analog Mode of Operation
GPS Receiver Circuit

Figure 3-9. GPS Diagram

OMAP
Processor
GPS IC