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Time To First Fix (TTFF)
TTFF is often broken down into three parts:
Cold: A cold start is when the receiver has no accurate knowledge of its
position or time. This happens when the receiver’s internal Real Time Clock
(RTC) has not been running or it has no valid ephemeris or almanac data.
In a cold start, the receiver takes 35 to 40 seconds to acquire its position.
Warm or Normal: A typical warm start is when the receiver has valid
almanac and time data and has not significantly moved since its last valid
position calculation. This happens when the receiver has been shut down
for more than 2 hours, but still has its last position, time, and almanac
saved in memory, and its RTC has been running. The receiver can predict
the location of the current visible satellites and its location; however, it
needs to wait for an ephemeris broadcast (every 30 seconds) before it can
accurately calculate its position.
Hot or Standby: A hot start is when the receiver has valid ephemeris, time,
and almanac data. This happens when the receiver has been shut down
for less than 2 hours and has the necessary data stored in memory with
the RTC running. In a hot start, the receiver takes 1 second to acquire its
position. The time to calculate a fix in this state is sometimes referred to as
Time to Subsequent Fix or TTSF.
Module Description
The R4 Series GPS Receiver module is based on the SiRFstarIV chipset,
which consumes less power than competitive products while providing
exceptional performance even in dense foliage and urban canyons. No
external RF components are needed other than an antenna. The simple
serial interface and industry standard NMEA protocol make integration of
the R4 Series receiver into an end product extremely straightforward.
The module’s high-performance RF architecture allows it to receive GPS
signals that are as low as –160dBm. The R4 Series can track up to 48
satellites at the same time. Once locked onto the visible satellites, the
receiver calculates the range to the satellites and determines its position
and the precise time. It then outputs the data through a standard serial port
using several standard NMEA protocol formats.
The GPS core handles all of the necessary initialization, tracking, and
calculations autonomously, so no programming is required. The RF section
is optimized for low level signals, and requires no production tuning.
Backup Battery
The module is designed to work with a backup battery that keeps the
SRAM memory and the RTC powered when the RF section and the main
GPS core are powered down. This enables the module to have a faster
Time To First Fix (TTFF) when it is powered back on. The memory and
clock pull about 660µA. This means that a small lithium battery is sufficient
to power these sections. This significantly reduces the power consumption
and extends the main battery life while allowing for fast position fixes when
the module is powered back on.
The backup battery must be installed for CGEE start. If the serial command
is used to place the receiver into hibernate while keeping VCC powered,
then the battery backup current is 15µA while the current through the VCC
line is about 170µA.
Power Supply Requirements
The module requires a clean, well-regulated power source. While it is
preferable to power the unit from a battery, it can operate from a power
supply as long as noise is less than 20mV. Power supply noise can
significantly affect the receiver’s sensitivity, therefore providing clean power
to the module should be a high priority during design. Bypass capacitors
should be placed as close as possible to the module. The values should be
adjusted depending on the amount and type of noise present on the supply
line.
The 1PPS Output
The 1PPS line outputs 1 pulse per second on the rising edge of the GPS
second when the receiver has an over-solved navigation solution from
five or more satellites. The pulse has a duration of 200ms with the rising
edge on the GPS second. This line is low until the receiver acquires an
over-solved navigation solution (a lock on more than 4 satellites). The
GPS second is based on the atomic clocks in the GPS satellites, which
are monitored and set to Universal Time master clocks. This output and
the time calculated from the GPS satellite transmissions can be used as a
clock feature in an end product.
