Linx Technologies RXM-GPS-FM - Manual (Page 6)

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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 up to 30 seconds to acquire its position.
Warm: 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: A hot start is when the receiver has valid ephemeris, time, and
almanac data. 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 FM Series GPS Receiver module is based on the MediaTek MT3339
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 FM Series into an end product extremely straightforward.
The module’s high-performance RF architecture allows it to receive GPS
signals that are as low as –161dBm. The FM Series can track up to 22
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 6µA. 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 the module to be enabled.
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 100ms with the rising edge
on the GPS second. This line is low until the receiver acquires a 3D fix.
The GPS second is based on the atomic clocks in the satellites, which are
monitored and set to Universal Time master clocks. This output and the
time calculated from the satellite transmissions can be used as a clock
feature in an end product with ±11ns accuracy.
Hybrid Ephemeris Prediction (AGPS)
AGPS is where the receiver uses the ephemeris data broadcast by the
satellites to calculate models of each visible satellite’s future location. This
allows the receiver to store up to 3 days’ worth of ephemeris data and
results in faster TTFF. Having this data reduces the cold start time to less
than 15 seconds. Contact Linx for details on this.