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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.