Table 3 4.  Precise Time Output Accuracy (95%)

for a Typical SPS C/A code Receiver

Receiver Mode and Output

S/A On

S/A Off

Stand Alone (4 SVs), Stationary or Low Dynamic, PTTI Port

274 ns

157 ns

Stand Alone (1 SV), Stationary, Known Position, PTTI Port

255 ns

147 ns

Coordinated Time Transfer, PTTI Port

59 ns

20 ns

Instrumentation Port

2 ms

2 ms

3.5.1  Warm Start, Cold Start, and Hot Start

Three different variations of TTFF are commonly defined and any one or all three

can be specified or required for a particular receiver.  A warm or normal start is

based on the assumption that the receiver has an estimate of current time and

position as well as a recent copy of the satellite almanac data.  Typically, time

should be known within 20 seconds of GPS time, position should be known within

100 kilometers, velocity within 25 metres per second, and the satellite almanac

should have been collected within the past few weeks.  TTFF1 for warm starts is

typically in the 2 to 5.5 minute range.

A cold start occurs whenever there is a problem with these key data elements.  This

is typical of a receiver as delivered from a manufacturer, supply depot, or repair

depot.  Date and time will not be maintained if the receiver "keep alive" battery has

been removed or drained.  If the receiver clock and memory remains active, the last

known position might be at a factory or depot thousands of kilometers from the

present position, and the almanac may be several months old. Under such

conditions, the receiver may have to systematically "search the sky" until it can find

a satellite and retrieve time and a current almanac.  A cold start can add at least

12.5 minutes to TTFF1 over that based on a warm start.

A hot start occurs when a receiver is provided with a standby feature to maintain

oscillator temperature, time, position, and individual satellite ephemerides (as well

as the almanac). When the receiver is commanded out of the standby mode, the

time required to achieve the next full position fix is usually Termed Time to

Subsequent Fix (TTSF) rather than TTFF.  Typically, TTSF is on the order of 10

seconds for standby periods of a few hours.

3.5.2  Receiver Warm Up

When a GPS receiver is initially turned on, time must be allowed for the receiver

crystal oscillator to warm up and stabilize at its normal operating temperature.  In a

GPS receiver it typically takes up to 6 minutes to complete this process.  If the

receiver is provided with a  mode that keeps the oscillator warm, this contribution to

TTFF can be avoided.

3 9