Ashtech Z-sensor Manual do Utilizador descarregar pdf (Página 4)

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.

1. Typical accuracy is one hundred times as good as

DGPS.

2. There is an initialization period following power-

on. T his initialization calculates the integer

number of carrier phase wavelengths. This is

known as Òfixing the integersÓ.

3. There is a non-zero probability that the

initialization will be wrong.

In assessing RT K performance we thus address all t hree

issues: reliability, speed and accuracy.

RELIABILITY - The best RTK receivers offer

greater than 99.9% reliability. To put this in context:

suppose you turned on your RTK receiver once per

day, t he receiver fixed int egers and from then on

maintained lock on at least 4 satellites all day, then

your receiver would fix int egers incorrect ly about once

every three years.

Both the Z-Sensor and GG-RTK receivers provide

greater than 99.9% reliability, as well as providing the

user with control of the reliability. The user may

choose from three Ò formal reliabilityÓ settings,

corresponding to probabilities: 95%, 99% and 99.9%.

The receiver guarantees that the achieved reliability is

greater than the formal reliability setting. The greater

the reliability the slower the initialization.

SP EED -

RT K initialization is split into two stages, the

acquisition phase (when the satellites signal is acquired)

and the integer fixing stage (when the integer numbers

of wavelengths are computed). T he GG-RTK receiver

is always faster than a dual-frequency receiver for the

acquisition phase (because it only needs to acquire the

single-frequency C/A code, which is easy). On short

baselines (<1km) the GG-RTK receiver is also faster

than dual-frequency receivers for the integer-fixing

stage. On medium and long baselines the Z receiver is

faster than GG-RTK for the integer fixing stage.

Typical times to fix integers are 30 seconds through 2

minut es.

ACCURACY - Once the integers are fixed

correctly, RTK accuracy is at the centimeter-level.

During the integer fixing phase, while the integers are

being fixed they are modeled as real numbers (or

floating-point numbers), and the position is referred to

as a ÒfloatÓ solution. Float solutions have accuracy

ranging from DGPS levels (meter level) to decimeter

level, depending on how long the receiver has been

tracking the signals.

If the integers are fixed incorrectly, the position will

have float-solution accuracy, but the statistical

indicators available in the field will make it look like it

has centimeter accuracy. This will persist until the

satellite geometry changes, and the receiver realizes its

mistake. At this time the receiver will return to float

mode, and then fix the integers correctly. The satellite

geomet ry changes when new sat ellit es come int o view

or, if this does not happen in time, when the satellites

move enough in t he sky (usually 2 - 10 minut es is

required).

FIELD TESTS

We present results achieved by the two new

product s, Z-Sensor and GG-RTK. T he initialization

results are summarized in the next two figures. These

figures represent the results of, literally, thousand of

tests. For each test the receiver is allowed to fix

integers, and the time it takes to do this is recorded,

then the receiver is reset. This gives us a measure of

the integer-fixing phase of RTK initialization. T ests

were done both for st atic and dynamic cases, t he t ime

to initialize is the same whether the receiver is static or

dynamic.

The first plot shows the results for the Z receiver. The

results are organized into three baselines: short (<1km),

medium (3 to 7 km) and long (19 km). For the long

baseline tests, dat a was logged to a PC and processed on

the PC to give an approximation of what we expect to

see in real time. PC-processed data is shown as a dotted

line on the plots. Real time data collected so far agrees

closely wit h t he dat a processed on t he P C, but , where

we have not collected enough real-t ime dat a t o give

meaningful statistical results (i.e. thousands of tests) we

show only t he P C-processed results.

40%

50%

60%

70%

80%

90%

100%

1 10 100 1000

Time since reset (seconds)

Z Receiver, RTK intialization, integer-fixing

phase . Short, Medium and Long baseli n es

The second plot shows results for the GG-RTK

receiver, collected and displayed in a similar way. The

Z Receiver results are shown, in light grey, on this plot

for reference.

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