Navman 11 User Manual

Page 80

Occasionally a type 2 message is sent

interspersed among the correction messages,

which provides a secondary correction. This is

done to allow a user to operate with old (up to

two hours) satellite ephemeris and satellite clock

data while the reference station is operating with

most recent data. This correction, called the ‘delta

correction’ is added to the normal correction for

that satellite. The reference station will usually

decode the satellite data before the user does

since it is constantly monitoring the data.

Data Link

The data link, which communicates the corrections

from the reference station to the user receiver,

can take a number of forms and operate at any of

several frequencies. The chief requirement is that

the messages be communicated reliably at a data

rate of at least 50 bps (continuous transmission).

Figure C-5 shows the user data link functions. In

its simplest form, the data link continuously carries

the DGPS data message without interruption at a

constant data rate of at least 50 bps. However, it is

transparent to the GPS receiver whether the data

is transmitted continuously or in bursts, or whether

protocol overhead is added.

For example, each message (or multiple

messages, or any fraction of a message) could

be transmitted as a short burst at 2400 bps, along

with a data link protocol preamble, parity, and even

error correction bits. This would be stripped off at

the receiver end and the differential correction bits

would be stored in the buffer, to be transferred to

the receiver at will. DGPS broadcasts intended for

general public use require that the data link be a

standard, published design.

For non-public use, however, the reference

station, data link and receivers could be part of

an integrated DGPS system. In such a case, the

data might be encrypted to limit the service to

paying customers. The format allows for such

operation. At the minimum rate of 50 bps, there

is considerable robustness in the data link. The

corrections are broadcast frequently enough so

that the loss of as many as three consecutive

corrections can be tolerated and still provide 5 m

accuracy.

GPS sensor

display and

output

measurement

processor

differential data

processor

satellite data

navigation data

position coordinates

GPS receiver

GPS satellite

signals

data link receiver

data formatter

Data link

DGPS data

link

demodulator

Figure C-5 User equipment block diagram

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