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E Comparison Of RT-2 And RT-20
MiLLennium Command Descriptions Manual 213
2. If the loss of differential corrections lasts longer than 30 seconds, the RT-20 filter is reset and all ambiguity
and monitor model information is lost. The timeout threshold for
RT-20 differential corrections is 30
seconds, but for Type 1 pseudorange corrections, the timeout is 60 seconds. Therefore, when the
RT-20
can no longer function because of this timeout, the pseudorange filter can produce differential positions
for an additional 30 seconds (provided
RTCM Type 1 messages were transmitted along with the Type 59
messages) before the system reverts to single point positioning. Furthermore, once the link is re-
established, the pseudorange filter produces an immediate differential position while the
RT-20 filter takes
an additional 14 seconds to generate its positions. The monitor models require 7 monitor observations
before they are declared useable, and this will take 14 seconds, based on a 1/2 Hz differential correction
rate. The monitor model must be healthy before solutions are logged to the
POSA/B and P20A/B logs, so
there is a delay in the use of real time carrier positioning to the user once the link has been re-established.
The
RT20A/B log uses matched observations only (no extrapolated observations), and these will be
available after three monitor observations are received, but will have about 1.5 seconds latency associated
with them.
Figure E-8 RT-20 Re-initialization Process
The RT-20 system is based on a time-matched double difference observation filter. This means that observations
at the remote site have to be buffered while the monitor observation is encoded, transmitted, and decoded. Only
two seconds of remote observations are saved, so the monitor observation transmission process has to take less than
2 seconds if any time matches are to be made. In addition, only remote observations on even second boundaries
are retained, so monitor observations must also be sent on even seconds if time matches are to be made.
REFERENCE
REMOTE
1234567
Reference
Doppler
Start generating
RTKA/B logs
Models
Ready
Generate
RTKA/B
logs
RTCM59 messages
required following
RESETRT20
reference phase
models and
and
PRTKA/B