Markus et al,
There is a data transmission superimposed on Sylt (and now on the other EU
Loran tx). which may explain your unknown lines. This extract from a paper
given by TU Delft at the US PLANS 98 conference explains it a bit (I have
full tech data if you really want it but it's too long to put on here)
Quote:
LORAN-C DATA TRANSMISSION
As Loran-C is a navigation system in itself, its navigation requirements
and parameters
restricts the use of the signals for transmission of information. The
additional data modulation
onto the Loran-C signal shall not influence normal Loran-C operation.
Therefore, the
following restrictions are imposed on the use of the Eurofix datalink:
- The blinking service must be preserved, which excludes the first two
pulses of each
Loran-C group from Eurofix modulation.
- The modulation is not allowed to induce tracking biases, which
requires a balanced
type of modulation.
- The modulation index must be kept small in order to prevent an
undesirable loss in
tracking signal power.
Based on these requirements, a pulse position modulation with a 1 uS
modulation index is
chosen. Only 6 out of 8 pulses per group will be modulated and the
modulation is always
balanced on a per GRI basis. The application of 3-level modulation (a 1 uS
advance, a prompt,
or a 1 uS delay) leaves a possible 7 bit of information per GRI. With
Loran-C GRI's varying
between 40 ms and 100 ms, the raw bit rate available for data transmission
ranges from 175 to
70 bps.
Normal Loran-C users only experience a slight signal loss of 0.79 dB.
Future Loran-C
receivers, which have knowledge of the Eurofix modulation, can easily
compensate for the
applied modulation, once the pulses are demodulated. This will cancel the
signal loss
completely. Note that the influences of Cross-Rate interference and
blanking, phenomena
inherent to the choice of the Loran-C signal structure, cause larger signal
degradation.
DGNSS message format
The differential information is sent to the user in an asynchronous message
format. The use of
standard RTCM type-1 messages requires too much time to transmit a complete
set of
corrections. To keep data latency within acceptable limits, a minimum RTCM
type-9
compatible message of 56 bits is applied, Table 1. Unfortunately, the
simple parity schemes
used in the RTCM messages do not suffice in the Loran-C signal environment
of Cross-Rate
interference and high ambient noise levels. Therefore. a different error
correcting strategy is
chosen. However, as standard and commercially available DGNSS receivers must be
facilitated, the received Eurofix data is converted into a standard RTCM
type-9 message.
Forward Error Correction
To ensure reliable broadcast data communication through Loran-C. Forward
Error Correcting
codes are applied. These codes provide an effective means to correct
occasional errors
(improved datalink availability) and validate the decoded data (integrity)
at the cost of an
increased message overhead. Figure 1 shows the modulation and encoding
currently used to
transmit data via Loran-C. In Eurofix a Reed-Solomon code ensures datalink
availability for
stations up to 1,000 km. Each 56-bit message (8 GRI's of 7 bits) is
protected by additional
Reed-Solomon parity GRI's. In recent experiments messages contained in 20
and 30 GRI's
have been tested. Depending on the Group Repetition Interval (40-100 ms) of
the Loran-C
station the effective datarate of these schemes will be 70-28 bps and 47-19
bps, respectively.
Message integrity is protected in three steps:
1. Before demodulation a received pulse is first compared with a stored
reference pulse,
built up by integration of the first 2 unmodulated pulses (Receiver
Autonomous Signal
Integrity Monitoring, RASIM). This way, the quality of the demodulated
pulses is preserved
[6].
2. The Reed-Solomon code inherently adds integrity to the messages. The
Probability of
Undetected Error. Pue for a 10-error correcting Reed-Solomon code (30-GRI
Eurofix
message) is 1/10! = 2.7*10-7.
3. As a final safety net a 14-bit Cyclic Redundancy Check (CRC) ensures a
lower bound
on the Pue of better than 2(-14)=6.1*10-5.
The final message integrity will be a combination (probably product) of the
above three
mechanisms. This level of message integrity outperforms the level of the
standard RTCM
messages without further integrity improvements.
IMPLEMENTATION OF DGPS SERVICE AT THE SYLT LORAN-C STATION
On February 5th, 1997, Delft University installed a DGPS reference station
at the Sylt
Loran-C transmitter site (Germany) on an experimental basis. From that date
on differential
corrections have been broadcast throughout Europe on the Sylt Secondary
rate 8940.
Corrections for all satellites in view are broadcast in RTCM type-9
compatible messages with
one satellite correction per message. The update rate for a 30-GRI message
at the Sylt's GRI
is once every 2.7 seconds per satellite (30*89.40 ms). Since September
23"', 1997, Sylt
operates as a Secondary in the new French chain 6731. The slightly lower
GRI number
increased the update rate to once every 2.0 seconds. When also the Master
rate of Sylt (7499)
will be used for data transmission the correction update rate will be even
further improved.
End quote.
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