Null Frame Messages

[DD2DR]

Good morning,
On the 12.2.2018, I've listen on 289kHz to DGPS stations and received some DGPS Null Frames.
In the Message  were named the DGPS stations from Crib / Australia and St Davids on the Bermudas. ( see Attachment )
Who provides this Messages ?
The signal was S5 and it seems that it comes from a station in the UK
Whats happend with that kind of Null Frames.?
I hope anyone can explain and has more informations.
TNX in advance
73 de DD2DR

[ChrisSmolinski]

According to this DGPS station listing: https://www.ndblist.info/datamodes/worldDGPSdatabase.pdf

There is a station in Denmark on 289.5 kHz with transmit ID 700. Perhaps that is what you were receiving?

[Uli_Bel]

Could it be , that multiPSK (your software) adds these information, as the software knows the frequency of your receiver and match this frequency with the listed stations according the database of Multipsk.
In this case it mentions that on this frequency, two stations are listed with different baudrate...
May be check it, with the software developer ?!

Otherwise you can check the standards for messages in the DGPS transmissions...

Ulli, ON5KQ

[skeezix]

There are null frames in the spec. From the document TRANSMISSION CHARACTERISTICS OF MARINE DIFFERENTIAL GPS (DGPS) STATIONS by Klaus Betke, 2001:

4.4 Message type 6 – GPS Null Frame
This message has no parameters (N = 0, header only) and is used as transmission fill, if required. If an uneven fill is needed, N = 1 is also possible. The 24 data bits in the following word are then filled with alternating 1’s and 0’s. In practice, however, type 6 messages with N = 1 are hardly ever observed.

From the USCG, BROADCAST STANDARD FOR THE USCG DGPS NAVIGATION SERVICE COMDTINST M16577.1

J. FAILURE/DEFAULT MODE

1. A failure represents the absence of either pseudorange corrections in the broadcast stream or the absence of any message transmission at all. If a radiobeacon can no longer transmit any information, this condition will be broadcast to the user as a Type 16 Message via adjacent marine radiobeacons. Shall any of the aforementioned conditions occur, an updated Type 7 Message for all surrounding marine radiobeacons will be automatically generated and immediately broadcast by the appropriate marine radiobeacons in the area. If the reference receiver can no longer generate pseudorange corrections, Type 6 Messages will be broadcast in which the message header will be set to indicate an unhealthy condition.

2. In the case where the reference receiver can not generate Type 6 Messages, a single tone will be broadcast. Modulator failures may result in the broadcast of alternating ones and zeros, a single tone, or no output at all. As a modulator failure could occur at any time, it is imperative that the user equipment suite be capable of detecting the absence of RTCM messages containing pseudorange corrections in the data stream and if available tune to a different marine radiobeacon in advance of the "PRC Time Out Limit" (see Section 6D). It may be matter of minutes before the standby reference station is put on line. The broadcast of alternating ones and zeros should not cause any false acquisitions since the subject broadcast will be listed as unhealthy by the Type 7 Message. Should it be anticipated that an out of tolerance condition will continue for more than two hours a "Notice To Mariners" will also be issued. Refer to Section 5.E. for obtaining 24 hour on-line operational information on the DGPS service.