Each DCP message will have header fields followed by a message body. Regardless of which interface a DCP message was received from, the LRGS internally formats the header into a 37- byte character string with the following fields:
- 8 hex digit DCP Address
- YYDDDHHMMSS – Time the message arrived at the Wallops receive station. The day is represented as a three digit day of the year (julian day).
- 1 character failure code
- 2 decimal digit signal strength
- 2 decimal digit frequency offset
- 1 character modulation index
- 1 character data quality indicator
- 3 decimal digit GOES receive channel
- 1 character GOES spacecraft indicator (‘E’ or ‘W’)
- 2 character data source code Data Source Code Table
- 5 decimal digit message data length
Following the header will be the message body, with a variable number of characters. The format of the message body varies widely depending on the manufacturer of the transmitter, data logger, sensors, and the technician who programmed the DCP. The body can be simple ASCII, sometime with parameter codes and time-stamps embedded, sometimes not. The body can also be in ‘Pseudo-Binary’ which is character encoding of binary data that uses 6 bits of every byte and guarantees that all characters are printable.
The single character labeled 'Failure Code' in the DCP message header indicates whether the message originated from a DCP or whether it is a DAPS-generated status message. Real DCP messages have a failure code of 'G' for good message, or '?' if the message contained parity errors when received by DAPS.
If the failure code is anything other than '?' or 'G', the message is generated by DAPS. These status messages have the DCP address of the pertinent platform and are typically sent immediately after the real DCP message from that platform. The body of the message will be a brief text message explaining the event.
Possible failure codes are as follows:
Real DCP Messages:
|G||Good DCP Message|
|?||DCP Message with Parity Error|
|W||Previous DCP message was Received on the wrong channel|
|D||Previous DCP message was duplicated (i.e. received on multiple channels)|
|A||Previous DCP message contained a correctable address error|
|B||Previous DCP message contained a bad (unknown) address|
|T||Previous DCP message was received outside its proper time slice (early/late)|
|U||Previous DCP message was unexpected|
|M||The DCP message for the referenced platform was missing (not received in its proper time slice)|
|I||Previous DCP message had an invalid address|
|N||The referenced platform has a non–complete entry in the DAPS Platform Description Table (PDT)|
|Q||Previous DCP message had bad quality measurements|
Signal Strength will be two ASCII digits and will be in the range of 32 to 57. Signal strength is the implied EIRP, assuming the pilot is a +47 dBm reference.
Frequency Offset will be two ASCII characters. The first will be a plus or minus sign. The second will be on ASCII digit 0 through9, or the capital letter 'A'. The sign indicates that the DCP is transmitting above or below (plus or minus, respectively) the channel center frequency. The digit indicates the amount of the offset in increments of 50 Hz. The character 'A' represents 500Hz, which is the worst case frequency error that DAPS can acquire.
Modulation Index will be one of the following three characters:
NNormal: (60° ± 5°)
L Low: ( 50° )
H High: ( 70° )
Data Quality will be one of the following three characters:
NNormal: Error rate better than 10-6
F Fair: Error rate between 10-4 and 10-6
P Poor: Error rate worse than 10-4