ID01140002_Attachment_B_DCS_DOMSAT_Requirements.pdf

Attachment B

WSCS II SON

The GOES DCS DOMSAT Ku-Band Link Requirements

Primary Site - Wallops CDAS, Va.

Backup Site – Suitland NSOF, Md

The GOES DCS DOMSAT link provides a communication service as a point-to-multi-point simplex 56kbit single channel satellite service operating over the Ku-band. The communication service shall be provided to operational downlink sites (the downlink sites already exist and are not a part of this procurement) located within the 48 contiguous states and shall require no modifications to either the software or hardware at these downlink sites. Each downlink site has a

Prodlin 1.8 meter antenna connected through a low-noise block down converter (LNB) to a

ComStream DBR401 demodulator (or equivalent, e.g. the Adaptive Broadband SDM-300L which is a plug-in replacement for the ComStream DBR401 in the existing downlink sites). The communication service must also support the data transfer protocol currently being used (see

Appendix X).

The data to be broadcast will originate at the NOAA/CDA Station at Wallops Island, Virginia, and so all transmission equipment must be located and maintained there. A downlink receive system is required at the NOAA/CDA station for monitoring the performance of the uplink.

Performance

The RFQ BER requirements shall apply to the 1.8m DCS receive antenna system.

Current DCS DOMSAT RF Parameters

1. Circuit#: 8819

2. Data Rate: 56 Kbps

2. Satellite: SES-1, Located at 101deg West

3. Satellite transponder: Ku Transponder 14

4. Downlink Frequency: 11997.525 MHz

5. Downlink Polarity: Horizontal Polarization

Backup Service

The current DCS DOMSAT system provides for a backup DCS DOMSAT transmit/ receive antenna system located at the NSOF in Suitland, Md. location. This system functions as a backup to the Wallops primary system and operates over the same satellite link, transmit frequency and with the same availability as the Wallops system. The Contractor shall be responsible for enabling or disabling the transmitters at either site, within 15 minutes following a request from NOAA..

GOES Data Collection System (DCS) Domsat Data Stream Format

DOMSAT is a point-to-multipoint broadcast, and thus no responses are sent from the receiver to the broadcaster. Three nested protocols are used to transfer DOMSAT messages. The broadcast data are encoded using:

1. HDLC Lap-B frame

2. X.25 Information Packets

3. Custom DOMSAT Message Protocol

4. DOMSAT Packet

5. DOMSAT Message

The figure on the following page illustrates the nesting. This figure is followed by a description of each layer.

1. HDLC (LAP-B) Frames

Frames are encoded using a non-standard subset of HDLC LAP-B.

The HDLC address field must always be 1 (or error is assumed and packet is ignored).

The HDLC Control field must always be 0 (or error is assumed and packet is ignored). The high-order bit being 0 indicates that only HDLC information frames are broadcast. Note that the

3-bit sequence number fields and the poll/final bit are always 0. The receiver should not attempt to perform sequence checking or any other processing using these fields.

Zero-bit insertion is used (standard HDLC) and a standard CRC16 is contained in the last two bytes of data.

Each HDLC frame contains an X.25 Packet with a maximum length of 256 bytes.

2. X.25 Packets

Packets are encoded using a non-standard subset of X.25.

The first byte contains various flag bits and a group number. This byte will always have a value of 0x10 (or error is assumed and packet is ignored).

The high-order bit (bit 7) contains 0, meaning that only data packets are sent (never an interrupt packet or a control packet)

Bit 6 always contains 0. No processing should be done on this bit.

Bits 5 and 4 always contain 01, indicating that a single byte follows the channel number containing sequence numbers and the 'more bit'.

The low-order 4 bits always contain the group number 0.

The X.25 channel field will always be 0. Combined with the 0-group number this means that all data travels on logical circuit 0.

The 3rd byte contains receive/send sequence numbers and the 'more bit'. The sequence numbers are always 0. The receiver should not attempt to perform sequence checking using these values.

The 'more bit' (bit 4) is used to indicate the end of a complete DOMSAT message:

More=1 means that the message is not yet finished. The receiver should expect more packets to construct the complete aggregate message.

More=0 means that this is the last packet for a DOMSAT message. The receiver can now process the complete message.

Each X.25 packet contains one DOMSAT packet with a maximum length of 253 bytes.

3. DOMSAT Packet Protocol

a. DOMSAT Packet

Each DOMSAT packet starts with a 2-byte (16-bit) message sequence number. The number is big-endian (i.e. the first byte is the high-order byte). In general, sequence numbers increment with each complete DOMSAT message (which may be composed of multiple packets).

Individual packets within a message will have the same sequence number.

Following the message sequence number is a single byte containing:

A 'Retransmitted Message' Flag (high bit).

A seven-bit packet sequence number.

The packet sequence number starts at 1 for the first packet within each message and increments until the complete message is received. There can be a maximum of 64 packets making up a complete DOMSAT message. Hence the largest

DOMSAT message can by 64 packets times 250 bytes per packet equals 16000 bytes.

The Retransmitted-Message flag is 0 if this is the first time that this message has been transmitted over DOMSAT. It is 1 if this message has been previously transmitted. Messages are retransmitted in response to a manual user request.

The 16-bit message sequence number starts with 0 (when DAPS is re-booted or a fail-over occurs) and continues to increment. It wraps from 65,535 to 0 and continues to increment.

A DOMSAT packet contains up to 250 bytes of DOMSAT message data.

b. Complete DOMSAT Messages

A complete DOMSAT message is constructed by concatenating the data fields of successive

DOMSAT Packets. The maximum message size (imposed by NESDIS, not by the protocol) is

16,000 bytes.

DOMSAT messages begin with a 37 byte ASCII text header followed by variable length data.

The header has the following structure:

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 hex digit uplink carrier status

5 decimal digit message data length