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B-7. Ground System Status (GSS) Processing

Ground System Status (GSS) processing is considered to be , for this loading analysis, a contact function that receives status data from the RGF and local command and control equipment. Processing of this status data is similar to that required for telemetry processing. Data is received in packet format, individual status bits or words must be extracted from the message, status parameters must be converted to engineering units or to discrete system states, and the state of the ground system is used as a constraint in satellite commanding and assessment of telemetry data quality.

B-7.1 GSS Assumptions

The reference architecture for satellite TT&C operations assumes a fairly comprehensive GSS processing capability that can support all contact requirements. The primary performance factors are data rate, data expansion, and processing load.

Data Rates. An estimate of RGF load can be extrapolated from the current Remote Tracking Station (RTS) interface with the Command and Control Segment (CCS). The current RTS/CCS interface includes a once-per-second status message consisting of 348 bytes (8 bits) transmitted at a data rate of 9600 bps, for an effective data rate of 2784 bps. Status data includes antenna tracking data, which must be stripped out during the contact and stored for post-contact processing. The RTS status line can also include occasional Command Buffer Status and Retransmit Request messages, raising the peak data rate to 2864 bps.

In the near future, the status message rate is expected to increase to support more autonomous antenna operations, although the transmission rate will remain at 9600 bps [6]. Table B-3 contains a list of the message types and their respective sizes:

Table B-3. Future RGF Status Messages

Message Type

Nbr Bytes
SV Command Buffer Status
2
1/Sec ARTS R/T Operations Data
197
1/Sec ARTS R/T Pass Support
41
ARTS Status
462
Post Pass Status
509

During nominal RGF operations, message traffic would consist of the first three messages in Table B-3, which total 240 bytes, and would be transmitted at an effective rate of 1920 bps. If a message is sent during contact with the RGF to change mode or configuration, then one ARTS status message is returned. After completing the contact, one Post Pass Status message is returned. Note that neither the ARTS Status nor the Post Pass Status messages are continuous, and that the post-pass message would not affect contact performance. To determine future RGF status processing load during the contact, it will be assumed that the future RGF status data will be transmitted at a nominal rate of 1920 bps, and that the peak status rate will include the ARTS Status message, for a total of 5616 bps.

An additional status component will be from the new command and control system and/or from local communications equipment, such as data switches, encryption/decryption devices, and LAN equipment. Since these status messages are expected to contain device IDs, device status, and time, rather than the complex bit location scheme used in the ARTS status messages, a local equipment status rate of 1200 bps would not be unreasonable. GSS rates can be expected to range from 3120 bps to 6816 bps; the maximum rate of 6816 bps is assumed for this loading analysis.

Data Expansion. A review of the message structure in reference [6] for ARTS messages resulted in the message statistics shown in Table B-4. The last column reflects assumptions about the format of local ground system status.

Table B-4. GSS Status Message Lengths

Word Size

Number of Words
(Nbr of bits)
ARTS R/T Ops Data Message
ARTS R/T Pass Support Message
ARTS Status Message
Local Status Message (Estimated)
1
41
37
50
0
3
0
4
0
0
4
10
10
0
0
5
0
1
0
0
8
7
11
32
50
16
1
1
0
50
32
21
0
0
0
56
2
1
0
0
64
7
0
0
0
Total # Words
89
65
83[2

]100
Total # Bits
1576
328
3696
1200
Avg Word Size (bits)
18
5
43

12

Averaging the four message types in Table B-4 results in the following data expansion calculations:

* Average word size is 10 bits

* Each status word is assigned an 8-bit tag; total of 18 bits per word (1.8:1 expansion)

* Time is processed as a 32-bit word, plus a 16-bit tag and occurs once per message packet, adding 48 bits per packet (1.8:1 expansion + 48-bits time)

* EU conversions transform 10-bit status words to 32-bit words (i.e., real numbers) with a new tag, and the raw word is discarded, resulting in a total of 42 bits/word (4.2:1 expansion + 48-bits time)

* Quality indicators are associated with each message packet, adding 8 bits of quality data plus 16 bits tag per message, for a total of 24 bits/message (4.2:1 expansion + 48-bits time + 24-bits quality)

* Alarms are directly associated with each status word and are 8 bits with a 16-bit tag, resulting in 24 additional bits for each word's information packet and a total of 64 bits/word (6.4:1 expansion + 48-bits time + 24-bits quality)

For GSS loading analysis, a data expansion of 6.4:1 for composite status messages results in the following maximum throughput requirement for nominal GSS processing:

ThroughputGSS = 6.4 * (6816 bps) = 43,622 bps

Processing Load. Processing loads for RGF GSS data are assumed to be similar to those for raw telemetry, since the majority of the data is binary in nature. For local GSS data, modern network and system management software is expected to provide converted and tagged equipment status, thus reducing the processing load.