from each DCP in the message block. This block is received in
the Mission Center as a sequential file.
In order to simplify the article, it will be addressed here only
the environmental data received from Cuiabá Ground Station.
In that case, the DCP messages transmitted to the Mission
Center follows the SDID (Station Data Interchange Document)
format, presented in TABLE 1.
octet nr. octet nr.
Tesp
0 | code | msg type
msg subtype 1
2 | satellite code satellite subcode 3
4 |source site code syst.typd number| 5
6 |receive site code spare 7
8 channel ID 9
12 Time stamp in milliseconds 13
14 15
16 measured up-link carrier 17
frequency
18 | message length channel status 19
20| sync. word 21
22 platform identification 23
24| platform sensor data (N x 4 Octets)
(1 «-N «- 8)
TABLE 1: DCP Message Format
Considering that the average of DCP messages received per
satellite pass over Cuiabá Ground Station is 100 (they will be
increased four times in the near future) and their transmission to
the Mission Center is not totally safe of errors, the DCP
message analysis shall be careful with the occurrence of both
truncated and wrong messages. Therefore it is not sufficient to
separate the DCP messages only based on both the recognition
of the pattern which begins each message and the message
length field, since the DCP data field is variable in length.
Problems such as synchronism loss might carry out wrong
conclusions about the next messages in the messages block.
The finite states automaton, presented in FIGURE 3, shows the
sequence of events (octets read sequentially in the message
block) which are analyzed in order to detect both wrong
messages and truncation data in different parts of a DCP
message. As it might be followed in the TABLE 2, the
presented events are sequentially searched for in the message.
134
EVENT MEANING
0 response code/message type
1 message subtype
2 satellite code
3 unexpected data
4 synchronism word
5 expected end of data
6 unexpected end of data
7 expected data
TABLE 2: Events Description
The states of the automaton, followed by the SCMCD
acquisition module during the DCP messages analysis, are:
INI - The initial state, in which the SCMCD acquisition module
stays until the event 0 occurs. Such occurrence means that
a beginning of a new DCP message might have happened;
TRI - The state in which the system waits for the event 1
occurrence in order to follow a beginning of a new DCP
message, started in the last state INI;
ZR1 - Analogue to the state TRI, in this case, the event 2 is
waiting for in order to complete the three firsts character
of the message header;
SCI - This state represents the search for the synchronism word
in the 20th header position. In that state the system waits
for the event 4 occurrence in case of 20th header positions
matching. If the header position in check is less than 20th,
the event 7 occurrence is accepted. That means, any
character is considered accepted data in the header until
the 20th position;
LEN - Working similarly to the SCI state, the LEN state deals
with the message length information which is used to
verify the end of that message followed by the beginning
of a new message. In this state, the system waits for the
event 0 occurrence immediately after the message length
being reached. While the message length has not been
matched, the event 7 occurrence is accepted because it
represents message data field;
TR2, ZR2, SC2 - These states are analogue respectively to the
states TR1, ZR1 and SC1 but they do not deal with the
first message in the messages block anymore. In order to
check the right length of the last message, the SC2 state
has to be reached. That means, the header standard of the
new message has been matched;
CFL - It is the state in which the system stays in case of a
situation of conflict has happened. This situation happens
when no matching of the expected event occurs in one of
the following states: LEN, TR2 and ZR2. Therefore, a
truncation has been identified in the message;
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
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