SMAD

22 Mission Characterization 2.1
2.1 Step 3: Identifying Alternative Mission Concepts 23
tives are to generate and transmit data. The process of defining how to transmit the
data between the spacecraft and various users and controllers on the ground is called
the communications architecture and is discussed in Chap. 13.
TABLE 2-2. Process for DefIning the Mission Concept of Operations. See Table 2-1 for
definitions and AreSat example.
Where
Step Key Trades Discussed
1. Deline data delivery process for Space vs. ground processing Sec. 2.1.1
- Mission and housekeeping data Levelofautononrv Chap. 13
Central vs. distributed proceSSing
2. Define tasking, scheduling, and control for Levelof~onomy Sec. 2.1.2
- Mission and housekeeping data Central vs. distributed control
- Long term and short term
3. Deline communications architecture for Data rates bandwidth Sec. 13.1
- Mission and housekesplng data TImeliness of communications
4. Define preliminary mission timeline for Replenishment and Sec. 2.1.3
- Concept development end-of-iHe options
- Production and daployment Deployment strategy for
- Operations and end-of-life mUltiple satellites
Level of timeline liexibHity
5. Iterate and document NlA NlA
The mission timeline differs from other elements of the mission concept in
Table 2-1. It represents the overall schedule for developing, planning, and carrying out
the mission. This defines whether it is a one-time only scientific experiment or longterm
operational activity which will require us to replace and update satellites. In
either case, we must decide whether the need for the mission is immediate or long
term. Should we give high priority to near-term schedules or allow more extensive
planning for the mission? Of course, much of this has to do with the funding for the
mission: whether money is available immediately or will be available over time as we
begin to demonstrate the mission's usefulness.
2.1.1 Data DeHvery
Space missions involve two distinct types of data-mission data and housekeeping
data. Mission data is generated, tmnsmitted, or received by the mission payload. This
is the basic information that is central to what the mission is all about For FrreSat, this
data starts out as infrared images on a focal plane and ends up as the latitude, longitude,
and basic characteristics of a forest fire transmitted to a fire fighter on the ground. The
mission data has potentially very high data rates associated with it. However, the need
for this data may be sporiKIic. Thus, FrreSat may generate huge quantities of raw data
during periods of time that it is passing over the forests, but there is little need for this
same level of data when it is over the poles or the oceans.
Ultimately, the processed mission data may go directly to the end user or through
ground stations and communication networks associated with mission operations. This
will, of course, have a fundamental effect on how the mission works. In the first case,
FrreSat would process its imagery and send the forest fire information as it is being
observed to the fire fighters in the field. In the second case, data would go instead to
an operations center, where a computer system or human operators would evaluate it,
compare it with previous data, and determine the location and characteristics of a
forest fire. Then, the operations center would tmnsmit this information to the fire
fighters in the field. The result is about the same in both cases, but the system's abilities,
limits, characteristics, and costs may be dramatically different
In contrast to the mission data, housekeeping data is the information used to
support the mission itself-the spacecraft's orbit and attitude, the batteries' temperature
and state of charge, and the status and condition of the spacecraft's parts. Unlike
the mission data, which is typically sporadic and may have huge data rates, the housekeeping
data is usually continuous and at a low data rate. Continuously monitoring
system performance does not require much information transfer by modem standards.
In addition, rather dian going to the end user, housekeeping data goes to the system
monitoring and control activity within mission operations. Although housekeeping
and mission data are distinct, we often need housekeeping data to make the mission
data useful. For example, we must know the spacecraft's orbit and attitude to determine
the ground lookpoint of the payload sensors and thereby locate the fire.
For both mission and housekeeping data, the data delivery system should be an
information management-oriented process. We want to take a large amount of raw
data, frequently from a variety of sensors, and efficiently transform it into the information
the users need and provide it to them in a timely manner. We do not know at
first whether sending FireSat data directly to the field or sending it first to a mission
operations center for interpretation and analysis is the best approach. But we do know
our choice will dramatically affect how well FrreSat works and whether or not it is an
efficient and effective system.
The principal trades associated with data delivery are:
• Space vs. ground-how much of the data processing occurs on board the spacecraft
vs. how much is done at mission operations or by the end user?
• Central vs. distributed processing-is one computer talking to another computer,
or does one large central computer on the spacecraft or on the ground process
everything?
• Level of autonomy'" -how much do people need to intervene in order to provide
intelligent analysis and minimize costs?
These trades are strongly interrelated. Thus, autonomy is important by itself, but is
also a key element of the space vs. ground trade. If human intervention is required (i.e.,
it can't be done autonomously), then the process must be done on the ground-or it
must be a very large spacecraft. We will discuss each of these trades below after we
have looked at the data delivery process as a whole. Autonomy is discussed in
Sec. 2.1.2, because it is also critical to tasking and control.
The best way to start looking at the data-delivery problem is a data-flow analysis.
This defines where the data originates and what has to happen to it before it gets to
where it needs to go. To examine the data flow we can use a data-jlow diagram as
shown in Fig. 2-2 for the FrreSat mission. A data-flow diagram lets us outline the tasks
which we need to do, even though we don't understand yet how we will do most of
them. For FrreSat we know that we need some type of information collection, probably
• The language here can be confusing. An autonomous operation runs without human intervention.
An autonomous spacecraft runs without intervention from outside the spacecraft. .