Jpl Mission Design Tool Mars

3. Spacecraft Design Drivers, Space and Orbit

3.3 Payload Design

The subject and payload are the most important design drivers as they are the reason the mission exists. The subject drives the location/orbit the spacecraft must go, the payload that must be accommodated, and all the subsequent design requirements that flow down into the spacecraft bus design. Classifications of payloads include observation, communications or navigation, in-situ, action at a distance, and human spaceflight [New SMAD]. The majority of CubeSat payloads are Earth observation, communication, navigation, science, and technology [Alen]; human spaceflight payloads and action-at-a-distance payloads are rarely accommodated on CubeSats. Observation payloads are self-explanatory as payloads that observe, typically through remote sensing. The subjects may be outward-facing toward deep space (on exoplanets, astronomical features, or our sun's effect on space weather) and inward-facing toward Earth (on climate, surveillance, land surface mapping, or water bodies). These observational payloads function at different wavelength bands.

NASA electromagnetic spectrum mapping to space phenomena. Image courtesy of NASA.

Communication payloads may be separated into unidirectional systems that gather information or two-way systems that gather but also transmit information. Navigation payloads track and transmit position information like the Global Positioning System or Software Defined Radio platforms. In-situ payloads measure signals in the environment directly that cannot be remotely sensed, like a magnetic field, gravity, or sample collection.

We will focus on observational payloads as the Artemis CubeSat kit comes with a visible-IR camera. The Ke Ao team at UH Manoa wants to take a picture of Hawai'i (observation) from Low Earth orbit and transmit the picture back to the mission operators. The payload is a Raspberry Pi visible spectrum camera.

Before seriously considering the spacecraft bus subsystem design as a result of payload selection, look at the constraints of the payload design. There are four types of constraints: fundamental, technological, mission, and programmatic [NASA & NOAA]. Fundamental constraints include diffraction limit, photon noise, Nyquist limit, which all refer to laws of physics that limit observation. Some missions are impossible because of these fundamental limits, even if technology gets better over time. Technological limits stem from the ability of state-of-the-art detectors to measure the subject, capped by detector size and performance; optical form, figure, fabrication, and alignment, or processor speed. Outside the theoretical limits of observation, the detectors or optics that we manufacture are bound to perform less ideally. To constrain the technology further, the mission constraints adhere to size, weight, and power constraints due to spacecraft design and launch vehicle selection. Detectors or payloads may not be miniaturized any further without significant sacrifice to the mission performance. Finally, programmatic constraints, like cost, schedule, risk, and regulatory requirements, may break a mission that is fundamentally feasible, technologically feasible and closes in mission design. This sequence of constraints moves from the highest level of science to the physical and societal reality of spaceflight missions.

For the Ke Ao project, the payload is not a past state of the art. The Raspberry Pi camera is technologically proven. The physical characteristics of the camera are well within reason for a 1U CubeSat mission and the program constraints are quite lenient as it is a vertically integrated project within a university environment.

The payload design process generates requirements for accommodating the payload. Image courtesy of NASA and NAP

Say you've found a payload that fits within all those constraints. Now you have the go-ahead to design the mission architecture. We've discussed solutions for the mission components but for the concept of operations, we'll focus on mission lifetime and the sequence of events once the satellite reaches space.

The Artemis CubeSat kit does not have a specific expiration date or concept of operation as it depends significantly on the mission objective and payload. The Ke Ao mission, an instantiation of the Artemis CubeSat kit, has an operational lifetime of 1 year and has the following concept of operations:

With the mission architecture defined, we can design spacecraft buses around the payload with the context of the operations and mission components. Regardless of the type of payload, the payload's data, size, weight, and power (SWaP) requirements must be accommodated by the spacecraft bus design. The payload-spacecraft interface requirements for an observational payload reside in mechanical, thermal, electrical, and subject-specific accommodation requirements [NASA & NOAA]:

Mechanical	Thermal	Electrical	Subject Specific
Size (Structures and Launch Vehicle)	Conducted and radiated heat flux to/from the payload (Thermal)	Power requirements (Power)	Sensor orientation and clear fields of view (Structures)
Mass (Structures and Launch Vehicle)	Thermal gradients and baseplate distortion (Thermal and Structures)	Output data rate and storage (CDH)	Pointing stability, agility (ADCS)
Moments of inertia (Structures)		Command, control, and telemetry (Communications and Ground Segment)	Contamination: particulates, outgassing (Environment Testing)
Uncompensated momentum (ADCS)		Electromagnetic interference (Environment Testing)	Level of autonomy and operations (Mission Architecture)
Launch loads (Environment Testing and Launch Vehicle)
Disturbances (Environment Testing and Orbit)

Each spacecraft bus subsystem and peripheral mission component is mentioned at least once in the payload accommodation requirements table. The spacecraft bus subsystems are affected by the payload in the following ways:

- Structures: the payload could require
  - a certain orientation within the spacecraft frame (facing away from the spacecraft center)
  - an unobstructed view into the space environment (most optics)
  - accommodation of specific size and weight
  - active mechanisms or deployable (like an extendable boom)

ASNARO-1 Satellite has many components that have unobstructed views or access to the space environment, like the star tracker, camera, imager, and antennas" Image Courtesy of NEC.

- Power: the payload could require
  - an orbit average and peak power on the power distribution (wire harnessing, motherboard, and daughterboard control)
  - a specific amount of power generated and/or stored (solar array and battery size)
  - voltage conversion and limits (circuit board design)
  - current regulation or limits (circuit board design)
    
    Schematic view of the Wideband Imaging Camera (WIC) instrument, showing the placement of the power supply with respect to the instrument. Image courtesy ofUCB/SSL.
- Command and Data Handling: the payload could require
  - processing speed (on-board computer processors; CPUs and GPUs, RAM)
  - data storage (on-board computer memory; SD card or hard drive)
  - specific data format (image, frequency spectra)
  - bandwidth or data rate to transfer across the network (RAM and software algorithm to prevent bufferbloat)
  - data resolution (the number of significant figures in measurements to retain)

- Communications: the payload could require
  - a link budget that closes and has margin (communication reliability and two-way transmission)
  - Command uplink data rate (critical operating modes during mission sequence)
  - Telemetry downlink data rate (prevent onboard memory from overflowing)

- Attitude Determination, Control, Navigation, and Sensing: the payload could require
  - accuracy and precision pointing, slewing, or tracking maneuvers (momentum control systems)
  - specific resolution of attitude or position estimate (estimation algorithms, sensors)

The primary mission objective of BRITE Constellation is to provide milli-magnitude (0.1% error) differential photometry of bright stars. On-orbit fine pointing performance of UniBRITE spacecraft (left) and BRITE-Toronto spacecraft (right); the payload plate scale is 26″/pixel (image credit: UTIAS/SFL).

- Thermal: the payload could require
  - a specific thermal operating range (optics like to be cold)
  - structural stability due to temperature (materials resistant to thermal expansion or temperature maintenance)

A unique combination of meticulous old-world skills and high-tech materials produced the finely sewn, super-strong and extremely lightweight thermal blankets that protected Cassini from the extreme hot and cold of deep space. Image courtesy of NASA.

- Propulsion: the payload could require
  - stationkeeping (intermittent correction of the orbit)
  - orbit changes (delta-V and propellant budget)

For payloads that require a certain inclined orbit, a spacecraft gets from the initial orbit to the final, lower-inclination orbit, by providing a downward thrust at one of the nodes. Image by Blog Spot.

For a list of other payloads, compiled by David Doody:

^{Examples of Mars Express Science Instruments.Ultraviolet and Infrared Atmospheric Spectrometer (SPICAM) on the left. A} ^{High-Resolution Stereo Camera (HRSC) pictured on the right. Images courtesy of NASA.}

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