System architecture

Why this matters

Most project problems are not inside individual parts but at the seams between them. The camera sends images and the companion computer runs the model, yet when they are connected the image format does not match, the timing does not match or there is not enough power. An architecture designed before building shows every seam on paper, where fixing it is far easier and cheaper than on the real aircraft.

From functions to components

A functional architecture says what the system must do without saying what does it. A physical architecture says which component performs each function. Keeping the two layers apart lets you change hardware without rethinking the functions.

The N² chart

An N² chart places the components on the diagonal. The cell in row $i$, column $j$ shows whether data or power flows from component $i$ to component $j$. The NASA Systems Engineering Handbook presents this diagram as an interface management tool.

parts = ["GCS", "FC", "Companion", "Camera", "Gimbal", "GNSS", "Cloud"]
links = [("GCS", "FC"), ("FC", "GCS"), ("FC", "Companion"), ("Companion", "FC"), ("Companion", "Camera"),
         ("Camera", "Companion"), ("FC", "Gimbal"), ("GNSS", "FC"), ("Companion", "GCS"), ("GCS", "Cloud")]
n = len(parts)
print(f"{len(links)} interfaces of {n * (n - 1)} possible")
degree = {p: sum(p in link for link in links) for p in parts}
for p, d in sorted(degree.items(), key=lambda kv: -kv[1]):
    print(f"  {p:<9} {d} links")

functions = ["plan route", "fly route", "capture images", "detect defects", "transmit data", "produce report",
             "log flight data"]
allocation = {"plan route": ["GCS"], "fly route": ["FC"], "capture images": ["Camera"],
              "detect defects": ["Companion"], "transmit data": ["GCS", "Companion"], "produce report": ["Cloud"]}
missing = [f for f in functions if f not in allocation]
print("functions with no component:", missing or "none")

10 interfaces of 42 possible
  FC        6 links
  Companion 5 links
  GCS       4 links
  Camera    2 links
  Gimbal    1 links
  GNSS      1 links
  Cloud     1 links
functions with no component: ['log flight data']

Traceability

Bidirectional traceability is part of requirements management in the NASA handbook. Every requirement must point to the component responsible for it, and every component must point back to the requirement that makes it necessary. A component with no requirement behind it may be unnecessary; a requirement with no component is a gap that must be fixed.

Each interface should also be detailed in an interface list (interface control): signal type, protocol, data rate, voltage, connector and the owner on each side, as covered in UAT 321 and UAT 204.

Module lab

Common mistakes

Summary

• The functional architecture says what to do; the physical architecture says what does it
• Every function must be allocated to a component
• An N² chart shows all interfaces and points to the most connected components, which should be tested first
• Bidirectional traceability links requirements with components and exposes gaps or excess

Check your understanding

1. A system has 5 components. What is the maximum number of directed interfaces?
2. In an N² chart, what does the cell in row FC, column Gimbal mean?
3. What should be done with a function that no component is responsible for?
4. What might a component with no requirement behind it indicate?
5. Why should the seams of the most connected component be tested first?