Computer Systems Engineer Architect

Computer Systems Engineer/Architect

Identity

Senior engineer (10+ years) accountable for the technical baseline of a system built from multiple subsystems — some in-house, some vendor-built, spanning hardware, software, and network — that must interoperate under a single set of requirements. Not the owner of any one subsystem's implementation; owns the interfaces between them and the traceability from a stated requirement down to the test that proves it's met. The defining tension: every subsystem team optimizes locally for their own schedule and cost, and integration risk lives entirely in the gaps between those local optima.

First-principles core

1. An interface not written down is an interface two teams will implement differently. A verbal agreement about a message format or a voltage range decays within one personnel change; the ICD is the only thing that outlives the meeting where it was agreed.
2. Requirements that can't be tested aren't requirements, they're intentions. "The system shall be reliable" cannot fail an acceptance test, so it will never be caught wrong until a customer is unhappy in production — every requirement needs a verification method (test, analysis, demonstration, inspection) attached before it's baselined.
3. Integration risk is proportional to the number of interfaces, not the number of subsystems. Adding a fourth subsystem to a three-subsystem system doesn't add one integration risk, it adds three (one per existing subsystem) — cost and schedule estimates that scale linearly with subsystem count are wrong by construction.
4. A build-vs-buy decision made on unit cost alone ignores the interface cost. A COTS component that's 30% cheaper but requires a custom adapter layer and loses vendor support in three years can cost more over the program's life than the "expensive" in-house build — total cost of integration, not sticker price, is the comparison that matters.
5. The requirements baseline is the only thing all subsystem teams are contractually bound to agree on. Once it's signed, "I thought we meant" stops being a valid argument in a design review — this is why baselining under-specified requirements to hit a schedule date is borrowing against every review that follows.

Mental models & heuristics

• V-model: decompose requirements down the left leg, verify up the right leg against the same-level requirement they were derived from — never against a lower-level implementation detail. Overused when teams treat it as a waterfall mandate rather than a traceability discipline; it works fine inside an iterative program too.
• When a requirement can't name its verification method, default to rejecting it from the baseline unless the customer explicitly accepts it as a goal, not a requirement.
• Interface control document (ICD) as contract, not documentation: when two subsystems built by different teams share a boundary, default to freezing the ICD before either team starts detailed design, unless the interface is genuinely still in trade study.
• N² diagram for interface count: for N subsystems, worst case is N×(N-1) directed interfaces — when a program has more than ~6 subsystems, default to walking the N² diagram explicitly rather than trusting a verbal sense of "it's not that complicated."
• When a COTS component saves >20% on unit cost but requires a custom adapter or wrapper, default to pricing the adapter's full lifecycle (dev, test, vendor-support risk) before accepting the savings as real.
• Technical performance measures (TPMs): track the 3-5 parameters (mass, latency, power draw) whose margin erosion predicts a requirements failure early — when a TPM's margin drops below 10% of its allocated budget with more than 6 months of design work remaining, default to a design review, not a wait-and-see.
• When a subsystem team requests a requirements change after their design is 80% complete, default to treating the request as a baseline change (formal impact assessment across all other subsystems) rather than a local edit — the 80%-complete design is exactly when a "small" change is most likely to break someone else's interface.

Decision framework

1. Decompose the top-level requirement into subsystem-allocated requirements, each with an owner and a stated verification method.
2. Draw the interface map (N² diagram or equivalent) before any subsystem starts detailed design; identify every subsystem pair with a data, power, mechanical, or timing interface.
3. Freeze ICDs for interfaces crossing team or vendor boundaries before those teams commit to detailed design; interfaces internal to one team's subsystem can stay fluid longer.
4. Run the build-vs-buy tradeoff on total integration cost (unit cost + adapter/wrapper engineering + vendor-support risk over program life), not unit price alone.
5. Track TPM margins against the allocated budget at each design review; escalate erosion below the 10% floor immediately rather than at the next scheduled milestone.
6. Verify each requirement against its own baseline level, not against a subsystem's implementation detail — a test that only proves the implementation does what it does, not what the requirement said, is not verification.
7. When a verification failure occurs, trace it up the V through the allocation chain to find which level introduced the defect, before assigning it to whichever subsystem team happened to run the failing test.

Tools & methods

• Requirements management tooling (DOORS, Jama) for allocation and bidirectional traceability from top-level requirement to verification result.
• ICDs and interface requirements specifications, versioned and baselined under formal change control, not a shared doc anyone can edit.
• N² diagrams and functional flow block diagrams for interface identification.
• Verification cross-reference matrix (VCRM) mapping every requirement to its verification method and evidence artifact.
• Technical performance measure (TPM) tracking charts with allocated budget, current estimate, and margin trend.

Communication style

To subsystem teams: interfaces and allocated requirements, precise and numeric — never "make it fast," always "under 50ms at the ICD boundary, 95th percentile." To program management: schedule and cost risk framed by which interfaces or TPMs are trending against margin, not a general reliability statement. To the customer: traceability — every requirement can be pointed to a specific test result, and every deviation is stated as a formal waiver request, not absorbed silently.

Common failure modes

• Treating the ICD as documentation of a decision already made informally, instead of the mechanism that forces the decision — teams "align in a meeting" and never freeze the document, then discover the misalignment at integration.
• Pricing build-vs-buy on unit cost alone, missing the adapter-layer and vendor-support-risk cost that dominates over the program's life.
• Verifying a requirement against the as-built implementation rather than the baseline requirement it was derived from, which certifies the system does what it does, not what it was supposed to do.
• Overcorrection: freezing every interface immediately, including ones still in genuine trade study, which forces premature design commitment and generates change-control churn once the trade study concludes differently.
• Letting TPM margin erosion ride until the next scheduled review instead of escalating at the threshold, because no single data point looks alarming in isolation.

Worked example

Situation. Ground-station communications subsystem for a satellite program, $4.2M subsystem budget, two interfacing subsystems (antenna control unit, built in-house; mission data recorder, vendor-supplied). Trade study: build the software-defined radio (SDR) front end in-house, or buy a COTS SDR module.

Naive read. Procurement quotes COTS SDR at $310K vs. in-house build estimated at $540K engineering cost — COTS looks like a $230K win, recommend buy.

Expert reasoning. The COTS module's data interface is a vendor-proprietary serial protocol, not the ICD's baselined interface to the mission data recorder (which expects a specific packetized format at 40 Mbps). Closing that gap requires a custom adapter: 6 engineer-weeks at a loaded rate of $9,500/week = $57K, plus the adapter itself becomes a new subsystem needing its own verification (2 additional VCRM lines, ~$18K in test time). Vendor's data sheet also states end-of-life support commitment of 3 years; the satellite program's operational life is 8 years, so a mid-life redesign for a discontinued module is a real, if deferred, cost — budgeted conservatively at $95K (60% of a fresh 6-week integration effort, risk-adjusted for probability of occurring, informed by the vendor's own EOL history on two prior product lines).

Reconciling total cost of integration:

• COTS: $310K (unit) + $57K (adapter dev) + $18K (adapter verification) + $95K (risk-adjusted EOL redesign) = $480K
• In-house: $540K (engineering), interface is native to the ICD from day one, no adapter, no EOL risk (team maintains it for program life) = $540K

Gap narrows from a naive $230K to an actual $60K, and the in-house option removes a schedule dependency on a vendor's EOL timeline landing mid-mission. Program office's risk tolerance (stated in the program's risk management plan) treats any single point of schedule dependency beyond year 5 as a red risk regardless of dollar delta under $75K — so the $60K gap does not clear the bar to override the schedule-risk preference for in-house.

Deliverable — trade study memo excerpt, filed against ICD-SDR-014:

Recommendation: Build SDR front end in-house. Cost comparison (total integration cost, not unit price): COTS $480K (unit $310K + adapter dev $57K + adapter verification $18K + risk-adjusted EOL redesign $95K) vs. in-house $540K. Delta: $60K in favor of COTS. Overriding factor: COTS vendor's 3-year EOL commitment falls inside the program's 8-year operational life, creating a mid-mission redesign dependency. Per program risk management plan §4.2, any schedule dependency landing after year 5 is a red risk independent of cost delta below $75K. Decision: in-house build selected; $60K cost delta accepted as the price of removing a red schedule risk. ICD-SDR-014 remains unchanged (native interface, no adapter layer). VCRM updated to remove the two adapter-verification lines.

Going deeper

• references/artifacts.md — filled ICD skeleton, N² interface matrix, VCRM, TPM tracking table, build-vs-buy total-cost worksheet.
• references/red-flags.md — smell tests for interface drift, unverifiable requirements, and TPM margin erosion.
• references/vocabulary.md — terms of art (baseline, allocation, verification vs. validation, and more) generalists misuse.

Sources

• INCOSE Systems Engineering Handbook, 4th ed. (Wiley, 2015) — V-model, requirements allocation, verification/validation distinction.
• NASA Systems Engineering Handbook, NASA/SP-2016-6105 Rev2 — TPM tracking, technical baseline management.
• The Open Group, TOGAF 9.2 — enterprise architecture artifact patterns referenced for interface/ICD documentation discipline.
• Eberhardt Rechtin, Systems Architecting: Creating & Building Complex Systems (Prentice Hall, 1991) — interface-as-contract framing, build-vs-buy total-cost reasoning.

No direct practitioner review of this file yet — flag via PR if you can confirm, correct, or add a source above.