Non Standard Day 310 — Robotics/Atmosphere

Non-Standard Day 310

Robotics / Atmosphere

The MIL-HDBK-310 worldwide climatic atmosphere: a geopotential altitude in, and the atmosphere's temperature, speed of sound, static pressure and density out. The standard tabulates a temperature and a density against geometric altitude for each climatic extreme, so the block converts first and then interpolates inside the interval the altitude falls in:

• h_gm = h·R_E/(R_E − h), with R_E = 6371010 m
• f = (h_gm − A_k)/(A_k+1 − A_k), then T = T_k + f(T_k+1 − T_k) – linear in temperature
• ρ = ρ_k(ρ_k+1/ρ_k)^f – logarithmic in density
• then P = ρ·(R_sT) and a = √(γTR_s), with R_s = R*/M = 287.05307204706463

Neither pressure nor the speed of sound is tabulated anywhere: both follow from the interpolated pair. Nothing clamps at either end – below the first breakpoint and above the last, both axes are extrapolated.

Ports

• h – the geopotential altitude, in metres. Any size [m,n]; the block works entry by entry.
• T – the temperature, in kelvin. Same size as the input.
• a – the speed of sound, in m/s. Same size as the input.
• P – the static pressure, in pascals. Same size as the input.
• rho – the density, in kg/m³. Same size as the input.

Parameters

• Atmospheric Model Type – which half of the standard is read.
  • Profile – a realistic atmosphere whose extreme occurs at one chosen altitude; for a vehicle traversing the atmosphere vertically.
  • Envelope – the extreme value at every altitude; for a vehicle staying near one altitude.
• Profile Extreme Parameter – which quantity is extreme in the profile tables: High temperature, Low temperature, High density or Low density. Read only when the model type is Profile.
• Profile Frequency Of Occurrence – how often the profile's extreme is met or exceeded: 1% or 10% of the time.
• Profile Altitude Of Extreme Value – the geometric altitude the profile's extreme sits at: 5, 10, 20, 30 or 40 km. The rest of the profile is the atmosphere that goes with that extreme, so this moves the whole table rather than one point.
• Envelope Extreme Parameter – the envelope's extreme quantity: High temperature, Low temperature, High density or Low density. Read only when the model type is Envelope.
• Envelope Frequency Of Occurrence – Extreme values (the worst case recorded) or 1%. The 5%, 10% and 20% envelopes are not offered, and neither are the two pressure parameters – see Notes.
• Sampling Time (s) – zero or less inherits the solver's rate; a positive value runs the block at that period.

Code export

All ten targets: Python, MATLAB, Java, Rust, C, C++, VHDL, Verilog, SystemVerilog and PLC Structured Text.

The selected table is unrolled into a comparison chain at export time rather than shipped as an array: which interval an altitude falls in is a chain of comparisons, and each branch carries only its own four constants – the breakpoint, the interval width, the temperature step and the density ratio. Each of those three combinations is one rounded operation on the table's own values, printed at 17 significant digits, so every backend computes with the same doubles as the block's own simulation. A generated program therefore carries one of the 48 tables and no search.

The three hardware targets are simulation-only: a fractional power and a square root have no Q16.16 form, so values convert at the port boundary and the arithmetic runs in floating point. And a sea-level pressure does not fit a fixed-point port at all – Q16.16 saturates past about 32767, while this atmosphere reaches 1.1×10⁵ Pa at sea level – so a hardware export is for the thin atmosphere above roughly 9 km.

Simulink bridge

Import and export, mapped to Aerospace Blockset's aerolibatmos2/Non-Standard Day 310. Six configuration values cross one for one: Atmospheric Model Type → model, Profile Extreme Parameter → profile_var, Profile Frequency Of Occurrence → profile_percent, Profile Altitude Of Extreme Value → profile_alt, Envelope Extreme Parameter → envelope_var and Envelope Frequency Of Occurrence → envelope_percent.

Two Simulink parameters are always implied: units is always Metric (MKS), this block being metric throughout, and action is always None, that parameter only choosing whether Simulink warns when an altitude leaves the table. spec is MIL-HDBK-310 and is not cosmetic: the same Simulink block serves the COESA Atmosphere Model and Non-Standard Day 210C library entries at a different spec, so naming the standard is what says which atmosphere crossed. The Simulink block defines no SampleTime parameter, measured, so the rate stays on the ICore side.

Notes

• Algebraic and stateless: the outputs depend on this sample alone.
• Not linear, so the block carries no state space and model reduction correctly reports it as unmergeable.
• The altitude is GEOPOTENTIAL, not geometric, and the tables are geometric: the block converts on the way in, as the reference does. At 20 km the two differ by about 63 m, which is a third of a table interval.
• Nothing clamps – below the bottom breakpoint and above the top one the two axes are extrapolated, linearly in temperature and logarithmically in density. Verified against R2026a from −10 km to 150 km.
• The 5%, 10% and 20% envelopes are not offered, and neither are the High pressure and Low pressure envelopes. The standard tabulates only the extreme quantity for those, so Simulink's block shows a single output port there – a port list that moves with a parameter, which this block's fixed four outputs cannot follow.
• A non-positive temperature answers zero on all four outputs, which the reference does not do. Only a long extrapolation above the table reaches it, and the speed of sound would otherwise take the square root of a negative number, which VHDL's SQRT asserts on rather than answering.
• MIL-HDBK-310 is MIL-STD-210C with nine tables changed, measured: 61 of the 70 tables the two standards share are byte-identical, and the nine that differ do so at one or two points each. Non-Standard Day 210C is the same block over the other set.
• Verified against R2026a over all 48 tables – 779 altitudes each, 148 thousand answers – where temperature and the speed of sound are bit-exact and density and pressure sit within one unit in the last place.