How JSR 385 could have Saved the Mars Climate Orbiter CodeMotion Milan October 2019

How JSR 385 could have Saved
the Mars Climate Orbiter
Werner Keil & Thodoris Bais
24-25 October, 2019
@thodorisbais@wernerkeil

Mars Climate
Orbiter
• Launched on 11
December 1998 
• 338 kilograms 
• $125 millions 
• To study Martian
climate, atmosphere
and surface changes

Result of the Mission

Mars Climate Orbiter
“The problem here was not the error; it was the
failure of NASA’s systems engineering, and the
checks and balances in our process, to detect the
error. That’s why we lost the spacecraft.”
Edward Weiler 
NASA associate administrator of space science, 
IEEE Spectrum: Why the Mars Probe went oﬀ course

More unit mishaps

Problem Statement

What is the Fundamental
Problem?
Primitive (Java) types are primitive types

What is the Fundamental Problem?
• static final double C = 1079252849; 
• static final double SPEED_OF_LIGHT = 1079252849; 
• static final double SPEED_OF_LIGHT_IN_KM_PER_H = 1079252849;
static final Quantity<SPEED> SPEED_OF_LIGHT =
Quantities.getQuantity(1079252849, Units.KILOMETRE_PER_HOUR);

Werner Keil Thodoris Bais
Spec Lead JSR-385 Expert Group Member JSR-385
Let’s meet

Rolling your Own Library
• Development
• Maintenance
• Upgrades & extensions

What is JSR-385?

JSR-385: Units of Measurement API 2.0
Description

This JSR is a major evolution of the Unit API 1.0 (JSR 363) specification. Focused
on the SI System redefinition, modularity and support for Java SE 8/9 and above.
JSR 363

This JSR specifies Java packages for modelling and working with measurement
values, quantities and their corresponding units.

JSR-385 Basic Concepts
• Dimensions

• Units

• Quantities

• Preﬁxes

• Converters

• Formats

• Systems of units

Quantities

–Wikipedia
“A physical quantity is a physical property of a phenomenon, body,
or substance, that can be quantiﬁed by measurement.”
–Ibidem
“A physical quantity can be expressed as the combination of a
magnitude expressed by a number - usually a real umber - and a
unit.”
Deﬁnition of a Physical Quantity

Speed of Light
c = 299,792,458 m/s
Value Unit

–Wikipedia
“A physical quantity is a physical property of a phenomenon, body, or substance,
that can be quantiﬁed by measurement.”
“A physical quantity can be expressed as the combination of a magnitude
expressed by a number - usually a real umber - and a unit.”
Deﬁnition of a Physical Quantity
–Ibidem
“The same physical quantity can be represented equivalently in many unit
systems.”

Speed of Light
c = 299,792,458 m/s
= 1,079,252,849 km/h

Units

Base Units

Derived Units

Multiples
• km

• ms

• Mm?

• ks?

Dimensions

Dimensions
• L: length

• T: time

• M: mass

• I: electric current

• Θ: thermodynamic temperature

• N: amount of substance

• J: luminous intensity

• (1)
Force: L · M · T-2

Summary Table

Doing the Maths

Equality and Equivalence
• kΩ ≠ mHz

• kΩ ≠ mΩ

• kΩ ≡ kΩ

• N ≅ kg·m·s-2

• kg·m-2 ≅ mg·mm-2
• 1 kΩ ≠ 1 mHz

• 1 kΩ ≠ 1 mΩ

• 1 kΩ ≡ 1 kΩ

• 1 N ≅ 1 kg·m·s-2

• 1 kg·m-2 ≅ 1 mg·mm-2

• 1 kΩ ≅ 1,000 Ω
Units Quantities

Quantity Addition
• 1 Ω + 1 Ω = 2 Ω

• 1 Ω + 1 mΩ = 1.001 Ω

• 1 mΩ + 1 Ω = 1,001 mΩ

• 1 Ω + 1 m “does not compute”

CodeMotion Milan School
October Exam on Maths
How much is 0 °C + 0 °C?
• 0 °C
• 0 °C
• 273.15 °C
Two absolute temperatures
0 °C + 0 °C = 273.15K + 273.15K
= 546.30K
= 273.15 °C

• 0 °C
• 0 °C
• 273.15 °C
An absolute temperature and a temperature change
0 °C + 0 °C = 273.15K + 0K
= 273.15K
= 0 °C

• 0 °C
• 0 °C
• 273.15 °C
Two temperature changes
0 °C + 0 °C = 0K + 0K
= 0K
= 0 °C

273.15 °C (546.30 K)
0 °C + 0 °C = 0 °C (273.15 K)
0 °C (0 K)
Answer

Multiplication
Units
•kg × m = kg·m
•kg × mm = kg·mm
•g × km = g·km ≟ kg·m
•m × m = m2
•km × km = km2
•mm × km = mm·km ≟ m2
•µm × m = µm·m ≟ mm2
Quantities
•2 kg × 3 m = 6 kg·m

Division
Units
•kg / m = kg·m-1
•kg / mm = kg·mm-1
•g / mm = g·mm-1 ≟ kg·m-1
•m2 / m = m
•m / m = 1
•km / km = 1
•km / m = km/m ≟ 1k
Quantities
• 3 kg / 2 m = 1.5 kg·m-1

Powers
Units
•(m)2 = m2 
•(km)2 = km2
•(m2)2 = m4
Quantities
• (3 m)2 = 9 m2
•

Celsius Revisited
Units
•(K)2 = K2 
•(°C)2 ≟ (K - 273.15)2
•m × °C ≟ m·(K - 273.15)
Quantities
•0 °C2 ≟ 74,610.9225 K2
•1 m°C ≟ 1 mK or 273.151K

Parsing Units & Quantities
•299,792,458 m/s 
•299792458 m/s 
•299792.458 km/s 
•299,792.458 km/s 
•299,792,458 km*s-1 
• 299,792.458 km×s-1
•299,792.458 km·s-1 
•299,792.458 km·s⁻¹ 
•299,792.458 km*s^-1 
•1,079,252,849 km/h 
•1,079,252,849 km/t
• 670,616,629 mph
Speed of Light

Systems of Unit
Systems of UnitSystems of Unit
• Metric System
• Imperial System
• US customary units
• Roman units of measurement
•Norwegian Units of Measurement

Demo Time

What Was the Fundamental
Problem Again?

JSR-385 Current Status
• JCP page

• https://jcp.org/en/jsr/detail?id=385

• GitHub repositories

• https://github.com/unitsofmeasurement

• Current status

• Final Release

Java Community Process

We’re running for an Associate Seat!

Becoming a JCP Member

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