How JSR 385 could have Saved the Mars Climate Orbiter JFall 2019

How JSR-385 could have
Saved the Mars Climate
Orbiter
Werner Keil & Thodoris Bais
#jfall19
@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

What went wrong

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

Lessons Learned
•Read the software speciﬁcations
•Use JSR-385 instead of primitives
•Fail-fast

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);

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

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

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. Focussed
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”

JFall School 2019
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
JFall School 2019

• 0 °C
• 0 °C
• 273.15 °C
Two temperature changes
0 °C + 0 °C = 0K + 0K
= 0K
= 0 °C
JFall School 2019

273.15 °C (546.30 K)
0 °C + 0 °C = 0 °C (273.15 K)
0 °C (0 K)
Answer
JFall School 2019

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
JFall School 2019

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
JFall School 2019

Powers
Units
•(m)2 = m2 
•(km)2 = km2
•(m2)2 = m4
Quantities
• (3 m)2 = 9 m2
•
JFall School 2019

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
JFall School 2019

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?

Setting up a project
<dependencies>
<dependency>
<groupId>tech.units</groupId>
<artifactId>indriya</artifactId>
<version>2.0</version>
</dependency>
</dependencies>

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

How JSR 385 could have Saved the Mars Climate Orbiter JFall 2019

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