The United States lost the Mars Climate Orbiter spacecraft in 1999 due to a confusion between English and metric units. The spacecraft's navigation team at NASA expected data about small orbital adjustment forces to be in metric units, but the spacecraft team in Colorado provided the data in English pounds of force without including units. As a result, the navigation team calculated the wrong orbital adjustments, causing the spacecraft to go off course and be lost. This incident highlights the importance of consistently using and specifying units when working with physical quantities and spacecraft data.

1. Article-1.6
Units &
Dimensions

2. Lost in space
In September 1999, the United
States lost the Mars Climate Orbiter
as it approached Mars. The loss of
the $125 million spacecraft was due
to scientists confusing English units
and metric units.
Two spacecraft teams, one at
NASA’s Jet Propulsion Lab (JPL)
in Pasadena, CA, and the other at a
Lockheed Martin facility in
Colorado, where the spacecraft was
built, were unknowingly
exchanging some vital information
in different units.
The missing Mars Climate Orbiter

3. Lost in space….
The spacecraft team in Colorado
used English units of pounds of
force to describe small forces
needed to adjust the spacecraft’s
orbit. The data was shipped via
computer, without units, to the
JPL, where the navigation team
was expecting the to receive the
data in metric measure.
The mix-up in units led to the JPL
scientists giving the spacecraft’s
computer wrong information,
which threw the spacecraft off
course.

4. DIMENSION

6. UNITS

7. UNITS
 A number alone is not sufficient to describe
a physical quantity. For example, to say that
"a pipe must be 4 long " has no meaning
unless a unit of measurement for length is
also specified. By adding units to the
number, it becomes clear, "a pipe must be 4
feet long ."
To specify and perform calculations with
physical quantities, the physical quantities
must be defined both in kind and magnitude.

9. Caution !!!

10. Base Units details

11. Base Units details

13. Since the Americans still use the English System
of Units, it is beneficial to learn the relationship

14. Dimensional Analysis
 dimensional analysis is a tool to understand
the properties of physical quantities
independent of the units used to measure
them.

17. 

19.  Before we discuss error in measurements, we
would like to discuss
Significant Figure

20. An indication of the precision of the measurement is obtained from the
number of significant figures in which the result is expressed.
example: 9. 15V
or 9.1569V
The more significant figures, the greater the precision of
measurement.
.
Significant Figure

30. Rounding Off
 Retention of Significant Figures - Rounding off Figures
 The rounding off procedure is applied to retain the required
number of significant figures.
 If the digit coming after the desired number of significant figures
happens to be more than 5, the preceding significant figure is
increased by one, 4.317 is rounded off to 4.32.
 If the digit involved is less than 5, it is neglected and the preceding
significant figure remains unchanged, 4.312 is rounded off to 4.31.
 If the digit happens to be 5, the last mentioned or preceding
significant figure is increased by one only in case it happens to be
odd. In case of even figure, the preceding digit remains unchanged.
8.375 is rounded off to 8.38
while 8.365 is rounded off to 8.36.

The rounding off is done only in the final answer when the
problem involves more than one step.

31. Most electronic instruments are
good to only three significant
digits. When in doubt, for most
engineering analyses, three digits
are usually the maximum that can
be expected.