Map Projections
There are three map projections used on the Solo
Spirit web site: Mercator, Polar Stereographic, and Orthographic
projections.
Mercator Projection
Features in a Mercator map are projected from the
center of the globe onto a cylinder placed over the globe and
touching it at the equator. The unwrapped cylinder forms the map.
In a Mercator projection, lines of latitude and longitude are
straight lines and intersect at right angles. The spacing between
lines of longitude is constant. However, the spacing between latitude
lines increases as the latitude increases. In fact, the distance
between latitude lines becomes infinite at the poles. The shapes
of small features are accurately shown on a Mercator map, but
the sizes of some features are distorted. The classic example
of size distortion on a Mercator map is the apparent large size
of Greenland relative to North America. An important navigation
feature of Mercator maps is that the true heading between two
points is shown as a straight line.
Polar Stereographic and Orthographic Projections
A Polar Stereographic projection is formed by projecting
features onto a plane that touches the globe at one of the poles.
The projection point is located at the other pole. The Orthographic
projection is formed by projecting features onto the same plane.
The projection point is infinitely far away. For this flight,
the South Pole is at the center of the map with lines of longitude
as straight lines radiating from the map center. Lines of latitude
are presented as a series of concentric circles centered on the
South Pole. As in the Mercator projection, the shapes of small
features are accurately shown on a South Polar Stereographic projection.
The sizes of features on this map type are distorted. The South
Polar Stereographic projection is often used for showing the polar
regions of the globe.
Reference:
Snyder, J. P., 1987, Map Projections – A Working
Manual, USGS Prof. Paper 1395.
Science Measurements
The Solo Spirit uses a GPS (Global Positioning System)
instrument to determine horizontal position (latitude and longitude),
altitude, ground speed, and heading, with the help of special
GPS satellites orbiting the Earth. The GPS records measurements
once every ten seconds. More information about the GPS can be
found at http://tycho.usno.navy.mil/gps.html.
The flight path of Solo Spirit shown on the maps on the web site
is based on the latitude and longitude measurements recorded by
the GPS. The other payload measurements are made by individual
temperature, pressure, humidity, and accelerometer sensors.
Latitude
Latitude lines are imaginary circles on the globe
that run parallel to the equator. The equator is a circle halfway
between the North and South Poles, and is defined as zero latitude.
For science data, latitudes north of the equator are positive,
whereas latitudes south of the equator are negative. The poles
are at 90 and –90 degrees latitude, respectively, for the North
Pole and the South Pole. Latitude values in the Solo Spirit data
files are determined by the GPS instrument. Latitude is given
in degrees and decimal minutes for reporting of the balloon's
position on the Solo Spirit home page.
Longitude
Longitude lines are imaginary lines that connect
the North and South Poles. Longitude is divided into 360 degrees.
The 0 degree longitude (the Prime Meridian) was selected by international
agreement to pass through the observatory at Greenwich, England.
Historically, both east and west directions have been called positive,
so it is best to always specify whether the longitude is east
or west of the prime meridian. For science data, east longitudes
are given as positive values and west longitudes are negative
values. Longitude values in the payload data files are determined
by the GPS instrument. Longitude is given in degrees and decimal
minutes for reporting of the balloon's position on the Solo Spirit
home page.
Altitude
The altitude measured by the GPS is the height of
Solo Spirit in meters above a reference ellipsoid. The specific
reference ellipsoid used is known as WGS84, which represents the
Earth as an ellipsoid with an equatorial radius of 6378.137 km
and a polar radius of 6356.752 km. Note that the altitude values
on the Solo Spirit web site are distances above the ellipsoid,
which are not always the same as the distance above the ground.
Ground speed
Speed can be measured relative to many things. The
payload ground speed parameter tells how fast the Solo Spirit
balloon is going relative to the ground. Ground speed is determined
from changes in Solo Spirit's position that are measured by the
payload GPS instrument.
Heading
The Palantir Technology Demonstrator Payload heading
parameter is the compass direction in which Solo Spirit is traveling,
expressed as 0 to 360 degrees clockwise from north. Thus, headings
of 90, 180, and 270 represent east, south, and west, respectively.
Heading is determined from measurements made by the Palantir Technology
Demonstrator Payload GPS instrument.
Total distance
The total distance parameter represents the cumulative
distance traveled by Solo Spirit. It is determined by computing
a series of distances between points along the WGS84 reference
ellipsoid. The points used in determining the total distance are
the latitude and longitude positions from the Solo Spirit GPS
instrument at the time of each downlink. The total distance value
on the Solo Spirit home page is based on the time of the most
recent Solo Spirit downlink.
Elapsed time
The elapsed time parameter is the difference between
the time Solo Spirit launched and the time of an Solo Spirit downlink.
The elapsed time value on the web page is based on the time of
the most recent downlink.
UTC
UTC (Universal Time, also known as GMT -- Greenwich
Mean Time) is the international standard time. It is the time
for the timezone of Greenwich, England at 0 degrees longitude.
The values of UTC in the data files and graphs are derived from
the Solo Spirit GPS instrument. Times are shown in the international
standard time format of yyyy-mm-ddThh:mm:ss for year, month, day,
hour, minute, and second.
For more information on UTC and time see:
National Institute of Standards and Technology
(NIST)
US Naval Observatory Time Service
Balloon time
The balloon time in the Solo Spirit flight path
file is the local time determined from the longitude of Solo Spirit.
It assumes that the timezone changes by 1 hour for every 15 degrees
of longitude. (In 360 degrees of longitude, you get 24 hours.)
The Solo Spirit time can be up to several hours different from
the actual local timezone because timezone boundaries often are
significantly different from longitude boundaries. Times are shown
in the international standard time format of yyyy-mm-ddThh:mm:ss
for year, month, day, hour, minute, and second. See the section
on UTC for more information about times.
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