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Introduction
The Aerobot Science Payload on the Solo Spirit Mission is on loan from the Jet Propulsion Laboratory. It is being flown under NASA Missouri Space Grant Consortium auspices. The payload contains instrumentation to measure atmospheric pressure, temperature, relative humidity, upwelling sky radiance, and vertical wind velocity, along with Solo Spirit's geographic position, altitude, speed and heading. These data are relayed every hour to the Earth and Planetary Remote Sensing Laboratory at Washington University for processing, validation and posting on the web. The experiments simulate an aerobot (unmanned balloon) mission that may fly in the atmosphere of Mars early in the next century. The Solo Spirit Mission gives us an opportunity to learn how to perform aerobot-type measurements and operations and to involve students in these activities. It is also a chance to learn about Earth's atmosphere. Overview of Payload
The Solo Spirit Payload was developed by the Jet Propulsion Laboratory Planetary Aerobot Program. The payload weighs approximately 7.5 pounds. It has outside dimensions of approximately 21 x 10 x 8 inches. It is designed to operate on 12-15 volt batteries and communicate on an hourly basis via an Inmarsat Mini-M satellite telephone. To conduct the science measurements during the Solo Spirit Mission, approximately 40 pounds of batteries have been added to the main battery loads carried by the gondola.
The left photo shows the package that houses the Aerobot science payload instruments. The right photo is of the satellite communications system used to downlink the data from the payload. Sensors on the Solo Spirit Aerobot Science Payload
Sensor Measurements Position Sensor (GPS) Time, global position, ground speed, heading Pressure Sensor Atmospheric pressure in Pascals Humidity Sensor Relative humidity in percent External Temperature Sensor Temperature in degrees Celsius Radiometer Upwelling sky radiance Vertical Wind Sensor Vertical wind speed relative to the balloon
Mission Operations
The Aerobot Science Payload on Solo Spirit transmits its data back to the Payload Operations Center (POC) at Washington University via an Inmarsat Mini-M satellite telephone. The atmosphere data are stored on board Solo Spirit and sent down approximately once per hour. The data are received, calibrated and converted to physical units (degrees C, Pascal, meters per second, etc.), and then posted on the Solo Spirit web site.
The left photo shows the electronics inside the Aerobot payload instrument package. The photo on the right shows the payload control panel inside the gondola (circled in red) and other equipment on Solo Spirit. The data are used to update the Solo Spirit flight path on a set of maps, including a topographic map of Earth, a map of country boundaries, and current weather satellite images. Atmospheric data for the last hour's measurements are plotted on several graphs. These maps and graphs are posted on the web site. The data files themselves are also posted on the web site for users to download and analyze in more detail. All maps, graphs and data files are updated on an hourly basis throughout the Solo Spirit mission. In addition, weather satellite images are updated once every three hours.
Map Projections
There are three map projections used on the Solo Spirit Aerobot Science Payload 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 Aerobot Science Payload on 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 Aerobot payload measurements are made by individul temperature, pressure, humidity, wind, and radiance sensors (see above).
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 Aerobot data files are determined by the GPS instrument. Latitude is given in decimal degrees for science data, and 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 Aerobot data files are determined by the GPS instrument. Longitude is given in decimal degrees for science data, and degrees and decimal minutes for reporting of the balloon's position on the Solo Spirit home page.
Altitude
The altitude measured by the Aerobot 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 Aerobot 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 Aerobot 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 Aerobot GPS instrument.
Heading
The Aerobot 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 Aerobot 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 Aerobot 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 Aerobot downlink. The cumulative data file on the Science Data page contains total distance values as a function of time throughout the Mission.
The total distance traveled is not the distance used for long distance flight records.
Elapsed time
The elapsed time parameter is the difference between the time Solo Spirit launched and the time of an Aerobot downlink. The elapsed time value on the web page is based on the time of the most recent Aerobot downlink. The cumulative data file on the Science Data page contains values for elapsed time throughout the Mission.
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 Aerobot Science Payload data files and graphs are derived from the Aerobot 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
International date and time format
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.
Temperature
The temperature parameter represents the outside air temperature in degrees Celsius. (0 degrees Celsius is 32 degrees Fahrenheit.) The temperature sensor samples the atmosphere once per minute. Note that the temperature measurements by Aerobot may be affected by heat from burning fuel on Solo Spirit. Pressure
The pressure parameter represents the air pressure at the altitude of Solo Spirit. The pressure is sampled once per minute. Pressure values are given in Pascals (Pa). The average atmospheric pressure at sea level is 101325 Pa, which is equal to 1013.25 mbar, 1.013 bars, or 29.92 inches of mercury. Relative humidity
The relative humidity parameter represents the water vapor content of the air outside Solo Spirit in a percentage from 0 to 100%. A 0% value would be dry air, and 100% is saturated air (maximum water vapor the air can hold). The amount of water vapor that air can hold is dependent on temperature. Note that the humidity measurements by Aerobot may be affected by burning fuel on Solo Spirit. The relative humidity is sampled once per minute. Sky radiance
The sky radiance sensor measures upwelling radiance from the surface and sky every ten seconds. The measurement averages over the visible and near-infrared wavelengths (0.4 to 1.0 micrometers) at which the Earth receives light from the sun. Looking down, the radiance detector observes light reflected by the surface, by clouds, and by atmospheric gas and particles. The sky radiance measurements are given as relative values between 0 and 1. Vertical wind speed
Vertical wind speed measures wind speed relative to Solo Spirit, in meters per second, sampled once per second. It is measured by a tachometer connected to a propellor that is oriented to detect vertical wind movement. To get the vertical speed of the air, you need to take into account the vertical motion of the balloon itself. Wind speed values may be positive or negative; a positive value means the wind is moving downward relative to the balloon.
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This page was last updated
July 14, 1998 19:30 UTC |
