The design philosoply of the U.S. defense establishment has generally been to build a system, study it to death to determine what else is needed, then have another series built that incorporates the desired changes. On the other hand, the former Soviet Union tended to make incremental changes to a system as desired. The NAVSTAR and GLONASS systems show this tendency. There are very few blocks of NAVSTAR satellites while there have been many blocks of GLONASS satellites. What is a "block"? A block is a production run of one particular design, incorporating some fundamental change or improvement.
Orbital Characteristics
The NAVSTAR satellites are arranged in six orbital planes spaced equally around the earth. The planes are tilted approximately 55 degrees to the equator. The satellites are generally allowed to "float" in their orbits and aren't rigidly held in position.Block I Satellites
Construction: The Block I satellites were built by Rockwell International. They can operate for 3.5 days between navigation message uploads from the ground.Antenna Subassembly: The L-band antenna array on the Block I satellite is composed of twelve elements arranged in two concentric rings. The elements are hollow cylinders, approx. 51 cm lomg and 7 cm in diameter. The elements are composed of.05 cm thick fiberglass. Each of the elements protrudes from a 15 cm diameter launcher. The launchers for the inner elements are approx. 26 cm tall, while the launchers for the outer rings are approx. 19 cm tall. The inner ring, approx. 15 cm in radius, has four equally spaced elements. The outer ring, approx. 44 cm in radius, has the remaining eight elements in an octagon configuration. The inner and outer rings are fed 180 degrees out of phase with each other, with 90% of the power supplied to the outer array. This antenna configuration results in a "dimpled" dimpled pattern. this pattern is designed to supply even power across the face of the earth.
Attitude Control: Attitude of the Block I satellites can be controlled by momentum wheels, magnetic coils and attitude control thrusters. Attitude control is ordinarily maintained by the momentum wheels and by torquing against the earth's magnetic field. The thrusters are used when the on-board computer senses unusually high momentum for over 109 minutes. The master control station is notified when this occurs.
Nuclear Detonation Sensors: The Block I satellites, starting with SV 8 have carried instrumentation that allows them to monitor nuclear explosions either in the atmosphere or in space in support of the Nuclear Non-proliferation Treaty and the Atmospheric Test Ban. This function was previously fulfilled by the VELA satellite system. There are many more satellites in the NAVSTAR system then VELA satellites, making detection much more reliable. Instruments carried on the Block I satellites include X-ray sensors, Background Sensors, bhangmeters and radio wave detectors.
Nuclear explosions emit an X-ray flash lasting less than 1 microsecond. This flash can be seen by the X-ray flash detectors on several satellites. By measuring the time delay of arrival of the flash at several satellites, the location of the explosion can be determined. Several of the GPS satellites carry background X-ray radiation detectors to provide an accurate record of the X-ray environment around the earth.
The Bhangmeters are optical sensors (radiometers) that look for the characteristic double flash of a nuclear explosion. When a light flash exceeds a preset level, the bhangmeter triggers and records the event. The bhangmeters are designed to reject false triggers and are calibrated by hitting the satellite with a fast-rising laser from the Sandia Laboratories facility in Albuquerque, NM.
Nuclear explosions also emit radio waves, usually called an electromagnetic pulse (EMP). EMP is caused by the primary bomb radiation kicking electrons loose from molecules in the atmosphere. The NAVSTAR satellites carry a special antenna to detect and measure EMP. A radio antenna developed at Los Alamos is used to calibrate the EMP detection system.
Block II Satellites
Construction: The Block II satellite is designed to provide reliable service over a 7.5 year life span. The satellites were built by General Electric Astrospace. The satellite design requires minimal interaction with the ground and allows all but a few maintenance activities to be performed without interruption to the signal broadcast. Periodic uploads of data from the control segment cause no disruption to service.Antenna Subassembly: The L-band antenna array on the Block II satellite is also composed of elements arranged in two concentric rings. The elements are approx. 62 cm lomg but are tapered over the remaining 13 cm. The launchers for both inner and outer rings are 25 cm long cones. Dimensions for the inner and outer ring launchers have identical dimensions. The improved Block II antenna provides slightly greater gain than the Block I antenna.
Block IIA Satellites
The Block IIA satellites are essentially identical to the standard block II with one exception. In the event the ground stations are unable to upload new navigation information, the satellites will continue to transmit the same navigation message for up to 180 days. From a user standpoint, signals from the Block IIAThe orbit of the satellite will change over this period of time. Without periodic navigation updates from the ground, the accuracy of the system will degrade over time. However, the system would at least still be operational.
Block IIR Satellites
The block IIR satellites feature autonomous navigation. That is, they can create their own navigation messages without uploads from the ground. This allows the system to maintain system accuracy for much longer periods between contacts with the ground.
Orbital Characteristics
Block I Satellites
Block II Satellites
Block IIA Satellites
Block IIB Satellites
Block IIV Satellites
Keplerian Elements
Information about spacecraft orbits are commonly called keplerian elements. AMSAT, the amateur radio spacecraft organization, has an excellent tutorial on keplerian elements. A two line format is used by NASA and NORAD. The AMSAT format is more verbose and is used by amateur radio operators.Keplerian elements for many satellites are available from the sources listed below. If these don't work, there is a satellite info page at Stanford with additional sources.
- archive.afit.af.mil
- kilroy.jpl.nasa.gov
- seds.lpl.arizona.edu (mirrors above sites)
Satellite Tracking Software
- SAT TRACK
- SAT TRACK is a realtime satellite tracking and orbit prediction program for UNIX/Linux platforms running color X-windows. It was written by Manfred Bester.
- SatTrak IV
- I don't know who makes SatTrak IV yet, but here it is on a web page maintained by William Roth, N7RYW. It's for IBM PC compatibles.
Observing Satellites
It is possible to see many satellites from the ground. There is a web-based earth satellite ephemeris service at Georgia State University that will tell you when you can see some of the brighter ones - including GPS satellites, I was surprised to see! There is a FAQ that discusses satellite watching.
For more information on satellites in general, check out these web sites:
Johnathan's Space Report
JSR is a weekly report describing all manned and unmanned space launches.
JPL
NASA's Jet Propulsion Laboratory has an excellent tutorial on satellites, spacecraft and the space environment.AMSAT-NA
AMSAT-NA, the Radio Amateur Satellite Corporation builds low cost satellites containing amateur radio equipment. Individuals interested in AMSAT should read their FAQ.
The University of Surrey
The University of Surrey has a spacecraft engineering research group that has participated in the design and launch of several satellites.
Czopek, Francis M. and Lt. Scott Shollenberger, "Description and Performance of the GPS Block I and II L-Band Antenna and Link Budget", ION GPS-93, p.37-43.
Shank, Christopher M., Lt, USAF and John Lavrakas, "GPS Integrity: An MCS Perspective", ION GPS-93, p.465-474.
Higbie, Paul R. and Norman K. Blocker. "Detecting Nuclear Detonations with GPS"
GPS World 5, no. 2 (February 1994) : 48-50