Variability of GPS satellite differential group delay biases

An important issue in determining the accuracy of global positioning system (GPS) satellite ionospheric measurements is the instrumental delay biases between the L-band frequencies in both the satellites and the receivers. These differential L1-L2 biases must be measured and removed from the GPS measurements before an accurate estimate of the total electron content can be obtained. The results from the measurements indicate that the day-to-day variations of the satellite differential biases are quite well over a five-week time span, with a variation of less than 0.3-ns differential delay (one sigma). A follow-up experiment conducted two years later showed that the satellite biases had not changed significantly over this longer time span. When the prelaunch calibration values are compared with the experimental bias estimates, two of the four satellite pairs show excellent agreement and two differ significantly, indicating that prelaunch calibrations should be used with caution.<>     

The Toroidal Imaging Mass-Angle Spectrograph (TIMAS) for the polar mission

The science objectives of the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) are to investigate the transfer of solar wind energy and momentum to the magnetosphere, the interaction between the magnetosphere and the ionosphere, the transport processes that distribute plasma and energy throughout the magnetosphere, and the interactions that occur as plasma of different origins and histories mix and interact. In order to meet these objectives the TIMAS instrument measures virtually the full three-dimensional velocity distribution functions of all major magnetospheric ion species with one-half spin period time resolution. The TIMAS is a first-order double focusing (angle and energy), imaging spectrograph that simultaneously measures all mass per charge components from 1 AMU e^–1 to greater than 32 AMU e^–1 over a nearly 360° by 10° instantaneous field-of-view. Mass per charge is dispersed radially on an annular microchannel plate detector and the azimuthal position on the detector is a map of the instantaneous 360° field of view. With the rotation of the spacecraft, the TIMAS sweeps out very nearly a 4 solid angle image in a half spin period. The energy per charge range from 15 eV e^–1 to 32 keV e^–1 is covered in 28 non-contiguous steps spaced approximately logarithmically with adjacent steps separated by about 30%. Each energy step is sampled for approximately 20 ms;14 step (odd or even) energy sweeps are completed 16 times per spin. In order to handle the large volume of data within the telemetry limitations the distributions are compressed to varying degrees in angle and energy, log-count compressed and then further compressed by a lossless technique. This data processing task is supported by two SA3300 microprocessors. The voltages (up to 5 kV) for the tandem toroidal electrostatic analyzers and preacceleration sections are supplied from fixed high voltage supplies using optically controlled series-shunt regulators.     

PEACE: A PLASMA ELECTRON AND CURRENT EXPERIMENT

An electron analyser to measure the three-dimensional velocity distribution of electrons in the energy range from 0.59 eV to 26.4 keV on the four spacecraft of the Cluster mission is described. The instrument consists of two sensors with hemispherical electrostatic energy analysers with a position-sensitive microchannel plate detectors placed to view radially on opposite sides of the spacecraft. The intrinsic energy resolutions of the two sensors are 12.7% and 16.5% full width at half maximum. Their angular resolutions are 2.8° and 5.3° respectively in an azimuthal direction and 15° in a polar direction. The two sensors will normally measure in different overlapping energy ranges and will scan the distribution in half a spacecraft rotation or 2 s in the overlapped range. While this is the fastest time resolution for complete distributions, partial distributions can be recorded in as little as 62.5 ms and angular distributions at a fixed energy in 7.8 ms. The dynamic range of the instrument is sufficient to provide accurate measurements of the main known populations from the tail lobe to the plasmasheet and the solar wind. While the basic structure of the instrument is conventional, special attention has been paid in the design to improving the precision of the instrument so that a relative accuracy of the order of 1% could be attained in flight in order to measure the gradients between the four spacecraft accurately; to decreasing the minimum energy covered by this technique from 10 eV down to 1 eV; and to providing good three dimensional distributions.     

A flat panel display upgrade solution replaces military CRTtechnology

Successful technology insertion programs must satisfy many system constraints in order to incorporate new capabilities into aging avionics systems while meeting program cost requirements. Such constraints frequently include form, fit, and functional replacement specifications, as well as power and electrical performance restrictions. This paper describes a technology insertion program undertaken with the goal of replacing the 30-year-old azimuth indicator display of a radar warning receiver system. This necessitated the use of electroluminescent (EL) display technology to replace the analog cathode ray tube display currently used in the system. Because of the prohibitively high cost of aircraft wiring modifications, the replacement display was required to be completely form, fit, and functionally equivalent to its replacement. The form, fit, and functional equivalency requirement imposed the following system constraints: (1) power consumption of less than 10 Watts; (2) the need to maintain the same stroke-deflection current electrical interface; and (3) the need to meet the maintenance and repair budget of the existing display unit. Additional requirements included night-vision compatibility and full sunlight readability. The display was also required to be MIL-STD-1553 Remote Terminal communication capable. All requirements posed a challenging technology insertion problem to program personnel. The case study described in this paper illustrates the approach to meeting the particular requirements of this technology insertion program