Over the past several years, we have witnessed significantly increased research activities, as well as user interest, in the Precise Point Positioning (PPP) GNSS processing technique. This interest has resulted in not only novel solutions to tackle the major problems in PPP, but also many novel applications of the PPP technique. One of the fundamental challenges of PPP is that due to transmitter and receiver hardware biases, the estimated ambiguity parameters are not integer-valued, and significant convergence time, i.e., tens of minutes, is required for operational precision, i.e., centimetre- to decimetre-level positioning. Convergence period therefore is the limiting factor in wider PPP usage [Bisnath and Gao, 2008]. Read the rest of this entry »
Telemetry from the Russian INTERCOSMOS 24 satellite recorded extremely low frequency (ELF) and very low frequency (VLF) electromagnetic disturbances in the outer ionosphere from an underground nuclear explosion that was detonated at the Novaya Zemlya Island on 24 October 1994 [Mikhailov et al., 2000]. The IC24 satellite observations were obtained at about 900 km altitude within a few degrees of ground zero. The disturbances were attributed to the magnetohydrodynamic excitation of the ionosphere’s E layer by the acoustic wave (Figure 1). Electrons accelerated along magnetic force lines amplify longitudinal currents and generate magnetic disturbances that may also be measured by geomagnetic observatory and satellite magnetometers [von Frese et al., 2009]. Read the rest of this entry »
In November 2006, the International GNSS Service (IGS) switched from relative to absolute antenna phase center modeling. This was one of the most profound changes in the strategy of the various IGS Analysis Centers (ACs) since the start of the IGS in 1994. Phase center corrections relative to the reference antenna AOAD/M_T (Dorne Margolin element with chokerings from Allen Osborne Associates) that caused systematic errors on long intercontinental baselines were replaced by absolute calibration values independent of a reference antenna. Those can be determined, e.g., by a robot able to rotate and tilt the antenna to be calibrated. The application of absolute receiver antenna corrections that were available long before 2006 was only possible due to the simultaneous consideration of the satellite antenna behavior. Read the rest of this entry »
An integration methodology for Carrier phase Differential Global Navigation Satellite System (CDGNSS) attitude determination and relative positioning is presented. It attempts to improve the integer ambiguity resolution performance in terms of Time-To-First-fiX (TTFX). The integer ambiguity resolution performance is evaluated using data from ground and flight tests. The results show that the integrated system can improve the integer ambiguity resolution performance by decreasing the TTFX. The improvement can be a shortening of TTFX by a few seconds to a few minutes. While an improvement is realized when only a single baseline of the attitude determination system is used, the performance is enhanced when multiple baselines are used. Read the rest of this entry »
Over the last several years, GPS technology has made rapid advances in receiver and antenna capabilities. GPS receivers are smaller, require less power, track more satellites, and have digital interfaces available to support multi-beam anti-jam antennas, which provide superior jamming immunity over legacy GPS receivers and antennas. GPS receiver and antenna technology will continue to advance. The competition between manufacturers will push the limits of technology, resulting in smaller and higher performance products.
Today manufacturers of military GPS receivers typically offer multiple form factors with a variety of interfaces. They may all feature common physical interfaces such as RS-422 or RS-232. However, the interface protocols and data message formats supported can vary from GPS receiver to GPS receiver. A typical military GPS receiver will support one or more Interface Control Documents (ICD) that identifies and defines the interface protocol and associated data message formats for the GPS receiver. Read the rest of this entry »
Unexploded ordnance (UXO) refers to explosive weapons such as mines, bombs, bullets, shells and grenades that failed to explode when they were employed. In North America, especially in the US, the UXO is the result of weapon system testing and troop training by the DOD.
National Geodetic Survey (NGS) has embarked upon an ambitious initiative [1] to compute a centimeter accurate gravimetricgeoid for the USA. While the release of the 2009 geoid series is imminent, as of now, NGS’ two most recent geoid models are USGG2003 and GEOID03 [2]. The former is referred to as a gravimetric geoid, having been computed using gravity data, DEM information and EGM96 (a spherical harmonic geopoential model to maximum and degree and order of 360). GEOID03, on the other hand, is referred to as a hybrid geoid because it is the result of the optimal fitting (by least squares collocation) of the gravimetric geoid, USGG2003, to GPS benchmarks (i.e. stations at which both GPS and leveling observations have been made) throughout the USA and along the US-Canadian border. Therefore, it is the hybrid geoid GEOID03 (and not USGG2003) which should be used for conversion between NAD83 ellipsoidal heights (usually observed using GPS) and NAVD88 orthometric heights (which tend to be obtained by traditional leveling techniques). According to [3] the rms fit between USGG2003 and the utilized GPS benchmarks was 2.4 cm. GEOID03, therefore, does not meet the NGS goal of a cm-accurate geoid. Read the rest of this entry »
Recording the atmospheric water vapor variation during the passage of a typhoon is particularly important, as it is the main source of precipitation and a dominant constituent of energy resources related to typhoon dynamics. As technology advances and new methods of making measurements for water vapor sensing are developed or made more economical, it becomes feasible to make comparative measurements of the same parameters using totally independent techniques. One such technique is ground-based GPS meteorology, which enables quick and inexpensive expansion of the number of global upper-air moisture observations for radiosonde and satellite verification. The ground-based GPS provides a temporally invariant and reliable way of sensing precipitable water vapor (PWV) in the atmosphere with reasonably high accuracy under all weather conditions. This study confirms that GPS meteorology is an efficient PWV sensing technique, capable of capturing the complex characteristics of water vapor distribution and its temporal variation during a progress of a severe typhoon. Read the rest of this entry »
An accurate orbit of Low Earth Orbit (LEO) satellites is indispensable for GPS meteorology and the recovery of the Earth gravity field. Therefore an efficient precision orbit determination (POD) technique has been a great research challenge. Most of the orbit determination procedures are designed as dynamic, because the kinematic orbit determination method depends entirely on the observational geometry and GPS signal continuity. In the dynamic approach, all the forces acting on the satellite orbits are computed and integrated numerically to estimate the initial state vectors (position and velocity) and other unknown force parameters (Grejner-Brzezinska, 1995; Bock, 2003). The position and the velocity of a LEO satellite at the following epochs can then be propagated from the initial states using the force models. Therefore, accurate force models are mandatory for the dynamic orbit determination. Read the rest of this entry »
GNSS (Global Navigation Satellite System) common errors are highly correlated within a short time-latency and a short distance, so an RTK correction message generated from a reference station (RS) can easily cancel the errors out. A decorrelation error, however, becomes larger when the correction age gets older or when the distance between the reference and a rover increases, so the single RS generally broadcasts an RTK correction message every second within a 10–20 km baseline. Several-second latency error or some dozens kilometer distance from the RS would make additional error, which are temporal and spatial decorrelation error, and it is large enough to cause a wrong ambiguity fixing and deteriorates the RTK performance. Read the rest of this entry »
Over the past several years, we have witnessed significantly increased research activities, as well as user interest, in the Precise Point Positioning (PPP) GNSS processing technique. This interest has resulted in not only novel solutions to tackle the major problems in PPP, but also many novel applications of the PPP technique. One of the fundamental challenges of PPP is that due to transmitter and receiver hardware biases, the estimated ambiguity parameters are not integer-valued, and significant convergence time, i.e., tens of minutes, is required for operational precision, i.e., centimetre- to decimetre-level positioning. Convergence period therefore is the limiting factor in wider PPP usage [Bisnath and Gao, 2008]. Read the rest of this entry »
