Different methods have been applied to characterize ionospheric total electron content (TEC). One of the most common comparative studies has been made between direct ionospheric measurements and various ionospheric model predictions. Brown et al.  used the TEC data from a wide range of latitudes and longitudes for a complete range of solar activity to evaluate the performance of six ionospheric models as predictors of TEC. These ionospheric models include (1) the International Reference Ionosphere (IRI), (2) the Bent model, (3) the Ionospheric Conductivity and Electron Density (ICED) model, (4) the Penn State model, (5) the Fully Analytic Ionospheric Model (FAIM), and (6) the Damen-Hartranft model. They made extensive comparisons between monthly mean TEC at all local times and model TEC obtained by integrating electron density profiles produced by the six models. They have found that the models can generally describe the diurnal variations of the ionospheric TEC but can exhibit large discrepancies from direct ionospheric measurements. They suggested that such discrepancies may be caused by inaccurate representation of the topside scale height. Comparative studies between the TECs from Ocean Topography Experiment (TOPEX) measurements for 1992–1997 and the Bent and IRI model predicted TEC values indicated a model underestimation bias of 1.7 (IRI) and 2.2 (Bent) TECU (1 TECU = 1016 el/m2) on average at high latitudes which reflect the absence of auroral contributions in the empirical models. The bias at midlatitudes on the other hand is very small [Codrescu et al., 2001]. A comparison between the Global Positioning Satellite (GPS) derived TECs and the IRI model predictions at midlatitude (Matera, 40.6°N, 24.4°E) during the low solar activity years (1996–1997) indicates an overprediction of GPS TECs with differences of as high as 50% (or 2–4 TECU) of the diurnal TEC depending on season (maximum in winter and autumn, minimum in summer) and time of day (large at nighttime and small at daytime) [Ephishov et al., 2000]. Sethi et al.  use the incoherent scatter radar data from Arecibo (18.4°N, 66.7°W) to evaluate the performance of IRI. They found that the IRI model overestimates the observed TEC for all local times during equinox and summer but shows good agreement in winter.
 Many comparative studies have been conducted between GPS TEC measurements and other direct measurements. Conkright et al.  compared TECs at Boulder, Colorado (40.0°N, 105.2°W), derived from observations of GPS and those obtained using the Faraday rotations from the Geostationary Operational Environmental Satellites (GOES) 2 geosynchronous satellite. They found relatively good agreement for diurnal variations and general agreement in seasonal cycle with sufficient smoothing over space and time. Furthermore, TECs derived from different GPS receivers at two nearby stations spaced about 50 km apart are quite consistent with RMS differences in TEC of 0.3–1.0 TECU. They found that the nighttime GPS TECs are higher than those from GOES 2. Ho et al.  compared the GPS-derived TECs with ionospheric measurements from the TOPEX altimeter. Their results indicated that the difference in the two TEC measurements is less than 1.5 TECU within a 1500 km range from a reference GPS station and that the RMS gradually increases with increasing distance from the station. The differences become relatively large during ionospheric disturbed periods. A similar comparative study using a restricted (within 5° of the zenith angle) TOPEX altimeter data set found that the GPS TECs were in agreement with the TOPEX measured TECs at the 2–3 TECU difference level in the midlatitudes (30°–55°) relative to a typical daily maximum TEC of ∼80 TECU [Mannucci et al., 1994].
 An International GPS Service (IGS) report documented in details on the comparisons between the TOPEX TEC and the GPS TECs from the five IGS Ionosphere Associate Analysis Centers (IAACs): Center for Orbit Determination in Europe (CODE), Astronomical Institute, University of Berne, Switzerland; European Space Operations Center (ESOC) of ESA, Darmstadt, Germany; Jet Propulsion Laboratory (JPL), Pasadena, California, United States; Natural Resources Canada (NRCan/EMR), Ottawa, Ontario, Canada; and Technical University of Catalonia (UPC), Barcelona, Spain (International GPS Service, Performance of IGS ionosphere TEC maps, IGS Ionosphere Working Group report, 16 pp., Technical University of Catalonia, Barcelona, Spain, 2003, available at http://maite152.upc.es/∼ionex3/doc/IGS_IONO_report_April2003_7.pdf.) The TOPEX TEC and the GPS TEC are compared for validations that are required to facilitate GPS TEC mappings to an official operation status. The different IAAC TEC maps have been computed with different approaches but with a common formal resolution of 2 hours in UT and 5° and 2.5° in longitude and latitude using the 2,500,000 TOPEX observations during the period from 15 December 2002 to 15 March 2003. It shows that the GPS TECs have a mostly positive bias (4 out of 5) ranging from 0.8 to 4.8 TECU and one negative bias of –1.0 TECU. The report noted that their comparison provides a lower boundary for the GPS TEC performance.
 In a companion paper [Huang and Roussel-Dupré, 2005], data collected from the Fast On-Orbit Recording of Transient Events (FORTE) satellite received Los Alamos Portable Pulser (LAPP) signals during 1997–2002 are used to derive TECs at Los Alamos, New Mexico. The FORTE satellite was launched on 29 August 1997. It is in a circular, 800-km-altitude orbit inclined 70° from the Earth's equator. The FORTE radio payload is a set of tunable wideband radio receivers (26–100 MHz) to provide data on the propagation of broadband radio signals through the ionosphere. Such data can be used to study ionospheric properties, such as changes in TEC, which produces variations in the amount of dispersion of a transient broadband RF signal [Jacobson et al., 1999]. The LAPP is an electromagnetic pulse generator coupled to a 30 m dish/antenna, located at Los Alamos, New Mexico (35.872°N, 106.327°W, elevation 2274.08 m). Readers can find detailed FORTE-LAPP data descriptions in the work by Huang and Roussel-Dupré .
 We have presented the variabilities of the FORTE-derived TECs at Los Alamos for diurnal, seasonal, interannual, and 27-day solar cycle in the companion paper [Huang and Roussel-Dupré, 2005]. In that paper, we have analyzed the effects of several technical aspects on deriving and converting TEC, including slant-to-vertical TEC conversion, quartic effects on transionosperic signals, and the thin shell assumption. We also examined geomagnetic storm effects on the TEC variance superimposed on the averaged TEC values. We concluded that those effects need to be particularly considered under certain circumstances but they do not affect the results significantly in general. By comparing FORTE-derived TECs to other TEC sources, our main objectives of this paper are (1) to validate the FORTE-derived TECs, (2) to explore if there are any significant discrepancies, and (3) to indicate specific time/conditions for these discrepancies.