Volume 53, Issue 4 p. 525-534
Research Article
Free Access

Total Electron Content Retrieved From L-Band Radiometers and Potential Improvements to the IGS Model

Yan Soldo

Corresponding Author

Yan Soldo

NASA Goddard Space Flight Center, Greenbelt, MD, USA

USRA, Greenbelt, MD, USA

Correspondence to: Y. Soldo,

[email protected]

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Liang Hong

Liang Hong

NASA Goddard Space Flight Center, Greenbelt, MD, USA

SAIC, Greenbelt, MD, USA

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Salem El-Nimri

Salem El-Nimri

NASA Goddard Space Flight Center, Greenbelt, MD, USA

ASRC Federal Space and Defense, Inc., Greenbelt, MD, USA

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David M. Le Vine

David M. Le Vine

NASA Goddard Space Flight Center, Greenbelt, MD, USA

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First published: 30 March 2018

Abstract

In recent years, several L-band microwave instruments have been launched into Earth's orbit to measure soil moisture and ocean salinity (e.g., Soil Moisture and Ocean Salinity [SMOS], Aquarius, and Soil Moisture Active/Passive [SMAP]). As the microwave signal travels through the ionosphere, the polarization vector rotates (Faraday rotation) and it is possible to estimate the total electron content (TEC) along the path by measuring this change. A comparison is presented of the TEC retrieved from Aquarius and SMAP over the ocean with the values provided by the IGS (International Global Navigation Satellite System Service (GNSS)). The TEC retrieved from Aquarius and SMAP measurements show good agreement with each other and, on a global scale, are in agreement with the TEC provided by the IGS. However, there are cases in which the TEC from the two satellite sensors are in good agreement with each other but differ significantly from the IGS TEC. The comparison suggests that the L-band instruments are a reliable source of TEC over the ocean and could be a valuable supplementary source of TEC values that could be assimilated in the IGS models, especially over the ocean, where GNSS ground stations are sparse.

Key Points

  • Vertical total electron content (VTEC) is retrieved using measurements from the Aquarius and the Soil Moisture Active/Passive (SMAP) satellite missions
  • Good agreement is observed between these two data sets
  • VTEC values predicted by the International GNSS Services (IGS) model seem to miss or only partially capture some of the features highlighted by the two satellite missions

1 Introduction

Aquarius and SMAP (Soil Moisture Active/Passive) are two satellite missions that were launched in recent years to measure ocean salinity and soil moisture. Both are equipped with microwave radiometers operating in L-band (1.4 GHz). It is expected that future missions (e.g., the Chinese Water Cycle Observation Mission) will be launched soon and with similar instruments.

Because Faraday rotation is significant at L-band, these radiometers have been equipped to measure the rotation angle using the ratio of the third and second Stokes parameters as proposed by S. Yueh (2000). The technique was verified using Aquarius data by Le Vine et al. (2013). Given the Faraday rotation angle (FRA) it is also possible to compute the total electron content (TEC) along the raypath from the surface to the sensor. In this manuscript, a comparison is presented of the TEC retrieved from Aquarius and SMAP over the ocean with the values provided by the International GNSS Service (IGS) (accessed from https://oceandata.sci.gsfc.nasa.gov/api/file_search/).

1.1 SMAP and Aquarius

The Aquarius/SAC-D (Satélite de Aplicaciones Científicas-D) satellite mission was developed by NASA and the Argentinian Space Agency (Comisión Nacional de Actividades Espaciales) (Le Vine, Lagerloef, et al., 2007). The main instrument, Aquarius, included three radiometers operating in the 1400–1427 MHz protected band (Le Vine, Lagerloef, et al., 2007) and aimed at estimating the global distribution of sea surface salinity. Aquarius/SAC-D was launched on June 10, 2011, and the mission was lost on June 7, 2015, due to a failure in the satellite platform (http://aquarius.umaine.edu/cgi/news_more.htm?id=51).

The SMAP satellite mission has one conically scanning radiometer that operates in the same 1400–1427 MHz range. Its main objectives are to retrieve the soil moisture and to determine the freeze/thaw state of the land surfaces (Entekhabi et al., 2010). Although the SMAP observatory was developed for applications over land, it provides global coverage, including the ocean. The SMAP satellite was launched on January 31, 2015, and radiometric measurements are available since March 31, 2015, providing a brief overlap with Aquarius. (Both Aquarius and SMAP include radar, but the analysis here is on the radiometer measurements. The SMAP radar stopped functioning about 5 months after launch.)

1.2 Faraday Rotation

The microwave radiometers on these missions measure Earth's thermal radiation. As the radiation travels through the ionosphere, the birefringent property of the refractive index of the electrically charged medium affects its polarization vector. In particular, the left-circular and right-circular components of the wave propagate at different velocities (see the Appleton-Hartree equation as expressed in Gerson, 1962; Ratcliffe, 1959), which causes a rotation of the polarization vector called Faraday rotation. The change (Faraday Rotation Angle, FRA) can be significant at L-band, depending on solar activity, time of day, and geomagnetic latitude (e.g., Le Vine & Abraham, 2002).

The FRA can be retrieved from a measurement of the second and third Stokes parameters (Le Vine, Lagerloef, et al., 2007; Yueh, 2000). This was demonstrated successfully by Aquarius (Le Vine et al., 2013). Given the Earth's geomagnetic field, which is reasonably well known (Thébault et al., 2015), it is possible to estimate the TEC of the ionosphere along the propagation path traveled from the FRA. This manuscript reports a comparison of the TEC retrieved by Aquarius and SMAP during the short period (3 months) when the two instruments were both operational. The two spacecraft are in sufficiently similar orbits to provide many instances where the radiometers look through approximately the same portions of ionosphere. One motivation was to test the reliability of the TEC retrieval and the calibration of the instruments. It became apparent that the agreement between the two sensors was good and that for the most part the agreement with the IGS product was also good giving credibility to the retrieval. However, there were cases where the TEC retrieved by the sensors were in agreement but differed substantially from the IGS model. Similar problems with the IGS TEC had been reported in the literature (Hernández-Pajares et al., 2009). This suggested that perhaps the TEC derived by SMAP and future L-band radiometers could be used as an additional source to help improve the IGS product, especially over oceans where the existence of ground stations is sparse. The comparison is limited to oceanic surfaces. Over land, spurious signals in the third Stokes parameter make the measurement less reliable (Le Vine & Abraham, 2016) and issues can arise due to the presence of radio frequency interference.

Section 2 provides an overview of the data sets used in this contribution and briefly describes how the FRA can be obtained from Aquarius and SMAP measurements. Section 3 describes the relationship between the TEC and the FRA. Section 4 includes comparisons among the TEC from Aquarius, SMAP, and IGS. The main conclusions of this study are outlined in section 5.

2 Data

2.1 Aquarius

Aquarius/SAC-D is on a Sun-synchronous orbit at 657 km altitude and with a 7-day repeat cycle (Le Vine, Lagerloef, et al., 2007). The Aquarius instrument consists of three real-aperture radiometers observing Earth's surface in push-broom fashion. The radiometers are at fixed look angles (measured with respect to the local nadir direction) of 25.8°, 33.8°, and 40.3° (Le Vine, Lagerloef, et al., 2007) and fixed azimuth angles (measured from the direction perpendicular to orbital plane and positive toward the direction of motion) of 9.85°, −15.29°, and 6.55° (Hilburn et al., 2013). The radiometers 3 dB footprints at the surface have dimensions 76 × 94, 84 × 120, and 96 × 156 km (Le Vine, Lagerloef, et al., 2007).

The timing of the Aquarius radiometers is based on 10 ms samples, of which 9 ms are allocated as radiometer integration times. Sixty of such samples are used to obtain brightness temperatures, which correspond to 540 ms of integration time (Le Vine et al., 2014).

The brightness temperature measurements can be used to retrieve the FRA using the second and third Stokes parameters (Yueh, 2000). In particular,
urn:x-wiley:00486604:media:rds20668:rds20668-math-0001(1)
where Q and U represent the second and third Stokes parameter, ΩF is the polarization rotation (i.e., the FRA), and the TOI and TOA superscripts indicate values at the Top Of Ionosphere and at the Top Of Atmosphere, respectively.
The third and fourth Stokes parameters at the ocean surface are expected to be small (Yueh, 2000). Assuming that they are negligible, equation 1 becomes
urn:x-wiley:00486604:media:rds20668:rds20668-math-0002(2)
which can be solved for the FRA as
urn:x-wiley:00486604:media:rds20668:rds20668-math-0003(3)

Equation 2 has been used in Aquarius processing to measure the FRA in situ and correct for the polarization rotation during the retrieval of sea surface salinity (Wentz & Le Vine, 2011). This procedure assumes that the FRA corresponds to the total polarization rotation angle between surface and the instrument. Whenever that is not the case, the left side of equation 3) is actually the total polarization rotation angle. To obtain the FRA, one would then need to account for the other causes of polarization rotation, such as geometric angle (Meissner & Wentz, 2006) or clocking angle (Le Vine, Jacob, et al., 2007). The Aquarius data processing already accounts for a small geometric angle (about 0.1° for beams 1 and 3 and about 0.5° for beam 2).

The FRA retrieved in this manner was validated by comparing with colocated altimeter measurements (Le Vine et al., 2013). Unfortunately, over land, spurious values of the third Stokes parameter can make the retrieval unreliable.

2.2 SMAP

SMAP is also in a Sun-synchronous orbit, at 685 km altitude and with 8-day repeat cycle.

SMAP is equipped with a conical-scanning real-aperture radiometer at 35.5° look angle (Spencer et al., 2010). The radiometer 3 dB footprint measures 39 × 47 km at the surface (Piepmeier et al., 2017), but the SMAP swath is 1,000 km wide due to its conical scanning motion (Entekhabi et al., 2010).

The timing of the SMAP instrument is based on the Pulse Repetition Intervals (PRIs) of the radar that are approximately 350 μs long. For each PRI, the radiometer integration time is 300 μs. The brightness temperatures are computed after averaging 32 PRIs, which correspond to 9.6 ms of total integration time (Piepmeier et al., 2014).

SMAP measurements of the brightness temperature are used to retrieve the FRA using the same procedure as employed by Aquarius (e.g., equation 3).

2.3 IGS

GNSS satellites transmit signals at two different frequencies to the GNSS ground stations. These two frequencies (1227.6 and 1575.42 MHz) are affected differently by the ionosphere and the two signals are therefore received with different phases. The phase difference between the two signals is then used to estimate the Slant TEC (STEC) along the path traveled by the signals. The Vertical TEC (VTEC) is estimated from the STEC (Hernández-Pajares et al., 2009).

However, the global maps of IGS VTEC are obtained from a relatively sparse network of ground stations, and the lack of ground stations is particularly significant over the oceans (Hernández-Pajares et al., 2009). In fact, IGS VTEC maps are distributed with a rather coarse spatial resolution (2.5° latitude by 5° longitude). The temporal resolution of the IGS maps is 2 hours. Also, a comparison with altimeter measurements suggests errors of several TEC Units (TECU) in the vicinity of large peaks in TEC (Hernández-Pajares et al., 2009).

In this study, the IGS VTEC data (downloaded from https://oceandata.sci.gsfc.nasa.gov/api/file_search/) have been interpolated, in space and time, to the Aquarius middle beam. The IGS computes the VTEC at the altitude of GNSS satellites, that is, 20200 km. In order to correct for the difference in altitude between IGS and the spaceborne radiometers, a correcting factor has been applied. This correcting factor changes along the orbit and it has been computed from the NeQuick model (https://t-ict4d.ictp.it/nequick2) as the ratio between the VTEC predicted by NeQuick at 657 km (Aquarius altitude) and at 20200 km (GNSS altitude).

3 Retrieval of TEC From Aquarius and SMAP

The total polarization rotation at a given frequency is determined by the TEC along the path of the electromagnetic wave and the geometry between the direction of observation and the geomagnetic field lines. In particular, the FRA, along the boresight ray can be written as (Le Vine & Abraham, 2002)
urn:x-wiley:00486604:media:rds20668:rds20668-math-0004(4)
where urn:x-wiley:00486604:media:rds20668:rds20668-math-0005 and νB(s) are the plasma frequency and electron gyrofrequency, respectively, and ΘB(s) is the angle that the path makes with the local magnetic field. A good approximation at L-band for the boresight ray is (Le Vine & Abraham, 2000, 2002)
urn:x-wiley:00486604:media:rds20668:rds20668-math-0006(5)
where B is the magnitude of the Earth's magnetic field, θ is the angle between the boresight ray and the normal to the surface, and VTEC is the TEC along the vertical path between the spacecraft and the surface.
The magnetic field lines are computed at the point of intersection between the line of sight and a sphere of radius 6771 km (mean Earth radius plus 400 km). This point is called the Ionosphere Pierce Point (IPP) and it is computed as follows:
urn:x-wiley:00486604:media:rds20668:rds20668-math-0007(6)
where ψIPP and φIPP represent the latitude and longitude of the IPP; ψ and φ the latitude and longitude of the spacecraft; the orientation of the radiometer is determined by the azimuth angle (γ) and the elevation angle (δ, which is the angle complementary to the look angle); and χIPP is an angle defined as
urn:x-wiley:00486604:media:rds20668:rds20668-math-0008(7)

where R is the Earth's radius and h is the satellite altitude.

The intensity and the orientation of the geomagnetic field are computed using the coordinates of the IPP and using the IGRF (International Geomagnetic Reference Field) model (Thébault et al., 2015), whose data are available at https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html.

I, D, and B indicate the inclination, dip angle, and intensity of the geomagnetic field at the IPP, and the TEC can be computed as
urn:x-wiley:00486604:media:rds20668:rds20668-math-0009(8)
urn:x-wiley:00486604:media:rds20668:rds20668-math-0010(9)
urn:x-wiley:00486604:media:rds20668:rds20668-math-0011(10)

Equation 9 becomes singular when the direction of the line of sight is perpendicular to the magnetic field lines, that is, when cosΘB is zero. Also, when cosΘB is near zero, small variations (e.g., due to radiometric noise) in the measured parameters induce large variations in the estimated STEC. In order to avoid these cases in our analysis, we computed VTEC only when ΘB is smaller than 75° or larger than 105°, that is, 15° away from the perpendicular direction.

4 Comparison

4.1 Geometry

Aquarius and SMAP do not observe exactly the same portion of the ionosphere, but when aligned, they are very close. Figure 1 illustrates the geometry of Aquarius and SMAP measurements. The two panels depict the same instant from two different perspectives (side and top views). The particular instant represented in Figure 1 was chosen to be when the Aquarius middle beam, which has approximately the same look angle as the SMAP radiometer, is colocated with the rightmost 3 dB footprint in the SMAP scan; in this configuration, Aquarius and SMAP observe approximately the same portion of the ionosphere.

Details are in the caption following the image
Geometry of the Aquarius and SMAP observations. Blue and red correspond to Aquarius and SMAP, respectively. The two panels depict the same instant, in which both satellites sample approximately the same portion of the ionosphere. IPP = Ionosphere Pierce Point; SMAP = Soil Moisture Active/Passive.

In Figure 1 (left), the spacecraft are moving away from the observer and into the page. The blue color is associated with Aquarius and the red with SMAP. The points N and N′ represent the subsatellite points for Aquarius and SMAP, respectively. The solid lines are the boresight rays. These intersect the Earth's surface in points B1, B2 and B3, corresponding to the Aquarius inner, middle, and outer beam, respectively. For SMAP, scanning motion makes its boresight ray rotate around the nadir direction of the satellite; Figure 1 (left) shows only one boresight ray, corresponding to the rightmost orientation (i.e., to scan angle equal to 270°). In Figure 1 (right), the observer is above the satellites, which are moving as indicated by the black arrows. The blue ellipses represent the location of the Aquarius 3 dB footprints at a given instant. The red ellipse represents the location of the SMAP 3 dB footprint; different ellipses correspond to slightly different times during one conical scan. Note that Figure 1 only shows few of the SMAP 3 dB footprints that the radiometer makes during the conical scan; in reality the radiometer measurements are oversamples in the scan direction (i.e., the 3 dB footprints overlap). Only a sector of the SMAP conical scan is represented in the figure. This is the sector that has been used in this study to retrieve the VTEC from SMAP; it was chosen so that Aquarius and SMAP would observe approximately the same portion of the ionosphere.

Since Aquarius is equipped with three radiometers operating simultaneously, for every instant we can obtain three estimates of the VTEC. In this study, the Aquarius VTEC are averages of the VTEC estimated from all three beams. During the time needed for Aquarius to measure one brightness temperature (1.44 s; Le Vine et al., 2014), the spacecraft has advanced by about 10.8 km along its orbit.

The scan angle in SMAP is defined as zero in the direction of the satellite's motion and positive counterclockwise. Only the data samples corresponding to scan angles between 225° and 315° (corresponding roughly to the geometry in Figure 1, right) have been used in study, so that Aquarius and SMAP would observe approximately the same portion of the ionosphere. For every conical scan, there are usually 58 data samples within the range [225, 315]° scan angle. This means that for every conical scan, we can estimate the VTEC 58 times. The values of SMAP VTEC reported in the following sections are averages of all the 58 VTEC values obtained from single data samples. In the time needed to take 58 data samples (0.97 s), the SMAP radiometer has rotated along the conical scan (by 90°) and the satellite has moved forward in its orbit by about 7.3 km. (In the SMAP radiometer timing sequence, some PRIs are dedicated to measurements and others to calibration. Each data sample is made up by 32 “measurements” PRIs and 16 “calibration” PRIs. Therefore, time elapsed while taking 58 data samples is 350 μs × (32 + 16) × 58 = 0.97 s).

As a result of the different instrument designs and integration times, the Aquarius VTEC correspond to 3 × 540 ms (= 1.62 s) of radiometer integration time, whereas the SMAP VTEC correspond to 58 9.6 ms (~0.56 s). This is for the case where no data samples have been discarded because the propagation ray was almost perpendicular to the geomagnetic field line (i.e, ΘB is close to 90°). The standard deviations are on the order of 1 TECU with SMAP and a little smaller with Aquarius as one would expect because of the larger integration time for Aquarius.

The spacing between VTEC retrievals (10.8 and 7.3 km for Aquarius and SMAP, respectively) is small compared to the scale of the variations in VTEC that will be presented in the following sections.

4.2 Colocations Between Aquarius and SMAP

The Aquarius and SMAP orbits are close to the day-night terminator, and they cross the equator at 6 p.m. local time (ascending passes) and at 6 a.m. local time (descending passes). Because of the different altitudes, the orbits are not synchronized, but periodically they are closely aligned. For purposes of this analysis, Aquarius and SMAP are considered as colocated when the SMAP footprint corresponding to 270° scan angle (i.e., the “rightmost” footprint) is within 100 km of the ground track of the boresight of the Aquarius middle beam (Figure 1; the 3 dB footprint of this beam is about 84 × 120 km).

4.3 Comparison Among Aquarius, SMAP, and IGS on Single Passes

Figure 2 shows a comparison of VTEC retrieved from Aquarius and SMAP and as predicted by the IGS model. In this figure, the values of Aquarius VTEC have been obtained by applying the formulas in section 3 for all three Aquarius radiometers and by averaging the resulting three values of VTEC (corresponding to each beam). In the case of SMAP, the values reported in this figure are the average of the VTEC values obtained during each scan, using all the data samples between 225 and 315° scan angle.

Details are in the caption following the image
Example of comparison of VTEC estimated from Aquarius and SMAP measurements during an ascending satellite pass (local evening) with the IGS TEC. This orbit took place on May 30, 2015, around 3 a.m. UTC. (left) VTEC as a function of latitude. (right) Location of Aquarius and SMAP ground tracks. VTEC = vertical total electron content; IGS = International Global Navigation Satellite System Service; SMAP = Soil Moisture Active/Passive.

In Figure 2 (left), the VTEC is plotted as a function of latitude. The blue circles and red triangles represent VTEC as retrieved from Aquarius and SMAP, respectively. The black and white dashed line is the VTEC from IGS interpolated at the locations and times of the Aquarius middle beam. The gap in the Aquarius data series between −20° and 0° latitude is due to the fact that all three radiometers are approximately perpendicular to the geomagnetic field lines and therefore their measurements are discarded. SMAP does not have a gap, but its VTEC retrievals become noisier around the same region. This is because although SMAP uses a wider range of directions (see Figure 1, right), around −10° latitude there are fewer measurements that can be averaged. Figure 2 (right) shows the geographical locations of the ground tracks corresponding to Figure 2 (left).

The VTEC from Aquarius and SMAP are close to one another, while the strong gradients of VTEC between latitudes −30° and +30° are represented only partially by the IGS. Similar differences between the IGS TEC at peaks in TEC have been reported in retrievals of TEC obtained from the Jason altimeter (Hernández-Pajares et al., 2009).

Figure 3 shows a similar comparison for a descending satellite pass.

Details are in the caption following the image
Example of comparison of VTEC estimated from Aquarius and SMAP measurements during a descending satellite pass (local morning) with the IGS total electron content. This orbit took place on May 30, 2015, around 3 p.m. UTC. (left) VTEC as a function of latitude. (right) Location of Aquarius and SMAP ground tracks. VTEC = vertical total electron content; IGS = International Global Navigation Satellite System Service; SMAP = Soil Moisture Active/Passive.

During the descending passes (6 a.m. local time at the equator) the VTEC values are smaller and without strong gradients. Again, in this case, there is a good agreement between Aquarius and SMAP. The IGS model appears to overestimate the VTEC over a large portion of the orbit.

4.4 Global Comparison Aquarius-SMAP

To get a representative comparison, all the colocations of Aquarius and SMAP have been computed. These occurred between April and June 2015, when both satellites were operational. Figure 4 shows the difference between the VTEC from Aquarius and from SMAP over several orbits. All the measurements in Figure 4 were made at approximately the same local times: around 6 p.m. for the ascending passes in Figure 4 (top row) and around 6 a.m. for the descending passes in Figure 4 (bottom row). Figure 4 (left column) shows the differences in VTEC for every instant SMAP and Aquarius were colocated. Figure 4 (right column) is smoothed versions of Figure 4 (left column). While the figures on the left allow to see the small-scale variations along the orbits and between orbits, the figures on the right help outline the general trends. Note that Figure 4 (right column) has a narrower color scale.

Details are in the caption following the image
Global comparison of VTEC values estimated by Aquarius and SMAP. Positive values (shades of red) indicate locations where Aquarius retrieved VTEC values higher than SMAP. All the colocations between Aquarius and SMAP are included.

The root-mean-square error (RMSE) between Aquarius and SMAP VTEC is 2.27 TECU for the ascending passes and 2.14 TECU for the descending passes. The bias is 0.66 TECU and −0.41 TECU for the ascending and descending passes, respectively. The difference between Aquarius and SMAP VTEC seems to change sign between north and south of the geomagnetic equator. This difference, small when compared to differences between Aquarius and IGS or between SMAP and IGS, would be consistent with the presence of a clocking angle in either Aquarius or SMAP measurements. A clocking angle would in fact offset the value of the FRA obtained from the instruments and affect the VTEC in opposite ways between north and south of the geomagnetic equator, where the term cosΘB changes sign. More precisely, a clocking angle of about 0.2° in either instrument would minimize these differences. Unfortunately, clocking angles this small are difficult to detect and the hypothesis that one of the two instruments is affected by a small clocking angle could not be verified for the time being.

4.5 Global Comparison Aquarius-IGS

Figure 5 shows a similar comparison between Aquarius and IGS, for the same orbits as in Figure 4.

Details are in the caption following the image
Global comparison of VTEC values estimated by Aquarius and from the IGS model. Positive values (in red) indicate that the VTEC retrieved from Aquarius is higher than the IGS values. The same locations as Figure 4 have been used.

In this case, the RMSE is 4.18 TECU (Figure 5, top row) and 4.65 TECU (Figure 5, bottom row). The bias is −0.87 TECU and −4.07 TECU for the ascending and descending passes, respectively. The error pattern in Figure 5 (top row; 6 p.m.) is similar to that shown in the example in Figure 2 in which IGS tends to underestimate TEC near large peaks. Figure 5 (bottom row) shows a pattern consistent with that shown in Figure 3 in which, in the morning (descending passes), the VTEC from IGS has consistently higher values than those retrieved by Aquarius and SMAP.

4.6 One-Year Comparison Between Aquarius and IGS

To get additional insight into the differences between the TEC retrieved from Aquarius and that reported by IGS, the two products were compared during one year.

Figure 6 shows the RMSE between VTEC from Aquarius and VTEC from IGS as a function of time for the year 2013. Each point (blue lines) corresponds to one ascending (Figure 6, top) or descending (Figure 6, bottom) pass of Aquarius over the ocean. The red line is the running average of the blue line, computed for one orbital cycle (i.e., 1 week = 103 orbits; Aquarius is in an exact repeat orbit that cycles in 7 days.).

Details are in the caption following the image
Comparison of VTEC values form Aquarius and from IGS over one year (2015). The blue line corresponds to each Aquarius half-orbit. The red line is a sliding window average of the blue points corresponding to one Aquarius orbital cycle (i.e., 1 week). The top figure includes the ascending (evening) passes; the bottom figure includes the descending (morning) passes. VTEC = vertical total electron content; IGS = International Global Navigation Satellite System Service; RMSE = root-mean-square error.

The RMSE averaged over one orbital cycle is relatively stable over the year at 4.5–5 TECU for both the ascending and descending passes. There is a slight hint of a seasonal cycle (smaller in the summer/winter and larger in spring/fall), but this is not at all clearly defined. The pattern for ascending and descending passes is similar. However, as shown above (section 4.4.5; Figures 2 and 3), the nature of the difference is not the same for the two half-orbits. In particular, the RMSE for ascending orbits is due to IGS not representing well the rapid increases and decreases in VTEC (similar to the example in Figure 2), while for descending orbits the RMSE is due to a bias between Aquarius and IGS.

5 Conclusions

L-band radiometers in space are important for monitoring soil moisture (SMOS and SMAP) and sea surface salinity (Aquarius). Faraday rotation is important at L-band and a correction for the Faraday rotation is necessary because the retrieval algorithms are polarization dependent. For this reason, these spaceborne radiometers have included a measurement of the third Stokes parameter and an algorithm to retrieve the actual rotation angle in situ. The technique was proposed by Yueh (Yueh, 2000) and validated by Aquarius (Le Vine et al., 2013).

The data presented here show small differences between Aquarius and SMAP. But significant differences can exist between the TEC derived from Aquarius and SMAP and that reported by IGS over the ocean. In particular, the IGS product tends to underestimate in regions of large peaks in the TEC. This has been reported before (Hernández-Pajares et al., 2009) and is a consistent pattern present in both the Aquarius and SMAP retrievals. Although Aquarius is no longer operational, SMAP and SMOS continue to operate and are potential sources of TEC.

It is possible that including TEC from SMAP over the ocean would improve the IGS prediction in the vicinity of peaks such as shown in Figure 2. However, this is only useful over ocean. Over land, the inhomogeneity of the brightness temperature of the scene causes spurious values of the third Stokes parameter and noisy retrievals of FRA and TEC (Le Vine & Abraham, 2016). Techniques to reduce this noise have been proposed (Le Vine & Abraham, 2017) but need to be verified.

Acknowledgments

The Aquarius data are available at ftp://podaac-ftp.jpl.nasa.gov/allData/aquarius/. The SMAP data are available at https://nsidc.org/data/smap/smap-data.html. The IGS data are available at https://oceandata.sci.gsfc.nasa.gov/api/file_search/, searching for N*TEC_IGS*. The NeQuick model can be found at https://t-ict4d.ictp.it/nequick2. The IGRF data are available at https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html. Funding for this research was provided by the NASA Aquarius project.