# Radio Sounding in the Magnetosphere and Topside Ionosphere

.

BW Reinisch^1^, DM Haines^1^, RF Benson^2^, K Bibl^1^, G Cheney^1^, SF
Fung^2^, IA Galkin^1^,

JM Grebowsky^2^, JL Green^2^, X Huang^1^, R Manning^3^, and WWL
Taylor^4^

1.  University of Massachusetts Lowell, Center for Atmospheric Research,
    Lowell, MA

2.  Goddard Space Flight Center, NASA, Greenbelt, MD

3.  Observatoire Paris-Meudon

4.  Raytheon STX, GSFC, Greenbelt, MD

**Abstract.** Two space borne instruments are being developed to
remotely sense plasma distributions in the magnetosphere and the topside
ionosphere. The first instrument is the Radio Plasma Imager (RPI); a low
frequency sounder designed to sweep from 3 kHz to 3 MHz, which will be
part of NASA's IMAGE mission to be launched in January 2000. While in
the magnetospheric cavity (7 R~E~ altitude), RPI will receive echoes
from the magnetopause and the plasmasphere and will measure the direct
response of the magnetosphere's configuration to changes in the solar
wind. With three orthogonal dipole antennas, the angle of arrival of
returning echoes will be determined with high accuracy. Similar to
modern groundbased ionosondes, RPI will operate like a low-frequency
radar system measuring range, location, plasma density, and motion of
the reflecting plasma structures.

The second instrument is the TOPside Automated Doppler Sounder (TOPADS)
which is being developed for the Ukrainian WARNING mission scheduled for
launch in 2001. At a high inclination circular orbit, TOPADS will
measure the topside vertical electron density profiles every 75-km along
its orbit. Three orthogonal 20 m tip-to-tip dipole antennas will be used
for reception, one of them for transmission. Operating as a
high-frequency radar, TOPADS will for the first time provide topside
plasma velocities by remotely tracking the motions of plasma
irregularities. Automatic processing of the ionogram data will provide
real time electron density profiles that will be made freely available
in real-time as a space weather diagnostics tool.

## 1 Introduction

![](media/image1.wmf){width="6.8in" height="0.19236111111111112in"}For
the first time, modern sounding techniques will be applied to the remote
sensing of plasma structures above the peak of the F2 layer. The Radio
Plasma Imager (RPI) will be flying on NASA's magnetospheric Explorer,
the IMAGE satellite, to be launched in January 2000, and the Topside
Automated Doppler Sounder (TOPADS) is being developed for the Ukrainian
WARNING mission. Radio sounding is a well-established technique that was
first deployed in the 1920's for ionospheric sounding from the ground
(Breit and Tuve, 1926). In recent years, advanced digital sounders were
developed for groundbased observations that provide detailed information
on the structure and dynamics of the bottomside ionosphere (Reinisch,
1996). Radio sounding relies on total reflection of radio waves from
plasma structures that have plasma frequencies f~N~ equal to the radio
frequencies f. It is, therefore, not possible on the ground to receive
echoes reflected from the topside ionosphere or the magnetosphere (i.e.,
from plasma less dense than the F-layer peak). Topside ionospheric
sounders, first developed by Franklin and Maclean (1969), recorded the
amplitudes and echo delay times of ionospheric echoes as function of
frequency in the same way it was done by the groundbased sounders of the
time. Data from the highly successful Alouette/ISIS topside sounders
(Jackson et al., 1980; Jackson, 1986) were the basis for nearly 1,000
scientific publications (Benson et al., 1998) The typical frequency
range of these analog sounders was 0.1 to 20 MHz corresponding to the
plasma frequencies in the ionosphere (Pulinets, 1989). For
magnetospheric sounding the required frequency range is 3 kHz to 3 MHz,
corresponding to electron densities N~e~ ≈ 10^5^ to 10^11^ m^-3^, since

The potential use of IMAGE data for space weather analysis and
forecasting is summarized in Table 1.

+------+----------------------+---------------------------------------+
| **   | **Data Product       | **Derived Quantities**                |
| Imag | (Level-1)**          |                                       |
| er** |                      |                                       |
+------+----------------------+---------------------------------------+
| **R  | -   Plasmagram       | -   Distance to Magnetopause,         |
| PI** |                      |     Plasmapause,                      |
|      | -   Echo Map         |                                       |
|      |                      | > Polar Cusp (when observed)          |
|      | -   f(t) Spectogram  |                                       |
|      |     (1/orbit)        | -   Magnetospheric shape (with        |
|      |                      |     model), structure,                |
|      |                      |                                       |
|      |                      | > Gross irregularities                |
|      |                      |                                       |
|      |                      | -   Storm conditions from a           |
|      |                      |     plasma/radio wave Perspective     |
+------+----------------------+---------------------------------------+
| -    | -   FUV auroral      | -   Structure and intensity of the    |
|   ** |     image at (121.6  |     Proton Aurora                     |
| SI** |     nm)              |                                       |
|      |                      | -   Structure and intensity of the    |
|      | -   FUV auroral      |     Electron Aurora                   |
|      |     image (135.6 nm) |                                       |
+------+----------------------+---------------------------------------+
| -    | -   Aurora Image     | -   Size and shape of Auroral Oval    |
|  **W |     (140-190 nm)     |     from intensities                  |
| IC** |                      |                                       |
|      |                      | > Of LBH bands                        |
+------+----------------------+---------------------------------------+
| **G  | -   Lyman alpha      | -   Integrated line-of-sight density  |
| EO** |     image (121.6 nm) |     map of the Earth's geocorona      |
+------+----------------------+---------------------------------------+
| -    | -   Plasmasphere     | -   Plasmaspheric density structure   |
|  **E |     He^+^ image      |     and plasmaspheric processes       |
| UV** |     (30.4 nm)        |                                       |
+------+----------------------+---------------------------------------+
| -    | -   Neutral atom     | -   Neutral atom image of composition |
| **HE |     image (10-500    |     and energy of the Ring Current    |
| NA** |     keV)             |     and near-Earth Plasma Sheet       |
|      |                      |                                       |
| **ME | -   Neutral atom     | -   Both instrument data combined to  |
| NA** |     image (1-30 keV) |     make a provisional Dst index      |
|      |                      |                                       |
|      |                      | -   Plasma Sheet and Ring Current     |
|      |                      |     injection dynamics, structure,    |
|      |                      |     shape and local time extent       |
+------+----------------------+---------------------------------------+
| -    | -   Neutral Atom     | -   Neutral atom composition and      |
| **LE |     Image (10-500    |     energy of the Auroral/Cleft ion   |
| NA** |     eV)              |     fountain                          |
|      |                      |                                       |
|      |                      | -   Ionospheric outflow               |
+------+----------------------+---------------------------------------+

**Table 1.** IMAGE space weather measurements

## 2 The Radio Plasma Imager on the IMAGE Mission

NASA's IMAGE mission is scheduled for launch from Vandenberg, CA, in
January 2000. The 13.5 hour orbit will be highly elliptical (Figure 1)
with 7 R~E~ altitude at apogee and 1,000 km at perigee. The scientific
objective of the mission is to determine the dynamic characteristics of
the magnetosphere in response to changes in the solar wind. Remote
sensing from a single satellite using imaging techniques can capture the
changing characteristics of the magnetospheric plasma structures, from
the magnetopause to the auroral ionosphere. IMAGE, which will be NASA's
first Medium-class Explorer (MIDEX) mission, will carry three different
types of sensors: neutral atom imagers, UV imagers, and the radio plasma
imager; J. L. Burch of Southwest Research Institute in San Antonio, TX,
is the principal investigator of the IMAGE mission. The list of
instruments and the instrument lead investigators is shown in Table 2.
This ensemble of instruments will address the following questions: (1)
What are the dominant mechanisms for injecting plasma into the
magnetosphere on substorms and magnetic storm time scales? (2) What is
the directly driven response of the magnetosphere to solar wind changes?
(3) How and where are magnetospheric plasmas energized, transported, and
subsequently lost during storms and substorms? The cartoon in Figure 2
illustrates the "field of views" of the different instruments, and it is
immediately evident that RPI "sees" plasma structures in a different way
than do the other instruments. The "plasmagram" shown on the left side
of Figure 2 displays echo delay time versus frequency. RPI employs an
active technique: short radio pulses are transmitted into all directions
and echoes received from plasma structures with f~N~ = f and gradient
vectors parallel to the incident wave normal.

![](media/image2.wmf){width="4.183333333333334in"
height="3.1881944444444446in"}

Figure 1. **Orbital characteristics of IMAGE**

+-----+---------------+----------------------------------+-------------+
| **I | **            | ### C                            | **Principal |
| mag | Measurement** | ritical Measurement Requirements | Inve        |
| e** |               |                                  | stigators** |
+-----+---------------+----------------------------------+-------------+
| #   | Neutral atom  | *FOV*: 90ºx90º (image ring       | T.E. Moore, |
| NAI | composition   | current at apogee).              | GSFC        |
|     | and           |                                  |             |
|     | en            | *Angular Resolution*: 8ºx8º      | D.T. Young, |
|     | ergy-resolved | (LENA), 4ºx8º (MENA), 4ºx4º      | SwRI        |
|     | images over   | (HENA),                          |             |
|     | three energy  |                                  | D.          |
|     | ranges:       | *Energy Resolution (\_E/E*): 0.8 | Hamilton,   |
|     |               |                                  | Univ. of    |
|     | 10-500 eV     | *Composition*: distinguish H and | Maryland    |
|     | (LENA)        | O in magnetospheric and          |             |
|     |               | ionospheric sources,             |             |
|     | 1-30 keV      | interstellar neutrals and solar  |             |
|     | (MENA)        | wind.                            |             |
|     |               |                                  |             |
|     | 10-500 keV    | *Image Time*: 4 minutes (resolve |             |
|     | (HENA)        | sub-storm development).          |             |
|     |               |                                  |             |
|     |               | *Sensitivity*: effective area 1  |             |
|     |               | cm^2^ for each sensor.           |             |
+-----+---------------+----------------------------------+-------------+
| *   | 30.4 nm       | *FOV*: 90ºx90º (image            | B.R.        |
| *EU | imaging of    | plasmasphere from apogee),       | Sandel,     |
| V** | plasmasphere  |                                  | Univ. of    |
|     | He^+^ column  | *Spatial Resolution*: 0.1 Earth  |             |
|     | densities.    | radius from apogee.              | Arizona     |
|     |               |                                  |             |
|     |               | *Image Time*: several minutes to |             |
|     |               | hours resolve plasmaspheric      |             |
|     |               | processes).                      |             |
+-----+---------------+----------------------------------+-------------+
| *   | Far           | *FOV*: 15ºx15º (SI) for aurora   | S.B. Mende, |
| *FU | ultraviolet   | (image full Earth from apogee),  | Univ. of    |
| V** | imaging of    | 1ºx360º for geocorona, and       | California, |
|     | the geocorona | 22.4ºx30º (WIC)                  | Berkeley    |
|     | at 121.6 nm   |                                  |             |
|     | (GEO) and the | *Spatial Resolution*: 70 km      |             |
|     | aurora at     | (WIC), 90 km (SI)                |             |
|     | 140-190 nm    |                                  |             |
|     | (WIC) and     | *Spectral Resolution*: separate  |             |
|     | 121.6 and     | cold geocorona H from hot proton |             |
|     | 135.6 nm (SI) | precipitation (\_\~0.2 nm near   |             |
|     |               | 121.6 nm); reject 130.4 nm and   |             |
|     |               | select 135.6 nm electron aurora  |             |
|     |               | emissions.                       |             |
|     |               |                                  |             |
|     |               | *Image Time*: 2 minutes (resolve |             |
|     |               | auroral activity).               |             |
+-----+---------------+----------------------------------+-------------+
| *   | Remote        | *Density range*: 10^5^-10^11^    | B.W.        |
| *RP | sensing of    | m^-3^ (determine electron        | Reinisch,   |
| I** | electron      | density from inner plasmasphere  | Univ. of    |
|     | densities and | to magnetopause).                | Ma          |
|     | m             |                                  | ssachusetts |
|     | agnetospheric | *Spatial resolution*: 500 km     | Lowell      |
|     | boundary      | (resolve density structures at   |             |
|     | locations     | the magnetopause and             |             |
|     | using         | plasmapause).                    |             |
|     |               |                                  |             |
|     | Radio         | *Image Time*: 1 minute (resolve  |             |
|     | sounding.     | changes in boundary locations.   |             |
+-----+---------------+----------------------------------+-------------+

**Table 2.** IMAGE instruments

![](media/image3.wmf){width="4.751388888888889in"
height="4.470138888888889in"}

**Figure 2.** IMAGE observations

**2.1 Radio Plasma Imaging**

**2.1.1 The RPI Instrument**

The RPI functions as a radar transmitting radio waves simultaneously in
all directions and measuring the angle-of-arrival, amplitude, Doppler
frequency, and delay times of all echoes (Green et al., 1998). It uses
three orthogonal dipole antennas for reception, two of which are 500-m
tip-to-tip antennas in the spin plane; these two dipoles are also used
for transmission. The third antenna along the spin axis is 20 m long and
is used for reception only. Right and left-hand polarized signals can be
transmitted by feeding the currents into the long-wire antennas ±90^0^
out of phase. This produces a fairly uniform radiation pattern with
maxima in the direction of the spin axis and a 3 dB minimum in the spin
plane. In a feasibility paper, Calvert et al. (1995) had shown that a 10
W transmitter together with low-noise receivers provide adequate signal
to noise ratios to determine range and direction of echo signals up to a
distance of at least 4 R~E~. . Unlike radio sounding on the ground,
magnetospheric sounding is not limited by the interference from
atmospheric and man-made sources, but by natural external noise, mainly
solar noise bursts and storms (Bougeret et al., 1984), auroral
kilometric radiation (Gurnett et al, 1979), and the non-thermal
(escaping and trapped) continuum (Gurnett, 1975). In comparison, the
receiver noise and cosmic noise are usually of less importance but may
dominate at certain times and locations. Figure 3 shows a block diagram
of RPI, and its specifications are listed in Table 3.

![](media/image4.wmf){width="5.469444444444444in"
height="5.645138888888889in"}

**Figure 3.** RPI system configuration

+---------------+------------------+-----------------------------------+
| **ITEM**      | **               | **Comments**                      |
|               | Specifications** |                                   |
+---------------+------------------+-----------------------------------+
| # R           | ≤ 40 W           | ≤10 W/monopole                    |
| adiated Power |                  |                                   |
+---------------+------------------+-----------------------------------+
| Antennas      | 3 orthogonal     | Two electronically tuned 500m     |
|               | dipoles          | dipoles in spin plane             |
|               |                  |                                   |
|               |                  | one 20m dipole along spin axis    |
+---------------+------------------+-----------------------------------+
| Frequencies   | 3 kHz--3 MHz     | 10^5^-10^11^ m^-3^                |
+---------------+------------------+-----------------------------------+
| Bandwidth     | 300 Hz           | Matched to pulse width            |
+---------------+------------------+-----------------------------------+
| Pulse Width   | 3.2 ms           | ⇒ 480 km range resolution         |
+---------------+------------------+-----------------------------------+
| Pulse         | 0.5, 1, 2, 4,    |                                   |
| Repetition    | 10, 20 pps       |                                   |
| Rate          |                  |                                   |
+---------------+------------------+-----------------------------------+
| Range         | 5 R~E~           | SNR depend.                       |
+---------------+------------------+-----------------------------------+

**Table 3.** RPI specifications

**2.1.2 Imaging the Magnetospheric Structures**

Electromagnetic wave propagation in a magnetoionic medium is anisotropic
and only waves with the so-called *characteristic* wave polarizations
can propagate. The two characteristic polarizations are generally right
and left-hand elliptical, and the two characteristic waves propagate
with different phase and group velocities, leading to Faraday rotation
effects, different plasma cutoff frequencies (reflection levels), and
different echo delay times for plasma sounder observations. Since the
magnetospheric plasma is inhomogeneous the characteristic polarization
slowly changes along the ray path without significant mode conversion,
unless steep gradients are encountered, or when f ≈ f~p~, where f~p~ is
the plasma frequency and f the wave frequency.

RPI uses a quadrature sampling technique to digitize the intermediate
frequency available at the receiver output, the same way it is done in
the groundbased Digisondes (Bibl and Reinisch, . This makes it easy to
adopt the technique of finding the wave normal of the echo signal
described by Shawhan (1970). The **E** field is measured with the three
orthogonal antennas at two times separated by a quarter cycle (t=Each
3-antenna measurement determines an instantaneous field vector **E**(t),
and the two quadrature samples give a pair of orthogonal **E** vectors,
**E**~i~ **=** (I~x~, I~y~, I~z~) and **E**~q~ = (Q~x~, Q~y~, Q~z~),
measured on the x, y, and z antennas. The cross product
**E**~i~x**E~q~** determines the orientation of the wave normal **n** of
the arriving wave (Figure 4):

![](media/image5.wmf){width="6.8in" height="0.45069444444444445in"}

![](media/image6.wmf){width="6.177777777777778in"
height="2.3125in"}**Figure 4.** RPI antenna system and the polarization
ellipse.

Here it is assumed that that the E field component in the direction of
the wave normal is negligible, which is a good approximation as long as
fp \<\< f at the satellite location. If the **E** vector rotates
clockwise when looking in the direction of the wave normal,
**E**~i~x**E~q~** points in the direction of **n**, otherwise it points
opposite to **n**. This ambiguity need be resolved with the help of the
plasmagram and models of the magneto- and plasmapause. The sense of
rotation of the **E** vector is determined by the direction of the
**E**~i~x**E~q~** vector. The accuracy of the angular measurement can be
expressed in terms of the voltage signal-to-noise ratio SNR):

![](media/image7.wmf){width="6.9in" height="0.28194444444444444in"}

For a signal to noise ratio of 100, or 40 dB, the angular accuracy is of
the order of 1^°^.

**2.2 RPI data displays**

The distance to the reflection point is approximately given by the
virtual range P' = \_ ct~d~ (c = free space speed of light, t~d~ = echo
delay time) if f~p~ \<\< f for most of the propagation path between the
spacecraft and the reflection point. Accurate target distances can be
calculated from the plasmagrams giving P' as function of frequency by
"inverting" the P'(f) plasmagram traces. Simulated traces of
plasmasphere and magnetopause echoes are shown in Figure 5 for an
assumed plasma distribution. The figure shows the "ordinary wave"
(Budden, 1985) echoes only; the extraordinary echo traces (not shown)
only differ slightly. The satellite altitude is assumed as 6 R~E~ where
the model plasma frequency is about 3 kHz (N~e~ = 10^5^ m^-3^). The
maximum frequency reflected from the magnetopause is 41 kHz,
corresponding to N~E~ = 1.8x10^3^ m^-3^. So-called profile inversion
programs (Jackson, 1969; Huang and Reinisch, 1982) are available that
calculate the N~e~(R) profiles from the P'(f) traces (X. Huang, personal
communication).

It is difficult to present RPI's multi-dimensional sounding data in one
image, since for each sounding frequency, echoes can arrive from
different directions with different delays (ranges). The most direct
display is the plasmagram, which is similar to an ionogram, but it does
not show the angles-of-arrival. A complimentary image will be the
echo-map, a simulation of which is shown in Figure 6. Here the measured
reflection points are projected into the orbital plane and a model
plasmasphere, magnetopause and cusp is superimposed. The plasma
frequency for each reflection point is indicated by the gray shading
(color in the planned browse product displays). In this simulation,
echoes were received from the plasmasphere/ionosphere, the magnetopause,
and the cusp. Uniform gray shading is used for the "ghost" echo
locations produced by the 180° ambiguity in the arrival angle.

![](media/image8.wmf){width="5.518055555555556in"
height="4.294444444444444in"}

**Figure 5.** N~e~ distribution (dashed curve) as function of altitude
(left scale), and the simulated plasmagram (right scale) with echoes
from the plasmasphere and magnetopause.

![](media/image9.wmf){width="4.7in" height="3.6083333333333334in"}

**Figure 6.** Simulated RPI echo-map with superimposed model of the
magnetosphere.

3.  **Topside Sounder**

A TOPside Automated Doppler Sounder (TOPADS) is being designed for
flight on the Ukrainian WARNING mission, tentatively scheduled for
launch in 2001. The design concepts for TOPADS are derived from UML's
highly successful groundbased ionosonde, the Digisonde Portable Sounder
(DPS) (Reinisch et al., 1997). The same imaging technique described for
RPI will be used by TOPADS. The frequency range of the topside sounder
must be different, of course, to measure ionospheric densities from
10^8^ to 10^13^ m^-3^, i. e., 0.5 to 30 MHz. A high inclination circular
orbit at 600 to 800 km altitudes is planned. The specifications for
TOPADS are summarized in Table 4. Three orthogonal 20-m dipole antennas
will be used for reception, and one of them for transmission. The
transmit antenna will be electronically tuned to optimize the
transmission efficiency. Ten watts of radiated power will produce
signal-to-noise ratios of 30 to 40 dB, making use of the digital
processing gain obtained by applying the pulse compression and Fourier
analysis techniques developed for the DPS. Use of a staggered sequence
of 33.3 s pulses will expand the Doppler frequency range to ±1 kHz,
large enough to avoid aliasing caused by the satellite motion.

*By measuring the angle--of-arrival and the Doppler shift of the
ionospheric echoes it will be possible to determine the vertical
electron density in real time, and also to calculate the velocity of
ionospheric irregularities. Figure 7 illustrates the different echoes
that can be expected at the satellite. By identifying and disregarding
the oblique echoes, the vertical echo trace can be automatically scaled
and the vertical N~e~ profile calculated (Reinisch and Huang, 1982;
Huang and Reinisch, 1982). The availability of topside profiles will be
a valuable input to space weather forecasting. By analyzing the Doppler
frequencies for the different echoes it will be possible to determine
the drift velocity of the plasma irregularities similar to the method
used by the groundbased Digisondes (Cannon et al., 1991; Smith et al.,
1998).*

+--------------+--------------+--------------+-----------------------+
| **System     | **Nominal**  | **Limits**   | **Rationale**         |
| Parameter**  |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Radiated     | \< 10 W, 5%  | 10 W         | Produces adequate SNR |
| Power        | or 10% duty  |              |                       |
|              | cycle        |              |                       |
+--------------+--------------+--------------+-----------------------+
| Frequency    | 2-18 MHz     | 0.5 - 30 MHz | Measure in-situ and   |
| Range        |              |              | F-layer plasma        |
|              |              |              | densities             |
+--------------+--------------+--------------+-----------------------+
| Frequency    | 2.5% steps   | 1 Hz to 500  | Provides 5% in plasma |
| Steps        |              | kHz          | density resolution    |
+--------------+--------------+--------------+-----------------------+
| T            | 10 s         | \> 4 s       | 75 km along orbit     |
| ime/ionogram |              |              | path                  |
| or echo-map  |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Maximum      | 3000 km      | 3000 km      | vertical and ducted   |
| Range        |              |              | echoes                |
+--------------+--------------+--------------+-----------------------+
| Minimum      | 40 km        | 10 km using  | No reception is       |
| Range        |              | short pulses | possible during       |
|              |              |              | transmission          |
+--------------+--------------+--------------+-----------------------+
| Range        | 256 x 2.5 km | 2.5, 5, 20   | Sample period = range |
| Inc          |              | km           | resolution            |
| rements/iono | 32 x 20 km   |              |                       |
|              |              |              |                       |
| /echo-map    |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Waveforms    | Compl. phase |              |                       |
|              | code         |              |                       |
|              |              |              |                       |
|              | Staggered    |              |                       |
|              | pulses       |              |                       |
|              |              |              |                       |
|              | Chirp        |              |                       |
+--------------+--------------+--------------+-----------------------+
| Pulse Rep    | 200 Hz or    | 50, 100, 200 | Provides required     |
| Rate         | SPS          | Hz or        | unambiguous range and |
|              |              |              | Doppler range         |
|              |              | SPS          |                       |
+--------------+--------------+--------------+-----------------------+
| Pulse        | 33.3 s       | 1 or 8 x     | 5 km resolution /     |
| Width/iono   |              | 33.3 s       |                       |
|              | 266.7 s      |              | 40 km resolution      |
| /echo-map    |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Rec.         | 30 kHz       | 30 kHz       | Consistent with XTR   |
| Ba           |              |              | pulse width           |
| ndwidth/iono | 4 kHz        | 4 kHz        |                       |
|              |              |              |                       |
| /echo-map    |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Rec.         | 500 nV       |              | Keeps receiver noise  |
| Sens         |              |              | below cosmic noise    |
| itivity/iono | 125 nV       |              |                       |
|              |              |              |                       |
| /echo-map    |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Coherent     | 80 ms        | 40 ms - 10 s | Provides processing   |
| Integr./iono |              |              |                       |
|              | 10 s         |              | gain & Doppler        |
| /echo-map    |              |              | resolution            |
+--------------+--------------+--------------+-----------------------+
| Doppler      | 12 Hz        | 0.1 Hz       | Given by coherent     |
| Res          |              |              | integration time      |
| olution/iono | 0.1 Hz       |              |                       |
|              |              |              |                       |
| /echo-map    |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Doppler      | ±1 kHz       | ±1 kHz       | SPS waveform          |
| Range/iono   |              |              | eliminates Doppler    |
|              | ±100 Hz      |              | ambiguities           |
| /echo-map    |              |              |                       |
+--------------+--------------+--------------+-----------------------+
| Rec.         | 100 s        |              | Specially designed    |
| Saturation   |              |              | monostatic radar      |
| Recovery     |              |              | receiver              |
+--------------+--------------+--------------+-----------------------+
| Amplitude    | 3 dB         | 3/8 dB       | typical display is 3  |
| Resolution   |              |              | dB                    |
+--------------+--------------+--------------+-----------------------+
| Angl         | 2o           | 1o if SNR \> | Identifies echo       |
| e-of-Arrival |              | 40 dB        | direction             |
| Res.         |              |              |                       |
+--------------+--------------+--------------+-----------------------+

#### Table 4. TOPADS specifications

![](media/image10.wmf){width="3.9993055555555554in"
height="3.2666666666666666in"}**Figure 7.** Vertical and oblique echoes
received by TOPADS.

**4. Summary**

Two modern radio sounders are being developed for deployment in the next
few years, one for magnetospheric the other for topside sounding. The
angle-of-arrival and Doppler measurement capabilities will make it
possible to "image" the positions of the plasma boundaries and
irregularities, and give information about the velocities of the plasma
structures.

## References

Benson, R.F., Reinisch, B.W., Green J.L., Fung, S.F., Calvert, W.,
Haines, D.M., Bougeret, J.L., Manning, R., Carpenter, D.L. Gallagher,
J.L., Reiff, P. and Taylor, W.W.L., Magnetospheric radio sounding on the
IMAGE mission, Radio Science Bulletin, No. 285, ISSN 1024-4530,
International Union of Radio Science, URSI, c/o University of Gent,
9-20, 1998.

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