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Bodo W. Reinisch

University of Massachusetts Lowell, Center for Atmospheric Research 450 Aiken Street, Lowell, MA 01854, USA


The capabilities of modern ionospheric sounders are illustrated using results obtained with the Digisonde 256 and the Digisonde Portable Sounder. Unlike any other ionosonde, these instruments routinely perform a variety of complementary operations in real time: automatic ionogram scaling, calculation of electron density profiles, high resolution Doppler and angle of arrival, plasma drift velocity, polarisation, and precision group height.


Today's advanced digital sounders1 are digital HF radars with real time analysis capabilities. This paper shows results from selected stations of the Digisonde network (Fig. 1), including the Digisonde 256 (D256) and the Digisonde Portable Sounder (DPS). Since the D256 has previously been described2,3 it suffices to present the main differences between the DPS and the D256 (Table 1). Low weight (40 kg), low power (500W peak) and pulse-to-pulse software control of the transmission waveforms make the DPS very attractive for routine monitoring and research.

Vertical Incidence Ionogram Sounding

The Digisondes measure all observables of the received signals at each of 128 or 256 ranges: Doppler spectrum (amplitude and phase), angles of arrival, and polarizations. The quiet daytime ionogram (Fig. 2) from El Arenosillo, Spain, illustrates the format applied for routine sounding: 100 kHz frequency steps, 128 x 5 km height increments. Small optically coded numbers represent the echo amplitudes: X polarisation in grey, O in black. The ARTIST-scaled3 traces are marked by the letters E and F. Superimposed on the ionogram is the electron density profile.

For each range, the DPS measures a full complex spectrum with 2N Doppler lines (N = 1,2...7) for both O and X polarisation. This information is compressed into a standard ionogram, and for routine archiving each echo amplitude is amended by Doppler, polarisation and incidence angle. The Ny Alesund ionogram in Fig. 3 shows strong spread on the "overhead" echoes as well as an off-vertical patch of ionisation (light grey) with foF2 = 5.4 MHz, compared to 4.3 MHz for the overhead ionosphere.

Converting the sequence of measured electron density profiles to plasma frequency contours 5 gives the diurnal variations of the E and F layers, as in Fig.4 for two days at Jicamarca, Peru. Spread F developed in the first night when the F layer peak height rose above 500 km. The DPS also measures the three components of the F region plasma velocity, Vvert, magnetic Vnorth and Veast (Fig. 5); the vertical velocity measured by an incoherent scatter radar (ISR) is shown for comparison.

As one of the standard operating modes, Digisondes measure the Precision Group Height (PGH) by transmitting 2 frequencies separated by D f. The phase difference W F measured on the received echoes is proportional to the group path:

where 0 < W F < 2p . The range ambiguity introduced by the factor 2p n (30 km for D f = 5 kHz) is resolved by the amplitude-versus-range profile in the ionogram. The DPS currently outputs PGH data with 1 km increments leading to the accuracy and resolution illustrated in Fig. 6.

Oblique Incidence Sounding

The same instrument can make vertical and oblique soundings synchronised by GPS receivers and special interface software to within 1msec. The oblique echo trace in Fig. 2 is an example for a 860 km link from Tortosa to El Arenosillo in Spain. For oblique sounding6 the DPS can operate with 100% duty cycle, compared to 1% for the D256. A 127 bit binary phase code provides 67msec effective pulse width. Applying the PGH technique to the oblique echoes gives unparalleled group path resolution of 3msec. Fig. 7 shows a 620 km DPS ionogram received at Millstone Hill with only two polarised receiving loop antennas; the DPS at Wallops Island transmitted 200 W peak at10% duty cycle.

Digisonde Drift Measurements

Like the incoherent scatter radar the Digisonde measures the Doppler shifts of signals arriving from different directions. The line-of-site "Doppler velocity" is defined as vr,i = (c/2f)di, where c is the speed of light, f the sounding frequency and di the measured Doppler shift of a signal arriving at an angle of incidence, i. The wide beam transmit antenna illuminates a large area of hundreds of kilometers in the ionosphere and the different reflection points are resolved by combining Fourier transform and interferometry techniques7, producing so-called skymaps8. In Fig. 8 (Millstone Hill), small circles indicate the angles of arrival (zenith and azimuth)9, the lines from the circles point toward the center (positive Doppler) or away (negative Doppler), and the line length is proportional to vr. Since there are always more than 3 sources a least-squares error calculation10,11 is used to determine the plasma bulk velocity12,13 v = (vx, vy, vz).



Table 1. D256 Vs. DPS





Vertical Inc. Ionogram.

128/256 Height

128/256 Heights

2.5, 5.0 and 10km

Oblique Inc. Ionogram.


Pulse or CW

Binary Phase Coding

Peak Trans. Power




Beam Forming




GPS synchronous.



1msec Accuracy

Phase Coding


Inter & Intrapulse

Pulse Compression in DPS


67/133ms 50,100,200Hz PRF

m x 67ms 50,58,100,200Hz PRF

Pulse Compr. in DPS m=1,8,127 or 255

Transmit Antenna


Circular or Linear


Receive Antennas

7 Loops

4 Loops

O/X Switching

Drift Measurements

2 Ranges, 2 Freqs. 128 Dopplers

128 Ranges, 8 Freqs. 128 Dopplers

Skymaps, Drift Velocity

Max. Doppler Range

25 Hz

100 Hz


Doppler Resolution




Prec. Group Height


1 km Accuracy





Real Time

N(h) Profile



Real Time

Output Storage

1/2" Tape, 30MB

QIC, 150MB

QIC also for DGS256









Input Power

110/220Vac, 2kW Avg

24VDC, 300W Avg.

Waveform dependent

Remote Control

Modem, asynchronous.

Modem, asynchronous.


Data Editing






Ionogram and drift data formats of the D256 and the DPS systems are compatible and all the software14,15,16 for data editing and processing, archiving and dissemination developed for the D256 data is applicable to the DPS data.


The Digisondes have been developed and built at the University of Massachusetts Lowell Center for Atmospheric Research. I thank H. F. Yang, J. S. Scali and D. M. Haines for preparing the figures and D. Kenney for editing this paper.


1. Reinisch, B.W., New Techniques in Ground-Based Ionospheric Sounding and Studies, Radio Science, 21, 3, pp. 331-341, May-June 1986.

2. Reinisch, B.W., The Digisonde 256 System and Ionospheric Research, URSI INAG Ionospheric Station Inf. Bulletin, #48, 1986.

3. Reinisch, B.W., K. Bibl, D.F. Kitrosser, G.S. Sales, J.S. Tang, Z.M. Zhang, T.W. Bullett and J.A. Ralls, The Digisonde 256 Ionospheric Sounder, World Ionosphere/ Thermosphere Study, WITS Handbook, Vol. 2, Ed. C.H. Liu, December 1989.

4. Reinisch, B.W. and X. Huang, Automatic Calculation of Electron Density Profiles from Digital Ionograms, 3, Processing of Bottomside Ionograms, Radio Science, 18, 477, 1983.

5. Zhang, Z.M., ARTIST Data Editing and Printing (ADEP) Manual, University of Massachusetts Lowell, Center for Atmospheric Research, 1992.

6. Reinisch, B.W., D.M. Haines and W.S. Kuklinski, The New Portable Digisonde for Vertical and Oblique Sounding, AGARD Proceedings Number 502, pp. 11-1 to 11-11, 1992.

7. Bibl, K. and B.W. Reinisch, The Universal Digital Ionosonde, Radio Science, 13, pp. 519-530, 1978.

8. Bibl, K., W. Pfister, B.W. Reinisch and G.S. Sales, Velocities of Small and Medium Scale Ionospheric Irregularities Deduced from Doppler and Arrival Measurements, Adv. Space Res, XV, pp. 405-411, 1975.

9. Scali, J., A Quality Control Package for the Digisonde Drift Analysis (DDA), Version 2.0, University of Massachusetts Lowell, Center for Atmospheric Research, 1993.

10. Reinisch, B.W., J. Buchau and E.J. Weber, Digital Ionosonde Observations of the Polar Cap F Region Convection, Physica Scripta, 36, 372-377, 1987.

11. Buchau, J., B.W. Reinisch, D.N. Anderson, E.J. Weber, J.G. Moore and R.C. Livingston, Ionospheric Structures in the Polar Cap: Their Origin and Relation to 250 MHz Scintillation, Radio Science, 20, 3, pp. 325-338, 1985.

12. Cannon, P.S., B.W. Reinisch, J. Buchau and T.W. Bullett, J. Geophys. Res., 96 (A2), 1239-1250, 1991.

13. Bullett, T.W., Mid Latitude Ionospheric Plasma Drift: Comparison of Digital Ionosonde and Incoherent Scatter Radar Measurements at Millstone Hill, Doctoral Thesis, University of Massachusetts Lowell, 1994.

14. Gamache, R.R. and B.W. Reinisch, Ionospheric Characteristics Data Format for Archiving at the World Data Centers, Scientific Report No. 3, GL-TR-90-0215, Air Force Geophysics Laboratory, Air Force Systems Command, USAF, Hanscom AFB, Bedford, MA, 1990.

15. Gamache, R.R., T.W. Bullett, Z.M. Zhang, B.W. Reinisch and W.T. Kersey, ADEP Database Record Structure, ULRF-468/CAR, University of Massachusetts Lowell, 1992.

16. Gamache, R.R., Data Format for Station Monthly Characteristics Reports and Validation of Ionospheric Model Study, ULRF-469/CAR, 1992.

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