M. D. COUSINS, R. C. LIVINGSTON, C. L. RINO, and 1. F. VICKREY

THE HILAT SATELLITE MUL TIFREQUENCY RADIO BEACON

The HILAT beacon, L-band/UHF antenna, and ground receiver system allow us to make efficient phase scintillation measurements. The L-band signal also accepts bi-phase modulated telemetry data from the other scientific instruments by means of the Science Data Formatter. Thus, the L-band sig­ nal serves the dual functions of telemetry channel and phase reference for the scintillation measurements.

BACKGROUND readily compensated for. At VHF, the error is less than 20/0 and it can be ignored. The primary objective of the HILAT satellite pro­ With adequate sampling, it is possible to obtain a gram is to understand the physics of the large-scale continuous phase record for the entire pass; however, electron density irregularities that cause disruptive scin­ the absolute reference level is ambiguous within a mul­ tillation effects on transionospheric radio signals. Ir­ tiple of 21r. To remove this ambiguity, three frequen­ regular variations in the electron density, when cies are transmitted at UHF. The maximum change sufficiently large, act like lenses that focus and defocus in the expected ionospheric integrated electron content the radio waves, causing rapid amplitude variations will cause less than a 21r change in the second differ­ (scintillation) and, ultimately, loss of frequency co­ ence of phase. Thus, the three-frequency measurement herence. can be used to remove the 21r ambiguity. Because of the lens-like action of the ionosphere, the signal phase shift near the disturbed region is pro­ portional to the integral of the electron density evalu­ OPERATION OF THE MUL TIFREQUENCY ated along the propagation path. The amplitude BEACON variation is very small. As the wavefield propagates away from the disturbed regions, amplitude scintilla­ Table 1 lists the frequencies and radiated power of tion develops. Rapid phase excursions, which distort the HILAT beacon. The beacon is made up of eight the high-frequency spectral components, accompany modules arranged in two mounting frames as shown the signal fades. Nevertheless, for weak to moderate schematically in Fig. 1. The oscillator module contains scintillation, the signal phase retains the main charac­ a temperature-compensated crystal oscillator and a teristics of the irregularities in the source region; more­ divider circuit for generating signals at 45.892 and over, the path integral is sensitive to the spatial 22.946 megahertz (MHz). All other signals are gener­ coherence of the irregularities. ated by multiplying and/or mixing the signals. For ex­ For scientific diagnostics, the signal phase is the ample, the UHF modulator mixes the 22.946 MHz quantity of primary interest. The spectral character­ signal with a 413.028 MHz signal (the ninth harmonic istics of the phase scintillation can be related to the of 45.892 MHz) to generate a double sideband spec­ corresponding spectral characteristics of the in situ ir­ trum. The 413.028 MHz signal is reinserted to provide regularities. The systematic changes with changing the three spectral lines at UHF. propagation angles (relative to the direction of the The L-band modulator provides antipodal modula­ earth's magnetic field) can be used to determine the tion of the carrier. The input is a 4098 bit per second spatial coherence (anisotropy) of the irregularities. data stream generated by the satellite science data for­ Measurements with separated antennas give a more di­ matter. The L-band power amplifier consists of a two­ rect measure of the irregularity anisotropy. stage tuned transistor amplifier with an output of 33 The HILAT beacon transmits phase-coherent sig­ decibels referenced to 1 milliwatt. This power level is nals at L band, UHF, and VHF. The L-band signal sufficient to achieve the necessary link margin for good is used as a phase reference to synchronously demodu­ telemetry decoding. late the UHF and VHF signals. Thus, the signal phase The power supply is a simple switching type, with as measured at UHF, for example, is actually the the switch frequency near 28 kHz and a nominal 28 difference between the UHF phase perturbation and volt input. Five different voltage outputs drive the bea­ the L-band phase perturbation divided by the ratio of con circuitry. With a power consumption of 20 watts, the L-band frequency to the UHF frequency. For weak the L-band output power is 32.5 decibels referenced to moderate disturbances, the phase perturbation to 1 milliwatt. The beacon will operate from - 25 to varies linearly with wavelength, and this effect can be 52°C.

Volume 5, Number 2, 1984 109 M. D. Cousins et al. - The HILA T Satellite Multifrequency Radio Beacon

Table 1-Signals transmitted by HILAT beacon. Radiated Power (decibels referenced Frequency (MHz) to 1 milliwatt) Purpose

1239.084 33 Telemetry phase reference 447.447 20 Scintillation/ total elec- tron content 413.028 23 Scintillation/ total elec- tron content 378.609 20 Scintillation/ total elec- tron content 137.676 23 Scintillation

L-BAND/UHF ANTENNA the L-band pattern adversely; moreover, the combined structure provided good phase dispersion characteris­ The main antenna is a nested bifilar helix structure. tics over the UHF frequencies . The latter property is The bifilar helix (volute) provides a compact antenna important because the second difference of phase for structure with good pattern characteristics for full the three UHF signals is used to derive an absolute earth coverage. It was found that nesting the L-band measure of the path integrated electron content. helix inside a contrawound UHF helix did not affect The VHF antenna is a separate " tripole" located at the tip of the x solar panel. The lack of a common Frame 1 Frame 2 phase center at VHF is not a serious problem because modules modules the phase center varies slowly over the orbit and, there­ fore, does not bias the more rapid phase scintillations. VH F to antenna A prototype of the spacecraft antenna together with the beacon electronics frames is shown in Fig. 2. The 137.676 VHF3X UHF 3X L-band volute is inside the UHF helix. The structure phase-locked MHz PLO is 18 inches high and 4 inches square at the base. oscillator (PLO) 413.028 MHz 137.676 MHz 45.892 413.028 THE GROUND RECEIVER SYSTEM MHz MHz 22.946 A functional diagram of the main HILAT receiv­ MHz UHF Oscillator UHF ing stations is shown in Fig. 3. The antenna is a four­ module modulator/ amplifier to element helix array at L band with a single UHF helix 45.892 MHz antenna L-band modulator in the center. The signals are amplified at the antenna 28 vo lts, 413.028 and transmitted to the scintillation receiver / demodu- direct cu rren t MHz from SI C 1239 Science MHz data input

L band 3X PLO 1239.084 MHz Power to other modules Phase modulated Interface to sc ience , 1239 MHz data formatter

Mon itor inputs from L band to other modules antenna

NOTE : A ll f requenc ies generated are multiples of 11.473 MHz Figure 1-Functional diagram of HILAT beacon. Figure 2-Prototype of beacon modules and antenna.

110 Johns Hopkins A PL Technical Digest M. D. Cousins et al. - The HILAT Satellite Multifrequency Radio Beacon

HILAT satellite t ,. Main VHF antenna antenna

...... LL "0 c I C13 > ..0 -1 ~~ ! Sc i nti Ilation Remote Figure 4-Main antennas at Sondre Stromfjord. g receiver/ ...... antennas c 0 demodulator (2) u c ~ 0 ...... ;; turnstile. Figure 4 is a photograph of the antenna sys­ (I) - ~Q) :J g ~ E +-' - tem at the Sondre Stromfjord, Greenland, site. C13 C ..;; g ~ 0 Q) ci) e; c c Data acquisition is controlled by a scheduler that f0- ·u 0 ,C/) u ~ determines the times of the passes and initiates track­ ing and signal acquisition at the appropriate times. The Data acqu isition data are initially recorded on a 26 megabyte hard disk system but baCked up on digital tape after each pass. Between , passes, the scheduler initiates a data analysis program that processes all the data recorded during the pass. Tape Control/ Graphics The storage capacity of the system is 15 to 18 passes. display printer For routine operation it is convenient to have an oper­ ator check the station daily, which permits recording all passes that achieve a maximum elevation angle of 20° or more. Figure 3-Diagram of HILAT ground station. The raw data tapes and the summary tapes are be­ ing archived at the Air Force Geophysics Laboratory lator. An L-band carrier recovery loop is used to gener­ for dissemination to the science team members. Three ate the reference signal for demodulating the telemetry automated stations are currently operational: one at data and synchronously demodulating the UHF and Sondre Stromfjord, one at Tromso, Norway, and a VHF signals. third at Ft. Churchill, Canada. The data acquisition system is a Hewlett-Packard A 700 general-purpose computer with appropriate in­ PRELIMINARY BEACON RESULTS terfaces for transmitting control signals, receiving te­ The Sondre Stromfjord HILAT station became lemetry data, and sampling the complex scintillation operational in September 1983; however, full opera­ channels. The computer also generates steering com­ tion did not start until late October. Figure 5 is a plot mands for the main L-band/UHF antenna and the of the VHF signal intensity and phase scintillation and UHF remote antennas. The VHF antenna is a fixed a map grid showing the trajectory of the pass that oc-

(a) VHF November 4,1983 (b) Northbound pass ~~ 5.---.---~---.---.----~--~--~--~----~--~--~ . ~:e -5 irl - 15 eC"O -- -25~==~==~==~==~==~====~==~==~==~==~==~ 8 4 Q) '"c '"ctJ . ctJ_ L"O 0 a.. ~ -4 -8

Longitude (o\jlJ) 0811 0812 0813 0814 081 5 0816 0817 0818 0819 0820 0821 0822 Universal time (hours) -Satellite positions (subsatellite points at 1 minute intervals) Figure 5-(a) Plot of detrended VHF signal intensity and VHF phase. (b) Map grid showing the south-to-north trajectory of the satellite.

Volume 5, Number 2, 1984 111 M. D. Cousins et al. - The HILAT Satellite Multifrequency Radio Beacon

30~------~------~------~------~ 18.0 +-' November 4, 1983 C ~ November 4, 1983 • UHF (scaled to VHF) c 0 u 17.5 • VHF c

20 +-'e en u C Q) 17.0 .~ Qi "0 e ~ "S- B I:) Q) 16.5 10 -5 ..... 0 E 16.0 -5.;: ca 0 C> 0 1.2 -.J 15.5 0810 0812 0814 0816 0818 0820 0822 Universal time (hours) 1.0 Figure 7-Total electron content derived from UHF phase data. 0.8

-.::t 0.6 sociated with localized, highly structured F-region en­ (f) hancements ("blobs"). For routine analysis, the data are summarized by moments, and spectra are comput­ 0.4 ed on a "sliding" 30 second data window with out­ puts generated every 15 seconds. Figure 6 shows the scintillation summary parameters generated during the 0.2 in-field data reduction. The UHF phase data have been scaled to VHF (using the theoretical linear wavelength 0.0 dependence) to show the consistency of the data 0811 0814 0817 0820 0823 channels. Universal time (hours) Figure 7 is a plot of the total electron content derived Figure 6-Amplitude and phase scintillation summary from the UHF signals. The systematic enhancement parameters. The arrows indicate the localized enhancement at the beginning and end of the pass is due to the that is given in Fig. 8. changing slant range. For a uniform layer, the total electron content varies with the secant of the zenith curred in the early morning local time sector. The event angle. The localized total electron content enhance­ of primary interest is the burst of scintillation near the ment near the center of the pass is coincident with the center of the pass. Such events are very common in scintillation burst. Such an enhancement is consistent the Sondre Stromfjord data and are believed to be as- with an enhanced F region as the source.

(a) 21T/ K (km) (b) 21T/ K (km) 10 0.1 10 0.1 10 0.1 10 0.1 40 40 ~---~1--~1 --1~~---~1--~1--1~ 0815:45 UT 0816:15 UT 0 O r------~ r------~

en ~Q) ~.- • Qi -40 ] -40 - - .0 - ~- ·u "0 • Q) ~ -80 ;; -80 I I I • I I I 0 40 ~ 40 .---~I----.-I_ --~J--~~-----~I--~I--~I~ CJ) CL ~ 0815:30 UT 0816:00 UT .(i; ~ ca c O r------~r_------~ ~ a ~ CL c ~.- ~.- -40 -40 - ~:- _ r -.-~_

-80 -80 1 I I I I I 0 0.1 10 o 0.1 10 100 o 0.1 10 0 0.1 10 100 Frequency (hertz) Frequency (hertz) Figure 8-(a) Phase spectra measured within the local scintillation enhancement (November 4, 1983). (b) Intensity spectra corresponding to the phase spectra of (a). Note the suppression of low frequencies due to Fresnel filtering.

112 Johns Hopkins APL Technical Digest M. D. Cousins et al. - The HILAT Satellite Multifrequency Radio Beacon Figure 8 shows the four phase and intensity spectra source that would be revealed by the particle detector through the scintillation burst obtained from the rou­ data. In the next segment, structure is thought to be tine summary data. The spectra have been fitted to a generated by a turbulence-like cascade. The drivers for multicomponent power law. The low-frequency sup­ this process may include both E fields and currents that pression of the intensity spectra due to Fresnel filter­ are measured by HILAT instruments. ing is clearly evident. The central portion of the phase Finally, the highest frequency structure is strongly spectra shows a power-law slope very near 3, but the affected by crossfield diffusion, which is affected by spectra have a somewhat shallower high frequency seg­ a variety of ionospheric phenomena, particularly the ment. At the lowest frequencies the spectra remain en­ presence of a conducting E layer. The unique comple­ hanced. ment of HILAT instrumentation will help sort out The low-frequency enhancement in the phase spec­ these various processes that affect the generation and tra is possibly a feature imparted by the irregularity decay of irregularities.

Volume 5, Number 2, 1984 113