NASA Technical Reports Server (NTRS) 19940018321: A study of satellite motion-induced multipath phenomena

6 Pages·0.37 MB·English

by  NASA Technical Reports Server (NTRS)

N94-22794
A Study of Satellite Motion-Induced Multipath Phenomena
R. M. AIInutt, A. Dissanayake, C. Zaks, and K. T. Lin
COMSAT Laboratories, 22300 COMSAT Dr
Clarksburg, Maryland 20871-9475, USA
Phone [phone]
FAX [phone]
Abstract propagation phenomena that handheld satellite
communications systems are likely to encounter.
This paper discusses one particular set of
Experiments have been undertaken at
COMSAT Laboratories to determine some of the experiments which involved the diurnal variation of
anL-band satellite signal received by an
propagation effects likely to be encountered by
handheld satellite communications devices. L-band omnidirectional antenna. The results reported herein
indicate that diurnal motion ofthe satellite, or its
pilot tones aboard geosynchronous satellites at 15°
and 40°elevations were used to examine diurnal corollary, physical movement of the antenna, can
signal variations measured by using a hemispherical cause signal variations inexcess of 9dB peak-to-
antenna. Itwas found that the receiver with a peak for low-gain antennas under certain conditions.
Apreliminary finding isthat close-in multipath effects
hemispherical antenna suffered daily peak-to-peak
may bethe cause ofthe variations.
signal level variations of upto 12dB compared to
only 2to 3dB for areceiver equipped with a
directional antenna. These results were highly System Description
repeatable, and extensive tests were conducted to
confirm the accuracy of the data. The results suggest Two receivers were set up, one employing a
that the diurnal variations were due to multipath 16-dB-gain dish antenna with a25°half power
effects caused by the motion of the satellite with beamwidth, and the other using a 3-dB-gain
respect to the receive antenna. Noting that the orbit hemispherical Global Positioning System (GPS)
inclinations of the satellites used inthe experiment antenna. A computer-controlled frequency tracking
were only on the order of 2to 3°,the results also system was used to keep the receivers locked on
suggest apotentially serious signal variation problem the signal. The GPS-based receiver was slaved to
for low-gain antenna-based communications the frequency of the dish-based system to avoid
systems using low earth orbit satellites, since the potential discrepancies during frequency tracking.
satellite elevation angles relative to earth change far The receivers were coupled to a 10-MHz crystal
more rapidly. reference source for coherency, and their down-
converters shared acommon local oscillator to
Introduction further maintain system coherency. The signal power
was measured by adetector within a 65-Hz
bandwidth to allow sufficient measurement range.
With the current and projected rise in the
use of mobile communications systems, significant The high resolution and linear outputs of the
receivers were sampled at 100 Hz, but since storage
efforts are being invested inpropagation studies at
space was limited, the data were recorded at arate of
the frequencies concerned, particularly L-band.
10Hz. The receivers were automatically calibrated
Many of these studies have involved taking signal
and re-centered on the signal after each minute of
propagation data from fast moving vehicles [1],[2].
However, little consideration has been given to recording, as the signal center frequency drifted
diurnally due to the Doppler effect. Concurrently,1
signal propagation for stationary orslow-moving
communications systems, principally the handheld second averaging of the noise level at afrequency
1-kHz below the pilot's center frequency was made,
variety. Handheld personal communications systems
and arecord of the noise level was stored to allow
will be power-limited and restricted to small operating
accurate assesment of potential gain fluctuations
margins. Only limited blockage and multipath
along the receive chains. For more information on
experiments have been used to develop preliminary
the receivers, see Reference 3.
fade margins and performance specifications. Sofar,
effects induced by the satellite movement have
been ignored. Afew experiments have been
conducted at COMSAT Laboratories to identify
: 337
Long-Term Signal Variations After ruling out all local effects, interfering
signals, and equipment problems, the signal level
The two antennas were positioned on aflat variations were determined to be due to diurnal
roofat the Laboratories, withthe rest of the variations inthe satellite position, satellite mutation
equipment in an air-conditioned room directly below effects and, to a lesser extent, changes in Doppler
the antennas. The signal source was a 1.537525- frequency. In an attempt to settle the location
GHz pilot tone re-broadcast via aglobal coverage dependency of the effects, the GPS antenna was
antenna on the INMARSAT-II F-4 satellite in moved to atop a 10-ft pole at the very edge of the
geosynchronous orbit at 55.5°west with an elevation roof and directed toward the satellite. Figure 4
angle of 40°.The pilot EIRP atthe satellite was about presents almost 24 hr. of measurement data. From
10 dBW. this figure, it is evident that the signal variations are
Figure 1shows the variation in signal power over 24 different from the previous ones and also greatly
hours obtained with the GPS antenna directed at reduced; the total daily peak- to-peak variation
zenith, and with the high-gain antenna lined upon recorded from the dish antenna and the GPS
the satellite. Both antennas had clear paths to the antenna are similar. Also, the effects were not
satellite, as they were mounted on aflat rooftop, diurnally repetitive. This could be attributed to the
about 2.5 m apart and 1.5 m above the surface. fact thatthere was alarge car parking lot 40ft directly
There are significant differences inthe signatures of below the GPS antenna. Some ofthe data were
the two signal levels shown in Figure 1.The peak-to- recorded over aweekend, when there were
peak signal variation experienced by the GPS-based comparatively fewcars were moving around. On
receiver is nearly twice that observed for the high- regular working days the distribution and number of
gain antenna system. To obtain the results shown in cars varies. This indicates that aconstant reflective
inFigure. 2,the GPS antenna was lined upalong the surface may not have existed during the
satellite look angle; the high-gain a_ntennawas left measurement period, which cou!dexplain the
unchanged. Inthis case the GPS antenna system disSimii,ai_ity_5_hedaily signal traces. Therefore, itcan
experienced as much as 4times greater peak-to- be argued that the recorded signal level was fairly
peak variations. Of equal significance isthe rapidity dependent on the reflective quality of its
signal variation measured by the GPS antenna; the surrounding; hence, the signal level isprobably
signal changed by upto 7dB within 1.5 hr, location-dependent. This suggests that the daily
compared to less than 1dB of change over the same observable variations in signal level recorded by the
time interval when measured by the high-gain hemispherical antenna are due to multipath effects,
antenna. which change over the course of aday because of
Measurements made over several days the motion of the satellite with respect to the receive
indicated that the signal level variations obtained antenna.
were highly repeatable from day to day. Figure 3 To support the above argument, asimilar
shows data from 4 consecutive days. The time series experiment was conducted at yet another location.
for each day of data is progressively shifted by the The receivers were moved to aflat, open grassy area
difference between asidereal day and acalendar day on COMSAT grounds bordering a highway. Data
(approximately 4min.). The correlation over 4days could only be recorded during working hours, since
on a sidereal basis is evident inthe figure. the equipment could not be left unattended outside
Extensive receiver checks were performed overnight. Approximately 6 hours of data were taken
to eliminate the possibility of equipment effects and as can be seen in Figure 5,the recorded data
which could contaminate the results. A stable signal bears little resemblance to that measured onthe
source, offset afew Kilohertz from the satellite's pilot roof. The changes insignal level at the GPS antenna
tone, was set up on the roof, and the respo0se of were rnuchsmaller andsmoother at the new location,
the GPS-based system was tested over 12hours. but as before, the peak-to-peak variations were
The received signal remained within 0.5 Hz of the always greater than those measured by the dish
transmit frequency and experienced less than 0.1 antenna.
dB power level variation. Noise level checks that To independently confirm the validity of the
were conducted over many days indicated the same results obtained using the INMARSAT-II F-4,
order of stability. Investigations into weather patterns another pilot tone from adifferent satellite
showed that even though there had been significant (INMARSAT-II at 15°W) was tested. The tone
changes inthe daily temperature cycles, no such frequency was at 1.537528- GHz, with an elevation
changes were detected inthe received signal levels of approximately 15°and anazimuth of 110°.
from either antenna. Itwas also determined that Measurements were made 0nthe roof, and data
there were no extraneous interfering signals. It were taken over several days (see Figure 6).While
became clear from these intensive investigations the long-term tre of the signal traces recorded by the
that the major variations inthe observed signal-to- dish and GPS antenna receivers were very similar,
noise ratios were soley the result of changes inthe the peak-to-peak variations over the course of aday
received carrier level. were significantly different; 4 dB for the dish and
338
over 10dB for the GPS antenna. As noted before,
changes insignal levels were again highly repeatable Acknowledgement
from day to day. Having almost identical signal
behavior when using asecond satellite source may The above measurements were funded by COMSAT
again suggest that the signal level variations aredue Mobile Communications. The views expressed in
to changes in multipath characteristics caused by this paper are not necessarily those of COMSAT.
motion of the satellite relative to the earth stations. It
was evident that the dish receiver was more affected
than in previous measurement campaigns, possibly References
because of apartial interception of its lobewith the
roof surface. This might suggest that the multipath [1] "'Measurement and Modeling of Land Mobile
was produced from an area directly infront of the Satellite Propagation at UHF and L-Band, Including
antennas, probably the roof surface. the Results from the Dedicated Stratospheric
These independent sets of measurements, Balloon Experiment of July 18, 1986," W.J. Vogel
bearing remarkable similarity interms of diurnal and U. Hong, Atechnical report submitted to the Jet
variation tothe data sets made using the INMARSAT- Propulsion Laboratory under contract 956520, May
IIF-4 satellite, seem to support that there is amuch 1987.
larger diurnal variation insignal level received by a
hemispherical antenna than by anarrowbeam [2] "'Multiband Propagation Experiment for
antenna. Italso could imply that the variation may be Narrowband Characterization of High
the result of closein multipath effects. Elevation Angle Land Mobile-Satellite Channels," G.
Another pilot tone at 1.5415 GHz aboard the Butt, B.G. Evans, and M. Richaria,
MARECS B-2 at 15°west has also been measured,
Letters, July 16,1992, VoI. 28 No.15 pp. 1449-50
and showed variations similar inshape to those for
the 15° elevation satellite discussed previously.
[3] "'Propagation Considerations on L-Band
Preliminary investigations of diurnal signal variations Handheld Communication Service Operations Via
measured through aglass window from inside an Satellite," R.M. AIInutt, A. W. Dissanayake, K.T. Lin,
office at COMSAT Laboratories have also shown
and C.Zaks, International Conference on
similar results. Antennas and Propagation, Edinburgh, Scotland,
April1993, Proc.
Conclusions
The satellite used for the measurements
wasgeosynchronous orbit with an inclination of 2.2°.
The elevation angle changes by less than +2.2° over
asidereal day. The movement of the satellite with
respect to earth is therefore slow, leading to
correspondingly slow changes inmultipath effects.
By inference, ifthe signal had emanated from alow
earth orbiting satellite, similar changes (but ata
higher rate) would be experienced by the user,
particularly when employing alow-gain antenna.
Increases insignal variations observed when
alow-gain antenna was lined up along the satellite
look angle suggest an increased contribution from
reflected signal components of the antenna's
surroundings, as alarge part ofthe antenna pattern
intercepted multipath-producing surfaces. The
results from the measurements indicate a strong
relationship between the physical orientation of the
antenna, and the profile/texture of the surrounding
surfaces, whether they were walls, flat areas,
cement, orfoliage.
Itappears that the user of a handheld
satellite communications terminal would have to be
aware of itslimitations inorder to use it satisfactorily.
Also, further propagation studies of the phenomena,
including an investigation into the nature of the
suspected multipath components, is prudent.
339
28
6,
Signal: 1.5 GHz pilot tone
24-
Satellite: Inmarsat II at 55.5"W
z2.
Elevation Angle: 40"
Surface Type: Tar and crushed cinder roofing
20
16
14
0 2 4 ¢_ 8 10 12 14 16 1'8 _ 2'2 24
Local Time 0-Iours)
t_'gure I. A Comparison of Received Satellite Signal Strength at 1.5 GHz
Between Directional (Dish) and Hemispherical (GPS) antenna; 14 October 1992
28
26-
Dish
24 Signal: 1.5 GI-Izpilot tone
Satellite: Inmarsat II at 55.5°W
22¸
Elevation Angle :40°
Surface Type: Tar and crushed cinder roofing
20-
18-
,.qo
._ 16-
r.,¢2
14"
12"
10
0 ½ 4 6 8 10 12 14 16 1'8 20 :_2 24
Local Time (Hours)
l_gure 2. A Comparison of Received Satellite Signal Strength at 1.5 GHz
Between Directional (Dish) and Hemispherical (GPS) antenna; 24 October 1992
340
._o 22-[ SEaletdvalittieo:nInAmngalresat: 4I0Ia°t 55.5°W
20- Surface Type: Tar and crushed cinder roofing A , [
18- GPS
_ 6-
14-
12-
Local Time (Hours)
Figure 3. A Comparison of 4 Days of Satellite Signal Strength Data Measured
(22 - 25 October 1992).
30
28-
g 6-
Signal: 1.5 GHz pilot tone
Satellite: Inmarsat II at 55.5"W
Elevation Angle :40°
Surface Type: Tar and crushed cinder roofing
{[20
GPS
g/2
18"
16"
14
0 4 6 8 I0 12 14 16 1'8 _ 22 24
Local Time (Hours)
Figure 4. A Comparison of Received Satellite Signal Strength at 1.5 GHz
Between Directional (Dish) and Hemispherical (GPS) antenna; 11 October 1992
341
28
26-
Dish
g 24-
Signal: 1.5 GHz pilot tone
Satellite: Inma.rsat II at 55.5°W
Elevation Angle :400
Surface Type: Dry Grass 6 inches High
18-
16-
14
ll 12 1'3 14 15 16 17 18
Local Time (Hours)
_'gure 5. A Comparison of Received Satellite Signal Strength on a Grass Surface
28 October 1992
28
26-
24
20
._ 22 Psf
18
_ 16
° 12 Signal: 1.5 GHz pilot tone _ lt,_. ,tA-, ._I ,_3'
Satelh'te: Inmarsat II at 15*W _/_ /_ lrlfl'_¢"
10
Elevation Angle : 15°
8 Surface Type: Tar and crushed cinder roofing
0 ½ 4 6 8 10 12 14 16 1'8 20 i2 24
Local Time (Hours)
l:t'gure 6. A Comparison of Received Signal Strength from a Second Satellite
7-9 December 1992
342