During the Fall of 1987 a land mobile satellite demonstration using the MARECS B2 satellite at 26 degrees W was performed. While all the data have not been digested, some observations are in order. First, the system worked remarkably well for the margins indicated. Second, when the system worked poorly, the experimenters could almost always identify terrain or other obstacles causing blockage. Third, the forward link seems relatively more reliable than the return link, and occasional return link problems occured which have not been entirely explained

Nicholas, David C.

English

NASA Technical Reports Server

N88-25698
LAND MOBILE SATELLITE PROPAGATION RESULTS
DAVID C. NICHOLAS, Ph.D., P.E., United States
ROCKWELL INTERNATIONAL CORPORATION
Avionics Group
CedarRapids, Iowa 52498
ABSTRACT
During the Fall of 1987, Rockwell International in cooperation
with COMSAT General and INMARSAT performed a land-mobile satel-
lite demonstration using the MARECS B2 satellite at 26 degrees W.
During this demonstration a van equipped with a satellite transceiver,
a 1.8-inch monopole antenna, a demonstration computer system, and
both LORAN-C and GPS navigators was driven over 2,000 miles in the
Northeastern and Midwestern United States. Elevation angles to the
satellite were 7 to 22 degrees. A narrow-band 200 b/s full-duplex data
channel was demonstrated in a half-duplex protocol in which the van
responded to an interrogation from the base station computer at
Southbury, Connecticut through the satellite every two or three
seconds. Results were summarized along with position approximately
every two minutes. Messages ranged from 40 to 67 bits with continuous
fill transmitted between messages on the base-to-mobile link. Burst
transmissions of similar size preceded by 88-bit preambles were used on
the mobile-to-base link. The overall success rate on 42,974 transactions
during a trip from Cedar Rapids, Iowa, to Washington, DC, and back
was 92.4 percent on the satellite-to-mobile link and 75.4 percent on the
mobile-to-satellite link, for a combined poll-response success rate of
69.6 percent.
INTRODUCTION
During September, October, and November 1987 a two-way data communications experi-
ment was conducted between an instrumented van and a simulated dispatch station. The
dispatch station was connected by dial-up telephone link to a base station computer located at
COMSAT General's Maritime Services Coast Earth Station at Southbury, Connecticut. A two-
way 200 b/s full-duplex radio link was then established through the INMARSAT MARECS B2
satellite located at 26 degrees West longitude to a mobile van. A half-duplex protocol simulated
a future lower-cost, half-duplex mobile unit.
The route of the experiment is depicted in Figure 1, which also shows elevation angles to
the satellite. The general purpose, system configuration, and overall results of the experiment
are discussed in [1]. The modem is discussed in [2]. Overall propagation performance results and
preliminary detailed results are discussed below.
SYSTEM DEscRIPTION
For purposes of the propagation experiments the forward link of the system may be
characterized as transmitting a continuous 200 b/s data signal through the satellite. The
satellite broadcasts this signal to the coverage area at the experimental frequency of 1541.3
MHz along with the other traffic in the system. The minimum power at the satellite corre-
125
https://ntrs.nasa.gov/search.jsp?R=19880016314 2020-03-20T06:53:40+00:00Z
W 
Fig. 1. Map of demonstration route 
sponds to 24 dBW EIRP which is the power normally assigned to four "standard-A" channels. A 
standard-A channel is normally used with a three-foot steerable parabolic antenna on a ship to 
provide FM voice service in a 50-kHz channel. 
For the eastbound trip, the mobile unit utilized a 1.8-inch monopole antenna over a 12-inch 
square ground plane. Due to a mechanical failure, a Dorne-Margolan Model DMC146-2-1 GPS 
antenna was pressed into service for the return trip. These were mounted on a passenger van 
type vehicle and are shown in Figures 2 and 3. It is probable that the monopole antenna pattern 
is uniform in azimuth, and has a fairly broad peak in elevation beam width with 3-dB points 
around 10 to 50 degrees elevation angle. It provides a peak gain of perhaps 4 dB for vertically 
polarized signals but theoretically incurs a 3-dB penalty for the circularly polarized INMAR- 
SAT signal. See [3] for example. 
Hence, the transmit and receive power budgets shown in Table 1 reflect a 0-dB gain 
antenna for the vehicle. 
Fig. 2. Monopole antenna Fig. 3. Crossed curved dipole antenna 
126 
Table1.Powerbudgets
FORWARD LINK RETURN LINK
CES to Satellite
6.42 GHz. El. Angle 22.7 °, d = 31.130 km
CES EIRP 64 dBw
Path loss = 200.9 dB
Absorption loss = 0.4 dB
Satellite G/T = -15 dB/K
Mean uplink C/N_ ---76.3 dB
Mean sat C/Io = 58.4 dB
Satellite to Mobile
1.54 GHz, El. Angle 10 °, d = 42,440 km
Satellite mean EIRP = 24 dBw
Path loss = 188.8 dB
Absorption loss = 0.4 dB
Mobile G/T = -27 dB/K
Mean downlink C/No = 36.4 dB
Interference loss = 0.5 dB
Total RSS random loss = 1.5 dB
Overall C/No = 34.4 dB
C/N (200 Hz) = 11.4 dB
Mobile to Satellite
1.64 GHz, El. Angle 10 °, d = 42.440 km
Mobile EIRP = 14.7 dBw
Path loss = 189.3 dB
Absorption loss = 0.4 dB
Sat G/T = -11 dB/K
C/N o = 40.9 dR
Sat C/Io -- 49.7 dB
Tpdr gain = 150.9 dB
Satellite to CES
4.2 GHz, El. Angle 22.7 °, d = 41,130 km
Sat EIRP = -25.8 dBw
Path loss = 197.19 dB
Absorption loss = 0.5 dB
CES G/T = 32 dB/K
Mean downlink C/No = 37.11 dB
Mean downlink C/No + Io = 35.4 dB
Interference loss = 0.5 dB
Total random RSS loss = 1.35 dB
Overall C/No = 33.55 dB
200 Hz = 23 dB
C/N (200 Hz) = 12.2 dB
Several circularly polarized antennas were procured, tested, and resulted in poorer per-
formance than the monopole antennas in prior laboratory tests. These tests included the
antenna actually used on the return trip. It is theorized that this may be due to the loss of the
horizontal component of the received wave due to low elevation angles, poorer matching, or
losses in these larger, more complex antennas.
The return trip antenna has maximum gain at the zenith and is characterized as having at
least -2 dBic gain at all angles above 5 degrees for the GPS frequencies. It is a circularly
polarized crossed dipole with the arms curved to fit the interior of a 4-inch hemispherical
radome.
The power budgets reflect a higher carrier to noise ratio for the return link than for the
forward link, yet the forward link was more reliable. This may be due to extra actual forward
link power, to the superior performance of the modem on the continuous forward link signal, or
to slightly greater interference and noise problems on the return link.
A companion paper [2], discusses the modem in detail. Unshaped (square) 180-degree
differential phase shift modulation is used. The modem is designed to rapidly acquire the 200
b/s signal in an initial acquisition bandwidth of about 2000 Hz. Thus, in acquisition, the signal-
to-noise ratio is about10 dB worse than during trackingl As explained in the companion paper,
the modem produces a.bit error rate of 10 -3 at a signal to noise ratio of about 8 dB. For the burst
transmission on the return link a preamble of 88 bits was most often used. The modem requires
a minimum preamble of about 40 bits, but results improve with preamble length, especially
with high noise.
For purpose of the propagation experiments, the base station interrogates the mobile unit
with a packet of meaningful data 42 to 67 bits long imbedded in a Continuous 200 b/s signal
which is mostly filler. This normally occurs every three seconds. The mobile unit then responds
with a burst of data at 1642.8 GHz consisting of an 88-bit preamble followed by a similar
variable length packet of 42 to 67 bits.
127
Each packet contains a 16-bit cyclic redundancy check (CRC) code. Messages with CRC
errors are ignored functionally and scored as a nonsuccess in the propagation statistics. Thus,
unless the CRC checks perfectly on the interrogation, the mobile unit does not respond. If a
multisegment transaction is involved, the next interrogation occurs as fast as possible after a
successful response. This maximum rate is about two seconds per transaction due to transmis-
sion times and processing and propagation delays.
EXPERIMENTAL RESULTS
As shown in Figure 1, the vehicle was driven at various times from as far West as Pella,
Iowa (about 7 degrees elevation angle) to Washington, DC (about 22 ° elevation angle) through
Indianapolis, Indiana. It returned via a more northerly route through Cleveland, Ohio, and
Chicago, Illinois. In addition, several side trips were made within Iowa. During most of this
time both LORAN-C and Global Positioning System (GPS) position data were available. Propa-
gation data were summarized every two minutes on disk in the mobile unit computer and in the
base station computer at Southbury, Connecticut. The Southbury data were then transmitted
nightly via modem to the dispatch station, usually at Cedar Rapids, Iowa. Mobile unit disks
were physically returned to Cedar Rapids.
While the base station report alone provides the overall round trip success of the transac-
tions, the mobile unit report allows one to separate the performance of the satellite-to-mobile
link from the performance of the mobile-to-satellite link.
The statistics in Figure 4 show the results for two minutes of September 30, 1987, on a trip
from Cedar Rapids to Pella, Iowa, in flat terrain on Intersate 80 headed West at the position
shown which is near highway mile marker 205. The statistics cover the period from 1:58 to 2:00
PM CDT on the van which is the interval from 2:58 to 3:00 PM EDT at the base station in
Southbury, Connecticut.
During this time the base station interrogated the mobile unit 49 times and received 44
responses with correct CRC codes. The mobile unit received and responded to 47 messages.
Thus, in this case 2 messages were lost on the forward link and 3 more were lost on the return
link. Of the three lost responses, unique words were correctly received for two of these, as
indicated by CRC errors while the other was lost entirely. Successful responses (S) and unsuc-
TIME 02:60 PM GPS time MOBILE UNIT
GPS LAT 41 41' 45" N GPS LONG 92 18' 09" W
LORAN LAT 41 41' 58" N LORAN LONG 92 17' 34" W
SEGMENT TYPE e 1 5 6 7 TOTAL
Transmits 12 5 0 30 e 47
Received 5 0 0 29 13 47
Re_ransmits 2 e e o e 2
J CRC Errors 2
Mobile Unit Statistics
TIME 03:_ PM GPS time BASE STATION
GPS LAT 41 41' 45" N GPS LONG 92 18' 69" W
LORAN LAT 41 41' 58" N LORAN LONG 92 17' 34" W
SEGMENT TYPE e 1 5 6 7 TOTAL
# Tronlmlits 5 e ® 29 15 49
# Received le 5 0 29 6 44
# Retr_nsmits e --e e 1 4 5
CRC Errors 2
SSSSSSSSSSSSSRSSSRRRSSSSSSSSSSSSSSSSSSSSSRSSSSSSS
TOTALS SINCE STARTING
2647 Tronmitted 18_4 Received
405 Retronsmitted 107 CRC Errors
Base Station Statistics
Fig. 4. Two minute reports
128
cessfulresponses resulting in a retransmit (R) are shown in Figure 4 as a string of 49 characters
in the base station statistics. These message totals are composed of five message types 0, 1, 5, 6,
and 7 ranging in length from 42 to 67 bits.
October 8, 1987, is considered in detail. This is a route segment from Indianapolis to
Pittsburgh. There were 7847 interrogations and 4715 successful responses for an overall success
rate of 60.1 percent. In terms of individual link performance, 7137 interrogations were received
by the mobile, so that overall forward link performance was 91.0 percent and reverse link
performance was 66.2%.
At a higher level of detail Figure 5 shows the overall round trip success rate for the 153
reports covering the Indianapolis to _"" _ .... ".... _-'-_-- ° • " •l_sburg, p_rLmo of the trip uu vew_r o, -98=, .rum ....J . JI.U
PM to 7:15 PM EDT. Most would be two-minute reports. Note that the success rate varies from
100 percent to near zero. A linear regression superimposed on the data shows that the overall
success rate was about 85 percent near Indianapolis decreasing during the day to 55 percent
near Pittsburgh reflecting a change in the terrain. A 10-point moving average is also shown.
1
0.9
O.B
0.7
0
0.6
Ilg
0
_ 0.4
0.3
0.2
0.1
° 0 _
_3 " [] A I:Y-' 0 u
o/ o- olo yOo°
o-O . .
® Z [] o " W o o _%°
[] ° [][]
( 17 0 El 0 I"I
- C] 0 El[]
[]
[] 0
[3 [] []
IOn [] lOP[]
in
[] []
n
157 554 71,5115 243 324 414 459
OCTOBER 8, 1987 EASTERN DAYLIGHT TIME
Fig. 5. Indianapolis to Pittsburgh results
At a finerlevelofdetailyetTable 2presentsa fourthorderMarkov model forthe success
of a transactiongiventhe historyofthe previousfourtransactions.Reduced ordermodels are
alsoshown.
We see thatpropagationfailuresareclustered-- so are buildingsand trees.
Finally,Figure 6 shows theoverallroundtripon October7,8,and 9,1987,eastboundand
October 26,27 and 28,1987,westbound.The antenna change occurredbetween the East and
West legs.The probabilityof a round tripsuccessforthe 42,974transactionscoveredby 853
two-minute reportsisshown as a 10 pointmoving average (about20 minutes) and alsoas a
linearregression.Figure 7 separatesthe forward (upper curve)and return !inkindividual
successprobabilitiesin the form of 30 point(aboutone hour) moving averagesforthe same
data.
129
Order
Table 2. Conditional probabilities of success -- percent
4
SSSS 89.18
SSSR 66.23
SSRS 78.68
SSRR 38.92
SRSS 75.60
SRSR 55.45
SRRS 59.O6
SRRR 23.50
RSSS 83.20
RSSR 51.95
RSRS 65.49
RSRR 30.38
RRSS 64.00
RRSR 32.30
RRRS 53.42
RRRR 12.87
3
SSS 88.49
SSR 62.12
SRS 75.34
SRR 35.18
RSS 71.27
RSR 41.70
RRS 55.40
RRR 14.40
2
SS 86.10
SR 55.27
RS 66.42
RR 18.18
1
S 82.69
R 28.89
1
1(
0.9
0.8
0.7
o_
._ 0.6
rr
u)
u) 0.5
UJ
o
o
0.4
0.3
I
0.2-
CEDAR
RAPIDS
0.10 1
I
INDIANAPOLIS WASHINGTON DC
PITTSBURGH
I I
101 201 301
CLEVELAND
I.
401 501 601
CHICAGO
RAPIDS
701 801
Fig. 6. Two-way success ratio - two minute intervals - 10 point moving average
130
1
lC
0.9 --
0.8 --
0.7 --
_0 0.6 -
m 0.5 -
0')
14.1
o
0 0.4 -
0.3-
INDIANAPOLIS
O.2- I I
CEDAR
RAPIDS PITTSBURGH
0.10 1
1 101 201 301
Iv
WASHINGTON DC
CLEVELAND
Lf_ I
CHICAGO
RAPIDS
701 801
Fig. 7. Forward (upper) and return (lower) link success ratios - 30 point average
OBSERVATIONS
While all the data have not been digested, some observations are in order. First, the
system worked remarkably well for the margins indicated. Second, when the system worked
poorly, the experimenters could almost always identify terrain or other obstacles casuing
blockage. Third, the forward link seems relatively more reliable than the return link, and
occasional return link problems occurred which have not been entirely explained.
ACKNOWLEDGEMENTS
This project was a joint undertaking of Rockwell International, COMSAT General, and
INMARSAT. At Rockwell about 15 people were involved over about a year, including S. D.
Smith and D. W. Sutherland who reduced the data for this paper.
REFERENCES
1. Gooch, G. M. and Nicholas, D. C., "Land Mobile Satellite Demonstration System." NASA, JPL
Mobile Satellite Conference, Pasedena, CA, May, 1988.
2. Henely, S. J. "Modem for the Land Mobile Satellite Channel." NASA, JPL Mobile Satellite
Conference, Pasedena, CA, May, 1988.
3. Johnson, R. C. and Jasick. H. Antenna Engineering Handbook, Second Edition. McGraw-Hill
Book Company, New York, 1984, p. 4-31.
131


Land mobile satellite propagation results

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server