In conjunction with the test and evaluation of the position fixing capabilities of the Army Manpack Loran Receiver AN/PSN-6, an extensive series of time difference and signal amplitude measurements were made within a 100 km map grid square encompassing Fort Monmouth, New Jersey. The test location is within the coverage area of the East Coast Loran-C Chain. The data were used to develop a simple smooth-earth model for the test area as well as to estimate the magnitude and distributions of deviations from this model. Local propagation processes associated with topographic features and the grid of overhead wires in the test area are shown to contribute to the deviations from the model

Pearce, D. C.

Walker, J. W.

English

NASA Technical Reports Server

GROUND EFFECTS ON LORAN-C SIGNALS 
D. C. Pearce and J. W. Walker 
Communications/Automatic Data Processing Laboratory 
U. S. Army Electronics Command 
ABSTRACT 
In conjunction with the test and evaluation 
of the position fixing capabilities of the 
Army Manpack Loran Receiver AN/PSN-6, an 
extensive series of time difference and 
signal amplitude measurements were made 
within a 100 km map grid square encompassing 
Fort Monmouth, New Jersey. The test loca- 
tion is within the coverage area of the East 
Coast Loran-C Chain. The data were used to 
develop a simple "smooth-earth" model for 
the test area as well as to estimate the 
magnitude and distributions of deviations 
from this model. Local propagation pro- 
cesses associated with topographic features 
and the grid of overhead wires in the test 
area are shown to contribute to the devia- 
tions from the model. 
INTRODUCTION 
As part of a broad program to develop a capability for navi- 
gation and position fixing, the Army is in the process of 
developing a manpack loran receiver. The position fixing 
function of the receiver is to provide a real time display 
of either the loran time difference coordinates or the geo- 
detic coordinates of the receiver position. This conversion 
between the time difference and geodetic coordinate systems 
is accomplished by a small computer within the receiver. 
The research described here was designed to provide a data 
base for development of simple conversion algorithms as well 
as to provide an error budget for the resulting conversion. 
Since a fundamental variable of a loran system is the prop- 
agation time of the 100 kHz signal and since the manpack 
loran receiver is designed to operate on the ground, the 
present research was planned to provide information on 
ground wave propagation at the surface of the earth. These 
propagation results are the subject of this paper. 
319 
https://ntrs.nasa.gov/search.jsp?R=19750019121 2020-03-19T21:06:34+00:00Z
BACKGROUND 
The fundamental equation describing the functional relation- 
ship between a loran time difference and the loran chain 
parameters is 
TD(P) =[IDS - Dm)/C] + ED, 
where TD(P) is 
D is the great 
t8 the point P, 
the loran time d 
circle distance 
Dm is the great 
i .fference 
from the 
circle d i  
at a field point P, 
slave transmitter 
stance from the 
master transmitter to the point P, C is the propagation 
velocity of the loran signal, and ED is the emission delay, 
that is the sum of the propagation time from the master to 
the slave transmitter and the coding delay introduced at 
the slave station. [See Footnote (a).] 
The non-constant propagation velocity,which varies with 
the density and amplitude of terrain features and the 
electrical properties of the overland path,severely limits 
the utility of Eq. (1) for time difference estimation. On 
the other hand, the ground wave propagation velocity over 
sea water is a well-known quantity,so Eq. (1) is very use- 
ful for this application. A complete knowledge of the 
propagation velocity for all propagation paths is necessary 
for rigorous use of Eq. (1). This procedure requires an 
extremely large volume of data and clearly is not practical 
for a manpack loran receiver. This fact furnished the im- 
petus for the development of simplified coordinate conver- 
sion algorithms. In essence, the approach was to develop 
a simple local conversion model based on calibration and 
to investigate the accuracy characteristics of that model. 
The reader is referred to the work of Johler [l] and to the 
references cited therein for details on the complete treat- 
ment of overland loran propagation. 
EXPERIMENTAL PROCEDURE 
The test area in New Jersey is the 100 km square, 18T WV 
of the Universal Transverse Mercator Map System. A spec- 
ially equipped four-wheel drive mobile unit was used during 
data acquisition. Present instrumentation includes 2 mili- 
tary loran receivers, a timing receiver system with rubid- 
ium standard, and ancillary items including printers, 
oscilloscopes, and power supplies. The military receivers 
used roof-mounted whip antennas whereas the timing receiver 
used a rotatable roof-mounted loop antenna. 
320 
The calibration procedure is relatively simple, namely to 
obtain time difference readings at sites of known geodetic 
control. 
approximately 100 time difference measurements were made 
with each receiver at each site. These measured values 
were then averaged to provide a time difference for each 
site. To eliminate separate site surveys, easily identifi- 
able topographic locations, such as road intersections, 
were used for geodetic control. Coordinates of all sites 
were determined from 7.5 minute USGS topographic maps. 
Criteria for site selection were positive identification, 
freedom from strong electromagnetic scatterers, and access- 
ibility with the mobile unit. In general, the absence of 
power lines was the most difficult criterion to meet. Sev- 
eral of the sites were at geodetic bench marks which pro- 
vided a higher order geodetic control. 
To obtain estimates of loran receiver performance; 
The distribution of calibration sites is shown in Fig. 1. 
The numbers shown in this figure are primarily for site 
identification purposes, but are also related to the time 
of calibration. Data at sites identified with numbers less 
than 1000 were obtained in December 1972, whereas those 
with identifiers greater than 1000 were obtained in July 
1973. This latter study was designed to give an increased 
calibration density within a 60 km square located in the 
SE corner of the primary test area. In addition, the ab- 
sence of nearby power lines was of extreme importance. It 
is estimated that for these locations, there were no wires 
within one kilometer of the site. For the December 1972 
measurements there were no wires within 300 meters of a 
site. This consideration of the proximity of overhead 
wires led to the classification of first and second order 
TD data, as indicated in Fig. 1. 
In October 1973, the timing receiver was used to obtain 
field strength information at the sites with identifiers 
less than 1000. Time difference measurements were also ob- 
tained to check the repeatability of earlier measurements. 
A study of the absolute phase variations of the Loran-C 
transmissions was initiated in October 1974. The sites for 
a ray path to the SS7-Y, Nantucket transmitter are desig- 
nated as phase track points in Fig. 1. All measurements 
were made relative to the reference point 1480 in Fig. 1. 
This point is approximately 1 km from the coast. The other 
sites are at approximately 10 km intervals along the ray 
path. The experimental procedure was to initialize at the 
reference point, make phase measurements at other sites, 
and then close the traverse at the reference site. Each 
321 
._.....  . .. i  . . .: 
:. .... '- I...i . I :  : i :  
2 i 4. 
. .  
. . .  
Fig. 1. Map o f  New Jersey Showing Locations o f  Loran-C 
Time Difference and Phase Data Measurements. 
322 
series of measurements required from 6 to 8 hours. Closing 
the traverse at the reference site provided an estimate of 
the frequency offset of the rubidium standard. 
The test area is a segment of the coastal plain which is 
essentially devoid of pronounced terrain features and con- 
ductivity discontinuities. 
tions of the loran signals were expected from these sources. 
ANALYTICAL PROCEDURE 
Thus no significant perturba- 
The model adopted for data analysis is a modification of 
Eq. (1). The underlying assumption for this model is that 
for modest coverage areas, a constant overland propagation 
velocity will provide a useful approximation. The equation 
is: 
TD(P) = (ED + a> + B ( D  S - Dm) + €(eS, em). 
In this expression, c1 and B are arbitrary parameters to be 
determined by least squares analysis of the measured data. 
The correction function, E(eS,em), is to account for sea 
water paths at a bearing angle 6 from the slave and master 
transmitters to the field point. This function is included 
to account for the significant difference between ground 
wave propagation velocity over land azd over sea water. 
The parameter c1 can be interpreted as an average time dif- 
ference offset characteristic of the test area. The parame 
ter B can be interpreted as the reciprocal of the local 
propagation velocity. However, in view of the simplicity 
of the model and the statistical method of analysis; strict 
physical interpretation of these parameters should be ap- 
proached with caution. [See Footnote (b).] 
For data processing, the variables DS and Dm were calculated 
using the method of Sodano and Robinson [ 2 ]  for the Clarke 
1866 spheroid [ 3 ] .  
The correction €(es,em) was constructed from tabulated 
functions of the sea water path length as a function of 
bearing angle from the Nantucket and Carolina Beach trans- 
mitters. The sea water path functions were prepared from 
maps in increments of 5* in the bearing angle. The path 
length for intermediate bearing angles was determined by 
linear interpolation. The sea water path length functions 
are shown in Figs. 2 and 3 for the Carolina Beach and 
323 
'-1 
1 
220 230 240 250 260 270 
BEARING ANGLE (DEG) 
Fig. 2. SS7-Y (Nantucket) Sea Water Path vs. Bearing Angle. 
n 
x E 
r 
a 
Y 
c 
d 
s 
W 
i- 
4 
W cn 
BEARING ANGLE (DEG.) 
Fig. 3. Master (Carolina Beach) Seawater Path vs. Bearing Angle. 
324 
Nantucket transmissions. 
tion procedure, the error associated with path length de- 
termination will be greatest for bearing angles greater 
than 2 5 O  for the Carolina Beach transmission. For this 
reason, all points with Carolina Beach bearing angles 
greater than 25' were not considered in the data analysis. 
This selection process left 61 calibration sites within 
the 100 km square. 
The correction function is 
In view of the simple interpola- 
In this expression, LS(Bs) is the sea water path length at 
a bearing angle B S  from the slave transmitter and Lm(em) is 
the sea water path length at a bearing angle Bm from the 
master transmitter, k = [(l/C,) - (l/Cs)] where Ck is the 
ground wave propagation velocity over land and Cs is the 
ground wave propagation velocity over sea water. The 
values used for these constants are: (l/C,) = 3.3416 ps/km 
and (l/Cs) = 3.3384 ps/km. 
value 0.0032 ps/km. 
the tables of Johler and Berry [ 4 ] .  
estimated from calibration of a geologically similar area 
in North Carolina where no sea water correction was required 
Thus the constant k has the 
The value of Cs was obtained from 
The value of CR was 
ANALYTICAL RESULTS 
As discussed previously, the data processing was designed 
to give the parameters c1 and 13 for the Nantucket and Dana 
slave configurations by the method of least squares. To 
evaluate the effect of coverage area size, the data were 
treated in two sets. One set included the total of 61 
points. The second set included the points within a 60 km 
square in the SE corner of the 100 km square. 
The analytical results are shown in Tables I and 11. Also 
shown are the RMS deviations of the least squares fit for 
each data set. 
325 
TABLE I. STATISTICAL PARAMETERS FOR 100 km SQUARE 
(61 Data Points) 
I Transmission 01 B RMS Deviation I Pair for Set 
ss7-Y 0.85 3.346 0.33 I ss7-z -0.19 3.339 0 . 3 3  
TABLE 11 .  STATISTICAL PARAMETERS FOR 60 km SQUARE 
(30 Data Points) 
RMS Deviation 
for Set 
ff B 
’ Transmission 
Pair 
An estimate of the experimental uncertainties was obtained 
by statistical analysis of the data acquired at each site. 
This procedure yielded an average value of 0.15 us attrib- 
utable to instrumental jitter. In addition, it has been 
estimated that the use-of topographic maps introduces a 
location uncertainty of the order of 20 meters. For the 
test area, this corresponds to a time difference error of 
about 0.1 vs. Therefore, experimental processes are esti- 
mated to contribute an uncertainty of the order of 0.18 US. 
The distribution of the magnitude of the time difference 
deviations is shown in Figs, 4 and 5 for the Nantucket- 
Carolina Beach (SS7-Y) and the Dana-Carolina Beach (SS7-Z) 
configurations, respectively. The solid curves are the 
normal distributions corresponding to the standard devia- 
tions calculated for each slave configuration. The areal 
distribution of the time difference deviations for each 
slave configuration is shown in Figs. 6 and 7. 
The results of the field strength study of October 1973 are 
shown in the contour plots of Figs. 8 and 9 for the Nan- 
tucket and Dana transmissions, respectively. The contour 
326 
A 8 DEVIATION ( PS) 
Fig. 4. SS7-Y Time Difference Deviation Magnitude Distr ibut ion.  
0 
1 I I I I . , 1 
0.4 0.5 0.6 07 0.8 0.9 0. I 0.2 0.3 
A= DEVIATION (CIS) 
Fig. 5.  SS7-2 Time Difference Deviation Magnitude Distr ibut ion.  
327 
Fig. 6. Map Showing SS7-Y Time Difference 
Deviations from Theoretical Model 
328 
Fig. 7. Map Showing SS7-Z Time Difference 
Deviations from Theoretical Model 
329 
Fig. 8. Map Showing SS7-Y Field Strength Distribution. 
Fig. 9. Map Showing SS7-Z Field Strength Distribution. 
330 
values are the output voltage of the amplitude strobe of 
the timing receiver when tracking the third cycle crossover 
with an input attenuation of 40 dB. The indicated conver- 
sion constant of 46 pv/m/volt was estimated from the an- 
tenna characteristics to provide a corresponding approxi- 
mate value of the field strength. 
The phase of the Nantucket transmission relative to the 
reference point as a function of the distance from the 
transmitter is shown in Fig. 10. The plotted points are 
an average of measurements on six different days. Thedevi- 
ations of these measurements from a least squares fitted 
straight line are shown in Fig. 11. The experimental re- 
sults have been corrected for loran chain variations from 
data furnished by the United States Coast Guard. The error 
bars in Fig. 11 represent typical uncertainty estimates 
arising from rubidium standard frequency offset. 
DISCUSSION AND CONCLUSIONS 
The simplified mathematical model presented provides a 
reasonably accurate description for a small segment of the 
coverage area, namely the 100 km square area. The observed 
magnitude of the deviations from the model are randomly 
distributed. Specific parameters of the model are sensitive 
to the size of the coverage area. In view of the simplicity 
of the model, strict interpretation of the parameters in 
terms of propagation properties is not possible. For exam- 
ple, the value of the parameter B for the SS7-Z slave shown 
in Table I ( 3 . 3 3 9  ps/km) is essentially the value expected 
for an all sea water path. [See Footnote (c).] 
The time difference deviations and field strengths exhibit 
a pronounced areal variation. Furthermore, the contours 
of both variables exhibit a preferred orientation in a 
NE-SW direction. Figure 12 shows salient topographic and 
geological features of the test area. These features also 
exhibit a preferred orientation in a NE-SW direction. Con- 
sequently, there appears to be some correlation between 
contour orientation and the topographic and geological 
features. 
The standard deviations of the time difference data are 
0 . 3 3  and 0.22 ps for the 100 km square and the 60 km square, 
respectively. These values exceed the 0.18 us estimated to 
arise from experimental sources. Clearly the results for 
the smaller test area are in better agreement with the 
theoretical model than are the results for the larger test 
area. The criterion for proximity of nearby overhead wires 
331 
400 
300 
n 
e a 
w 
u) 
& 
-J 
Y 
I200 
8 
100 
0 
400 420 440 460 480 
DISTANC€ FROM NANTUCKET (km) 
380 
Fig. 10. 
SLOPE = 3.352 
SS7-Y Phase Variation vs. Distance, 
380 +oo 420 440 460 480 
DISTANCE FROM SS7-Y (km) 
Fig. 11, SS7-Y Phase Deviation vs. Distance. 
332 
also influences the deviations from the theoretical model. 
Although the time difference data does not allow a defini- 
tive separation of the contributions of area size and prox- 
imity of overhead wires, both factors, as well as previ- 
ously discussed topographic effects, appear to contribute 
to the deviations from the idealized model. 
Fig. 12. Map of New Jersey Showing Salient 
Topographic and Geologic Features. 
Preliminary measurements of absolute phase along a ray path 
from the Nantucket transmitter across the 100 km test area, 
yielded a linear variation of phase with distance from the 
transmitter. The least squares slope is 3.352 ps/km, with 
a standard deviation of 0.3 us. These observations are 
considered to be consistent with the time difference meas- 
urements for the 100 km square. Since this particular ray 
path is located in a region of nearly constant conductivity 
and is devoid of terrain irregularities, it is assumed that 
the major contribution to the observed deviations arises 
from scattering associated with overhead wires. 
333 
These results are of importance to the position fixing 
accuracy of ground-deployed loran receivers. 
tributed time difference deviations of the order of 0.3 vs 
from an idealized model have been observed. This study 
suggests that scattering associated with topographic fea- 
tures as well as from man-made sources such as overhead 
wires contribute to the deviations. The results yield a 
realistic standard deviation of position location accuracy 
of the order of 60 meters. 
Randomly dis- 
ACKNOWLEDGMENTS 
The authors are indebted to Lt(jg) A. A. Sarra, Jr., Coast 
Guard Loran Station, Carolina Beach, North Carolinasand to 
LCdr A, R. Trivers, Coast Guard Electronic Engineering Cen- 
ter, Wildwood, N. J., for assistance in obtaining informa- 
tion on the East Coast Loran Chain during October 1974. 
The assistance of Mr. H. Garber and Mr. A. H. Zanella, 
USAECOM, in obtaining the field data is gratefully acknowl- 
edged. 
A portion of the funding support for this research was pro- 
vided by the Office of the Project Manager, Navigation and 
Control Systems (NAVCON), Fort Monmouth, New Jersey. 
REFERENCES J 
Johler, J. R., (1971), Loran radio navigation over 
irregular inhomogeneous ground with effective ground 
impedance maps, OT/ITS Research Engrg, Report 22 
(Supt. of Documents, U. S ,  Government Printing Office, 
Washington, D. C. 20402). 
Sodano, E. M. and T. A. Robinson,(1963), Direct and 
inverse solutions of geodesics, Army Map Service, 
Technical Report No. 7 (Rev.). 
Department of the Army (1967), Grids and Grid Refer- 
ences, Department of the Army Technical Bulletin 
TM 5-241-1. 
Johler, J. R., and L. A. Berry,(1967), Loran-D phase 
corrections over inhomogeneous irregular terrain, 
ESSA Technical Report I E R  59-ITSA 56 (Supt. of Docu- 
ments, U. S. Government Printing Office, Washington, 
D. C. 20402). 
334 
Additional Backrrround. 1nforma.tion 
(a) The assumption of identical propagation velocities 
over the paths Ds and Dm is implicit in Eq. (1). 
(b) The interpretation of B as the reciprocal of a local 
propagation velocity is valid only to the extent that 
the propagation velocities over the paths Ds and Dm 
are identical. 
(c) Furthermore, a change in the parameter a of 4 us for 
two fits of the same area is devoid of physical sig- 
nificance. This behavior is related to the fact that 
Ds and Dm are large quantities so that in the least 
squares process, small variations in the parameter B 
are compensated for,by large variations in the parame- 
ter a. Additional physical constraints as discussed 
by Doherty [SI are necessary for a realistic physical 
interpretation of a statistical model. 
[SI Doherty, R. H., (1972). A Loran-C grid calibration 
and prediction method, OT/TRER 25 (Supt. of Docu- 
ments, U. S, Government Printing Office, Washing- 
ton, D. C. 20402). 
335 


Ground effects on Loran-C signals

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