The possibility is discussed of using Doppler techniques for determining the location of ground based emitters causing radio frequency interference with low orbiting satellites. An error analysis indicates that it is possible to find the emitter location within an error range of 2 n.mi. The parameters which determine the required satellite receiver characteristic are discussed briefly along with the non-real time signal processing which may by used in obtaining the Doppler curve. Finally, the required characteristics of the satellite antenna are analyzed

Rao, B. L. J.

English

NASA Technical Reports Server

X-750-73-271
R-FI EMITTER LOCATION TECHNIQUES
(NASA-TM-X- 7 0 4 8 3 ) RFI EMITTER LOCATION V7333109TECHNIQUES (NASA) 13 p HC $3.00 CSCL 17B
Unclas
G3/0 7  19347 )
B. L. J. RAO
SEPTEMBER 1973
Reproduced by
NATIONAL TECHNICAL
INFORMATION SERVICE ,
US Department of Commerce
Springfield, VA. 22151
-GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
https://ntrs.nasa.gov/search.jsp?R=19730024376 2020-03-23T00:32:40+00:00Z
X-750-73-271
RFI EMITTER LOCATION TECHNIQUES
B. L. J. Rao
Communications and Navigation Division
September 1973
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland
/
pPECZDING PAGE BLANK NOT FILM..
RFI EMITTER LOCATION TECHNIQUES
B. L. J. Rao
Communications and Navigation Division
ABSTRACT
This document discusses the possibility of using doppler
techniques for determining the location of ground based emit-
ters causing radio frequency interference (RFI) at the low
orbiting satellites. An error analysis indicates that it is
possible to find the emitter location within an error range of
2 n. mi. The parameters which determine the required sat-
ellite receiver characteristic are discussed briefly along
with the non-real time signal processing which may be used
in obtaining the doppler curve. Finally, the required char-
acteristics of the satellite antenna are analyzed.
iii
PRECEDING PAGE BLANK NOT FIL E -
CONTENTS
Page
ABSTRACT ................ .... ...... iii
INTRODUCTION ......................... 1
DOPPLER TECHNIQUES .................... . 1
ERROR ANALYSIS ........................ 4
RECEIVER REQUIREMENTS ................... 5
Maximum Doppler Frequency Expected . . . . . . . . . . . 5
Maximum Rate of Change of Doppler . . . . . . . . . . . . . . 6
SIGNAL PROCESSING ...................... 6
ANTENNA ANALYSIS ...................... 7
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . 8
V
RFI EMITTER LOCATION TECHNIQUES
INTRODUCTION
Many attempts have been made to measure the spectrum 
utilization with mobile
or airborne receivers. 1 , 
2 This method is slow and not economically feasible
on a globe scale. Recently there is a growing interest in finding 
RFI at orbital
altitudes. 3, 4 There is very little experimental work reported 
to date and no
attempts were made so far to find the geographical location of different 
RFI
sources. Recently NASA/GSFC proposed an RFI experiment to 
determine the
frequency, type of modulation and the location of the ground based 
RFI sources,
which may interfere with the low orbiting satellites. The purpose 
of this docu-
ment is to discuss the feasibility of using doppler techniques in determining the
location of the ground based RFI emitters. The doppler measurements 
are as-
sumed to be made at the low orbiting satellite using a suitable receiver. 
The
frequency bands of interest are 180-174 MHz, 240-478 MHz 
and 1535-1665 MHz.
It is well known that the "doppler effect" enables a satellite to locate 
C. W.
transmitters on the surface of the earth.5, 
6 The same method with some modi-
fications could be used to find the RFI emitter location. Several 
other methods
of radio location are considered, 7,8 but rejected for different reasons.
Two methods of using the "doppler effect" in determining the emitter 
location
are discussed first. Then, a brief discussion on error analysis is included. 
It
is concluded that it is possible to find the emitter location within an error range
of 2 n. mi. using doppler effect. The parameters which determine the 
satellite
receiver characteristics are discussed briefly along with the now-real time sig-
nal processing, which may be used in obtaining the doppler curve. Finally, 
the
satellite antenna requirements are briefly analyzed.
DOPPLER TECHNIQUES
The relative motion of the satellite causes the frequency received by 
the satel-
lite to differ from the transmitted frequency from the earth. The bases 
for the
position estimation are the unique relationships existing 
between successive RFI
frequency measurements at the satellite and the location 
of the source relative
to the satellite. The frequency measurement may be considered to determine
the complete doppler curves of a source during one or more passes. 
From an-
other viewpoint, the frequency measurements can be used to spatially 
locate and
orient a number of geometric surfaces upon which the source is known to 
lie.
1
When the satellite orbit is known, a measurement of the doppler curve in the
vicinity of zero doppler shift allows a determination of the time and the line of
sight range to the transmitter. Consequently the position of the transmitter
could be determined relative to the satellite. This principle is well known in
navigation for several years. 5, 9 It is recognized recently5 , 1 0 that there is a
tremendous redundancy in the information contained in the shape of the doppler
curve. The position of the transmitter can be determined not only from the time
and slope of the curve at the point of maximum slope, but also from a compari-
son of the doppler shift at any two points on the curve. Thus in principle, each
pair of data points on the doppler curve determines the transmitter location and
each of these determinations are statistically independent (except for some bias
errors due to atmospheric refraction, which will be discussed later). This ob-
servation could be used to reduce the effect of random errors in measuring the
doppler curve.
The mathematical relationships between the doppler frequency, range rate be-
tweenthe satellite and source, and the source location geometry are discussed
below.
The phase angle of the received signal at the satellite is given by:
2i
0(t) = wOot + tSE . + o (1)
where oo = 27rf 0 f 0 being the transmitted frequency
E = S - E is the range between satellite and the source
s = (Xs, ys, zS) is the position vector of the satellite
RE = (XE, YE, ZE) is the position vector of the emitter (source)
o = A constant phase angle
The instantaneous frequency of reception is given by:
1 d dSE
f(t) fo SE2r dt Xo dt
Therefore
f(t) - fo 1 dSE (2)
Xo dt
Since f(t) - f, represents the doppler frequency and d/dt!sE is the range rate,
equation (2) gives the relation between these two. So, knowing the doppler fre-
quency, the range rate is also known.
2
The range rate could also be written as follows:
V d RS - s90 (3)
R dt S S -
where VR is the measured range rate
s is the velocity vector of the satellite
Rs and Vs are assumed to be known
So only unknown in equation (3) is RE. If we know the longitude and the latitude
of the emitter, we know RE . So there are only two parameters to be determined.
In principle, these unknowns can be determined by measuring range rate at two
satellite locations. However this is possible only if the emitter frequency fo is
known. Since fo may not be known exactly (but assumed to be a constant during
the measurement period), one could assume a constant bias error in the range
rate measurement. If the bias is in the units of velocity say VD , then the equa-
tion (3) becomes
RSE +VD - V = o (4)
IRsEI R
From at least three measurements of doppler (or VR) at three different satellite
locations, the equation (4) could be solved for VD and the two other unknown pa-
rameters of the source (longitude and latitude).
Therefore, in principle just three measurements of the doppler frequency at
three satellite locations will determine the source location. Knowing the com-
plete or partial doppler curve over one complete pass contains large amount of
redundancy. This redundancy may be used to reduce the random errors intro-
duced by noise or other statistical errors by proper signal processing. More
details on the error analysis will be given in a later section. The advantage of
this method lies in the fact that the frequency of transmission need not be known
precisely, only a partial doppler curve need be known and the large amount of
redundancy which exists even in partial doppler curve could be used in reducing
random errors.
When a complete doppler curve is available or when the maximum rate of change
of doppler is known, a much simpler approach could be used in determining the
source location. The procedure is to record the doppler signal as the satellite
passes nearby the source. At the point of closest approach (Ro), the rate of
change of frequency with time is a maximum and is related to Ro as follows:
9
3
R= (5)
o (dfd/dt) max
where Vs is the satellite velocity
0 = transmitted wavelength
fd = observed doppler frequency
Ro = point of closest approach.
In equation (5) Vs is assumed known, 0 corresponds to the received signal fre-
quency at the time at which maximum doppler rate occurs. So, the accurate
measurement of this frequency is a prerequisite for this method. Also, the un-
known source location imposes the condition that the complete doppler curve be
known so that the time at which maximum doppler rate takes place will be in-
cluded in that curve. Another disadvantage of this method is that the large
amount of redundancy which exists in the doppler curve is not used. Conse-
quently, the method discussed earlier gives better accuracy. The method dis-
cussed later in this section uses simple signal processing.
ERROR ANALYSIS
The probable causes of errors in obtaining doppler curve are enumerated below.
(1) Random noise
(2) Ionospheric refraction
(3) Frequency drifts of the source oscillator
(4) Frequency drifts of the satellite oscillator
It was mentioned earlier that by using the large amount of redundancy which
exists in the complete doppler curve, it is possible to reduce the errors intro-
duced by the random noise or any other random errors introduced into the dop-
pler curve. In general this error is comparatively negligible with other errors.
The errors introduced by the ionospheric refraction decreases with the increase
in frequency. 5 At the lowest frequency of interest (140 MHz) the ionospheric re-
fraction contributes about 5 Hz to the doppler curve. The corresponding error
in source location is about in. mi. 6 For higher frequency bands the refraction
errors are negligible.
When the source transmitted frequencies drift with time, they appear in the dop-
pler curve and introduce errors in source location. However, if one assumes
that the transmitter frequency remains constant during a single pass, it was ex-
plained earlier, that it is possible to correct for long term instability. This is
done by assuming the source frequency as an unknown parameter, and solving
4
for three unknowns (frequency, latitude and longitude of the source) using three
doppler measurements, as explained in detail in the previous section.
The above remarks concerning the source frequency drifts will also apply to sat-
ellite oscillator drifts. Since, oscillators with good short term stability are
available, there seems to be no problem in this respect.
RECEIVER REQUIREMENTS
Receiver requirements depend on the type of signal received (AM or FM etc.),
signal bandwidth, the amount of doppler and the rate of change of doppler fre-
quency. It also depends on any signal processing contemplated at the satellite
or on the ground. Since the amount of doppler expected will influence the re-
quired receiver bandwidth, and the rate of change of doppler will influence the
receiver capability of tracking a particular source, these two parameters are
calculated for a typical low orbiting satellite (ITOS).
Maximum Doppler Frequency Expected:
The required receiver bandwidth depends not only on the signal bandwidth but
also on the expected maximum doppler frequency. The maximum doppler fre-
quency is given by:
Vs R
fd max s (6)C R+h
where fdmax = maximum doppler frequency
fo = carrier frequency of the source
Vs  = satellite velocity
C = velocity of propagation (161,880 n. mi. /sec)
R = Radius of the earth (3440 n. mi.)
h = satellite altitude
For ITOS satellite, h = 1460km = 790n.mi. The velocity of that satellite could
be calculated from 11
VS R+h
where M = 6.274 x 104 n. mi. 3/sec2
Substituting for R and h in the above equation results in Vs = 3.84 n. mi. /sec
5
From equation (6) the maximum doppler frequencies are obtained for three typi-
cal frequencies in the three bands of interest. The results are shown in the fol-
lowing table.
fo fdmax
140 MHz ±2.7 KHz
400 MHz ± 7.74 KHz
1600 MHz +30. 96 KHz
Maximum Rate of Change of Doppler:
From equation (5) it is evident that the maximum rate of change of doppler de-
pends on R 0 , the range between the satellite and the RFI source at the closest
approach.
For ITOS type satellite R o may vary from 790 to 2460 n. mi. depending on the
source latitude substituting these limits for R o in equation (5), the following
values are obtained for maximum rate of change of doppler frequency
(d fd/dt) max
Frequency (d fd/dt)ma x
fo Ro = 790 n mi. Ro = 2460 n. mi.
140 MHz 16.2 Hz/sec 5.2 Hz/sec
400 MHz 46.4 Hz/sec 14.8 Hz/sec
1600 MHz 185.6 Hz/sec 59.2 Hz/sec
SIGNAL PROCESSING
The expected interference at low orbiting satellite may vary with the geographi-
cal location, the interfering signals may have different modulations, signal band-
width, and polarization. In some regions the carrier frequencies may be very
close to each other. In addition, due to the doppler effect, the received frequen-
cies will be constantly changing. For these reasons, if a receiver is designed
with a predetermined bandwidth, it is quite likely that there may be several sig-
nals within that bandwidth. To properly resolve the different signals and deter-
mine their doppler curves, the following signal processing is recommended.
It is assumed, at the outset, no signal processing will be attempted at the satel-
lite, as it complicates the satellite. For RFI experiment, the satellite carries
6
a wideband antenna and a tunable receiver. The required antenna characteristics
will be considered in a later section. The design details of the satellite receiver
will not be given here, but the following capabilities are assumed. The received
signals are first filtered in a preamplifier, and the first frequency conversion
bring the signals to the intermediate frequency range, where they are further
filtered. A second conversion brings the spectrum to the baseband range (say
1 KHz to 26 KHz for 25 KHz bandwidth or 1 KHz to 201 KHz for 200 KHz band-
width). These signals are multiplexed and sent to ground station for further
processing.
The proposed ground station processing is shown below:
BASEBAND
SIGNAL SIGNAL PRESENCE
SAMPLER FFT DETECTION &
FREO. ESTIMATE
FREQ. ESTIMATE
RECEIVER WITH FINAL ACQUISITION
ANALOG PHASELOCK OF SIGNAL FREQ.
TAPE LOOP & AMPLITUDE
Figure 1. Ground Station Signal Processing
The channel center frequency is translated to zero frequency and the resulting
signal is filtered by a low pass filter. The signal is analogue recorded on tape
and also sampled for detection of signal presence. The sampler takes complex
point samples at a rate of twice the composite signal bandwidth. These samples
are processed using a complex fast Fourier transform (FFT) to detect the pre-
sence of a signal and roughly estimate its carrier frequency (say within 1 KHz).
If a signal was detected, the estimated frequency is used to control the phase lock
loop receiver for final acquisition of signal frequency and amplitude as shown in
Fig. 1.
ANTENNA ANALYSIS
In this section a brief discussion is given on the required antenna characteristics
for RFI measurement. Since, the frequency range of interest is from 0.1 to
7
1.7 GHz, broadband antennas are mandatory to minimize the number of antennas
required.
An antenna with small beamwidth is desired to find the source location by spacial
discrimination. However, the difficulties associated with the use of such anten-
nas preclude this approach as explained below. To obtain small width, physically
large antenna would be required. Due to small beamwidth, a low probability of
RFI source signal would occur as the spacecraft travels at a high orbital veloc-
ity, the spacecraft's stability and pointing accuracy would be critical, and it
takes long time to complete earth coverage. Since only a narrow swath could
be covered per orbit. For these reasons it is required to use a simple antenna
which has an isotropic pattern from horizon to horizon.
For ITOS type of satellite, the beamwidth required to cover the earth could be
shown to be 108.40.
Another parameter to be considered is the required polarization. Since, a large
percentage of earth based signals are linearly polarized and the fact that the
Faraday rotation of linearly polarized signals below 1 GHz passing through the
ionosphere results in a polarization ambiquity, it is recommended that the cir-
cularly polarized antennas be used.
Log conical or cavity backed spirals have the required characteristics of broad-
bandwidth, broad-beamwidth and circular polarization.
REFERENCES
1. Anzic, G. "Aerial RF noise measurement in urban areas at UHF frequencies"
AIAA 3rd Comm. Satellite System Conference, Los Angeles, April 1970.
2. Zamites, C. J. et al. "Measurement of Interference Levels in the UHF
Band from Aircraft Altitudes" IEEE Trans. on Electromagnetic comp.
Aug. 1970, pp. 88-96.
3. Hoffman, L. A. et al. "Radio frequency interference at orbital altitudes"
IEEE Trans. on EMC, March 1966, pp 1-8.
4. Jenny, J. A.; Weiss, S. J. "The effects of multipath and RFI on the track-
ing and data relay satellite system" ESL Inc., Tech memo No. ESL-TM 215,
March 1971.
8
5. Guier, W. H. et al. "A Satellite Doppler Navigation System" PROC. of IRE,
April 1960, pp 507-516.
6. Maxwell, J. C. "A doppler satellite system design for animal tracking"
NTC '71 Record, pp 259-263.
7. Ryle M. and A. Hewish "The synthesis of large radio Telescope" Monthly
Notices of Royal Astronomical Soc. Vol. 120, No. 3, 1960, pp 220-230.
8. Sparagna J. et al. "Passive ECM: Emitter location techniques" The
Microwave Journal, May 1971.
9. Skolnik, M., "RADAR HANDBOOK" pp 32-26 to 32-27.
10. Arndt A. E. et al. "System study for the random acess measurement
system (RAMS), GSFC X-752-70-376.
11. Skolnik, M., "RADAR HANDBOOK" pp 32-6.
9


RFI emitter location techniques

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