Radio astronomy experiments have demonstrated the feasibility of making precise position measurements using interferometry techniques. The application of this method to navigation and marine geodesy is discussed, and comparisons are made with existing navigation systems. The very long baseline technique, with a master station, can use either an artificial satellite or natural sources as position references; a high-speed data link is required. A completely ship-borne system is shown to be feasible, at the cost of poorer sensitivity for natural sources. A comparison of Doppler, delay and phase-track modes of operating a very long baseline configuration is made, as that between instantaneous measurements and those where a source can be tracked from horizon to transit. Geometric limitations in latitude and longitude coverage are discussed. The characteristics of natural radio sources, their flux, distribution on the sky, and apparent size are shown to provide a limit on position measurements precision. The atmosphere and frequency standard used both contribute to position measurement uncertainty by affecting interferometric phase

Hulburt, E. O.

Johnston, K. J.

Knowles, S. H.

English

NASA Technical Reports Server

APPLICATIONS OF RADIO INTERFEROM ETRY TO NAVIGATION 
S. H. Knowles 
K. J.  Johnston 
Naval Research Laboratory 
E. 0. Hulburt 
Center for Space R e ,  erch 
ABSTRACT 
Radio astronomy experiments have demonstrated the feasibility of making pre- 
cise position measurements using interferometry techniques. The application 
of this method to navigation and marine gcodesy i s  discussed, and corr,parisons 
a r e  made with existing navigation systems. The very long baseline technique, 
with a master station, can use either an artificial satellite o r  natural sources a s  
position references; a high-speed data link i s  required. A ccmpletely ship-borne 
system is shown to be feasible, at the cost of poorer sensitivity for natural 
sources. A comparison of doppler, delay and phase-track modes of operating a 
very long baseline configuration i s  made, a s  that between instantaneous measure- 
ments and those where a source can be tracked from horizon to transit. Geo-- 
metric limitations in latitude and longitude coverage a re  discussed. 'le charac- 
teristics of natural radio sources, their flux, distribution on the sky, and 
app~ren t  size a re  shown to provide a limit on position measurement precision. 
The atmosphere and frequency shndard used both contribute to position meas- 
urment u~certainty by affecting interferometric phase. 
INTRODUCTION 
The Navy, a s  well a s  civilian mariners, have been looking for the ideal naviga- 
tion system for centuries. Recent developnlents in radio frequency navigation 
techniques, together with inertial navigation, have made navigation much more 
reliable, but a study of future possible techniques i s  still of considerable in- 
terest. One such technique i s  radio interferometry. The radio interferometer 
technique combines radio signals from two antennas spaccd a distance apart to 
obtain the angular resolution of an anteni~a whose diameter equals this distance. 
In principle the radio interferometer is  similar to the Michaelson interferometer 
used at optical wavelengths with both signals being combined coherently (with 
phase preserved) to produce an interference pattern. However, while the earth's 
atmosphere limits optical coherent interferometers to baselines of a few meters 
because of atmospheric phase degradation, this effect i s  much less serious a t  
radio wavelengths, and baselines of thousands of kilometers have been used. 
Radio interferometers have determined positions of radio-emitting galaxies more 
precisely than can be done by optical means, and have measured the distance 
between fixed antennas with a precision of czntjmeters. The attractive charac- 
terictics of radio interferometry for a navigation system include, besides its 
https://ntrs.nasa.gov/search.jsp?R=19750003207 2020-03-23T03:06:26+00:00Z
high inherent precision, a possible savings on the cost and complexity of a satel- 
lite navigation system, and an all-weather capability. 
The radio interferometer technique was developed several years ago by radio 
astronomers a s  a means to overcome the inherentlv poor angular resolution 
available from a rzdio astronomy antenna. Figure 1 shows the basic principle 
of operation. A radio interferometer consists of two antennas spaced a distance 
apart, pointing at the same radio emission source, which may be either natural 
o r  artificial. The two incoming radio signals a re  amplified separately, and then 
brought toghether and correlated. The maximum correlation occurs when the 
signal from one antenna is  delayed an amount equal to the excess travel time 
along a line from the radic, solirce to the otlier antenna; the source must be smal- 
ler than the resolution of the interferometer. Thus, the interferometer meas- 
ures, for a source at a distance much greater than the baseline, the dc4 nroduct 
of the baseline and the source position vector. Early interferomete: s L , * 4  ' ~ e  
two antennas located close enough together so  that the two signals, a*  I . . > 
oscillator needed to beat to base band, could be communicated by ti: .Ins c f ,' les. 
This limited interferometer baselines to several kilometers. To oi -rn ,,his, 
independent master oscillators were used at each station and the signals brought 
together by means .f microwave link o r  high-speed tape recording. This tech- 
nique is  known as  the Very Long Baseline Interferometer, o r  VLBI, technique, 
and has enabled antennas placed on opposite sides of the earth to form 2 success- 
ful interferometer. The longest baseline used so far i s  that between Westford, 
Massachusetts, U. S.A. and Semeiz, 27.S. S. R. (near Yalta), a distance of about 
8000 kilometers. 
As mentioned in the peceding paragraph, the principle of interferometer opera- 
tion i s  straightforward. When applied to ? navigation system, however, there 
a r e  many possible choices to be made. Figure 2 shows some of these. Either 
natural o r  artificial (artificial satellite) illuminating sources can be used. The 
two most suitable classes of natural sources a re  the quasars, which have a broad 
spectrum, and the water-vapor sources, which em : narrow band radiation near 
1.35 centimeter wavelength. The natural sources are  in general weaker than an 
artificial source, and are  variable in signal intensity, but of course spare us  the 
cost of an artificial satellite. The basic operating configuration can be either 
the VLBI one, with a shore-located master station, o r  a shipboard configuration, 
using two antennas on the bow and stern of a ship. If an artificial signal source 
i s  used, a "one-station1' interferometer configuration i s  also possible, with the 
satellite generating the signal reference. The basic observable in all interfer- 
ometers is  relative phase. However, in VLBI measurements, the oscillator 
stability is  usually too poor to measure phase directly, and either delay o r  rela- 
tive doppler rate i s  measured. An instantaneous measurement of one radio 
source always gives a line of position cn the earth's surface. There are  several 
ways to obtain the two intersecting lines of position needed to determine ship 
position. This includes observing two radio sources, &sewing both delay and 
doppler fmm one source, observing one source a t  different positions on the shy, 
or,  for the shipboard interferometer, rotating the ship. Rotating the ship adds 
no information for a VLBI system. The same is  true for simulbneous use of 
more than one master station. 
Although land-based iaterferometry is a m  dcme on a mutine basis, there a r e  
aeverzl practical problems to be solved in implementing a shipboard, real-time 
system. Figure 3 shows in symbolic form a block diagram of a possible system, 
using natural sources and the VLBI concept. Note that both ship and master 
station muire two antemas, one a t  each station pointed a t  the same natural 
radio source, ard me each pointed at  the data link satellite. Since both antennas 
oa the shfp n:ust be kept pointed accurately, they must be mounted on an iner-tial 
platform, Second, random pha3e excursions must be kept to less than 1 /4 of the 
observing aavelengtb (or about one centimeter) over the integration internal of 
several seconds. This may be done using a three-axis accelerometer a s  input 
to a phase-compensatiw network. Finally, the correlation peak show's  up as one 
point in a plane i:i a-hich one dimension is time delay, and the other is differen- 
tial doppler. The width in both plalias is affected by the ship's posi:ion uncer- 
tainty, the frequency-stz~dard's accuracy, and ssip's velocity uncertainty. A 
special-purpose, real-time computer i s  i-equi red to do th's search. 
The cmfiguralfoa diagram for the shrpboard system is shown in Figure 4. The 
requi .?merit for phase stability is easier to meet than for the VLBI system, since 
only differential acceleration must be measured, but the ship's heading must be 
accurately h o r n ,  since this system measures angles. In addition, there is a 
problem obtainiug sufficient signal from natural sources with b-o antennas small 
enough to be easily incnmted on board ships. Figure 5 illustrates this, Two me- 
metex antennas can produce a usable signal for water-vapor sources in about 
two minute9 integratior-, but a t  least 10-meter antennas w d d  be required for 
a quasar system. This is not a pmblem for rirtificial sources. 
ArVficial satellites may be used with interferometers in various ways; in some 
asps, this i s  an extensim of techniques already in use, For emmple, coherent 
doppler tracking of a space probe corresponds closeig to the interferometer 
phase-track concept. One important use of interferometer techniques d e v e l m  
by radio astronomers is to point out tire feasibility of measuring the range, a s  
well as range-rate, to a target to within one carr ier  cycle, oy the use of wide- 
bad modulaticn techniqur =. With a precise clock on board the satellite, the 
n n g e  to the satellite cpn b . measured without a real-time master station, mak- 
~ n g  a "one-station" interferometer. A system with less dependence on the 
satellite orbit results i C  the satc~:ite is observed simultaneously with a master 
station, and triangulatron is wed. The range can be tracked during a satellite 
pass, thus enabling detem..in?tion of both coordinates of aobiie station wsition. 
Use of a s h i m &  interferometer with an aritifcial satellite d i e s  determina- 
tion of ane angular -dinate at the same time its ram@z i s  measured, thm 
wabling instantaneous position detenniaatiaa. 
Altborrgb exact obsewatim geometry i s  often complex, Figure 6 shown approxi- 
m!e p r e c i s h  that em be & a i d  for various modcs if a favcnbte geometry is 
assumed (a signal-to-nsise ratio of 10 is assumed for all sources), All meas- 
urments in the bottom half of tbe graph are lixnited by atmospheric constraints to 
a precisiaa of about three meters. From this graph the inherent fiigb precision 
of radio interferemeter measurements m y  be appreciated, 
Thus the caecept of radio interferomctric navigation may add to tbe complement 
of techniques already available for navigatia~. Its high inherent precision makes 
it especially suitable for high-accuracy applications. Altbopgb some p a c t i d  
problems remain to be solved for a shipboard canfiguration, tbese are believed 
to be easily masterable if the system i s  plrsued. 
- ALL INTERFEROMETERS 
- VLBI  INTERFEROMETER COmELATOR (MULTIPLIER) PRODUCES OUTPUl FOR 
---. CABLE-CONNECTED INTERFERMTER 'CORRELATE0 FLUX -A 
Figure 1. Btleic Intorferomoter Conflgurntlons 
TYPE OF SOURCE 
TYPE OF 
INTERFERC] 
TYPE OF 
ANALYSIS 
SOURCES 
SHIP-BOARD 
CABLE 
TERFEROMETER 
ONE - STATION 
INTERFEROMETER METER 
MASTER 
STATIONS IN' 
I 
( PHASE ( 



ARTIFICIAL SATELLITE DOPPLER M O M  ( VLBI  I 
100 km 
t.0crn 8 ' QUASARS. VLBI PHASE - TRACK MODE ( 6 cm ! F \\ ' -------------.---------- -- ---- c --\- - - ---- . '\ i 
PULSARS, DELAY %?ODE 
S/N OF 1G ASSUMED 
\ 4 \ \ WWKT CASE 4 1 4 1 4 4 ,  
ARTIFICIAL SATELLITE. SHIPBOARD (250' SHIP) I 
H20, VLBI PHASE TRACK MODE (1 35m) ' ' 
------------------------------------ 
VL6I  
- \ +Om DELAY MOOE 6'Ooorn DEPENDS ON SOURCE 
v, 
Figure 6. Inherent precision for a signal-to-noise ratio of 10 for various iiter- 
ferometric navigation systems. S-band operation i s  assumed for artifi -ial satel- 
lites, and X-band for the quasar system. 
1 t t t t 4 ,  
OUASARS.VLBI DOQPLEi3 MODE 
&! MASUREMENTS *\ 
3 - . . . / ' 
a ' H20.VLBI DOPPLER MOOE ', ", ", y t o r n -  \ ' \  
'\ \ 
'\ ' '\ 6 ARTIFICIAL SATCLLITE . VLBI D E L ~ Y  ~ q ( t b q l ~ ~ z  EW ASSUMED 
- ------------------ &- - ------------- 3 ' \ ' . ' 
V) '. ' ' 
w10Ocm - \ ' '. ' ' \, '\ 
A S A R S .  VLBI DELAY MODE ( I GHZ BW ~ \ S U M E ~ ' ~ - .  
---------------------- +-- -4------- 
ARTIFICIAL SATELL~TE VLBI PHASE -TRACK MODE (h<m b. ----- - 
- -,-,-,-' --- , --------&-- 4 
QUASAR. SHIPBOAR0 (250'SHIP) 
I I 
H20 SHIPWARD 
I (250' SHIP) 
QUESTION AND ANSIVER PERIOD 
MR. CKI: 
Are there any questions? 
MR. BLACK: 
H a ~ l d  Black, of Johns Hopkins APL. I misunderstood what you intended to 
say, when you said that a doppler system didn't tvork on the Equator. 
DR. KNOWLES: 
A doppler system will not determine !atitude for  you on the Equator. 
MR. BLACK: 
That i s  n ~ t  correct. The Navy Navigation Satellite System, which is a doppler 
system, works beautifully on the Equator. It gives both components of position. 
Now, you have to qualify your statement somehox. 
DR. KNOWLES: 
Yes, you a r e  right, I a m  sorry. I should have qualified it ,  that applies to 
natural sources that travel from east  to west. I t  doesn't apply to artificial 
satellites, which can get an inclination in there. 
llm. BLACK: 
Thank you. 
MR. SWANSON: 
Eric Swanson, NELC. I would like to make a comment. The whole use of VLBI 
for navigation, a s  you quite properly pointed out, requires a very wide down 
link. Given that the inherent function of ~lavigation actually demands a very 
slight band width - i t  can be argued the real need is zero on transmission - you 
a r e  now talking of a method that will impact the spectrum by some megacycles 
or worse. 
Could you comment on that? 
DR. I<NO\VLES: 
Yes. It is a disadvantage of it, but I am not clear a s  to the statement that the 
real need for band width on a navigation system i s  zero. 
And particularly, on the other point that on a satellite system thcre i s  definitely 
a finite band width in there. 
This is,  by the \bay, one thing which I didn't mention on the VLBI technique. 
\Vhen you use it  on artificial satellite techniques it is really more an extension 
of other satellite techniques. If its essential contribution i s  trying to tell you 
to use a transmitter with a wide band width coding on it, so that you can get a 
lock in on the absolute phases of the transmission, the problem is  that you 
could get by ttith a zem h9rlc-l width if you were not a t  all concerned about ambi- 
guity problems. In particular, on the water vapor sources, we can get very 
accurate position measurements with the n a r r o ~ ~  band width, and you could let 
that go to infinitely narrow and still get position. 
The rc.ason you need the band width is to provide you with longer waves ss well a s  
shorter ones, to let you zero in  on your position. 
I am not clear that that is the answer to your question. I \vill just leave that up 
for thought. 
MR. CHI: 
Thank you very much. 
The next paper, No. 6, which has been withdrawn, and in place of it  i s  a paper 
under the title of "Application of Radio Interferometry to Clock Synchronization, '' 
by Dr. William Hurd, of Jet  Propulsion Laboratory. 


Applications of radio interferometry to navigation

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