CA1071307A - Flux valve heading repeater compensation systems - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A flux valve compass heading repeater system is provided with a compensating system which, when connected to a three-legged flux valve, provides fully compensated, three-wire output signals of the synchro data transmitter type for direct use in apparatus requiring precision three-wire heading data.
The compensating system includes control circuits for The compensating system includes control circuit for generating sine and cosine components of magnetic heading and for compensating them for typical compass errors such as those induced by changes in operating latitude and two cycle and index errors. Latitude compensation is accomplished by a novel proportional automatic gain control; two cycle cardinal heading error compensation is accomplished by a compensation circuit having only a single manual control, while index error compensation is similarly accomplished by a compensation circuit requiring only a single by a compensation circuit requiring only a single manual control.

Description

7~3~7 BACKGROUND OF THE I~VENTION
1. Field of the Invention ~ he invention pertains to means for the compensation of undesirable errors or changes in the signal characteristics of flux valve data repeater systems and more particularly relates to apparatus for the correction D~ variations in the outputs of flux valve compass data repeater systems, including errors due to variation in the horizontal co~ponent of the earth's magnetic field, index angle errors, and cardinal and intercardinal heading errors.

2. ~
When navigating at high latitudes with flux valve magnetic compass systems, difficulty i6 experienced because of the decreasing strength of the horizontal component of the earth's magnetic field, especially at high latitudes. A Mux valve magnetic com-pass is normally arranged to sense only the horizontal component of the earth's field. As a consequence at high latitudes, the strength of the sensed component is proportionally lessened, and the compass system experiences decreasing sensitivity, resulting in heading information of diminished accuracy.
Prior art systems have sought to solve this compensation problem of providing an input of the magnetic compass data repeater substantially independent of variations în the strength of the horizontal component of the earth's field, as by con-trollîng the gains of amplifiers or the effective values of impedances in the separate channels of the data transmitter system in a relatively complex manner, but generall~ in inverse relation to the signal strength as measured at the flux valve itself. Such prior art arrangements are described in D.A. ~spen in the United States patent 3,548,284 for "Synchro Data Transmission ~pparatus -. : .
Having Discrete Gain Changing to compensate for Undesirable ::

; - 2 ~

~ 7~3~7 1 Si~nal Gradi~nt Variation~ sued D~cember 15, 1970, and in J.R. Erspam~r and G.W. Sn~d~r in the United Statss patent

3,646,537 for an ~Automatic Gain Con~rol for an Electromechanical Transduc~r~', issued Febr~ar~ 29, 1972, both pat~nts being assign~d to the Sperry Rand Corporation. Whil~ th~s~ concepk~
have b~n use~ul in providing ade~uate ma~n~ti~ field comp~n~ation in mos~ circumstanc~s, the compensating 6ignals actually com-p~nsate only for vasiatio~ in th~ horiæon~al magn~c 1~1d co~pon~nt and gen~rally do not additionally correct fully for gain changes caused by camponent variations or du~ to te~nperature or powQr 8uppl y voltage drifts or to component aging. Furth~r~ore, ~he charac~ris~ic~ o~ ~he individual gain con~rvl el~m~nts of the individual cha~nels of the data system ~ay var~ without ; propar correctiva relativ~ adju~tm~nts wh~reby two-cycle transmission ~rr~r~ are induced within automa~ic gain control ~tages.
Th~ improved system disclosed by JOR. E~sp~er and G.W.
SnydQr in th~ United Xta~es patent 3~784,753, i~sued January 8, 1974 or a ~M~ltipl~xsd Gain Control for a Synchxo Data ~ran~
mls8ion Sy~te~" sou~h~ more ~ully to overcome th~s~ prior art defect~ by a relativ~ly c~plex and ~xpen~ive correction circuit.
~hough i~ g~nerally overcame ~uch defects, it was found that 80n~
und~sirabl~ two cycle ~rror collld be gen~rated in it~ relatively complex automatic gain co~rol staga, and that a simpl~ way was naeded ~o~ id~ically chan~ing ~h~ gains of both of the sine and cosin~ chann~ls of ~he data ~ran~mi~sion s~tem, but retaining th~ advantages of the conc~p~ of pa~Qn~ 3,784,753.
Pr~or art ~yst~m~ hav~ additionally sought to provid~ : :
- correction f or the cardinal heading error in compa~s data 30 tra~mis~ion ~y~tems b~ u~ of networXs i~cludi~g pr~cisio~ : :
dif~r2~tlal synchros ox g~n~ea dual po~n~iome~rs which mu~
~rack each other with high pr~cision if the~ ar~ not the~01v~s : ~

.

1L3~7 1 to introduce errors. According to the pre~ent invention, the ~xpense of obtaining such sel~ct~d synchros or pr~cision pot~ntiometers is de~irably eliminated- Index anglQ exror was similarly corrected in prior compas~ data ~ra~i~sion sy~t~m~ by using precision synchros or gang~d dual po~tiometers of ~i~ilar ~uality. It is found increasingly d~sirable to ~uh~titut~ ~impl~
and less expensive ne~works permitting ~ingle adju~tm~nt co~trol for eac~ of th~s~ correc~ion purposes and, at kh~ ~ame ~im~, retai~ing a high degr~e o~ pracision.

SU~:I~HE:3~
The pre~nt invention prov.ides m~ans or eorrection of und~sirabl~ change~ in the signal ampli~ud0~ in multlple chann~l flux valve data repeater syskem~ partly by ~h~ employment of a simpl~ common au~omatic gain co~trol i~ a circuit configuration which not only c~mp~nsates for ear~h's magn~tic field strength ~ :
change~, but also corr~cts for the effect~ of other error sourc~s witho~t introduci~g the ~rrsrs of prior art sy~tems. ~he novsl control circ~it o~ the pr~sent in~e~tion monitor~ the d~ta rep~at~r contr~l signals near th~ inputs ~o the utilization d~vice, ; 20 rather than ~rely at th~ output~ o~ th~ flux valve. B~ monitoring ~h~ inputs ~t the utilization device, and ~y u~ing ths dat~ :
r~a~er ~xcitatlon voltage a6 a switchins~ referenc~, the gain control, being part o a clo~ed aed back loop, compensate~ not only or chanyes in the ope~ating latitude~ but also for gain chang~6 caused by variations o:E cnpon~ t parame~ers and ~r o~her eff2cts without itself introducing 2~ew errors. According to a primary a~p~ct o~ the pr~s~n~ inYen~ion~ slectrically cros~
coupl~d ~atwork m~an~ provide8 corr~ction in ~h~ e and co~in~
chann~l~ of the flux valv~ data tran~mi~ion ~ys~m fo~ t~o cycle cardinal ~rror correctlon by the ~etting o~ only a ~ingl~ adjust-.:

me~. A similar arrang~men~, again r~uiring only one adju~tmen~, ' ~:37~7 is employed for correction of any index angle error. In a modi-fication of the compensation system, the two latter compensations are accomplished by direct current signals applied in predetermined ratios directly into the inductive windings of the flux valve.
According to a broad aspect of khe present invention, there is provided in a magnetic compass data transmission system for navigable craft, the combination comprising: magnetic field detector means including a plural-ity of induc~ive windings responsive to the directi~n and magnitude of the earth's magnetic field with respect thereto for providing a first plurality of alternating signals representative of said earth's magnetic field direction and magnitude, current servo means responsive to said first plurality of alternating signals for generating first and second unidirectional signals representative of said earth's magnetic field direction and magnitude, gain ; con~rol means responsive to said first and second unidirectional signals Eor generating third and fourth unidirectional signals representative of said earth's magnetic field direction and substantially independent of the magni-tude thereof, said gain control means inc~uding: first and sec~nd varia~le conductivity circuits having respective first and second conductivity states~
switching means fer causing said first and second separate variable conductiv-2a it~ circuits simultaneously to change from one to the other of said first and second conductivity states at a controlled duty cycle, filter means residing respectively in said first and second variable conductivity circuits for smoothing the respective currents flowing through said switching means for forming said third and fourth unidirectional signals, circuit means for separately modulating said third and fourth unidirectional signals with a reference alternating signal for forming first and second output alternating signals representative of said earth's magnetic field direction, control means responsive to said first and second output alternating signals for control of said gain control means, said con~rol means including: comparator means for comparing a signal representative of said first and secoDd output alternat-~k 5 '~

. .
. . - - .

~L~7~L3~
ing slgnals to a reference unidirectional signal, and variable pulse-width generator means responsive to said comparator means for supplying control].ed pulse-width modula~ed signals for controlling said controlled duty cycle, and utilization means additionally responsive to said first and second output alternating signalsO
The invention will now be described in greater detail with reference to the accompanying drawings, in which:
Figures lA and lB illustrate, partly in block diagram form, the principal elements of the invention and their electrical interconnections;
Figure 2 is a portion of Figures lA and lB showing details of the novel automatic gain control circuit, Figure 3 is a detailed circuit diagram of the novel index error angle compensator of Figure lB, Figure 4 is a detailed circuit diagram of the novel two cycle compensator also of Figure lB, and .:
Figure 5 is a block diagram of an embodiment of the invention alternative to that of Figures lA and lB.
In Figures lA and lB, the novel compensated compass sys~em includes a magnetic azimuth detector or flux valv0 11 which may be of the general type disclosed in the M. C~ Depp United States Patent 2,852,859 for a "Flux Valve Compensating System", lssued September 23, 1958 and assigned to Sperry Rand Corporation. Other details of such flux valve devices are disclosed in the D. J. Kesselring United States patent 3,573,610, issued April 6, 1971, in the D.J. Kesselring et al United States patent 3,641,679, issued Pebruary 15, 1974, and in the United States patent application S. N. 380,523 for "A
Flux Valve Apparatus for Sensing Both Horizontal and Vertical Components of an Ambient Magnetic Field", filed July 18, 1973, issued as patent 3,873,914 .
March 25, 1975 and assigned to Sperry Rand Corporation. Flux valve 11 is excited by alternating current source 2, which may be a conven ~ional 400 Hzo ~. :
3Q oscillator or signal generator and which is coupled to excitat ion winding 12 ~
of the flux valve llo "

.. - 5a - ~

. - . , , . ................ . , . . ~,:

3C~7 As disclosed in the afore~e~tioned Depp and Kesselring :
patents, ~lux valve 11 has three wy~-connected inductive windirlgs 13, 14, and 15 on a corresponding wye-shaped cor~, the windin~
- legs meeting at a conunon grourlded t~?nninal F. The t~srminals of windir~gs 13, 14, and 15 opposite ~erminal F are r~spec~ively labelled A, B, and C. Terminals A~ B, and C may, if de~ire~d, b~ supplied wlth single cycle c~slpensation ~ignal~ from a ~ingle cycle e~mpensat~r ~not shown) o~ the~ gex~ral type ~hown in ~h~3 aforemerltioned U.S. pa~ent 2~852,85g.
~erminal P. of flux valve 11 i5 connected via a blocking capacitor 16 to on~ winding 20 of a ~cott tee ~ansform~r 21, while terminals B and C ar~ colmected via rç~pective blocking ~:
capacitors 17 and 18 to th~ r~spective ends of a s~cond input winding 22 of Sco~t tee transformer 21. Winding 22 has a cent~3r tap ~o~nected ~o th~ ot:her end of winding 20.
A~ is well known, the sig~lal output~ of windings 13, 14 and 15 have a fr~uerlcy double that applied ~o excitatiola winding 12. q~he fr~au~ncy doubl~d coæin~ output od~ winding 23 of ~ran~;-form~r 21 and it~; fr~ue~ncy doubled sine output in winding 27 20 are connected to c:urrent s~rvo loop 31. Additio~ally supplied to curr~t ~:~rvo loop 31 via lead 29a is the outpu~ o~ frequency doubler 29~ Since fre~uency doubler 29 is excit~d by gen~rator . .
2, its output on lead 29a will hav,l3 an BOû Hz fr~au~ncy and ser~es as a r~erence sigllal ~ource for servo 31.
~ d~scribed ~r~ detail in th~ D<. ~. Bak~r, F. ~. Kallio U.S. pat~nt 3~678, 593 ~or a "Cosnpass Syst~n and C~nponenks '~herefor Having Automatic Fi~ld Canc~llation", issued July 25, ~:
197~ 1:o ~;p~rry Rand Corpo;ratio~, c:urrer~t s~rvo 31 suppli~s outputs o~ ad~; 32 and 33 which ara 3irec~ current signal ; resp~ctively 30 proportional in amplitude to th~ sin~ and co~in~ of craft ma~netic .~, - : . . .

~713~7 1 heading 5Hmsin ~ and Hmcos ~). Accordingly, the horizontal compon~nts of the ear~h's magnetic field sen~d by th~ flux ValVQ
windings 13~ 14 and 15 are re~olved into ~in~ and cosine com-pon~t valu~s that are then con~ert~d by current s~vo 31 into proportional direc~ currents on leads 32 and 33. As taught in ~h~ afor~mention~d Bak~r et al patent, theæe dir~c~ curr~nt components are fed back vla l~ads 10 and lOa into windings 13 and 15 of flux valve 11, which curr~nts tend ~o cancel the earth'~
magnetic fleld ~her~inO Th~ f~ed back arr~ng~m~n~ and its many advantag~s are di~cuss~d in d~tail in th~ aforementionQd patent 3,678,593, including closed loop operation affording high accuracy outpu~3 in the form Gf analog direct current outputs proportional to the sine and cosine of craft m~gne~ic headiny.
Accordi~gly, th~ 80Q Hz., thre~-wire magnetic azinuth in~ormation d~riv~d by the horizontal magnetic fi~ld detector or flux valve 11 i~ converted to dir~ct curxent signals pr~portional to the ~ine ana cosine of craft haading by th~ cooperation of Scott teQ transformer 21 and current s~rvo 31. ~h~ magnitud~s of th~ outputs o~ 10ads 32 and 33 ar~ thus a function of craft 20 magnatie azimuth or h~3ading and ~h~ int~nsity o ~he hc:rizontal component of the earth's magne~tic ~i~ld~ q!he variation in m~s~nitud~ of th~ sin~ and cosinQ outputs c~u~ed b~7 any chang~
in magnetic field s~r~ngth Hm afects ~a~ly th~ output gradient (vol~s p~r azi~lluth degree~) and do~s nvt change the ~rigonQmetric relationship of the! input magn~ic headi3~g angl@~ yJ and the outpu~
voltag~s of curr~nt servo 31, which may th~r~for2 be expr~sed as f~llow~:
V32 = Kl sin and V -- K cos y (2) wh~r~ Xl allow~ for th~ gain o~ curr~t s~rYo 31 and has dimerlsions of vs~lts p~r oerst~d.

,, .. . ... . . . . . .

7~3~7 The signals V32 and V33 on leads 32 and 33 s~rv~ as ~wo inputs to the! automatic gain con~rol circuit 34, which circui~
also re:ceives certain fed back signal~ on leads 56 and 57. P.s will b~ furth~r discus~ed, ~h~3 fed back signals ari~e at the outputs of buffer amplifi~rs ~2 and 53 after the outputs of automatic gain csntrol 34 are proc~s~:ed at least b~ dual channel modul~or 45. q!o underskar~d the ope~ation of the gain control clrcuit 34, the presenc~s of lthe ind~x error comp~nsator 37 and the two-cycle comp~nsator 48 may be ignor~d ~or th~ mc~ment as a 10 matter of conve~ nc~s.
~ h~ final output of the ccxnpass system supplied bs~ leads 61, 62 arad 63 to an aircraft na~rigation system or other utilization device 64 is usually r~luired to be u3e?ful in a three-wire synchro data transmitter ~yste~l and to cons:i st of proportional voltage~s be~ween pairs o~ such leads, as be~tw~n leads 61 and 62, 62 and 63, 63 and 61. 1!h~se may noJninally be 11..8 volts, for exampla, and must b~ maintain~d ak a constant gradi~n~ in th~ inter~st o~ me~ting res~uired compass accuracy over a wlde rang~ o horizontal magrL~ic fi~ld s~rengths ~m~
.~ 20 B~3cause th~ otltput of ~lux valve 11, and th~refor~ the output of curr~nt ser~o 31, ha~; a gradi~nt which is directly proportional in magnitud~ to ~h~ horizon~al magnetic fi~ld str~ngth which, o~ cour~e, vart~3s with lati~u~b, ~he automatic gair~ control s~ag~s 34 is re~auir~d to hold ~he syst~m output ~ignals at 1~ads 61, ~:
62, and 63 a~ ~hç~ de~ir~d nominal 11. 8 volt leg-to-leg constant gradient .
For this purpos~ls, th~ direct curre3nt outputs on le3ads 35 and 36 of gain control 34 ar~ suppli~d to ~h~ conventional dual - chann~sl modulator 45~. 2a~h of ~h~ ~wo i;ndividual channel~ o which are ~uppli~d by l~ad 2a with th~ ~00 Hz. re~Qr4~nc~ ~ignal ou~pu~
of gelal3rator 2. q!he dir~ct curr~l~t signals on l~ads 35 and 3 ,, .
.
, .

~q~7~3~7 are modul a~ed by th~ 400 Hz. alt~rnating current signal in the con~entional manner so that 400 ~z. signals app~ar on leads 46 and 47, proportional rQspQctively to th~ sine and cosina of ~he magnetic heading of ~h~ craft. Af~r individually separate supply to buff~r ~plifier6 52~ 53, ~qually amplified ver~ions o~ these signals appear on leads 54 ~ 55 ~o which the f ~d back leads 56 and 57 ars resp~c~iv~l~ cQnn~ct~d.
~ h~ aut~natic g3i~ control 34, ~hown in grea~c~r d~tail in FiçJure 2, monitor~ the gradi~n~ a~ ~he ou~put laad~ 54,, 55 of 10 bu~ r ampli~EiQrs 52, 53, r~spactivelyO c~mp~eæ th~ r~ult to a r~f~renc:e voltage level, th~ varie~; th~ system gain accordingly ~T con~rol of ~he gain o$ automatlc gain con~rol circui~ 34. I~
the gradie~t at the outputs of bu~r ainplifiers 52, 53 of Figur~
lA is 1~38E; than a pr~3det~rmined level,, the ~roltage gain of circuit 34 is increased to bring the output o~ the ~uffer ~nplifi~rs 52, 53 up to the prop~r level. The outpu~ of the buf~r amplifiers 52, 53 and the voltage g~adi~nt is ~milarly cor~trolled~ rrhe sigf~al l~vels at outpu~ leads 35, 36 of ~h~ auto~atic gain con~rol ;:
34 are ultima~ely pa~e3d through the--o~put Sco~t tea ~rax~o~mer 60. Th0 output~ of tran~form~r 60 are ther~3fox~ fully indepe!,ndent of any ear~h's magnetic fi~ld str~ h variation~
Thus;
V35 = K2 sin ~ (3) . . ~- :
and:
V36 = K2 C08 1~ (43 ~ ~ .
wher~ K2 i~ a new proportionality con~tan~.
- Automatic gain con~rol circui~ 34 i~ d~sign~d ~o preve~lt the :Lntro~uction o~ ar~y staIld-vff or Imbalanc~ b~tween trar 8-mi~siola chan~l~, reæultis~g in cyclic ~rror~, intQ th~ cra~t 3Q he~ading output da~a. ~he :Lndividual gain~ o~ ~he æine a~d co~ln~ ~ ~
c:hannel~ ar~ now identically controll~d and ~h~re æ~ no of-sE3t -:;
~'.

, 7~3~7 voltag~s induced into th~ dir~ct curr~3nt ~ignals repr~sen~ g ~ine and CoSil~e of craft magnetic h~ading.. A6 is seen in more detail in Figure 2, the direct curr~nt signals at leads 32 and 33 are, a~ b~fore, provided by th~ coopera~ive action of flux valve 11, Scott ~els transformer 21, ~d currerl~ ~ervo 31, and are r*sp~c-tiv~31y propor~lonal irl ~npli~ude to sin ~ and cc~s ~ . Outpu~
lead 32 is coupled in s~ries ~hrough r~si~or 75, junction 76, re~istor 77 and input l~ad 35 to o~e charm~l of the~ dual modulator 45. At input lead 35 i~ a capacitor 78 coupled to ground and 10 ~orming a low pass filt~r wi~h resi~or 77. ~ikewise, the ~econd o~tput lkad 33 is couplç~d in s~rie~ through re~istor 79, junctio~
80, resistor 81, an~ input l~ad 36 ~o a s~cond cha.nsl o:E dual modulator 45. At input lead 36 i6 a capacitor 82 forming a low pas~ ~ilt~r with re~istor 81. Switching or chopper transi~tors 83 and 84 are~ r~spectiv~ly coupled to g:round ~r~n juI~c~ions 76 and 80 and control curr~t flow through th~ir ~nitt~r and coll~ctor el~c~rodes in accordanc~ with their re~pec~ bas~
voltagas.
q~he dir~ct curront signal~ on leads 32 and 33 are choppad ~y tran~i~tor~ 83 and 84, resp~ctlvely and, aft~r ~moothing by low pa~;s f~l~ers 77-78 and 81-82,, forrn dir~ct curr~ts tha~
are individually ~odulat~d il~n ~ual channel modulator 45 by the 400 Hz. r~f~r~rac~ signal on l~ad 2a. The~3 dual charmel ou~pu~
voltages arQ dir~ct~d by ~;cott te!~ transformer 60 aæ three-wire synchro data to a navigation syæt~n or oth~r u~ilization d~vic~
64.
For.pllrpos~ of controlling th~3 au~omatic ç~ain control circuit 34, th~ 400 HP.. modulated output curr~nts on leads 54 and 55 ar~ rQsp~cti~ely couple!d by leads 56 and 57 to a con~ta~t 30 amplitud~; variable phas~ circ~lit comprising r~sis~or 95 and capas:itor 96 coupled in ~ri~s with lf~ad~ 56, 57 a~ junction 97~
' .
, - 10 ' 1~7~

Circllit 95-96 is of the general kind discus~,ed in ~he D. A.
Espen U.5. patent 3, 548, 284 entitlad "Synchro Data Transmis~ion Apparatus Having Discrete Gain Changing to Co~np~,nsate for Undesirable Signal Gradient variation~l, issu~d Dec~nbar 15, 19'10 and in ~he D. A. E:~pen U.S. patent 3,617~863, e~ntitl~d "Constant ~mplitude Vari~ble Phase Circuit", issued Novs~mber 2, 1971~ bo~h patents being assignlsd to Sp~rr3~ Rand. q!he constant arnplitud~, variable~ phase signal found ak junctioll 97 is r~ctifl~d by diode 94 and app~ars as a variabl~? unipolar 10 voltag* at one input of a conventional int~grating op~rational ~nplifier 92 having its output coupled ~y ~s~pacltor 91 to iit~ . .
sam~ input. 'rO the second input o:f ampLifier 92 is coupl~3d through r~si~tor 93 a~ able positiva unidirectional reference voltage fro~n a suitable source ~not shown) conn~cted to t~rminal 98. As shown in the drawingO amplifier 92 a~d its associated circu,it act as cor~v~3ntional comparator me~ for in effect -~
comparing l~he output gradiont on l~ads 54, 55 with ~he fixed level ~olta~e at te~minal 98 yi~lding an int~grat~d output as a . ~ function of the dtffer~nce o the two voltags 1~3vel~ at l~ads ~ 97 an~ 98.
l~he positive ~ignal at the output of device ~2 ls coupled through resistor 88 to one input of amplii~r 87, ko th~ oth~
input of which i~ suppli~d at tQ~ninal 90a and through r~sistor 89 the~ 400 Hz. excita~ion signal from gen~rator ~. Under control of i~s varying amplitllde dir~ct current and constant amplitude alt~rnating input current~, circlit ~37 acts as a conventional variable~ pul~e-width g~ rator for supplyin~ a 4GO Hz. variable pulse-width signal a~ junc~ion 74.
The variable p~lse-width signal is coupl~d in parallel ~raD
junc~i~n 74 through the re~pe~c~iv~ r~ tors 85~ 86 ~o the base el~ctrod~ of choppar ~ransistor~ 83 and ~4 to co~krol the r~lation 13~7 1 of the conduction to non-conduction times of ~hese ~witching transistors. The transis~or~ 83, 84 ar~ synchronously conducting at the ~am~ time and then are both non-conducting for a con-troll~d pexiod of time d~pending upon th~ pulse width of the ~utput of a~plifi~r 870 As th~ non-conduc~ing part of th~ c~cl~
is increa~ed in time dura~ion, ~he to~al curx~nt per cycle pas~ing r~m lead 32 to l~ad 35, for exampl~ is incr~a~ed. I~
other words, proportionat~ly 1~5s of th~ curr~nt available on lead 32 1~ d~mped ~o ground. In this ma~ner, ~he ~oltag~ ~etw~n l~ads 54, 55 is mad~ independent of an~ amplituda variatio~ in th~ total flux valv~ data as well a~ amplitud~ variation~
r~sulting fram other di~turbing ~ctors in th~ signal ch2nnQl6 between cur~ent s~rvo 31 and buff~r am~lifi~rs 52 and 53 Accordingly, the ~hre~-wir~ ou~put suppli~d to utilization d~vice 64 of Figuxe 1 by tran~form~r 60 is maintained nominal1 con~tant from leg to l~g, such as at 11. 8 volts .
In th~ co~ple~ systen as illustrat~d in Figura 1, the o~tputs V35 and V36 of ~h~ autamatic gain control 34 on lQads ; 35 and 36 m~y b~ firs~ proc~ss~a ~ the novel index error angl~ :
compan~ator 37 prior ~o 400 Hz. modulation. ~or this purpo6~, the conpensa~ion circuit of Figure 3 is employ~d~ The index error angl~ comp~n~a~ed by circult 37 is present because of the normal di~icult~ of achieving perQct alignment be!ttw~e~ the aircra~t fore-at axis and the affective ~lectrical fore-aft axis of th~ 1ux valve 11. Accordillgly, ind~x angle error compen~ator 37 is provid~3d to permit a matlual correction ~o be made `a~ter system installation by p~rfo~ming, in essenca, ~h~
SamQ funs:tion as might b~ provided b~ a rela~ ly ~xp~nsiva servo differen~ial which som~ prior art systens hav~ employed.
However, since installation accuracies are usually within ~10, th~ compensation unckicn may be ac:curately p~rformed by th~

. .

~1~7~3~

relatively in~xpensive circuit of Figure 3 w~rein only a single pot~ntiometer shaft need be adjusted. I~ will be appar~n~ tha~
th~s correction is made by the novel compensator herein disclosed to the value of angle ~when it is still in the trigonolaetric fonn of sin ~ and cos ~J data.
Accordia~gly, ~h~s apparatus of Figure 3 acc;~pts two inpu~s K2 sin y) and K2 cc~ and interrlally generat~s two values -K2 cos ~l~and -K2 ~ sin ~ he K2 sln ~ valu~ and th~ -K2 ,~
cos(1l) value ar~ added according to th~ wall known trigono~n~tric 10 identity ~o ~orm K2 sin (yJ+~g~ wh~e y~ = ~* ,B Ina~T be us~3d to reprQs~n~ a corr~ctad value o~ h~ K2 cos ~ valui~ and the~ -K~ ,~? sin~)valu~s ar~ similarly add~!sd to fonn K2 C05 () In ac:co~d wi~h ~he t~achings o:E~ the~ pr~s~Ilt invention, th~
terms must be identical in both the SinQ and cosine output channels to ef~ect precise comp~llsation; th~ same source far the ~t~?rm is used in the two channels o~ the circuit. :
In greater d~tail, ~he circuit o~ Figure 3 has, in operation, n~3gativ~ valued dir~ t voltag~3s r~presentlrlg K2 sin,~
and K~2 cos ~as r*sp~ctive inputs on l~ads 35, 36, and thes~ are 20 r~spectiv~ly supplied directly to inputs of convQntic)nal ~mity gain output amplifiers 145 and 155 at th~ right sid~ of thQ figurQ.
q~he same two dir~ct negakiv~ vol~ages are u.ssd in the remaining or major part o~ th~ circuit to produce~ comp~nsa~ing vol~ages also for inæ~3rJcion into atnplifiers 14~, 155. ~?or the la~ter purpo~3, th~ sin yiterm on lead 35 i s coupled through a convQntional irlverting amplifie~r 103 ~o tho swi tching transistor 107.
~rnplifier 103 has i~s output terminal 104 coupled through a resis~or 102 ~o its input terminal 101 and additionally has a s~cond inpu~ termi~al coupled ~o gra~nd through r~istor 105.
30 The -cos ~ t~rm on lead 36 is coupl~d dlrec~ly to switch~ng tran~is~or 109. Tran~;istors 107 and 10~ are mad~ al~ernat~l~

30~

fully conducting and ully non-conducting so that, first, th~ o output of amplifier 107 appears on lead 10}3a and ~h~n, th~
signal passed by switching transistor 109 app~ars on le!~ad 108b.
Since both of th~ lead~; 108a and 108b ar~ coupl~3d to the ~ u~t-able contact 113a o potentiom~ter 113, it is seen that ~h0 signal s alternat~ly passed b~ switching transistors 107 and 109 are alte,rnately applied to contac~ 113a :Eor ~ime-sharing purpos~s in the shar~d amplifi~r 120.
The ~witching transistors 107 and 109 are ma~e alterna~ly 10 conducting under colltrol of a sine wav~ signal app~aring on lead 2b; this signal i s conveniently obtained from the 400 Hz.
genera~or 2 of Figur~ lA, though o~her r~gular ~table-r~[u~ncy slgnals ma~7 alt~rsatively b~ ploy~d. In practice~, th~ 400 Hz.
c~rcle signal on lead 2b is applied ~ lead 106 to control the conduckion of transistor 107. So that time ~haring may b~
employed, th~ ~ignal on lead 2b is coupl~ via lead 111~ th~
180 phase shi:Et~r 112, and lead 110 to control th~ op~ation of switching transistor 109., In this mann~r, 1;h~ signals on l~ads 35 and 36 a~
20 aIternat~ly suppli~d at the sel~ct~?d contact point of p~t~ io- .
mQter 113, ~he latter having it~; opp~3~ed tern~inals 113b and 113c coupl~d to inputs of operational ampl~ier 120. The outpu~
ter~tinal 121 o~ amplifi~r 120 i8 coupl~d to its input at terminal 113b via r~sistor 115, and ter~ninal 113c is conne~c~ed through r~istor 114 to ground in conventi.onal fashion. 1~he inpu~ of ampli~ r 120 is thus time shared and its ou~p-~t o~ terminal 121 is suppli~d to a second pair o~ switching transistors 122, 123, l~h~se ~rans1 stors b~l~3 arr~nged for co~trolling ~he series signal flow through th~ re!l~pec:~ive r~sistors 1269 127 ~o ~0 amplifi,l3rs 128, 12~ he ~ff~ctive gain of amplifier 120 is changed accordin~ ~o the setting o t,h~ single control 37a~

~,:

~7~L3~7 1 which control is manuall~ set in accordance with the knowrl magnitude of th~ ind~x ~rror dl3tennin{~d as a result of cor~-v~ntional ground swirlging operations.
Conductivity of switching transistor 122 occurs simulta~oously with the collductivi~y o swi~ching tran~istor 109.
In lik~ manner, conductivi~ of switching transis1:or 123 is made simul~an~ou~ with thQ p~rio~s of conduc~iYi~y of switching transistor 107~ This operation ls accomplished b~ controlllng ~hR c:or~ductivi~y o:E switchi~g ~raasis~or 123 accordi~g lto ~he 10 slgnal on lead 2b wh~n supplied dire~ctl~ to swi~chillg tran~istor - . 123 v~a lead 125. q~he d~sir~d synchronous op~sration c~f sw:i tching tr~;istor 122 is accomplished b~ providing the 180 phas~ shi~ed : ~:
signal from circuit 112 via le~d 124 to transistor 122. In this : .
mann~r, both chanrl~ls o th~ circuit tima share th~ use of l:h~
c~wno~ acRplifier 120, ensuring t:hat identical corr~ctions are appl.ied to ~he two chaxmels; i.e., that th~ amount o~ ~he sin~3 t~rm added to th~ co~ine t~xm is identical to the amount of the cosine term ~ubtracted in th~ ~in~ channel . It i s :~urth~r 0~8~rVeld tha~ adju~nlsnt of the single control 37a allow~ adj~st-20 m~t o:f~ pot4~ntiamQt~r 113 50 that both channels ar~ identically ~et in accord with l:he magnitude of the inaex error.
~ he time shared currents alt~rna~ly flowing thr~ugh switchiLng transis~ors 122, 123 are al~erna~ely ~pplled ~o the conv~ntional unit~ galn amplifi~rs 1~8, 129, a~d the r~3sp~c~ive out-put~ on the t~3rrrlina~s 132, 133 ;~low through resi~tor~ 141, 150 to the sam~ re~p~c'cive i~put t~rminals o amplifiers 145, 155, as ar~
conn~c~ed to the re~pe3ctiv~ lQad~ 35, 36. q~e outputs o~ pli~iers 145 and 155 may ba ~noo~h~3d ~ ~h~ actlon of appropriate low pa~s filt~rs 90 a~s ~0 r~05rQ an~ 400 Hz. modula~ion from th~ outputs 30 app~aring in ~h~ resp~ctive ou~p~t l~ads 38, 35~. In ~h~ embodimen~
illustrat~3d, ~he ~ilters ar~ placed at the inpu~ o~ amplifi~rs 128 and 129 and compri se re8is~c>rs 1~6, 127 ar~d capaci~ors 12~3a, - 129a, r~pec~ively.
' ' `
.. .. .. . . . .... . . . .

7 IL3~7 ~ he mat~ematical rela~ion expressing the ind~3x error as a function of ~h~ sine and cosin~ o~ magnetic heading i8:

where~ tan~l =~ (5 y~' = th~ compensat~d output, ~1~ = the ~ ncc)rr~cted input" and f~3 = the~ ta~gent o the irld~x ~rror, Thus~ by ad,~usting ~he gain o~ th~ t~ne ~hare~d amplifier 120 in accord~c~3 wi~h th~ valu~ of ~ xpres~ion (5) i~ satis:~ied as 10 ~ol lows ., Th~ GU1:pUt vf arnplif ier 128 i s:

vl32 = -K2 ,~3 COE; ~) t6) when ~ra~i~tor8 109 and 122 are conduc~ing, and ~:he outpult of ampli~ier 129 is:
Vl 33 2 ,B sin ~ ) when tran~ ;tors 107 ar d 123 are conduc:~ing. q~he addition at amplifl~r 145 produc~s an output:

V38 = K2 (si~ ~ - ~ cos)~ ) (8 while ~he addition a~t amplifier 155 produc:es ar~ ou~put, V39 - K2 (c~ in ~) ) 20 or:
V38 ~C2 ~in ~ (10) and:
V39 K2 cos ~
~ha~e direct curr~nt signal~ are~ r~ady :Eor conversion in dual ~hannQl modulator 45, providin5~ a~ th*y do dir~c~ current signals in t~rms of )~ ' containing the de~ired ind~x angle Qrror cc~
pansation as set :Eorth in sgua~io~ (5~. ~I?hus, th~ dual channel - modulator 45 supplie~s on its outpu~ l~ads 500 51 400 Hz. signal~;
:
: ~ who8e ~nplitudes are Y46 K3 sitl ~ (12) ~T47 = K3 c09 ~D ' (13) _16--~' :L~713~

and these signals ~erv~ as inputs to ~he two cycl~? 0rror compensator 48.
q~wo cycle ~rror in a magn0tic field ~en30r, including th~
flux valv~ typ~ of sensor disclosed herein, is induced by the pres~3nce of a soft iron mass or masses in the vicinity of the flux valv~ which t~snds ~o distor~ th~ ear~h's amblent magn~
field th~reat. A~ the narne~ impli~s, the error is a æinusoidal error and has two complet~ cycl~s within 360 o azimu~h ro~ation of th~3 craf~. In gen~ral, ~he av1srag~ location of the soft iron 10 mass relative ~o ~he 11~x valve desl;lsrmina~æ the dir~ction of its ~foctiv~ vector. For cvnv~nience, th~ two cycl~ compensa~ion is acc:ompli~h~d by ~?f~c~iv~1y breaking down ~he to~al v~ctor into orthogonal c~pOn~nts, one term~3d th~ cardinal two cycle error component and the othar th~3 intercardinal error compon~3nt..
1!h~ cardinal two cycle error has extr~rnum valu~s at heading ar~gle value~ 0, 90, 180, and 270. Ir ~erca~dinal two cycle~ .
error, on ~he oth~r hand, has e~ctr~num ~ralu~s at 45, 135, 225 and 315 azimu~h ~valu~æ. In the inve~tion o~ Figure 40 the~
la~ r error i5 readily corr~sct~3d ~y placing an a~justabl~

s~ries resi~tor 199 in the fe~d back path of a.c. ampli~ier 200 that i~ ~xcit~d bs~ 1~3ad 46. Adjustment o control 4ab in accord wi~h data taken duri~g installa~ion compa~s 8wings then corr0cts ~he output at lead 50 in ~ha appropriate~ mann~r.
~he in~rcardinal ~wo cycle headin~ ~srror i ~ compen~at:~d changin$~ the gain balance? b~twe~n t~e sin~ and cosin~3 ch2mn~1 supplying ou~puts on ~h~ respacti~e 10ads 50, 51O
Corr~ction of th~ cardinal heading ~rror is accomplish~d ~-:
b~r the sis~pl~ circuit o~ ure 4, using a ~ingle ad~justant 48a and a co~non circuit stage ln a manner minimizing ~rror sources 30 and c~racterized ~ simpli<:~ity. ~he amount o adjust~ nt of corl~rol 48a ig also de~ermined b~ th~ in~alla~:ion grouxld ~winging proc~ss. ~he K2 ~ signal on 1~3ad 46 is suppli~d ~17- ~

- . . . . . ,:

lL~7~L3~7 via lead 175 through r~sis~ors 177 and 180 to ~he r~sp~ctive inputs of differential op~rational arnpliier 18~ Th~ value K2 cos ~' frcm l~ad 47 is added ~hrough resistor 178 at terminal 183 to ~he K2 sin (~ ~ ~erm; simila~L~y the ter~n ~C2 siny;l ' fxom lead 46 is added through resistor 180 at ~rminal 185 to ~he K2 co~ ~' tersn. Amplifi~ 18S3 has a resistor 187 coupl~d betw~0n its outpu~ 189 and the input ~rminal 183 in co~v~sntional a~3hion.
A variable re~lstor 186 with an adju~table control 48a is coupl~d betweer~ terminal 185 and ground. ~he variable resi~tor 186 constitut~s ~h~ gle card~nal h~ading error ad~ustmerlt, it~ variatlon a~fect:in~ ~he e~fec~ive gairl ~l) o~ amplifi~r lB6.
According to th~ tting o~ con~rol 48a, a compen~atiny voltage ~ppear~ at the output 189 of arnplifier 18~3:
V1~3g = - y(sl~ co~ ) (14) q!h~ single value Vl~g ic coupl~d at junction 190 to branching lead~ for supplying thi~3 signal through resi~or~ 192, 193 to the input~ of resp~cti~e asnplifi~rs ~00 and 201. As not~3d previously, ampl~ier 20û has a variable r~sistor 199 coupled betwe~n its output 203 and lts input lead 195., l~he othe~r input to ampli~i~r 200 1 s connec~3d through rosi s~or 196 to ground. A
~urther ampli~i~r 201 is suppli~d wl~h the ~ignal K2 C08y~ from th~s lead 47 through resis~or 194 and is similarly provid~d wi~h a r~si~or 202 connecting its output 204 to i~ input lead 1970 It similarly employs a r~si~3~0r 198 coupled betw~en a second input and ground. Amplii~rs 200 and 201, through th~ re~pectiv~
conne~c~ors l9S and 197, serv~ a~ adding and inv~3rting circuits so that the K2 ~in ~' term on lead 46 has added to it the aorr~ct~on t~rm app~aring a~ termlnal 190 and ~:he summation is found on output l~ad 50. Irl a similar manner, the K2 cos ~ ' signal suppli~d on l~ad 47 is add~d to ~he compensatin~ signal on junction 190 b~ ampliier 201 and its associatad circuit, an inv~rted ~ignal being g~n~ra~d on output lead 51~ In this:

~7~ 7 mann~r, th~ volta~e V50 is:
V50 = K4 ~in ~ ' + y (sirl ~1) ' * cos ~ ' j] (15) and tha~ on output lead 51 is:
V51 = X4 ~s ~ ' + y~sin 56 ' ~ cos y ~ ~ (16) In e~uations (15) and (1~ hç~ n2w value K4 may includ~ the ~ff~ct of the adjustment of resis~or 199. ~hus, the voltag~ on ou~pul~
lead 50 is sin t)' ' and the vol~age on output lead 51 is cos Jl ' ', where ~' ' r~pr~sen~s ~ ' corr~3c~ed bo~h :~or cardlnal and inter-cardinal headislg error5. Fr~m ~uations ~15) and ~16)~ it is 10 eviden~ that the value o~ the corracted anglep~ ' is e~pressed by th~ f ollowing equa~
- ta"~ ~ sin Y~ + ~(::~?s ~ (17 y - (1 +~) co~ Y sir~
y~' ' b~ing th~ final output heading valu~ componsnts correctQd for cardinal and ints~rcardinal two s::ycle ~rrors. It is se,en that amplifier I88, th* ~ffective ~ain ~ of whic~ is controlled by the sQtti~g of the variable potentiom~ter 186, coopera~e!s in th~
circuit in y~n~rating th~ ~unction ~(sin ~' + cos ~ , and this fu~ction i s added in the si~ and cOsine channel s by kh~ raspective action of ~mpliiers 200 and 201 and 1thsir associated circuits.
20 It is observed that c~rrec~ion of the cardinal hoading two cycle error is accompli~hed by manual op~ration o a single adjustmant. . .
Furthermore, the single stage aseociated with amplifi~r 188 mi~imizes potential ~rror ~c~urc~s and aids in simpllf~Ting the adjustment procedure.
It will b~ und~r~tood that the invention ma~ be~ ~nployed ~ .
in alterllative forms and tha~ the canpass system of Figures lA
and lB ~ay be modifi~d within the scop~ of the claims a?pended ~ `~
h~re~ , or example, as illustra~ed ill Figure 5~, In ~he embodi- :
- ment of Figures lA and lB, the index compensation and two cycle 30 error comp~n~a~ien signals are gene~rated fro~n ~h~ si~e and cosine ou~puts of th~ current sorvO 31 and are re-applied downstream in th~ two chann~ls to be sur~ed wi~h ~heir origina:L or uncompen~ated :;
19 , `' .

30~
1 values~ In the modifica~ion illustrated in Figure 5, the sine and cosine outputs of the current servo 31 are used in essentially the same manner to generate the compen~ating signal~
as direct current signals; however, the summing of these signals with the original data is accomplish~d directly at the flux valve 11 by feeding back the compensating signals as direct currents into the flux valve legs the~selves so as to, in effect, compensate the outpu~ of the flux valve itself broadly in accord-ance with the concept of the above referenced Depp patent 2~85?,859.
Referrlng now to Figure 5, similar reference numerals are used to designate elements corresponding to those found in Figures lA through 4; elements not found in the latter figures are identified by reference numerals i~ the three hundreds. It will be seen that the embodiment of Figure 5, like tha~ of Figures lA and 1~, employs in serial array a reference signal generator 2, a flux valve 11, blocking capacitors 16, 17, and 18, an input Scott tee transformer 2~, a current servo 31, an auto-matic ~ain control 34, a dual channel modulator 45, power ~ amplifiers 52 and 53, an output Scott tee transformer 60, and a :
utilization device 64, In a manner generally similar to that employed in Figure 1~ with respect to current servo leads 10 and lOa, the respective correction currents are fed to summation poin~s 320t 321 of Figure 5 so that they flow through the respec-tive legs o flux valve 11 to ground, being blocked from flowing into the Scott tee transformer 21 by capacitors 16, 17, and 18.
In the lower portion of Figure 5, the index compe~sation circuit 37 is schematically illustrated~ The ~imilarity with the corresponding structure of Figure 3 will be i~medi~tely apparent and the simplification of the illustration correspond-ingly apparent. Thu~, thc sin ~ and cos ~ direct current signal 20.

:~7~31)~7 1 outputs of the current servo 31 on leads 32, 33, respectively, are alternately applied after one is inver~ed by inverter 340 to the input of a variable gain amplifier 120a through switch means 300 corresponding to transistor switches 107, 109 of Figure 3.
The gain of amplifier 120a is illustrated schematically as being controlled by an adjustment knob 37a correspvnding generally to the gain control of ampli~ier 120 of Figuxe 3 by knob 37a and potentiometer 113 in accordance with ~he value ~ . The ou~put of amplifier 120a is similarly alternately switched to two branch lea~s as in Figure 3 by means of switch 301 corre~ponding generally to transistor switches 12~ 123 o Figure 3. The control of switches 300 and 301 o Figure 5 is the same as that of Figure 3, but is illustrated for convenience sche~atically in Figure 5 by switch control means 302 controlled, for example, by the ~00 Hz. ~ource 2. In Figuxe 3, the outputs o~ switches 122 and 123 are applied to two branching circuits including ampli-fiers 128 and 129 for modifying or summing with the original sln ~ and co~ ~ direct current outputs of the current servo 31 through amplifiers 145 and 155. On the other hand, two output 20 branches of switch 301 of Figure S are applied correspondingly to control direct current flow for supplying compensating currents in the proper ratio to the 120 spaced inductor coils 13, 14, and lS of f lux valve 11 for efective ~umming with the orlginal sources of the sin ~ and cos ~ signa}s of the current servo 31.
In figure 5, these direct current ratios are determined by the selected resistors 303, 304, and 305, in the ratios indicated.
The currents ~rom resistors 304 and 305 are applied to flux : :
valve windin~f 15, while that rom resistor 303 is supplied to ~ ~.
flux valve winding 13. I~ desired, the resistor capacitor circuits 306 and 307 ma~ be used ~o reduce transient efects of switches 300 D 301.

1~ 3~7 1 Thus, as in Figure 3, the apparatus of Figure 5 sexve~
to provide index error compensation throu~h the time sharing of a single amplifier 120a between the sin ~ and cos ~ channels by alternate operation of the switches 300, 301, the gain of the amplifiex 120a being controlled by a singl~ control element 37a in accordance with the magnitude of the error. Such operation in~ures that the amount o sin ~ current ~upplied to the flux valve legs 13, 14, 15 and contributing to khe cos ~ output channel of current servo 31 is identical to the amount o~ cos ~
current subtracted from the ~lux valve legs 13, 14, 15 and con-tributing to the sin ~ output channel of current servo 31.
A modif ication of the cardinal and intercardinal two cycle error compensator of the heading repeater system of Figure 4 is shown in Figure 5. Again, the ~ignifican~ feature of the embod~ .
iment re~ides in the manner in which the compensating signals are ~ummed with the primary signals; ice~, at the flux valve 11 rather than at the output of the current ~ervo 31. In Figure 5~ -:
for the cardinal two-c~cle error compen~ation, the sin ~ and cos ~ direct current outputs of current servo 31 on leads 32 and 33 are summed together in a summing circuit, schematically illustrated at 310, prior to supply to the input of variable gain amplif ier 188a~ The summing circuit 310 of Figure 5 corresponds to the summing network 177-180 of Figure 4, while the gain of the ampllfier 188a is illu~trated schematically a~ being varied by the adju~tment of knob 48a in accordance with the magnitude ~
corresponding to the tangent of the desired correction. As in Fi~ure 4, the output of amplifier 188a is coupled through branch- -ing l~ads and the respective cooperating re~istor~ 311 and 312 to summation circuit~ 32Q and 321. Instead of being added back into the uncompen~ated s~n ~ and cos ~ channels at amplifiers 200 and 201 in Figure 4, the compensation currents are employed in tha ~:

.

~713~7 1 strengths indicated in the drawing o Figure S so as to be fed directly into the winding legs 13 and 15 of flux valve 11. The currents are effectively summed with the flux valve winding outputs which contribute to the sin y and cos ~ signal outputs of the current servo 31.
The intercardinal two cycle error compensating slgnal is similarly applied to the ~lux valve windings. The direct current sin ~ signal output of ~he cur.rent servo 31 is applied to vari-able gain amplifier l99a, the gain of which is varied by knob 48b in accordance with the magnitude of the required corrections, amplifier l99a of Figure 5 corresp~nding to the variable imped-ance 199 and amplifier 200 of Figure 4. The output of amplifler l99a is modified by resistors 313 and 314 in accord with the ratios indicated in Figure 5 for application to the respective summation ele~ents ~2Q and 321 and thus to the windings 13 and lS of flux valve 11, so that the intercardinal two cycle cor-rection signal is effectively summed with the flux valve wind- :
ing outputs contributin~ to the ou~puts of t~e curxent servo 3~1.
Thus, in the modlfica~ion o Figure 5, the index and two . ~ :
cycle error compensation signals are generated fr~m the sin and co~ ~ direct current outputs o~ tha current ser~o 31 and are then ed back into the appropriate flux valve inductor windin~s in the re~uired ratios so that the flux valve output supplied to the current servo 31 is compensated. It will be noted in the ~igure 5 embodiment that the feedback compensation signals are generated from the current servo outputs prior to the latitude compensation automatic gain stage 34. This is desirable - because the compensating direct current signals supplied to the valve windings are essentially associated with the direction ; 30 o ~he magnetic f~eld sensed by the inductors and in this sense ~ are not related to the lat~tude gain compensation.

':

23.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. In a magnetic compass data transmission system for navigable craft, the combination comprising: magnetic field detector means including a plurality of inductive windings responsive to the direction and magnitude of the earth's magnetic field with respect thereto for providing a first plurality of alternating signals representative of said earth's magnetic field direction and magnitude, current servo means responsive to said first plurality of alternating signals for generating first and second unidirectional signals representative of said earth's magnetic field direction and magnitude, gain control means responsive to said first and second unidirectional signals for generating third and fourth unidirectional signals representative of said earth's magnetic field direction and substantially independent of the magnitude thereof, said gain control having respective first and second conductivity states, switching means for causing said first and second separate variable conductivity circuits simultaneously to change from one to the other of said first and second conductivity states at a controlled duty cycle, filter means residing respectively in said first and second variable conductivity circuits for smoothing the respective currents flowing through said switching means for forming said third and fourth unidirectional signals, circuit means for separately modulating said third and fourth unidirectional signals with a reference alternating signal for forming first and second output alternating signals representative of said earth's magnetic field direction, control means responsive to said first and second output alternating signals for controls of said gain control means, said control means including: comparator means for comparing a signals representative of said first and second output alternating signals to a reference unidirectional signal, and variable pulse-width generator means responsive to said comparator means for supplying controlled pulse-width modulated signals for controlling said controlled duty cycle, and utilization means additionally responsive to said first and second output alternating signals.

2. Apparatus as described in Claim 1 wherein said variable pulse width generator means is driven by said reference alternating signal.

3. Apparatus as described in Claim 2 wherein said detector means is excited by said reference alternating signal.