; CRIB SHEET TO REPRODUCE GEOTAIL REMOTE SENSING RESULTS
; This subroutine reads into idl an ascii file containing the following info.
; channelheadsector (chs) hour, min, sec of arrival of particles of almost
; 90 degree pitch angle at the detector, gyroradius, gyrophase delta i.e., the
; angle between the direction of the parcticle and the y' axis that is on the
; plane perpendicular to the instantaneous magnetic field direction.
; Note that g = 270 - delta, where g=gyrophase, if B is out of the page,
; for ions.
;
pro get_d_vs_dt,epsilon,hh,mm,ss,rho,delta,dist,times
times=hh*3600+mm*60+ss-(11.*3600.+19.*60.)
dist=rho
for i=0,n_elements(rho)-1 do begin
  dist(i)=rho(i)*sin((delta(i)-epsilon)*!PI/180.)
endfor
return
end

pro run
;
intro_ascii,'remote_sense_A.txt',delta,rho,hh,mm,ss,nskip=13,format="(25x,f6.1,f8.1,3(1x,i2))"
;
angle=fltarr(73)
chisqrd=fltarr(73)
for ijk=0,72 do begin
  epsilon=float(ijk*5)
  get_d_vs_dt,epsilon,hh,mm,ss,rho,delta,dist,times
  yfit=dist & yfit(*)=0.
  chi2=dist & chi2(*)=0.
  coeffs=svdfit(times,dist,2,yfit=yfit,chisq=chi2)
  angle(ijk)=epsilon
  chisqrd(ijk)=chi2
endfor
ipos=indgen(30)+43
chisqrd_min=min(chisqrd(ipos),imin)
plot,angle,chisqrd
print,angle(ipos(imin)),chisqrd(ipos(imin))
;
stop
;
; once you determine epsilon0 for minimum chi2 then:
epsilon0=angle(ipos(imin))
get_d_vs_dt,epsilon0,hh,mm,ss,rho,delta,dist,times
yfit=dist & yfit(*)=0.
chi2=dist & chi2(*)=0.
coeffs=svdfit(times,dist,2,yfit=yfit,chisq=chi2)
print,'angle=',epsilon0
print,'d0, V=',coeffs
print,'tcross=',-coeffs(0)/coeffs(1)
print,'chisqrd=',chi2
plot,times,dist,psym=1
oplot,times,yfit
;
stop
;
end