subroutine staton        june, 1976              l.weiner
this subroutine will emulate the stationkeeping logic
used in les8 and les9. this program simulates an 8th order
filter with input of 4 solar transit times from the calling
program lesthp and returns the thrust level and duration.
thrust is quantized to begin and end on tabular points
of integration output.  thrust is adjusted accordingly.
nb:  program may only be used with forward integration due
to the memory requirements of the filter.

--calling parameters--
i8or9 = 8 for les8
9 for les9
jd    = the current date
fract = the current time in pep
hmx   = the interval for constant step size integration

--input parameters--
jdtrns(i),frtrns(i) = julian day number and ephemeris time
fraction of day of sun transit
itrns(i) = +1  90 degree sun transit
= -1 270 degree sun transit
thlev =  thrust level     (micropounds)

--output parameters--
jdfire(i,j),frfire(i,j) = julian day number and fraction of
day for station-keeping thrusting
j = 1 start, j = 2 end
i =1,2,3,4 successive half orbit firing time
itrns(5) = number of transit points accumulated (0,1,2,3, or 4)
when =4 reset to 0
sthrst  = thrust level and direction
+ for positive roll face thrusters   (micropounds)
- for negative roll face thrusters   (micropounds)
itrns(6) = 0   no thrusting
= 4   station keeping thrusting, prev thr finished
= 5  station keeping thrusting, prev thr unfinished

--other variables--
irout = 0 first time staton is called, 1 oth.
timep = time since sun perigee
thrust = thrust duration (given by the statonkp algorithm)
thlev = actual thrust level used in the satellite for staton
thrusx = adj thrust duration so that thrust begins and ends on
tabular points
vthrst = adjust thrust level so thrusx*vthrst=thlev*thrust

change thrust duration and level to match integral number
of tabular points.  thrust must span at least 2 tabular intervals
in order for sbout and lesthp to know whether we are in the
start-up integration procedure.