1 - NGA

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perlcynthlon-See perflurre.
perlfocus-See perlcentor.
perigee-to-porigae porlod-Se e
mnomdlstlc period.
perlgaa-lhrr pint et whiih a sslelh orbii is
the last distance frum the centar of !ha
gravitational fmtd of the earth. Opposite of
spogee.
MIL-HDBK-B50
Perlhalion-l%e fminf in the atiiii orbii of a
planet wtkh ie the rtaerasi to the Sun, when the
Sun is the cantar of attraction. Opposlta of
●phailon.
porllune-The point of cbsesl approach of an
orbiing tmdy to the Mwn. Opposite of aplune;
apocynthlon. Also cd(ad perbynthlon.
period of saialflte-See orbital parlod.
I perlodlc ●rrors-in a cernplsta set of
Obearvaticms thera corresponds to every
.:, . A
igdiwidual error mother error which is nwesasriky
more or’i-” atpal and opposite. In a limited
series the carrcallsriert may not be quite axed,
but the error of the mean of n observations
-Y be 6xp6cted IO ba I/n of that of a single
measure, or less.
perlodlc parturbationa-Perfurbe! ions to the
orbii of a plane! or satellite which change
direcric.n in regular or perkrdii fashw in lime,
such that tha average effect over a long period
d time is zero.
pfwlodlc tarme-in tha mathematical
SZfJfaSSbn of an orbti, tenna which vary with
time In both magnitude and direction.
pertod-1. The inlarvel needed to cornolate e
~cia. 2. The interval batwean pesesgas “at a
tbrad point d a given phase of e eirr@
hsrmonk weve; Ihe reciprocal of frequency. Sae
also ●nomaliatlc parlod; nodlcal period;
orbltel period; alderoai pariod; aynodlc
period.
pormanwrt bsnch mark (PBM)-A bench
mark d es nearly permanent character as ft is
pmctiila to establish. Ueuslly designated
simpfy as e bench mark or BM. A permanent
bench mark is intended to msintein its elevet”on
wkh raference to an adopted datum wifhoul
185
changa ovar a long period of time. Also catted
monumwtted bench mmrk.
perpondicuiar ●quatlott-(traveme) A
rmndiin equation to reduca to zem the
algebraic sum of the pmjaotibrrs 0( the separate
Ii cd a traverse ~ pqmdcarlera to a
faed line with vdich the travaree fcnrre a cbsad
figure.
parparrdlculer-A perpandiiuler line, pfene,
etc. A distinction is sometimes made between
pe~ndabr and normal, the fermar applying
to a fine at right angiea to a etrdght he or
pfana, and the kdtar referring to a ~me a! right
angies 10 a curve or cutved surface.
personal equation-lhe time interval
between the serrcory @rception of a
phenomenon and the mo!er reaction thereto. A
personal equation may be either pceittie or
nagetiie, es an observer may anticipate tha
occurrence of an event, er wait until ha actually
eeee it occur before reeking a record. Ths is a
systermtic error, trasted ae the constant type.
personal ●rror—An error caused by an
imtiiiduet’s personal habits, hk inability 10
perceive or measure .iimenaionsl values exactly,
or by h!s tendency to reecr mentally and
physically m a uniform manner under aimitsr
conditions. i! may be a eysterndc error, il it
occurs regutsriy or a blunder tf il occurs once. A
cerlein amount of mimr paraoml errer can be
included with random armrs.
paraonal parallax—See Inetrumont
paraiiax, definition 2.
perepecthre ●xis—See axis of homoiogy.
porspectlve contar—The point of origin or
termination of bundles of perspective rays. The
two suh poirris usustty associated with e survey
photograph are the intarior parapactive cantar
and ihe exterior perspective center. In a perfect
Iena-camsre system, perspective rays fmrn the
interior perspective cent er to the phclographz
images ancloss the same engfes as do the
mrraspcnding rays from tha exterior perspective
center to the objects photegrephed. In a Ians
having distortion, this is true only for a particular
zone of the photograph. In a perfectly adjusted
Ians-cwmrra system, the axterbr and interior
perspective centers correspond, respectively, to
the fronf smd rear nodel points of the camera
Ians. Also celled canter of projection.