They knew the invasion was coming, but not where.
Things are developed as they are developed. The long ramps were developed first, the aerial launches and portable ramps were developed as the need come about.
If you look at the V1 and V2 bombardment of Antwerp, which is an example of a closer target. I think it shows neither system was accurate enough to hit the invasion beaches.
Though i'm sure someone will post that they were just a few weeks from developing a missile to do just that with 100% accuracy.
Speak of the devil, and here I am.
The V1 guidence system that was about to become opperational was called Ewald II - Sauekirsche. Ewald-II was the pulse transmitter on the missile. Saurkirsche (Sauer Cherry) the receivers in the ground stations. The ground stations were under construction with one completed. I can find the name of the location of the Station in Holland when I get home to my books.
Ewald-II worked by the V1 sending out a single coded pulse on a pre determined frequency at an predetermined time. Three ground stations received this pulse, and transmitted the resultant time of arrivals to one of the three stations. The Three time differences formed the parameters of a simultaneous equation that was used to solve for the exact position of the V1. This technique is called trilateration. Modern GPS works much the same way, though in reverse.
A course correction was transmitted as a single burst to the receiver "Mosel" which was an endless loop magnetic tape (a German invention) which had 5 heads, this decoded the instructions to provide both control and further jam resistance. Ewald-II was a midcouse correction system designed to be resistant to jamming, however if the V1 was used at short ranges and the course correction applied only a short distance, say 10km, before the impact point it would be capable of considerable accuracy. Drift of the V1 using its dead reckoning odometer system seems to have been about 2% so a course correct 10km before target should produce an error of about 200m on top of the 100m or so of radar instruments.
Hence an accuracy of about 300m or so seems achievable iin this situation (short range attack against beachead)
In addition there was the "Radischien" passive radar homming receiver which could home on to Chain Home, Chain Home Low, SCR-270 and Loran stations as well as MAX-P which could home on to allied centrimetric radars. (Originally designed for SAM attacks on to H2S) Radischien was tested against dummy radar targets and got to within 5 m via BV-246 glide bimb
An earlier V1 system "Ewald - Minerva" only used two ground stations, which tracked the Ewald transmitter as it begand broadcasting at various points of the flight via triangulation/durection finding. Initially about 5% of missiles had this system. The idea was to correct for the wind drift of subsequent missiles. Towards the end of the war nearly 50% were equiped with this kit suggesting the Germans were increasingly suspicious of the information they were getting from their agents.
The A to E versions of the V1 had a cruise speed of only 580km/h (361 mph). F version, launched first in march 1945 had a cruise speed of 630km/h (390 mph). These speeds were already at the limits of what a piston engined aircraft running on 100/150 could achive at low altitude.
in the same month, an experimental v1 reached 815km/h using a modified engine with improved fuel injection. such fast missiles would be very hard to intercept, even by jets.
The V1 was about to become a new and potently effective weapon.