VERLORT

It has been written by Ken Anderson, at one time the supervisor of the AN-FPQ6 radar at the NASA Tracking Station at Carnarvon, Western Australia. By the time the author arrived at the Carnarvon Station the Verlort radar had been removed. Consequently he never saw that radar and the following article is entirely based on the information from the excerpt of Dr Stodola’s book. The author had extensive experience of radar systems during service in the Royal Australian Navy and had spent four years as supervisor of AN-FPS16 radars at Woomera, South Australia.

Synopsis:

The Very long Range Radar [VERLORT] was a modification to the wartime SCR584, innovative radar used to automatically direct anti-aircraft guns. In the anti-aircraft role the radar had a maximum tracking range of 33 kms[ approximately 18 nautical miles], a capability far short of that needed to track satellites at ranges 100 times greater. The basic system was modified, by a team led by Dr E.K. Stodola, to have a maximum unambiguous range of 4000 kms whilst still utilising a pulse repetition frequency [PRF]} of some hundreds of pulses per second [PPS]. This meant that the system had to be able to carry out nth time around tracking. That is, although the PRF inter-pulse periods corresponded to radar ranges of 377 kms [204 naut., miles], 301kms [163 naut., miles], 290 kms [157 naut., miles], and 264 kms [143 naut., miles], the Mercury spacecraft, at an altitude of 405 kms [220 naut., miles], would never be nearer than that distance from the radar. Hence, the radar echo received during the present inter-pulse period is that from n pulses ago

The modified system utilised different PRFs, that in use being selected by cam-operated switches in the mechanical range gear- box. These cams were arranged so that when the echo being tracked approached the time at which the next transmitter pulse was due, a different PRF was selected such that the inter-pulse period gave an echo at a time different from that of the next transmitter pulse and thus allowed the radar to provide an unambiguous range to the spacecraft. In order to determine which zone, that is the number of pulses between the transmitter pulse and the resultant echo, the system, at regular intervals, deleted a transmitter pulse. On an auxiliary range display the range trace for the deleted pulse was displayed at the top of a screen, the next pulse slightly below that, and so on. The operator was required to move vertical marker to the trace without a return pulse-that is the zone corresponding to the coarse range of the spacecraft. The fine range display showed an apparent distance, always some fraction of the zone, to the target. That is, the actual range was n zones plus some fraction of a zone. The range system comprised a fine and a coarse section and it was here that the design team provided very elegant solution to the problem brought about by the maximum rate at which it was possible to move mechanical gears, some 55 kms/second [60kyds/sec.,]. The coupling between the two sections was accomplished by use of an electro-magnetic device, a synchro, which enabled the coarse section to be moved, or slewed, at 11 million metres/sec., [12 million yds/sec.,]. When the operator moved the marker referred to above to the empty zone it was this section of the range gear  which actually moved. The range error signal from the radar drove the range gear by means of a servo- motor attached to the fine section of the range gearing.

In addition to the changes to the range system modification detailed above the 1,83 metre[6 ft.,]  diameter dish of the SCR584 was replaced by a 3 metre [10 f.,] dish. This gave the VERLORT a beamwidth of 2.5 degrees and, thus, higher gain, and more precise angle tracking capability.

Detailed Article

The Very Long Range Radar or VERLORT was a modification of the highly innovative SCR584  tracking radar, strictly, able to automatically track in angles but requiring the operator to make adjustment to the range rate, developed by the MIT Radiation Laboratory in 1943 and used in conjunction with a MOD-9 Predictor to control anti-aircraft guns. That radar could operate in either search mode, with maximum range of 74 kms, or track mode when its maximum range was 33 kms. In order to track artificial satellites it would obviously require substantial modification, both a vastly increased range capability and better angle tracking. The most visible modification made was to replace the 1.83 metre dish with one of 3 metre diameter, This resulted in a narrower beamwidth of 2.5 degrees as opposed to the 4 degrees of the original, and, concomitantly, higher gain. But, the most significant modification was the provision of a range system with a maximum range of 4000 kms and the capability to automatically track a target. Please note that the SCR584 specifications were in yards. Throughout this note distances are expressed in metres.

 A  pulse radar measures the target’s position many times per second, at a frequency called the Pulse Repetition Frequency [PRF]. At each PRF pulse the radar’s transmitter is caused to operate so as to generate an extremely short radio frequency pulse, in the case of the Verlort, at 2.8 Gigahertz [GHz] in what was then called the S-Band.  In selecting the PRF there is a conflict between the requirement for the maximum unambiguous range, which requires a low PRF, and the need for a high PRF in order to minimise the time between measurements of the target position. This conflict arises because the maximum unambiguous range of a radar is the distance a radio wave can travel to the target and return during the PRF inter-pulse period. So, to meet the 4000 kms unambiguous range required by the Verlort specifications a very low PRF of about 38 PPS would obtain. The designers of the Verlort, led by Dr E.K. Stodola of Reeves Instrument Corporation, invented a new type of ranging system which combined the great maximum range and a high PRF.    In the Verlort the following PRFs were available:- 409.5 PPS, 512,3 PPS, 530.5 PPS, and 585.5 PPS.

 The time between successive pulses, the inter-pulse period, is the time in which the radar can receive an echo pulse. The time between the transmission and reception of the pulse can be determined to a high degree of precision, and, because the speed of radio waves is known, a range to the target may be determined.  The inter-pulse period for the four PRFs above represent, respectively a radar range of 377 kms, 301kms, 290 kms, and 264 kms.

The Mercury Spacecraft, which was fitted with an S-band beacon, a device which, upon receipt of a particular radar signal, responded by transmitting a relatively high powered pulse, orbited the earth at a height of 405 kms. At this altitude it would rise above the horizon, of a station at sea level, at a range of 2308 kms. However, although, at that range, the signal sent to the spacecraft by the radar was more than adequate to interrogate the S-band beacon the signal from the beacon received at the radar was so low that it was undetectable by the radar. The radar received signal would not be high enough to track until the spacecraft was 1500 kms from the radar. Note however that a range of 1500 kms is 3.98 times range of 377 kms set by our lowest PRF. The radar would acquire a signal from the spacecraft at the 1500 km range but would report its range as 369 kms, i.e. 377 x 0.98. [This arises because the transit time to the target and return is such that the echo we are seeing at this instant actually results from the pulse three ago.] As the target got closer the range reading would reduce until, at an actual range of 1123 kms [377 x 3] but a reported range close to zero, the received echo would coincide in time with the next transmitter pulse-at which point the track would be lost, for, at the instant, the transmitter pulse occurs, the receiver is disconnected from the antenna and the transmitter connected to it. It is near this point the ingenious solution provided by the designer of the Verlort ranging system comes into play. The range system they designed was divided into a fine range and coarse range sections. These consist of two separate mechanical gear- boxes coupled by synchros, electrical devices which utilise a rotating electro-magnetic field to as their coupling device. The electro-magnetic coupling meant that although the fine range section could only be slewed, that is rapidly changed in range at some 55 km/second, the coarse section could be slewed at 11 million metres/sec. This aspect will be discussed further below.   In the coarse section of the range gear there were cam operated switches which selected different PRFs as the range changed. So, if, when our target reached a range of 1141 kms, when it is being reported at a range of about 18 kms, the appropriate range gear switch selected a PRF of 585.5 PPS immediately after the previous 409.5 PPS pulse the target return, after four 409.5 PPS pulses, would be shown at a reported range of 95 kms –that is because it is at an actual range equal to 4.35 times the 264 kms which is the unambiguous range of the radar at the new PRF. In the Verlort the range system utilised a second pulse train running at about 38 PPS. This deleted a normal radar transmitter pulse at each 38 pulse point. Deleting a transmitting pulse meant that there would be no target response for, in our example, 4 PRF inter-pulse periods, thus defining the target range “zone.”  In order to make this information available to the operator an extra A-scope display was added to the SCR584 console. [An A-scope displays the radar range as a line travelling, usually, left to right across the screen and the radar responses as, again usually, upward deflection of that line.] On this added display the trace which represented the transmit interval for the deleted transit pulse was at the top of the screen. The next trace was slightly lower, the next lower again and so on. This allowed the operator to readily decide which range zone the target was in and to slew the range gear to that range zone. It was at this point that the ability to slew the coarse section of the range gear at extremely high rate came into its own, The operator used a hand-wheel to move a vertical marker to the appropriate scan on the added A-scope. The synchro coupling allowed this to be done and also automatically allowed the much slower fine range system to align itself. The range error voltage from the modified range tracker drove the range gearbox through a servo-motor attached to the fine system.

BREAK POINT 1.

 The transmitter was modified from the original SCR-584 design as the S-band transponder or beacon fitted to the Mercury and later spacecraft only responded to two specifically spaced interrogation pulses -the actual delay between the two pulses was of the order of  3 or 4 microseconds. There was also  finite delay in response of the beacon to these two pulses which had to be calibrated into the ground radar system.

BREAK POINT 2

The angle tracking system of the Verlort would have retained the basic SCR-584 conical scanning system in which a slightly offset dipole antenna at the focal point of the dish was rotated at high speed [1200 RPM]. This resulted in the antenna beam describing a cone in space. In the radar synchronous detectors generated voltages proportional to the signal strength received when the beam was at its extreme right hand and left hand positions. From these were developed the azimuth error voltage. Similarly, when the beam was at its highest and lowest positions an elevation error signal was developed. These voltages caused the antenna servo system to drive the antenna so as to bring the azimuth and elevation differences to zero, and when that condition was reached the target was centred in the conically scanned beam, and thus, on the bore sight line of the antenna. At this point the target position in space is defined as at such and such azimuth, elevation, and range with respect to the known position of the radar. It should be noted that Stodola says that the “radar for satellite tracking required extensive variation from the SCR584, including greatly improved range and angle tracking.”  No description of changes to the original angle tracking system can be found.

Acknowledgement. The author has to thank Fred Carl at InfoAge [www.infoage.org/stodola-1988.html] who very kindly provided scanned copies of the article by Dr Stodola

“ Some examples of post World War II radar in the USA',
by E. King Stodola;
in 'Radar Developments to 1945', 
Edited by Russell Burns, 
Published by Peter Peregrinus Ltd., London, United Kingdom, 
on behalf of the Institution of Electrical Engineers.” 

Discussions, notes etc

The Verlort radar (see figs. 8-3(a) and 8-3(b)) fulfilled the S-band requirement with only a few modifications. Significant ones were the addition of specific angle-track capability and additional angular scan modes. This from

MERCURY PROJECT SUMMARY (NASA SP-45)

8. WORLDWIDE NETWORK SUPPORT

at history.nasa.gov/SP-45/ch8.htm

DISCUSSION:

The Verlort was developed from the SCR-584 by a team led by E. K. Stodola at the Reeve Instrument Corporation. Stodola says that that “the radar for satellite tracking of artificial satellites required extensive variations from the SCR-584, including greatly improved range and angle tracking systems and numerous antenna refinements.”…and goes on “..a new type of ranging system having good accuracy  and operational at high repetition rates without range ambiguities and interference between transmitted and received pulses was essential.”

 Stodola includes a chart showing that line-of –sight range of 22500 km to 51000 km was readily within the state of the art. This includes ground receiver thresholds and shows that the best state of the art receiver had a threshold of –107 dbm. That means that the Noise Figure of the receiver would be 5 db, as opposed to the original SCR-584 figure of 15 db, with a threshold of –72 dbm. Calculations show that, at the point at which the Mercury S/C rose above the stations horizon [2308 kms], that the radar transmitted signal [Transmitter power 250 kW ] could provide a signal level 39 db above the beacon threshold the signal from the beacon received at the radar would not reach a trackable signal to noise ratio of 6db until the spacecraft was at a range of 1500 kms [800 nautical miles.] In February 1962 issue of “National Geographic”p.184 there is an article entitled “Tracking America’s Man in Orbit.” In this article there is an illustration “Mercury Tracking Station and its functions.” And this, inter alia, describes the “Long-range eye, the Verlort radar, can see 700 miles…. This is reasonably correlated with the previously calculated performance of the Verlort. Beacon interrogation and response for 2308 km [1246 Nmi= 31 db_nmi

Calculation of signal received at spacecraft

Radar Tx power=84 dbm + Ant Gain 38 db=122 dbm

Path loss to satellite at horizon break=  -168.7db

Therefore received signal = -46.7 dbm

              Beacon Threshold= -88dbm, therefore   S/N= 41 db

Beacon signal at radar with 0db gain beacon antenna

P_outBeacon = 2kW = 62dbm   Path Loss = -168.7db    = -106.7dbm

Assume Nf= 5db  Threshold= -107dbm so S/N = 0.3db

If we also assume that Beacon Ant gain=6db

Then Srx=103 dbm and S/N= 4 db

Notes on the evolution of ranging systems from the mechanical VERLORT to the fully digital AN FPQ6

Stodola’s solution of the range problem was extremely ingenious; it would be wonderful to find even a photograph of it. The historical progression from it to the IRACQ [Increased Range ACQuisition] modification of the AN-FPS16 to the fully electronic ranging sub-system of the AN=FPQ6 is interesting.

IRACQ

 The AN_FPS 16, which had a mechanical range system [with digital encoders as the output devices] not that different from that in the Verlort, had a modification called IRACQ [Increased Range ACQuisition] to overcome the nth time around problem, where the requirement for high repetition rates and the conflicting need for long range.  This modification comprised an auxiliary range tracking sub-system which utilised a voltage controlled crystal oscillator [VC XO] as the basic timing element. A split gate was generated which allowed the derivation of a range error voltage which was the VCO control voltage. As, for example, the range decreased the oscillator frequency was increased, thus slightly shortening the period to the next gate and, in this way, tracking the target. The gate replace that of the AN-FPS16’s own precision range system in the angle tracking sub-system of the radar and thus, allowed tracking in angles whilst the operator manually slewed the [mechanical] range gear, and to do so took some 18 seconds or so, for a closing target, from minimum range to maximum as the spacecraft response transitted the so called Big bang, the transmitter pulse for the next pulse period. IRACQ also generated Tx trigger pulses which were delayed from the normal time so as to avoid the generation of a Tx pulse co-incident with the target pulse. IRACQ did not provide any read out of range. [See: Skolnik “Radar Handbook” 1^st edition, fig 44 page 21-40 for a block diagram of such a system.]

The AN-FPQ6 Radar

Used a fully electronic digital ranging system with an unambiguous range of 32700 nautical miles.  To achieve that figure required that the system perform a number of functions. Firstly, the range system had to carry out a “find” process in which the target is located in range. Note that the unambiguous range of the radar is divided into, in the case of a PRF of 160 PPS, 64 zones of 512 nautical miles. To quote the RCA “Handbook of Operation and Maintenance instructions For the Radar sets AN/FPQ-6 and AN/TPQ-18” “When carrying out the automatic find process the system delays two successive Tx pulse by 16,000 yds. Concurrently it generates two range gates, with the second displaced 16,000 yds from the first. The number of transmissions relative to the delayed ones are counted by an Auxiliary Zone Counter until two video pulses are detected in the delayed range gates. At this point the zone counter contains the zone number of the target.” That value is transferred to the zone-or if you like, higher order digits of  the main range counter. The range between the latest Tx pulse and the target is represented by the lower order bits of that counter, in simple terms the range counter contains the sum of the integral number of zones and that part of a zone representing the apparent range to the target from the latest Tx pulse.

Secondly, the system carries out a “verify” process in order to confirm that the correct zone was determined in the “find” process. To carry out the “verify” one Tx pulse and the Range Gate in the assumed zone are delayed by 16,000 yds. The delayed range gate is then checked for the presence of a target return. This process is repeated until four video pulses are detected. Verification is completed at that point.

Thirdly, automatic tracking is maintained.

A trap for young players working on AN FPQ6 range system arose from the fact that the development of the this system was carried out by two teams who worked on, for want of better terms, the first half and the second half of the system. One team numbered the 2 4 binary digits 0-23, the other numbered them 1-24. So, when troubleshooting from the circuit diagrams you had to be aware of which side of the divide you were coming from-you suddenly had to be looking at the wiring for bit x rather than x+1 or x-1. Very confusing at 2 a.m.