Ziplip Shiplip


I suspect that in the armed forces, as everywhere else, the old adage ‘knowledge is power’ is fast being replaced by the new mantra ‘information is power’. We thus find around us, people who do things without telling us why they do it. Add to this, a whole host of devices that cipher, decipher, scramble, jam, home-on-jam, code, decode etc. to ensure that information of use is not compromised. Clearly, the balance of power today shifts to the one who can intelligently and extensively exploit the electromagnetic spectrum while denying the same to the adversary.

Given this fact, we in the navy have devised our own home-grown variety of communication procedures. Specifically, this article aims to provoke a discussion on pros and cons of the ‘ZIPLIP’ procedure (radio silence) followed by the naval air arm on V/UHF circuits. In most cases, the procedure has been extended to cover routine flying operations and even use of Motorola (here, I mean the less endowed cousin of MotoG) during seamanship evolutions. Like the Sicilian mafia and their ‘Omerta’ (code of silence), helicopter operations during tactical exercises are executed under a heavy shroud of voice security.

As a staunch votary of the rotary, I will restrict my discussion to ‘ZIPLIP’ on V/UHF circuits for two reasons. Firstly, these are the circuits most commonly used by helicopters during all phases of the mission; and secondly, because ‘ZIPLIP’ is most rampant here. But first, let’s hit the refresh key.

A Few Basics

Electromagnetic Wave Propagation.   E/M wave propagation in the metric/decimetric or V/UHF band is essentially restricted to line of sight ranges as they are limited in their propagation by the curvature of the earth. The exception to this fact of science occurs during unusual environmental conditions such as temperature / humidity inversion, presence of the Sporadic E layer or Sudden Ionospheric Disturbances (SID) etc. when freak ranges far outside expectations may be obtained. Thus under normal circumstances, V/UHF transmission and reception would take place along a straight line from the transmitter to the receiver, but a little more than the optical horizon, caused by the varying density of the atmosphere and diffraction around the curvature of the earth. Simply put, more the number of times that the E/M wave ‘knocks’ on the earth’s surface, more it gets attenuated. Also, as per the inverse square law of radiation in free space, power density at any given point is inversely proportional to the square of the distance from source. That is why low power of the order of 25-40 watts is used for V/UHF communications.

Space Wave Ranges.   Over the sea surface, this approximates to line of sight ranges as per the radio horizon empirical formula:-

R = 1.25*(√ Ht + √Hr)

where R = range in nautical miles

Ht = Height of transmitting antenna (in feet)

Hr = Height of receiving antenna (in feet)


R  = 4*(√ Ht + √Hr), if metric units are used (km and m).

As an example, let us examine the radio horizon obtained in the case of a frigate operating helicopters. Starting from about 16 nm when the helicopter is on deck, the usable space wave range opens out to around 65 nm when the helo is flying at 2000 feet above sea level. But mind you, these are communication ranges between the particular ship and helicopter. Between two airborne platforms, these figures could multiply rapidly owing to the greater heights involved.



History (and Skolnik’s Radar Theory) tells us that intelligence derived from signal / voice traffic analysis and direction finding can provide both tactical and strategic advantages. Patterns of communication (sudden activity, silence etc) can be used to identify the state of readiness and location of forces. A database of communications activity can be used in establishing a potential enemy’s radio discipline and movements.

So What? Extending the line of sight, an aircraft flying at high altitude with modern COMINT equipment or a satellite with a highly directional, very low-power receiver in this band could intercept V/UHF transmissions at very large ranges since all frequencies above 30 MHz can escape into free space. In fact, Voyager 1 and 2 reportedly communicated with the Earth from Saturn on a power of 30 Watts from about 1.5 billion kilometres away! However, in such cases, huge directional antennae and extremely low-power receivers were used. The signal power received by the antennae of NASA’s Deep Space Network was only of the order of 10-17 Watt! Fortunately, these are resources that we do not have to routinely contend with, at least in peacetime.

Undeniably, technology has reached a stage where the possibility of V/UHF transmissions being intercepted at long ranges with adequate degree of accuracy for DF purposes cannot be contested. However, extending the phenomenon of anomalous propagation for DF application beyond the radio horizon by a surface/air unit does not necessarily hold water. That’s because the direction measured could be as much as 180 degrees in error due to the effect of scattering / direction changes.

Ships and Helicopters.   In effect, higher you fly the aircraft; more are the chances of your V/UHF transmissions being intercepted / DFed at long ranges by surface units, limited as it would be, by the transmission power output and sensitivity / directivity of the search receiver. In the nominal height band from sea level to (say) 2000 feet, within which our helicopters carry out most of the ASW/ELINT/search missions, the corresponding ranges would be of the order of about 16-65nm. Now, with the array of capabilities at our command, would we allow the RED ships to lurk within this zone undetected?

My answer is yes – during peacetime we sometimes experience this due to exercise limitations. And about this time, communication security is usually jettisoned to seek immediate engagement. As such, for helicopters engaged in low-level missions at night, take-off / landing or box-pattern flying, the results accrued when compared with the safety and clarity of two-way R/T or data link, offer little benefit, if any. In fact, the benefits of resorting to ‘ZIPLIP’ on voice circuits for communication security in such a scenario may actually be more applicable to aeroplanes that fly at higher altitudes or to HF circuits that have global reach. But tell that to the birds. Especially, from the fraternity which do not have wings that go round in circles!

Ships, Helicopters and Fixed Wing Aircraft.       When aircraft or satellites fitted with modern COMINT equipment are in company, the sky is the limit. This is due to the relative transparency of free space to frequencies above 30 MHz. Such predators from friendly or adversarial forces should not be precluded from our calculations even if they are not snooping 24/7.

Evidently, the need for safeguarding our V/UHF radiations is very real in operations at sea.

Deprive or Enable?   Now we are faced with two options. Either deprive the ‘technologically challenged’ helicopters of their ‘airtime’, or enable ‘ZIPLIP’ by providing alternate aids. A few helicopters in the navy with data link capability can operate more securely since their tactical or navigational data can be exchanged with surface consorts via data links or secure data nets with burst transmission. A secure transponder can also enhance safety in ‘ZIPLIP’. The ACARS (Aircraft Communications Addressing and Reporting System) system on civil airliners uses the aircraft’s VHF communication system (131.55 MHz) to exchange messages with their operators in a compressed data format which can be uplinked or downlinked in about a second. This system has been in force for years and was used to warn civil airliners against cockpit intrusions during the crucial moments of 9/11. However, not all our helicopters have this kind of equipment. Hence, in today’s competitive E/M environment, they have to contend with being incommunicado – no commentary made, he’s saying nothing at all! (Credits – Dire Straits ‘Communique’).


‘ZIPLIP’ in Real Life!

Here are some interesting, real life case studies in ‘ZIPLIP’. Since nobody got hurt, we can afford to see them in a lighter vein.

Case 1.          Ships operating in company during a tactical exercise. On a sudden arousal, an Alouette is launched (read scrambled) for VERTREP / mail transfer. Receiving ship not yet settled on Foxtrot Corpen (flying course). Location for vertrep changed from helo deck to foc’sle via Motorola (no ZIPLIP here!). FOD parade and crew briefing in progress. Winds, as yet, unsuitable. ‘KILO’ at the dip. Before you could announce “All positions, bridge” the helo was on-top, ‘ZIPLIP’ and all! Time, tide and formation wait for no one! The confused signalman slowly inched up ‘KILO’. Both ship and helicopter doggedly held on to the imaginary zip across their lips while hips, arms, caps etc. were waved energetically to waive-off the ‘intruder’! Result – plenty of hazards, confusion galore and frayed tempers all around. Protests raised…and replied with the common rebuttal – “ZIPLIP in force!” My lips are sealed, but trust me!

Case 2.          Ship settled on Foxtrot Corpen. ‘ZIPLIP’ in force. Flying particulars passed on to the crew in cockpit scribbled on a piece of paper ripped off the LSO’s pad. No ‘lip service’ here, we mean business! Helo starting up. Wait…there’s a change in flying particulars! The swirling rotors are casting dancing shadows on the flight deck. . Another piece of paper carrying the amended flying particulars passed on to the cockpit. The chockman ducks under the accelerating rotors with just inches to spare! Pilots straining to read the information on that piece of paper in the dim light. Would the ship, in this case, warn him about an occasional roll upto the limit? Hmm, unlikely, isn’t it? Cheers! We have succeeded in bringing down the radio horizon innovatively from about 16 nm to zero! And how!

Case 3.          Somewhere in the Arabian Sea… (I guess that’s really all you know when you are launched from State 2 or State 1) An ominously crimson sky and daylight fast receding into a moonless night. Helo to be launched for ESM probe. It is that ‘decisive’ phase during ‘Encounterex’ (culminating phase of a tactical exercise) when the first platform to open up an emitter ends up on the wrong end of a paper missile. After a hurried briefing, the crew walks the catwalk (via FLYCO) to the helicopter carrying the familiar slop-chit inscribed with the flying particulars. Of course, ‘ZIPLIP’ is in force. Aircraft opens out 20…30…60 nm from the ship on its probe, with the ship’s radar silenced and the aircraft’s radar on ‘tactical’ policy. No operational HF set, weather radar, transponder or data link facility onboard. It is time to pass the first ‘ops normal’ call. As per briefing, the ‘encounter specialist’ lifts up to Strata 2 and transmits ‘Papad’ (fries or chips for my foreign readers). For those who came in late, this was the predetermined code for ‘Ops normal’. The ship replies feebly with ‘Pickle’, code for ‘roger’. Agreed, the message is conveyed (albeit in mumbles). But no information about the helo’s position is given out by either end in this exchange of spices. A serious game of blind man’s bluff has begun, with ‘ZIPLIP’ to boot. Weather is a serious consideration at night. Will the ziplipped controller in this situation alert the pilot of hazardous weather? Or, can he? Have you mapped your VHF ranges? Like my instructor in test pilots school used to say “Profound question. What’s your next question?” Further, imagine the consequences if the helo was to have a serious emergency during this sortie. ‘YOGA’ plus ‘ZIPLIP’, without the trappings of technology, can be a dangerous cocktail. Depending on whether he can climb at all, by the time the helo is able to clear ‘papad’ and receive ‘pickle’, the crew would in all probability end up in the drink without positional information at the controller’s end. Maybe this is what my friends meant when they said, “ship mein bahut papad belne padte hai!” (figuratively, your butt will get ‘pickled’ onboard ships).


We have seen how routine exercises involving ships and helicopters could have turned nasty with an embargo on two-way R/T and lack of suitable alternatives to ‘communicate’. In my interactions, junior aircrew admitted that ‘ZIPLIP’ has a serious ring to it. To the uninitiated, it effectively means ‘no R/T’. To those who have ‘been there, done that’, it tacitly discourages R/T. But what are the dividends? Do they score over safety? Do we have a culture which will ensure that all participants, whether on ship or in the air, abide by a common safety paradigm? Perhaps transponders, data link, tactical messaging systems or, in the least, NOFIX procedures, are needed to enable ‘ZIPLIP’ without compromising on safety. Food for thought.

Let us not carry home the wrong message. The need to avoid frivolous / irrelevant exchanges on R/T is one of the basics taught during ab-initio flying training. It is known as R/T discipline. But, haven’t we all witnessed instances some time or the other when unrestrained R/T has been used in perfect CAVOK conditions to pass irrelevant information to and fro? If everyone involved in flying operations understands R/T discipline, the need for ‘ZIPLIP’ under all situations may even cease to exist. But looks like the boundary between relevant and irrelevant voice communication has been smudged somewhere down the line, contributing to the necessity to enforce ‘ZIPLIP’ across the board. In peacetime, shouldn’t safety of the aircraft take precedence over routine exercise requirements? During specific low-intensity maritime operations or in wartime, the advantages of R/T silence may actually outweigh the risks to achieve tactical surprise. But that’s another day, another time.

The technology-driven cat and mouse game will go on. The reality is that we may indeed have to adopt ‘ZIPLIP’ in some ‘avatar’ or the other. But we need adequate checks and balances to ensure that safety is not compromised. The risks involved should be understood and the aids / measures to alleviate those risks have to be firmly in place for the fleet and the fleet air arm to abide by. In future, data link rather than voice transmission will increasingly be used for air-to-ground and air-to-air communications; because in such cases, higher data rates can be used while at the same time reducing crew workload. Surely, in this age of technology, we should be able to work towards a ‘ZIPLIP’ procedure that is within the ambit of ‘maritime domain awareness’ while ensuring minimum risk consistent with mission accomplishment.

Some of you may probably opine that I am raising an alarmist note when modern navies have been practicing ‘ZIPLIP’ for years. Good for them. Give me their safety culture and aids. Then we will talk. I personally don’t like borrowing jeans that don’t fit me.

Ideas are welcome. In fact, if anyone knows the actual ‘ZIPLIP’ procedure and how it can be implemented safely across the fleet today, it would be a good idea to elaborate for everyone’s benefit. Me? When I am in doubt about safety, God bless super-refraction, I’ll go for the PTT.

I rest my case. My lips are zipped.

Postscript: Much has changed in the navy since 2007 when this article was first written. There is data link, Maritime Domain Awareness networks, Automatic Identification System, transponders etc. But so is Ziplip. Legacy platforms that haven’t undergone upgrades will still face the ominous silence of Ziplip. It is not a thing of the past yet. For example, why did we let the MF Beacon onboard ships die a natural death through disuse & neglect? It is a vital lifeline for offshore pilots, even those that fly modern, fully-automated helicopters. Food for thought?


©KP Sanjeev Kumar, 2019. All rights reserved. I can be reached at This article was originally written by the author for Indian Navy’s Flight Safety Journal ‘Meatball’ in 2007. Views are personal.

3 thoughts on “Ziplip Shiplip

  1. Sir,

    Archetypal to military operations, Ziplip/RT restrictions must be explicit in scope with adequately defined ocassions to revoke. A precdence may be available in under quoted analogous scenario during FARP (Forward Area Arming and Replenishment Point) operations, reproduced from a Tactical operations manual. Please note opening sentence and closing lines for key points:-

    ” 2-17. The use of radios must be kept to a minimum to reduce the enemy’s ability to target and engage electronic emissions. However, each FARP (active and silent) should have two FM radios capable of secure voice or secure data burst transmissions. This allows simultaneous monitoring of both the command and administrative and logistics networks. The internal network (alternate network) would provide FARP personnel with information about the current status of inbound aircraft and ammunition requirements. The command-designated network would provide information that may affect the FARP’s operation. Because FM radios are limited by line of sight and range, the distance and/or location of the FARP may prevent FARP personnel from monitoring and/or transmitting on the designated command frequency. In such cases, using aircraft as retransmission or relay is an option as long as the factors of METT-TC (= mission, enemy, terrain and weather, troops available, time available, civilian considerations) are considered. These radios should be used to transmit only when—
    •The FARP is under attack.
    •The FARP relocates or ceases operations.
    •The FARP is not operational at the scheduled time.
    •A request is made to resupply Class III/V products.
    •The status of the FARP changes. (In this case, the radio is used to report damage or contamination.) ”

    Apparently, relevant ecosystem, phase of operation and extent of desired control in that phase mandates a customised, flexible procedure precluding templated solutions.

    “Ziplip’ could then be effective without being otherwise !.


    1. Thanks Peeush for that insightful feedback. Out in the middle of the ocean, the thin line between safety and operational expediency sometimes gets blurred in our eagerness to stick to rules and protocols. Then there is the saying ‘loose lips sink ships’. As rightly pointed out by you, there cannot be a templated solution and one must be alive, alert and modulate our response based on the situation. I know of cases where the calm, soothing voice of a controller could have saved the crew’s life but for Ziplip. But that’s for another story. Thanks again and happy landings. Kps

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