Freelance Traveller

The Implications of Jump Detectability

by Garry Ward

This article was originally posted to the pre-magazine Freelance Traveller website in 2007, and reprinted in the July/August 2021 issue.

Across the years of playing Traveller, several aspects of Jump have come into question. I’ve put this together to consolidate into one spot the various aspects of Jump as I define it.

Observation #1: Jump is an effect, not a space.

Jump space may be referred to in the popular media, but those who understand the mathematics and mechanics behind it know that it is an effect, not a space. Someone referring to Jumpspace is demonstrating ignorance of the way the drives work.

Since Jump is an effect and not a space, it is the technique used to produce the effect that controls the size of the jump drive unit, the distance traveled, the duration of the travel and the power consumed. Different techniques can produce the same effect or better or worse effects.

Observation #2: Initiation and Termination Signatures

Whenever a jump initiates or terminates, there is a visible energy discharge. This discharge can be rated by using the dTonnage of the ship and the distance it is jumping. This rating is not a power value, that is, it is not in megawatts or megajoules, it simply quantifies the visible discharge.

From this rating an observer can potentially determine a range of possibilities covering either the size of the ship and or the destination of the ship, depending on what the observer may already know.

Example: A Class S scout initiates a jump. Given that an observer knows that it is a Class S scout initiating the jump, then if the Jump Signature (JS) rating is 600 (100 dTon times J6), the observer can presume that the destination of the scout is one of the stars that are 6 parsecs away. If there is only one star 6 parsecs away, the destination is obvious. If there are 3, then the observer must make a guess (as educated as possible, given the observer's knowledge of those stars) as to which of the 3 is the actual destination.

Example: An observer notes a Jump Signature of 600. The observer was unaware of any ship at that point on the surface of the Jump Sphere (see later). This JS can imply any of the following:

• A 100 dTon ship leaving or arriving from 6 parsecs away.
• A 200 dTon ship leaving or arriving from 3 parsecs away.
• A 300 dTon ship leaving or arriving from 2 parsecs away.
• A 600 dTon ship leaving or arriving from 1 parsec away.

Given this information and knowledge of the nearby stars (out to 6 parsecs), the observer can begin to make some educated guesses. If the observer has access to suitable sensors, they can scan the area of the JS to determine if a ship is currently present. However, the JS should be treated for at least a turn as Area Jamming (per TNE’s Brilliant Lances). If the sensor scan reveals a ship in that area and if the sensors can ascertain the dTonnage of the ship, then the observer can safely assume an arrival and using the dTonnage of the arrived ship to guess the origin.

If the scan reveals no ship in the area (which may be true or which may simply be that the sensor check roll was failed), the observer may assume that a departure occurred. In this case, the observer must list the potential set of dTonnages and distances that could produce the JS rating observed and given their knowledge of adjacent stars eliminate those that are inapplicable. Of the remaining, the observer must still make a final guess.

Observation #3: Every world is surrounded by a Jump Sphere.

A Jump Sphere is simply a theoretical boundary line demarking the set of common, safe points at which ships may initiate or terminate jump. Traveller rules have used 100 diameters from a world as the safe point where a ship may initiate or terminate Jump. In my context, I presume this to be from the center point of mass for the world. However, since this 100 diameters can be measured through any point of the surface of a planet, I realized that it is in effect the radius of theoretical sphere which, at any point along that surface, a ship may safely Jump.

Along with this realization, I noted that the Traveller world size codes come up short. From TNE page 181 I found that Small Gas Giants started at 40,000km diameter (20,000km radius) and that Large Gas Giants started at 120,000km diameter (60,000km radius). CT Book 6: Scouts, doesn’t provide a diameter range, just notes that to determine the orbital distance of a satellite from a LGG, roll 2D-4 (1 to 8) or for a SGG, roll 2D-6 (1 to 6).

Since the official size codes stopped at A, incremented at intervals of 1600km (Editor’s note: Originally, size codes were defined in thousands of miles, that is, a world of size 8 was about 8,000 miles in diameter), and in the TNE books, gave a range size that was that ranged +/- 800km, I could add 5 codes, B through F taking the average world sizes up to 24,000km. TNE gave the smallest radius of a LGG as equal to the largest radius of a SGG, setting an average radius for a LGG as (60+120)/2 or 90,000km radius or 180,000km diameter for figuring the average Jump Sphere. For a SGG that would be (20+60)/2 or 40,000km average radius or 80,000km average diameter.

From this, I could calculate the Jump Spheres as shown in Table 1.

Note that for an Earth-sized world (code 8), the surface of the Jump Sphere is 20.588 trillion (10¹²) square kilometers. Theoretically, anywhere on that surface is where a ship can either arrive or depart.

For the average size SGG, the Jump Sphere has a surface area of 804 trillion square kilometers.

Since any given point along this vast sphere is where a ship can arrive or depart, tracking them can become complicated for several reasons.

Range Effects

Between size code 7 and C worlds (which would include Earth-sized worlds), the surface of the Jump Sphere is within the Extreme range band of Passive EMS sensors (1.92 million km) which, per the Brillant Lances rules for difficulty by range would require a Impossible Task Roll for a sensor array on the planetary surface. Between size code 4 and size code 6 worlds, the Jump Sphere is within the Long range of passive EMS sensors, which makes Task Resolution Roll Formidable. Size Codes 2 and 3 worlds have a Sensor Task Resolution Roll of Difficult and size code 1 is Average. Worlds with size codes D, E, F as well as the SGG and LGG would have the Jump Sphere Surface beyond Extreme range.

The next complication is how much of the surface of the Jump Sphere the sensor covers. Table 2 is based on the assumption of a passive EMS array on a planetary surface whose cone of coverage is to 45 degrees to either side of the array center. Yes, I know Gas Giants don’t have surfaces, but this is theory. While the percentage of the surface of the Jump Sphere covered remains consistent at 25%, the square kilometers in that 25% increase radically. Given Traveller ship sizes, the total square kilometers of coverage be significant.

For example, one of my favorite ships is the Tigress, at 500,000 dTons. That sounds incredibly huge. 500,000 dTons translates to sphere with a diameter of 237.34 meters or a cross section of 44,241.69 square meters. This impressive number, however, is only 4.42% of a square kilometer. From a sensor array on the surface of a Size 1 world, a Tigress at that world’s Jump Sphere would present a target that represents zero point 000000000055% (fifty-five trillionths of one percent) of the area scanned.

It isn’t so much a matter of the ship being missed, but one of so much input information to work through that it just hasn’t been gotten to yet. Presuming that the computers associated with sensors are massively parallel processors helps, but still leaves the possibility that a ship could arrive and not be noticed for some time.

This has all been based on the presumption that we are dealing with a single passive EMS array on the planetary surface. Four of them will obviously cover 100% of the surface of the Jump Sphere. Eight of them will provide overlapping coverage and effectively reduce the amount of observational data that must be processed. However, in increasing the number of sensor arrays, we encounter the next issue: Politics and Economics.

Politics and Economics

On a highly populated world, with a single, world wide government, establishing a world wide base sensor stations to monitor the entire surface of the world’s Jump Sphere will be easy. Balkanize the world and even if there are sensor stations around the world, the chances are they will belong to different and possibly antagonistic governments who won’t exchange the information.

As population goes down, the chances of an extensive set of sensor stations in undeveloped regions goes down, again creating blind spots over the surface of the Jump Sphere. Granted, these blind spots will move as the world rotates, but they will still be present.

Obviously as tech level goes down, the type of sensors available will change and with that the ranges and difficulties will change.

Politics and economics also affects sensor coverage for Captive/Conquered worlds (Gov Code 6). While you would expect such worlds to have extensive sensor nets in order to counter any off world support of insurgencies, the economics of the capturing world may limit the funds available to establish said net. As the number of Captive/Conquered worlds under the control of another goes up, the further the funds to establish and run sensor nets must stretch. Hence the capabilities of the sensor net around a Captive/Conquered world will be in proportion to its importance to the capturing/conquering world.

Company/Corporate worlds will have sensor nets based on their economics. Sensor nets, as valuable as they may be still are indirect costs from an bookkeeping viewpoint, which means that a corporate entity will invest as little as possible. Fewer sensor installations with higher Task Resolution Rolls are the most likely.

Orbital Sensor Arrays and SDBs

All of this assumes that the sensor stations are on the surface of the world. At tech levels 9 or higher the idea of orbital sensor arrays becomes viable. Traveller has long included the concept of the System Defense Boat which combines the orbital sensor platform with offensive capability. A SDB, using its passive EMS array as it orbits a world, can cover an assigned portion of that world’s Jump Sphere. How effectively will depend on the range from which the SDB must operate and how many SDBs are involved.

For example, using the 400 dTon Dragon SDB, at TL 12, we can calculate how much of the Jump Sphere for various world sizes its passive EMS array can cover, given range and assumption that the sensor code covers a 45 degree arc from the ship’s center line.

From tables 1 and 3, we can construct Table 4 showing how many SDBs are needed to cover each standard sized world’s Jump Sphere to various degrees of safety and what the costs will be.

From Table 4, you can see that, for a size code 1 world to completely cover the Jump Sphere with short range passive EMS scanning would require 8 Dragon class SDBs at an initial cost of 1,332.72 MCr. On the presumption that annual ongoing costs are 10% of initial build, then what ever is on this size code 1 world needs to be worth 133.27 MCr per year to protect.

An Earth-sized world (Size code 8) would require 456 SDBs in orbit to cover its Jump Sphere with Short Range monitoring at an ongoing annual cost of 7,596.5 MCr. This, however, will mean that every square kilometer of the surface along which ships may arrive or leave will be watched with an Average sensor task roll to detect any ship. It also means that every square kilometer of this same surface will be under the converging fire zones of multiple SDBs, either with lasers or missiles.

As noted earlier, the local politics and economics will have a major effect on this coverage. For a single-government world with a robust local economy and large population, expending nearly 8 GCr a year on defending their Jump Sphere may be achievable. However, reduce the population or worsen the local economy and that cost can become a major drain.

Worse yet, balkanize the world. Now, every SDB in orbit isn’t just a planetary defense system to protect the Jump Sphere, it is also a weapons platform aimed back at the world below.

However you justify reducing the number of SDBs in orbit, as they are reduced, the effective range at which they need to monitor the surface of the Jump Sphere (unless you want to just accept larger and larger holes in the sensor coverage) goes up.

A Size 8 world with only 114 SDBs in orbit will have each one perform sensor checks at a Difficult rate. With only 29 SDBs in orbit, the sensor checks become Formidable and with only 8 SDBs in orbit, the sensor checks become Impossible.

A Gas Giant can be even more difficult to protect, regardless of the tradition of SDBs hiding in the upper reaches of the Gas Giant waiting for unsuspecting ships trying to refuel.

As you can see from the tables above, with 960,000 kilometers for extreme range on the passive EMS sensors and the SGG’s Jump Sphere distance at 8 million kilometers, nearly 10 times the extreme range, an SDB in the traditional location will have beyond Impossible rolls to detect arriving ships.

To cover an SGG’s Jump Sphere just within Extreme range sensor coverage would require 278 SDBs. To bring the sensor checks within average difficulty would require 17,778 SDBs in orbit around the SGG. If the government that wants to defend the SGG has 300 GCr a year to spend, it can be done.

Now, all pirates and smugglers have to do is pay attention to which worlds can’t afford to or won’t invest in protecting their Jump Spheres.