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Continuity

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FundamentalsFundamentals
Title Continuity
Edited by GMV
Level Basic
Year of Publication 2011
Logo GMV.png

The continuity of a system is the ability of the total system (comprising all elements necessary to maintain craft position within the defined area) to perform its function without interruption during the intended operation. More specifically, continuity is the probability that the specified system performance will be maintained for the duration of a phase of operation, presuming that the system was available at the beginning of that phase of operation[nb 1].


Failures and Continuity at GNSS constellation level

The description of the type of failures and its impact on continuity has been adapted from the GPS Standard Positioning Service (SPS) Performance Standard[2].

The types of failures can be grouped as:

  • Hard Failures
  • Wear-Out Failures
  • Soft Failures
  • Satellite O&M Activities


Hard Failures

This description has been adapted from the GPS Standard Positioning Service (SPS) Performance Standard [2]

Satellites can suffer failures that result in the cessation of the GNSS signal transmissions. Such failures are known as "hard failures". The cessation of signal transmissions need not be sudden as a result of the hard failure, it can be gradual (e.g., a steady drop in transmitted signal power would be a gradual cessation). Some hard failures result in an immediate cessation of signal transmissions, while others result in a delayed cessation (e.g., if a satellite fails such that it can no longer accept new uploads of navigation message data, it will gracefully degrade until the signal becomes unavailable). Many different types of hard failures are possible.

Hard failures are subdivided into two main categories:

  • Long-term failures (LT) : LT hard failures are basically those failures which result in an irrecoverable loss of the signal from the satellite. The normal remedy for LT hard failures is the lengthy process of launching a replacement satellite.
  • Short-term failures (ST): ST hard failures result in only a temporary loss of signal from the satellite. The usual remedy for ST hard failures is the relatively rapid process of switching the satellite configuration over to using a redundant subsystem instead of the failed subsystem. All critical satellite subsystems have on-board redundancy.

Whether the hard failure of an otherwise functional satellite occupying a slot[3] results in a loss of continuity or not depends on the Control Segment issuing a warning in advance of the signal interruption. If the nature of the hard failure is such that the Control Segment issues the warning in advance of the interruption, then there is no loss of continuity. If the hard failure results in a sudden or rapid loss of the signal from the satellite such that the Control Segment cannot issue a warning in advance, then there is a loss of continuity.

An alternate means of avoiding a loss of continuity exists in those situations where a second satellite occupies the same slot in the constellation. So long as the second satellite does not cease its signal transmissions, the sudden or rapid loss of the signal in either of the two satellites occupying that slot does not cause a loss of continuity.


Wear-Out Failures

This description has been adapted from the GPS Standard Positioning Service (SPS) Performance Standard [2]

Satellites are subject to wear-out failures. Wear-out failures differ from hard failures in that wear-out failures are generally predictable (i.e., “schedulable”). Hard failures are generally not predictable. Wear-out failures are a characteristic of the satellite "end-of-life" (EOL) operating phase. They do not occur on recently launched satellites nor do they occur on satellites in the "middle age" operating phase. Wear-out failures are all ultimately LT failures, but it is frequently possible to prolong the usefulness of satellites in the EOL phase by the Control Segment expending substantial effort.

It is possible for a wear-out failure during the EOL phase to cause a loss of continuity, but this requires one of two unlikely errors on the part of the Control Segment. It is extremely improbable that the Control Segment would fail to predict the wear-out failure in advance. Much more probable, but still unlikely, the Control Segment could underestimate the effort needed to prolong the life of a satellite in the EOL phase. If the Control Segment chooses to prolong the life of an EOL satellite but later comes up short and cannot expend the necessary effort due to unforeseen circumstances, then -- unless a warning was issued -- a loss of continuity will occur.


Soft Failures

This description has been adapted from the GPS Standard Positioning Service (SPS) Performance Standard [2]

Integrity failures are known as "soft failures" in that while a failure has occurred, the GNSS signal continues to be available without an alert indication (alarm or warning) that the failure has occurred. Because the signal continues to be available to users, soft failures do not -- in and of themselves -- constitute a loss of continuity.

Although soft failures do not constitute a loss of continuity themselves, they can certainly trigger a loss of continuity. Certain soft failures are autonomously detectable on-board a satellite. If the satellite detects and reacts to that soft failure by transmitting an alert, it is actually the alert that makes the signal unavailable to users and thereby causes the loss of continuity. Soft failures are not predictable, so there is no way to issue a warning regarding them in advance. The same principle applies when the Control Segment detects and reacts to a soft failure. The loss of continuity occurs when the Control Segment reaction causes the signal to become unavailable to users without advance warning. This principle is similar to the one which applies to fault detection alerts issued by the RAIM algorithm in a GNSS receiver. A loss of continuity occurs if the fault cannot be excluded and a "do not use" alert is displayed to the user.

In the case of a loss of continuity triggered by a soft failure, the Control Segment will provide notification via a warning as soon as possible after the event.


Satellite O&M Activities

This description has been adapted from the GPS Standard Positioning Service (SPS) Performance Standard [2]

Certain types of routine satellite operations & maintenance (O&M) that are almost certain to cause a large User Range Error (e.g., station keeping maneuvers and atomic clock maintenance) are not commonly referred to as failures. However, from a strict integrity perspective, most of them do result in Misleading Signal-in-Space Information (MSI) and could thus be correctly classified as a form of failure. These types of "O&M-induced failures" are unique compared to all other failures in that they are planned in advance by the Control Segment. Being planned, the Control Segment can prepare for them and can take preemptive actions to ensure that any MSI the O&M activity causes is Alerted Misleading Signal-in-Space Information (e.g., by performing an upload prior to the start of the O&M to set the health status of the satellite to indicate the signal is unhealthy and thereby make the signal unavailable to users). Taking this preemptive action precludes Unalerted Misleading Signal-in-Space Information and thus prevents any impact to signal integrity. Although the preemptive action of taking the satellite off-line before the O&M activity is good for signal integrity, it is not necessarily good for signal continuity since it is an interruption in service that could potentially lead to a loss of continuity.

Since these "O&M-induced interruptions in service" are normally planned well in advance, the Control Segment can also take further preemptive actions to prevent any impacts to signal continuity. The typical required preemptive action is issuing a warning regarding the planned interruption in advance of the start of the signal scheduled outage period. From a strict continuity perspective, a warning only needs to be issued for a scheduled interruption affecting a satellite occupying a slot that is not backed up by a second satellite in the orbital slot (or location in the case of an expanded slot configuration).

The Control Segment can easily cause a loss of continuity of the signal by failing to issue the required warning in advance of the scheduled interruption in service. Such a loss of continuity is considered to be reasonably probable.


Measuring the Continuity of the Navigation Solution

The previous section focus on the continuity of the signal in space and constelation continuity. This will have an impact on the continuity of the navigation solution which constitutes a quantitative estimate whether the navigation solution can be computed without interruption[4]

The continuity is usually measured as the probability that the system performance is kept under the operational requirements during a certain amount of time.

Therefore common Continuity measurements are:

  •  %/h: Probability that the operational performance is kept over a one hour period.
  •  %/15s: Probability that the operational performance is kept over a fifteen seconds period.


Notes

  1. ^ This definition was adapted from the 2008 US Federal Radionavigation Plan[1]

References

  1. ^ US Federal Radionavigation Plan, DOT-VNTSC-RITA-08-02/DoD-4650.5, 2008
  2. ^ a b c d e GPS Standard Positioning Service (SPS) Performance Standard, 4th Edition, September 2008.
  3. ^ The orbital planes of a constellation are usually divided in slots that should contain at least one operational satellite
  4. ^ [http://www.gmat.unsw.edu.au/wang/jgps/v3n12/v3n12p29.pdf A Performance Analysis of Future Global Navigation Satellite Systems, C. Seynat, A. Kealy, K. Zhang, Journal of Global Positioning Systems (2004), Vol. 3, No. 1-2: 232-241