“Standard” Ambiguity Resolution
For both navigation and timing applications it is essential for a GPS receiver to calculate the distance from the receiver to the satellite. This is achieved by simply calculating the time taken for the signal to reach the receiver from the satellite and multiplying it by the speed of light.
The time of receipt is a simple measurement performed by the receiver. Satellites transmit timestamps in each sub-frame of data. For a crude measurement of Time of transmission the receiver could rely solely on these timestamps for the calculation – this is often the case for navigation applications. However, for accurate timing applications it is essential for a GPS receiver to resolve time to the millisecond.
As the instant of measurement actually occurs anywhere between sub-frame timestamps, it is necessary to “count the bits” between the sub-frame boundary and the instant of measurement. As the sub-frame data is clocked out at a known rate it is easy to establish the time from the sub-frame boundary, Tx. So to resolve the true Time of Transmission to the measurement instant simply add Tx to the sub-frame timestamp – this is known as ambiguity resolution or “resolving the millisecond ambiguity”
So as long as you can extract the data signal ambiguity can be resolved. But what happens in weak signal environments (below -142 dBM) where this data can’t be extracted? – then you need SigNav’s Advanced Ambiguity Resolution (AAR).
Advanced Ambiguity Resolution
Many receivers claim to be able to provide timing with as little as one weak signal satellite visible. However what they don’t say is these receivers need at least 1 strong signal satellite visible (above -142 dBm) AND be programmed with a position accurate to 75km for the receiver to start to operate when it is first installed.
In contrast, through patented signal processing schemes SigNav’s Advanced Ambiguity Resolution can both start AND operate in exclusively in weak signal environmets even if its position is unknown.
Satellites & Signal Strengths required for Start & Sync.
| Programmed Position Accuracy | subATTO AAR Receivers | Other Receivers |
|---|---|---|
| PA < 3 km | 1 x Weak | 1 x Strong |
| 3 km < PA < 75 km | 4 x Weak | 1 x Strong + 3 x Weak |
| PA >75 km | 4 x Weak | 4 x Strong |
Note:
- Weak Satellite Signal < -142 dBm
- Strong Satellite Signal > -142 dBm
But what does this mean in the real world?
When shipping low cost products in large volumes, the last thing manufacturers want is to provide complex installation procedures, individually configure each unit before shipping or have to provide significant amounts of support to install a product – any one of these scenarios will quickly evaporate slim consumer product margins, rapidly spirally a company towards significant losses.
AAR has been specifically designed to cater for consumer applications. It offers low complexity, end user installation. Receivers only need to know which continent they are on to start to operate (rather than in which city). They can start in a wider range of environments increasing your products market penetration by up to 20%. Operating in more environments and being easier to install leads to less support calls, greater customer satisfaction and of course greater profits.