Introduction
When a GNSS receiver is powered on, it faces a fundamental challenge: it does not know which satellites are visible, what frequency offset the satellite signal has due to Doppler shift, or where in the PRN code sequence the received signal currently is. The acquisition process solves this two-dimensional search problem. Once a signal is acquired, tracking loops maintain lock over time. Together, acquisition and tracking determine how quickly a receiver can produce its first position fix and how well it maintains performance in challenging environments.
The Acquisition Search Space
To acquire a GPS L1 C/A signal, a receiver must find the correct combination of:
- Code phase: The offset of the received PRN code relative to the locally generated replica, searched in steps of approximately half a chip across 1,023 possible chip positions (for GPS C/A code).
- Doppler frequency: The satellite radial velocity relative to the receiver creates a frequency shift of up to 5 kHz for GPS. The receiver searches Doppler bins typically 500 Hz wide across this range - approximately 20 bins.
The total search space for one satellite is therefore approximately 1,023 x 20 = 20,460 cells. A serial search testing one cell at a time with a 1 ms coherent integration would take over 20 seconds per satellite. Modern receivers use parallel correlator banks, FFT-based acquisition, or purpose-built ASICs to test all cells simultaneously, reducing acquisition time to milliseconds.
Coherent vs Non-Coherent Integration
Correlating the received signal against a local PRN replica over a 1 ms interval provides approximately 43 dB of processing gain - enough to detect GPS L1 C/A under open-sky conditions. For weaker signals (urban canyons, indoors), longer integration is needed:
- Coherent integration: Extending the coherent integration period beyond 1 ms is limited by navigation message data bit boundaries (20 ms for GPS C/A, 4 ms for Galileo E1) and by residual frequency uncertainty. Pilot channels on modern signals (GPS L5Q, Galileo E1C) carry no data bits, allowing coherent integration for seconds.
- Non-coherent integration: Squaring and summing multiple 1 ms coherent integration results avoids the data bit problem but introduces a squaring loss of approximately 3 dB. Non-coherent integration can extend to tens of seconds, enabling acquisition in weak signal environments.
Start Modes: Cold, Warm, and Hot
| Mode | What the Receiver Knows | Typical TTFF | Search Strategy |
|---|---|---|---|
| Cold Start | Nothing - no almanac, no position, no time | 30 to 60 s (open sky) | Search all PRNs, all Doppler bins, all code phases |
| Warm Start | Almanac and approximate position/time | 15 to 30 s | Predict visible satellites and expected Doppler; search fewer bins |
| Hot Start | Recent ephemeris, precise position, recent time | 2 to 5 s | Predicted code phase and Doppler within narrow window; minimal search |
Assisted GNSS (A-GNSS)
Assisted GNSS dramatically accelerates Time To First Fix (TTFF) by delivering almanac, ephemeris, approximate position, and precise time to the receiver via a network connection rather than downloading them from satellites. This is how smartphone GPS achieves sub-5-second first fixes that would otherwise require 30+ seconds of cold-start acquisition.
Two standards govern A-GNSS assistance data delivery:
- SUPL (Secure User Plane Location): Assistance data delivered over IP via a mobile network location server. Used by most smartphones.
- 3GPP LPP (LTE Positioning Protocol): Control plane assistance delivery, used in some cellular positioning architectures.
From Acquisition to Tracking
Once the acquisition correlator detects a signal above threshold (typically a correlation peak exceeding 3 to 4 sigma above the noise floor), the receiver hands off to tracking mode. The transition involves:
- Narrowing the code phase estimate using fine correlation.
- Initialising the Delay Lock Loop (DLL) and Phase Lock Loop (PLL) or Frequency Lock Loop (FLL).
- Synchronising to the navigation message frame boundary to enable data bit decoding.
- Decoding and validating the satellite ephemeris and clock parameters from the navigation message.
Only after ephemeris data is available can the receiver compute the satellite true position and use the pseudorange measurement for a position fix. This ephemeris download typically takes 30 seconds from a cold start - the dominant contributor to TTFF in the absence of A-GNSS assistance.
Summary
GNSS acquisition is a 2D search over code delay and Doppler frequency, solved today in milliseconds through parallel correlator architectures. Tracking maintains lock using feedback loops once acquisition succeeds. A-GNSS assistance eliminates the search bottleneck for smartphone receivers by delivering all necessary data over mobile networks. The tracking loops themselves - DLL, PLL, and FLL - are the subject of the following lesson.