The constellation
GPS satellites fly in six orbital planes tilted 55° to the equator, about 20,200 km up, and circle the Earth twice a day. Each one's antenna floods the whole visible Earth, out to its limb. Drag to rotate, and click the globe to move the ground point.
From the ground point
What a receiver at the ground point can see. Dilution of precision (DOP) turns the spread of satellites across the sky into a multiplier on ranging error: satellites bunched together make it large, satellites spread out make it small.
Sky plot
North up, zenith in the centre, horizon at the edge. Select a satellite to inspect its signal.
| PRN | Azimuth | Elevation | Range | Doppler (L1) | Received power | C/N₀ |
|---|
Signal generation, –
Everything a GPS satellite transmits is derived from one onboard atomic frequency standard at 10.23 MHz. The C/A code, the 50 bit/s navigation message and the L1 carrier are all integer ratios of it.
Reference clock
| Output | Ratio to 10.23 MHz | Nominal | With clock error | Error |
|---|
C/A code generator
Two 10-stage shift registers. G1 feeds back from stages 3 and 10, G2 from stages 2, 3, 6, 8, 9 and 10. Each satellite's code is G1's output XORed with two G2 stages chosen by its PRN (the taps). Both registers start at all ones, and the code repeats every 1023 chips (1 ms).
Output chips
Navigation message
Five 300-bit subframes of ten 30-bit words each, 6 s per subframe at 50 bit/s. Every subframe opens with the TLM word (preamble 10001011) and the HOW word (time of week). Subframes 1–3 carry this satellite's clock and ephemeris. Each word ends with 6 Hamming parity bits.
| Field | Value | Units |
|---|
Code ⊕ data, then BPSK onto L1
The code and the current navigation bit are combined modulo 2. The result flips the phase of the 1575.42 MHz carrier by 180° at each change. There are 1540 carrier cycles per C/A chip.
80 chips around now
L1 carrier at a phase reversal 3 ns window
About 4.7 carrier cycles in 3 ns. The carrier drops 180° at the chip edge (t = 0).
L1, L2 and L5
The same clock drives three carriers: L1 = 154 × 10.23 MHz, L2 = 120 ×, L5 = 115 ×. Each signal's spectrum is set by its chipping rate: 1.023 Mchip/s codes give a lobe 2 MHz wide, 10.23 Mchip/s codes give one 20 MHz wide.
L1 1575.42 MHz
L2 1227.60 MHz
L5 1176.45 MHz
Power spectral density at the minimum received power in IS-GPS-200 and IS-GPS-705 (Block IIF), in dBW/Hz. C/A uses this satellite's actual code. P(Y) is encrypted and the L2C and L5 codes are long, so random chips at the correct rate stand in for them; a spectrum depends only on the chipping rate and modulation, not on which code is sent. L1C (GPS III) is not shown.
Link budget, –
About 27 dBW leaves the satellite's antenna, and roughly 184 dB of it is lost across 20,000–26,000 km of free space. What reaches the ground is weaker than the thermal noise in the signal's own bandwidth, and the receiver pulls it out by despreading.
| Stage | dB | Note |
|---|
Signal vs noise in 2.046 MHz
Transmit power and antenna gains are typical values, adjustable in the side panel. The atmospheric loss grows as the satellite gets lower.
Doppler at the ground point
Each satellite moves at almost 4 km/s. The component toward or away from the receiver shifts the carrier by up to about ±5 kHz. It's largest near rise and set and passes through zero near closest approach.
L1 Doppler over the next 6 hours while each satellite is above the mask
Export I/Q for the receiver
Generate the digitized signal a receiver at the ground point would record: every satellite in view, at the C/N₀ from its link budget, mixed to a 1.25 MHz IF and quantized to 2 bits. Load the file into the GPS receiver simulator with its Load I/Q file button. It will acquire, track, decode the navigation messages built here, and fix a position at this ground point.
About 0.5 MB per second of signal at 2.048 Msps. The receiver needs roughly 36 s to collect three subframes from every satellite before its first fix.