GNSS - Receiver Data and Parameters

[SV Navigation Data]

50Hz data, SV almanac, SV ephemeris, SV clock error, GPS time to UTC, ionospheric model parameter, user range accuracy(URA)

[Receiver Start-Up]

• To get a fix, the receiver needs a valid almanac, initial receiver location, time, and ephemeris data.

• Cold/warm/hot start and reacquisition: depending on how many valid start-up data the receiver has

[Almanac]

• SV's coarse orbit, status information, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time (UTC).

• Valid for up to 180 days (6 months)

• Is used to find SVs currently overhead shortening acquisition time. In older models almanac is required to acquire SVs. Newer models acquire SVs w/o waiting for almanac.

• A valid almanac is required for booting up in warm/hot start.

• Takes 15 minutes for complete reception of almanac data from SV (for a brand new receiver w/o previous almanac data

• Execution of a cold start automatically results in a new almanac download.

• Stored in non-volatile memory

• An example of almanac

ALMANAC FOR SATELLITE 2 :

PRN number for data ............. 2

Health of SV .................... 0

Reference Week of Almanac ....... 797

Eccentricity .................... 0.0139475

Corr: inclination angle (rad) ... 0.00254631

Mean Anomaly @ ref time (rad) ... -1.04289

Argument of Perigee (rad) ....... -2.56822

Rate right ascension (rad/sec) .. -8.08034E-09

Right ascension @ ref time (rad) 1.74861

Sqrt semi-major axis (m^1/2) .... 5153.62

Clock correction term 1 ......... -0.00015831

Clock correction term 2 ......... -3.63798E-12

Reference time almanac .......... 466944

Semi-Major Axis (meters) ........ 2.65598E+07

Corrected Mean Motion (rad/sec) . 0.000145858

Inclination angle (rad) ......... 0.950477

[Ephemeris]

• Used to precisely calculate the position of the satellite.

• Updated every 2 hrs. Valid for 4 hrs.

• Each SV broadcast its own ephemeris data.

• Contains precision corrections to almanac data.

• Is required for accurate positioning.

• Without valid ephemeris data, it takes roughly 18 to 30 seconds to extract the ephemeris data from SV navigational information. -> Roughly 35 seconds are required for TTFF w/o valid ephemeris.

• An example of ephemeris data

EPHEMERIS FOR SATELLITE 2 :

PRN number for data .................. 2

Issue of ephemeris data .............. 224

Semi-Major Axis (meters) ............. 2.65603E+07

C(ic) (rad) .......................... 1.88127E-07

C(is) (rad) .......................... -1.00583E-07

C(rc) (meters) ....................... 321.656

C(rs) (meters) ....................... 87.6875

C(uc) (rad) .......................... 4.36418E-06

C(us) (rad) .......................... 2.70829E-06

Mean motion difference (rad/sec) ..... 5.04521E-09

Eccentricity (dimensionless) ......... 0.0139305

Rate of inclination angle (rad/sec) .. 4.11089E-10

Inclination angle @ ref. time (rad) .. 0.950462

Mean Anomaly at reference time (rad) . -2.62555

Corrected Mean Motion (rad/sec) ...... 0.000145859

Computed Mean Motion (rad/sec) ....... 0.000145854

Argument of perigee (rad) ............ -2.56865

Rate of right ascension (rad/sec) .... -8.43857E-09

Right ascension @ ref time (rad) ..... 1.75048

Sqrt (1 - e^2) ....................... 0.999903

Sqr root semi-major axis, (m^1/2) .... 5153.67

Reference time ephemeris (sec) ....... 240704

CLOCK FOR SATELLITE 2 :

PRN number for data ......... 2

Week number...... ........... 797

Predicted user range accuracy 32

Health of satellite ......... 0

L1 - L2 Correction term ..... 9.31323E-10

Issue of clock data ......... 224

Time of clock data .......... 240704

Clock offset ................ -0.000158074

Clock drift ................. -2.50111E-12

Rate of clock drift ......... 0

[Receiver's Rough Position]

• Rough estimate of the receiver location is required to calculate Doppler frequency shift of each SV signal thus narrowing the frequency search interval.

[Receiver's Clock (Time)]

• GPS time: derived from an ensemble of Cesium clocks in Colorado

• UTC time: maintained at US Naval Observatory in Washington D.C., in agreement with UTC by +-10ns

• Knowing receiver time reduces the acquisition time.

[Doppler Effect]

Carrier Doppler: carrier frequency change.

Code Doppler: The Doppler effect that modifies the received satellite carrier frequency also affects the transmission sidebands that contain the code. The effective code clocking frequency is modified by the Doppler effect just as is the carrier, but linearly scaled by frequency (that is, for the P-code, 10.23/1575.42 = 1/154).

[Coherent and Non-coherent Integration Intervals]

The correlator count accumulation is a coherent integration process that generates a signed integer that is a measure of the code match in the presence of noise and jammer. The great advantage of this process is that the expected value of the count contribution from jammers and noise is zero. The count variance from noise and jammer is greater than zero. A long coherent integration is desirable to reduce noise and jammer variance (i.e., increase signal SNR). However, the interval is limited by error in the carrier phase reference. Non-coherent accumulations using the absolute values of shorter coherent integrations (count accumulations) make the process more carrier phase reference-tolerant at the expense of reduced SNR gain. The selection of coherent and non-coherent intervals is a compromise to balance these factors

[Code Smearing]

The received GPS signal is buried in thermal noise and, if present, jammers. The signal must be integrated over an extended period of time (up to a half second) to assure signal detection in these conditions. The projectile and the GPS satellite are in motion during this interval. Therefore the arrival state of the code is also in motion (temporally) during the signal detection interval. The receiver's replica code generator must be appropriately slewed to match this motion during the signal detection. Otherwise the code alignment point will move (smear) in the correlator accumulators.

[Image Rejection]

The mixing operation in RF down conversion is double sided. The intermediate frequency (IF) is the difference between the RF input frequency and the local oscillator frequency (LO). Say the desired RF signal is at the LO IF; then the image frequency is at the LO - IF. Image rejection is the amount that RF filtering suppresses the receiver's response at the image frequency.

[In-Band Spurious Response]

The mixing operation in RF down conversion requires the generation of a local oscillator signal. The mixing operation itself generates sum and difference frequencies of the RF and LO applied. The down conversion frequency plan and implementation must assure that spurious frequencies generated in this process do not fall in the IF passband. If they do, the result is the equivalent of a built-in CW jammer.

[Correlation Integration Period]

The received GPS signal is correlated with a local replica code generating a correlator count of code state matches minus mismatches. The count value is periodically sampled and the counter reset. The integration period is the interval between count samples.

[Latency of IMU Data]

The effect of vehicle motion dynamics on the received satellite signal and on the IMU sensor is instantaneous. The processing and transport time to get the IMU sensor information regarding the vehicle dynamics to the GPS tracking loops is the latency. If the latency is too large, then the effects of the vehicle dynamics on the satellite signal will have already caused the carrier tracking loop to slip cycles or drop lock before the IMU aiding data that could have prevented it can reach the loop.

[Tacking Loop Bandwidth]

This refers to the code and carrier tracking loops. These loops operate as lowpass filters with the tracking of the loop intended to keep the signal in the lowpass filter passband. The tracking loop bandwidth is the effective noise bandwidth of this lowpass filter.