GLONASS Performances

Equivalent to the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS) of GPS, GLONASS provides a standard precision (SP) navigation signal and a high precision (HP) navigation signal, also referred to as Channel of Standard Accuracy (CSA) and Channel of High Accuracy (CHA), respectively.

However, as in GPS there is Standard Positioning Service Performance and the Precise Positioning Standard, in GLONASS there is no document specifying the performances provided by each service. According to Sergey Revnivykh, the deputy head of the GLONASS Mission Control Center, a GLONASS performance document will be released in the 2012-2013 time frame.

It has been stated that at peak efficiency, the SP signal offers horizontal positioning accuracy within 5–10 meters, vertical positioning within 15 meters, a velocity vector measuring within 10 cm/s, and timing within 200 ns. However these specifications are outdated, based on the performances provided by GLONASS in the year 2000, before starting the Modernization Plan.

Currently, accuracy comparisons provided by the Russian System of Differential Correction and Monitoring, show that GLONASS is slightly less accurate than GPS.

GLONASS Availability

The low number of operational satellites in the constellation (in 2001 there were only seven satellites) along with a ground segment limited to Russian territory, have been the main reasons of GLONASS poor availability performance. The modernization of the Ground Segment, including new monitoring stations (some of them outside Russia) and especially the increase of the number of satellites in the constellation (in the present, GLONASS constellation consists of 29 satellites in orbit although only 23 of them are operational), have led to almost worldwide coverage and 100% availability in the Russian territory.

The table included below shows the increasing of the number of satellites and the availability performance (calculated as the percentage of time during which the condition PDOP = 6 is valid at mask angles = 5 deg) from the beginning of 2007 to beginning 2012.

Date	Operational Satellites in constellation	Integral Availability Global	Integral Availability on Russian territory
2012/01/23	23	99.9 %	100 %
2012/01/01	24	100 %	100 %
2011/01/01	22	99 %	100 %
2010/01/01	15	73.6 %	85.3 %
2009/01/01	16	87.4 %	96.6 %
2008/01/01	12	48 %	57.6 %
2007/01/01	9	19.2 %	26.3 %

According with the above table, in January 23 there were 31 GLONASS satellites in orbit, being 23 of them in operational status. In the following figure is represented correspondent the worldwide Availability Performance (calculated as the percentage of time during which the condition PDOP = 6 is valid at mask angles = 5 deg) where can be seen the non-availability of the system in some regions of the globe.

Figure 1: Integral availability of GLONASS navigation (PDOP=6) during the 24 hours period (mask angle =5 deg) Date: 23.01.2012

GLONASS Accuracy

In the beginning of the century, one of the main objectives of the GLONASS program was to ensure performances similar to GPS by the end of 2011. GLONASS traditional poorer performance was the culmination of several factors, such as, poor on board atomic clocks or less accuracy in GLONASS broadcast ephemeris. The improvements carried out on the space, ground-based and user equipment segments have paid off, having recovered GLONASS. As it is shown in the figure below, in 2006 GLONASS accuracy at 1 sigma was in the order of 35 m, and in 2011 the accuracy of GLONASS reached less than 3 meters, hence similar to the GPS one.

Figure 2: GLONASS Accuracy Improvement

GLONASS Accuracy Comparison

With regard to GLONASS accuracy, according to the Russian System of Differentional Correction and Monitoring (SDCM), as of 23 January 2012, GLONASS horizontal precision is in the order of 4-7 m whereas the vertical error is in the order of 10-15 meters:

Station	GLONASS Error of navigation def. (p=0,95)			Mean number of NSV
	latitude (m)	longitude (m)	altitude (m)
Bellinsgauzen	4.80	5.23	11.44	8
Gelendzhik	5.60	6.28	14.08	8
Irkutsk	6.35	6.39	10.52	8
Kamchatka	5.73	5.25	12.72	8

Analyzing the accuracy obtained with GPS in the same stations it can be derived that GLONASS is slightly less accurate than GPS. In the same way, the mean number of GLONASS satellites in view is lower than GPS:

Station	GPS Error of navigation def. (p=0,95)			Mean number of NSV
	latitude (m)	longitude (m)	altitude (m)
Bellinsgauzen	2.74	2.70	8.09	11
Gelendzhik	2.46	2.09	5.62	10
Irkutsk	2.32	2.02	5.16	10
Kamchatka	3.13	2.05	6.10	10

Combined Services Performances

By combining GLONASS with other GNSS systems, such as GPS, GALILEO, COMPASS, SBAS and GBAS, improved performance in the following domains can be expected:

• Availability: Using as an example GLONASS in combination with GPS, the number of operational satellites will increase from 8-9 satellites to 18-19 (as it is shown in the table below). This is especially important in urban canyon environments, where the presence of large buildings leads to frequent shadowing of signal.
• Position Accuracy: Allied to an increased availability in restricted environments (urban) is a better geometry of spacecraft or enhanced positioning performance.
• Integrity: GNSS based integrity systems and techniques, such as SBAS, RAIM and GBAS, would benefit from the addition of new constellations, including GLONASS, in terms of lower achievable protection levels and/or integrity risk.
• Redundancy: Safety of Life applications require a full backup solution to be protected in the situation where the primary system fails. The combination of independent systems will lead to the required level of redundancy.

In the following table is depicted a comparison example of the navigation error (at 95% probability) provided by GLONASS only solution and GLONASS in combination with GPS, as well as the number of satellites in view in four different reference stations:

Station	Error of navigation def. (p=0,95)						Mean number of NSV
	latitude (m)		longitude (m)		altitude (m)
	GLONASS	GLONASS+GPS	GLONASS	GLONASS+GPS	GLONASS	GLONASS+GPS	GLONASS	GLONASS+GPS
Bellinsgauzen	4.80	2.69	5.23	2.29	11.44	6.26	8	19
Gelendzhik	5.60	2.83	6.28	2.60	14.08	6.86	8	18
Irkutsk	6.35	3.08	6.39	2.86	10.52	5.98	8	19
Kamchatka	5.73	3.03	5.25	2.40	12.72	6.07	8	18